Chemistry Quiz: Design Chemical Change Investigations
20 questions · exam conditions
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Design Chemical Change InvestigationsQuestion 1 of 20

A teacher demonstrates that steel wool can gain mass after being left in air for a week. Students want to design an experiment to test whether the mass increase is due to a chemical change (reaction with oxygen) and whether moisture affects the change. Which investigation design is best?

Leave steel wool in different places around the room and compare them after a week; do not measure mass because rust can be seen.
Place equal masses of steel wool in two labeled containers: one with dry air (with a drying agent) and one with moist air (a small cup of water) (independent variable: moisture). Keep container size, steel wool mass, and time the same (controlled variables). Measure mass at the start and at set times, and record color/texture changes. Include multiple trials.
Spray steel wool with perfume and, if the smell changes over time, conclude oxygen reacted chemically with it.
Heat steel wool strongly to make it react faster, then use a pH probe to measure the pH of the steel wool to confirm a chemical reaction.
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Chemistry Quiz

Chemistry Quiz: Design Chemical Change Investigations

Practice Design Chemical Change Investigations in Chemistry with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Design Chemical Change Investigations, giving you a quick way to practice the rules, question types, and explanations that matter most for Chemistry.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

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Question 1

A teacher demonstrates that steel wool can gain mass after being left in air for a week. Students want to design an experiment to test whether the mass increase is due to a chemical change (reaction with oxygen) and whether moisture affects the change. Which investigation design is best?

  1. Leave steel wool in different places around the room and compare them after a week; do not measure mass because rust can be seen.
  2. Place equal masses of steel wool in two labeled containers: one with dry air (with a drying agent) and one with moist air (a small cup of water) (independent variable: moisture). Keep container size, steel wool mass, and time the same (controlled variables). Measure mass at the start and at set times, and record color/texture changes. Include multiple trials. (correct answer)
  3. Spray steel wool with perfume and, if the smell changes over time, conclude oxygen reacted chemically with it.
  4. Heat steel wool strongly to make it react faster, then use a pH probe to measure the pH of the steel wool to confirm a chemical reaction.
Explanation: This question tests your ability to design scientific investigations that test whether chemical changes occur, including identifying variables, planning appropriate observations and measurements, and ensuring fair testing with controls. Designing an investigation to test for chemical change requires four key elements: (1) A clear testable question (Does moisture affect steel wool's reaction with oxygen?), (2) Identification of variables—what you'll change (independent: moisture level), what you'll measure or observe (dependent: mass increase, color changes), and what you'll keep constant (controlled: steel wool mass, time, containers), (3) A safe, feasible procedure with clear steps that produce observable evidence, (4) A plan for what evidence to collect—which observations or measurements will answer your question. This systematic approach ensures your investigation actually tests what you want to know! The setup compares dry and moist conditions to link mass gain to rusting. Choice B provides complete investigation design with clear variables, appropriate controls, feasible procedure, and evidence collection plan that addresses the testable question. Choices A, C, and D lack measurements, use irrelevant perfume, or improper pH testing. The investigation design recipe: (1) STATE THE QUESTION: 'Does moisture cause chemical rusting?' (2) IDENTIFY VARIABLES: Independent (moisture), Dependent (mass, texture), Controlled (masses, time). (3) OUTLINE PROCEDURE: Expose in containers, measure. (4) EVIDENCE PLAN: Record changes over time. Fair testing: identical setups except moisture ensure valid conclusions—well done!

Question 2

A student adds a teaspoon of table salt (NaCl) to 100 mL of water and says, "The salt disappeared, so it must have undergone a chemical change." Your teacher asks you to design an investigation to test whether dissolving salt in water is a chemical change or a physical change. Which investigation design best answers the testable question using observable evidence and a fair comparison?

  1. Stir the salt into water and decide it is a chemical change if the water tastes salty; do not take any measurements.
  2. Measure the mass of an empty evaporating dish, then add the salt solution and evaporate the water gently (e.g., on a warm hot plate). Compare the mass and appearance of the recovered solid to the original salt. Independent variable: whether water is removed; dependent variable: mass/identity (appearance) of recovered solid; controls: same starting salt mass and water volume; evidence: recovery of the original solid with no new substances observed. (correct answer)
  3. Heat the salt solution until it boils vigorously and record the highest temperature reached to prove a chemical reaction occurred.
  4. Add vinegar to the salt water and look for bubbles; if bubbles form, conclude dissolving salt was a chemical change.
Explanation: This question tests your ability to design scientific investigations that test whether chemical changes occur, including identifying variables, planning appropriate observations and measurements, and ensuring fair testing with controls. Designing an investigation to test for chemical change requires four key elements: (1) A clear testable question (Does mixing A and B cause a chemical reaction?), (2) Identification of variables—what you'll change (independent: substance type, temperature, concentration), what you'll measure or observe (dependent: temperature change, gas production, color change), and what you'll keep constant (controlled: volumes, time, equipment), (3) A safe, feasible procedure with clear steps that produce observable evidence, (4) A plan for what evidence to collect—which observations or measurements will answer your question. This systematic approach ensures your investigation actually tests what you want to know! In this case, the investigation should test if dissolving salt is chemical by attempting to recover the original salt through evaporation, with variables like whether water is removed (independent), mass and appearance of recovered solid (dependent), and controls like starting masses and volumes, including a procedure to measure masses before and after evaporation. Choice B provides complete investigation design with clear variables, appropriate controls, feasible procedure, and evidence collection plan that addresses the testable question. Choices A, C, and D fail because A lacks measurements and fair testing, C measures irrelevant boiling temperature without recovery evidence, and D introduces an unrelated substance (vinegar) without addressing the dissolving process. The investigation design recipe: (1) STATE THE QUESTION clearly: What are you testing? Be specific—'Does dissolving salt in water produce new substances?' not just 'What happens?' (2) IDENTIFY VARIABLES: Independent variable (what you'll change—make it ONE thing to change so you know what caused effects), Dependent variable (what evidence you'll collect—temperature? color? gas? be specific), Controlled variables (list 3-5 things you'll keep exactly the same—amounts, time, temperature, equipment). (3) OUTLINE PROCEDURE: Simple steps that safely produce the evidence you need. Usually: mix or treat substances, observe during and after, record specific measurements or observations. (4) EVIDENCE PLAN: Exactly what will you measure (temperature with thermometer before and after) or observe (color change—describe initial and final colors; gas production—count bubbles or note vigorous fizzing). The design is complete when someone else could follow it and get the same results! Fair testing through controls: imagine you're testing whether temperature affects reaction between vinegar and baking soda. If you use different amounts of vinegar at different temperatures, you won't know if changes come from temperature or amount—two variables changed! Fair test: same volumes (50mL vinegar, 5g baking soda) at different temperatures (10°C, 25°C, 40°C), measure fizzing time as dependent variable. Now temperature is the ONLY thing different, so any differences in fizzing time must come from temperature. Controls make results interpretable—without them, you can't draw conclusions!

Question 3

A teacher demonstration shows a strip of magnesium placed into vinegar. Students disagree about whether any chemical change occurred because the magnesium "just gets smaller." Which investigation design best tests for chemical change with clear evidence and controlled variables?

  1. Place magnesium into vinegar and watch; if it disappears, it was chemical. Do not measure anything because the result is obvious.
  2. Place magnesium into vinegar and into water, but use different sizes of magnesium in each cup to make the reaction easier to see.
  3. Set up two trials: (1) 25 mL vinegar + a measured mass of magnesium ribbon (e.g., 0.20 g), (2) 25 mL water + 0.20 g magnesium as a control. Independent variable: liquid type (vinegar vs water); dependent variables: gas production (bubble rate or gas captured), mass of magnesium remaining after a set time, and temperature change. Controlled variables: magnesium mass/surface area, liquid volume, container type, starting temperature, timing. Repeat trials. (correct answer)
  4. Test the vinegar with pH paper before and after, and if pH stays the same, conclude no chemical change occurred.
Explanation: This question tests your ability to design scientific investigations that test whether chemical changes occur, including identifying variables, planning appropriate observations and measurements, and ensuring fair testing with controls. Designing an investigation to test for chemical change requires four key elements: (1) A clear testable question (Does mixing A and B cause a chemical reaction?), (2) Identification of variables—what you'll change (independent: substance type, temperature, concentration), what you'll measure or observe (dependent: temperature change, gas production, color change), and what you'll keep constant (controlled: volumes, time, equipment), (3) A safe, feasible procedure with clear steps that produce observable evidence, (4) A plan for what evidence to collect—which observations or measurements will answer your question. This systematic approach ensures your investigation actually tests what you want to know! This setup compares vinegar to water with magnesium, using variables like liquid type (independent), gas production and mass remaining (dependent), and controls like masses and volumes, with timed repeats. Choice C provides complete investigation design with clear variables, appropriate controls, feasible procedure, and evidence collection plan that addresses the testable question. Choices A, B, and D fail because A lacks measurements and controls, B uses inconsistent sizes, and D measures pH without direct evidence of change. The investigation design recipe: (1) STATE THE QUESTION clearly: What are you testing? Be specific—'Does magnesium react chemically with vinegar?' not just 'What happens?' (2) IDENTIFY VARIABLES: Independent variable (what you'll change—make it ONE thing to change so you know what caused effects), Dependent variable (what evidence you'll collect—temperature? color? gas? be specific), Controlled variables (list 3-5 things you'll keep exactly the same—amounts, time, temperature, equipment). (3) OUTLINE PROCEDURE: Simple steps that safely produce the evidence you need. Usually: mix or treat substances, observe during and after, record specific measurements or observations. (4) EVIDENCE PLAN: Exactly what will you measure (temperature with thermometer before and after) or observe (color change—describe initial and final colors; gas production—count bubbles or note vigorous fizzing). The design is complete when someone else could follow it and get the same results! Fair testing through controls: imagine you're testing whether temperature affects reaction between vinegar and baking soda. If you use different amounts of vinegar at different temperatures, you won't know if changes come from temperature or amount—two variables changed! Fair test: same volumes (50mL vinegar, 5g baking soda) at different temperatures (10°C, 25°C, 40°C), measure fizzing time as dependent variable. Now temperature is the ONLY thing different, so any differences in fizzing time must come from temperature. Controls make results interpretable—without them, you can't draw conclusions!

Question 4

A student is investigating whether exposure to air causes sliced apple to undergo a chemical change (browning), and whether lemon juice slows that change. This matters for food quality in a culinary class. Which investigation design best tests the effect of lemon juice on the chemical change while keeping a fair test?

  1. Put lemon juice on one apple slice and nothing on another, but use slices from different apples and leave them for different amounts of time; judge the result from memory.
  2. Cut one apple into equal-sized slices. Assign slices to groups: no treatment (control), water (comparison control), and lemon juice. Independent variable: treatment type; dependent variable: degree of browning after a fixed time (use a color chart or take photos under the same lighting and score darkness). Controlled variables: slice size/thickness, time exposed to air, temperature, volume of liquid applied, lighting for observations. Use multiple slices per group and record data at set time intervals. (correct answer)
  3. Measure the mass of the apple before and after adding lemon juice; if mass changes, browning is chemical.
  4. Freeze the apple slices first to stop all changes, then add lemon juice and decide whether browning would have happened at room temperature.
Explanation: This question tests your ability to design scientific investigations that test whether chemical changes occur, including identifying variables, planning appropriate observations and measurements, and ensuring fair testing with controls. Designing an investigation to test for chemical change requires four key elements: (1) A clear testable question (Does mixing A and B cause a chemical reaction?), (2) Identification of variables—what you'll change (independent: substance type, temperature, concentration), what you'll measure or observe (dependent: temperature change, gas production, color change), and what you'll keep constant (controlled: volumes, time, equipment), (3) A safe, feasible procedure with clear steps that produce observable evidence, (4) A plan for what evidence to collect—which observations or measurements will answer your question. This systematic approach ensures your investigation actually tests what you want to know! The design uses equal apple slices with treatments including controls, variables like treatment type (independent), browning degree (dependent), and controls like size and time, with timed data collection. Choice B provides complete investigation design with clear variables, appropriate controls, feasible procedure, and evidence collection plan that addresses the testable question. Choices A, C, and D fail because A lacks consistency and measurements, C measures irrelevant mass, and D uses freezing without testing the actual condition. The investigation design recipe: (1) STATE THE QUESTION clearly: What are you testing? Be specific—'Does lemon juice prevent chemical browning in apples?' not just 'What happens?' (2) IDENTIFY VARIABLES: Independent variable (what you'll change—make it ONE thing to change so you know what caused effects), Dependent variable (what evidence you'll collect—temperature? color? gas? be specific), Controlled variables (list 3-5 things you'll keep exactly the same—amounts, time, temperature, equipment). (3) OUTLINE PROCEDURE: Simple steps that safely produce the evidence you need. Usually: mix or treat substances, observe during and after, record specific measurements or observations. (4) EVIDENCE PLAN: Exactly what will you measure (temperature with thermometer before and after) or observe (color change—describe initial and final colors; gas production—count bubbles or note vigorous fizzing). The design is complete when someone else could follow it and get the same results! Fair testing through controls: imagine you're testing whether temperature affects reaction between vinegar and baking soda. If you use different amounts of vinegar at different temperatures, you won't know if changes come from temperature or amount—two variables changed! Fair test: same volumes (50mL vinegar, 5g baking soda) at different temperatures (10°C, 25°C, 40°C), measure fizzing time as dependent variable. Now temperature is the ONLY thing different, so any differences in fizzing time must come from temperature. Controls make results interpretable—without them, you can't draw conclusions!

Question 5

A student drops an effervescent antacid tablet into water and observes fizzing. They want to investigate whether changing water temperature affects how fast the chemical change happens. Which investigation design best tests the rate question with appropriate variables and controlled conditions?

  1. Testable question: "How does water temperature affect the reaction rate of an antacid tablet in water?" Independent variable: water temperature (e.g., 10°C, 25°C, 40°C). Dependent variable: time for fizzing to stop (s) or time to dissolve completely (s). Controlled variables: water volume (mL), tablet brand/mass, container type, stirring method (none or constant), and starting tablet condition (whole). Do multiple trials at each temperature and compare average times. (correct answer)
  2. Use hot water for one trial and cold water for another; if hot water fizzes faster, conclude temperature causes a chemical change, without measuring time.
  3. Change both the tablet brand and the water temperature at the same time; record whichever trial looks most exciting as the fastest.
  4. Crush the tablet in one cup and leave another tablet whole in a different cup at the same temperature; if the crushed one reacts faster, conclude temperature is the cause.
Explanation: This question tests your ability to design scientific investigations that test whether chemical changes occur, including identifying variables, planning appropriate observations and measurements, and ensuring fair testing with controls. Designing an investigation to test for chemical change requires four key elements: (1) A clear testable question (Does mixing A and B cause a chemical reaction?), (2) Identification of variables—what you'll change (independent: substance type, temperature, concentration), what you'll measure or observe (dependent: temperature change, gas production, color change), and what you'll keep constant (controlled: volumes, time, equipment), (3) A safe, feasible procedure with clear steps that produce observable evidence, (4) A plan for what evidence to collect—which observations or measurements will answer your question. This systematic approach ensures your investigation actually tests what you want to know! Vary water temperatures, control tablet mass and volume, measure fizzing or dissolution time to assess temperature's effect on reaction rate. Choice A provides a complete investigation design with a testable question on rate, independent variable (temperature levels), dependent variable (time to stop fizzing), controlled variables (volume, mass, stirring), multiple trials, and average comparisons. Choices B, C, and D lack rigor: B skips measurements, C changes multiple variables, and D tests crushing instead of temperature. Recipe: question temperature's impact, change it systematically, time reactions precisely, control other factors, outline procedures, and average data—terrific! Controls ensure time differences are from temperature alone; varying extras confounds results—keep going, you're a design expert!

Question 6

A metal paperclip is left in tap water for several days and develops an orange-brown coating. The class wants to investigate whether this change is chemical (rusting) and whether salt water changes the amount of rust formed. Which experimental design best tests this with clear variables, controls, and evidence?

  1. Place identical paperclips in equal volumes (100 mL) of tap water and salt water (independent variable: salt concentration), in identical cups, for the same time. Keep temperature and exposure to air the same (controlled variables). Measure rusting by recording mass change of the dried paperclips and/or rating color coverage from photos taken daily. Include multiple trials per condition. (correct answer)
  2. Put one paperclip in salt water and one in tap water, but check them whenever you remember; if one looks rustier, conclude salt causes a chemical change.
  3. Scrape the rust off and identify it using a high-end spectrometer to prove it is a new substance.
  4. Heat paperclips in a flame, then put them in water to see if rust forms faster; measure the flame color as the dependent variable.
Explanation: This question tests your ability to design scientific investigations that test whether chemical changes occur, including identifying variables, planning appropriate observations and measurements, and ensuring fair testing with controls. Designing an investigation to test for chemical change requires four key elements: (1) A clear testable question (Is rusting chemical, and does salt affect it?), (2) Identification of variables—what you'll change (independent: salt presence), what you'll measure or observe (dependent: mass change, color coverage), and what you'll keep constant (controlled: volumes, time, temperature), (3) A safe, feasible procedure with clear steps that produce observable evidence, (4) A plan for what evidence to collect—which observations or measurements will answer your question. This systematic approach ensures your investigation actually tests what you want to know! The design compares tap and salt water with consistent setups to quantify rust. Choice A provides complete investigation design with clear variables, appropriate controls, feasible procedure, and evidence collection plan that addresses the testable question. Choices B, C, and D lack consistency, require advanced tools, or use irrelevant heating. The investigation design recipe: (1) STATE THE QUESTION: 'Does salt increase rusting?' (2) IDENTIFY VARIABLES: Independent (salt concentration), Dependent (mass, photos), Controlled (paperclips, time). (3) OUTLINE PROCEDURE: Expose, measure daily. (4) EVIDENCE PLAN: Record changes precisely. Fair testing: identical conditions except salt isolate its effect—superb design!

Question 7

Two unlabeled white powders in the lab are suspected to be either baking soda (sodium bicarbonate, NaHCO3_3) or powdered sugar. A teacher wants students to determine whether each powder undergoes a chemical change when mixed with vinegar (acetic acid solution). Which experimental design best tests the question using appropriate variables, controls, and evidence collection?

  1. Add vinegar until "something happens" for each powder, but use different amounts of powder and vinegar each time; decide the more dramatic sample is the chemical change.
  2. Put 1 g of each powder into separate test tubes (independent variable: powder type), add 10 mL of vinegar to each (controlled volume), and observe for gas production (bubbling/foaming), temperature change (thermometer), and formation of any new solid. Include a control tube with 10 mL vinegar only. Record observations and repeat trials. (correct answer)
  3. Dissolve both powders in water first, then taste them to identify which one reacted chemically with vinegar.
  4. Measure the mass of the powders only, without mixing them with vinegar, to determine which one would react chemically.
Explanation: This question tests your ability to design scientific investigations that test whether chemical changes occur, including identifying variables, planning appropriate observations and measurements, and ensuring fair testing with controls. Designing an investigation to test for chemical change requires four key elements: (1) A clear testable question (Does each powder react chemically with vinegar?), (2) Identification of variables—what you'll change (independent: powder type), what you'll measure or observe (dependent: gas production, temperature, new solid), and what you'll keep constant (controlled: powder mass, vinegar volume, tubes), (3) A safe, feasible procedure with clear steps that produce observable evidence, (4) A plan for what evidence to collect—which observations or measurements will answer your question. This systematic approach ensures your investigation actually tests what you want to know! The setup tests baking soda's reaction (fizzing) versus sugar's lack thereof using consistent amounts. Choice B provides complete investigation design with clear variables, appropriate controls, feasible procedure, and evidence collection plan that addresses the testable question. Choices A, C, and D lack fair testing, use unsafe tasting, or skip the mixing step entirely. The investigation design recipe: (1) STATE THE QUESTION: 'Which powder reacts with vinegar?' (2) IDENTIFY VARIABLES: Independent (powder type), Dependent (bubbling, temp), Controlled (masses, volumes). (3) OUTLINE PROCEDURE: Add vinegar to powders, observe. (4) EVIDENCE PLAN: Record specific observations. Fair testing: vinegar-only control isolates the powder's effect—excellent strategy!

Question 8

A student heats a small amount of baking soda (NaHCO3_3) in a dry test tube and sees moisture droplets and a white powder left behind. They want to investigate whether heating baking soda causes a chemical change rather than just melting or drying. Which investigation design best tests for chemical change using measurable evidence and controls?

  1. Heat baking soda until it changes, then assume it is chemical because heat was used.
  2. Heat equal masses of baking soda (e.g., 2.0 g) for the same time at the same flame setting in multiple trials, and keep an unheated sample as a control. Measure mass before and after heating, observe for gas production (e.g., bubbles when the released gas is directed into limewater), and compare the heated residue's reaction with vinegar to the unheated sample (dependent evidence: gas formation/amount, mass change). (correct answer)
  3. Heat baking soda and then measure how hot the flame is; if the flame is hot enough, conclude a chemical change occurred.
  4. Heat baking soda in an open dish and decide whether it was chemical based only on whether it smells different, without any control sample or measurements.
Explanation: This question tests your ability to design scientific investigations that test whether chemical changes occur, including identifying variables, planning appropriate observations and measurements, and ensuring fair testing with controls. Designing an investigation to test for chemical change requires four key elements: (1) A clear testable question (Does heating baking soda cause chemical change?), (2) Identification of variables—what you'll change (independent: heating), what you'll measure or observe (dependent: mass change, gas production, reactivity), and what you'll keep constant (controlled: mass, time, flame), (3) A safe, feasible procedure with clear steps that produce observable evidence, (4) A plan for what evidence to collect—which observations or measurements will answer your question. This systematic approach ensures your investigation actually tests what you want to know! The design compares heated and unheated samples for decomposition evidence. Choice B provides complete investigation design with clear variables, appropriate controls, feasible procedure, and evidence collection plan that addresses the testable question. Choices A, C, and D use assumptions, irrelevant measurements, or no controls. The investigation design recipe: (1) STATE THE QUESTION: 'Does heat decompose baking soda?' (2) IDENTIFY VARIABLES: Independent (heating), Dependent (mass, gas test), Controlled (amounts, trials). (3) OUTLINE PROCEDURE: Heat, test residue. (4) EVIDENCE PLAN: Measure and compare. Fair testing: unheated control shows changes are due to heat—impressive!

Question 9

A custodian warns students not to mix a bathroom cleaner labeled "contains sodium hypochlorite (bleach)" with a toilet bowl cleaner labeled "contains acid." The students want to investigate (safely, in tiny amounts and with teacher approval) whether mixing a dilute bleach solution with a dilute acidic solution produces evidence of a chemical reaction. Which investigation design best tests the question with clear variables, controlled conditions, and observable evidence?

  1. Mix 5 mL of dilute bleach with 5 mL of dilute acid in a test tube, and separately mix 5 mL of water with 5 mL of dilute acid as a comparison. Keep volumes and containers the same (controlled variables). Measure temperature before and after mixing and observe for gas production (bubbling) or odor changes while wafting from a distance. Repeat for 3 trials to improve reliability. (correct answer)
  2. Mix random amounts of bleach and acid in different containers and decide a reaction happened only if the mixture turns a "weird" color, without recording amounts or using a comparison mixture.
  3. Look up online whether bleach and acids react, then write the conclusion without doing any observations or measurements.
  4. Heat the bleach and acid mixture strongly on a hot plate to see if it reacts faster, then measure the boiling point to confirm a chemical change.
Explanation: This question tests your ability to design scientific investigations that test whether chemical changes occur, including identifying variables, planning appropriate observations and measurements, and ensuring fair testing with controls. Designing an investigation to test for chemical change requires four key elements: (1) A clear testable question (Does mixing dilute bleach and acid cause a chemical reaction?), (2) Identification of variables—what you'll change (independent: presence of bleach), what you'll measure or observe (dependent: temperature change, gas production, odor), and what you'll keep constant (controlled: volumes, containers, amounts), (3) A safe, feasible procedure with clear steps that produce observable evidence, (4) A plan for what evidence to collect—which observations or measurements will answer your question. This systematic approach ensures your investigation actually tests what you want to know! Here, the design compares a bleach-acid mixture to a water-acid control to safely observe reaction signs. Choice A provides complete investigation design with clear variables, appropriate controls, feasible procedure, and evidence collection plan that addresses the testable question. Choices B, C, and D fail due to random amounts without controls, no hands-on testing, or unsafe heating without proper evidence. The investigation design recipe: (1) STATE THE QUESTION clearly: 'Does bleach react with acid?' (2) IDENTIFY VARIABLES: Independent (bleach vs. water), Dependent (temperature, bubbling), Controlled (volumes, trials). (3) OUTLINE PROCEDURE: Mix safely, observe wafting. (4) EVIDENCE PLAN: Record changes quantitatively. Fair testing: same conditions except the independent variable ensure reliable results—keep practicing!

Question 10

A student mixes cornstarch with water and notices it becomes thick and behaves strangely when stirred quickly. They are unsure whether a chemical reaction occurred or if it is just a physical mixture. Which investigation design best tests whether mixing cornstarch and water causes a chemical change?

  1. Mix cornstarch and water, then decide it is a chemical change because it feels different than water.
  2. Mix a measured amount of cornstarch (10 g) with water (30 mL) and record thickness; then add vinegar and baking soda to see if bubbles form, concluding the original mixing was chemical if bubbles appear.
  3. Prepare two samples using the same masses and volumes (controlled variables): (1) cornstarch + water mixture and (2) water-only control. Measure temperature before/after mixing, observe for gas production, permanent color change, or formation of a new solid that cannot be separated by filtration. Attempt to separate the cornstarch from water by filtering and drying the solid to compare its appearance/mass to the original cornstarch. Repeat trials. (correct answer)
  4. Use a microscope to look at the cornstarch grains and, if they look "different," conclude a chemical reaction occurred without any comparison sample.
Explanation: This question tests your ability to design scientific investigations that test whether chemical changes occur, including identifying variables, planning appropriate observations and measurements, and ensuring fair testing with controls. Designing an investigation to test for chemical change requires four key elements: (1) A clear testable question (Is mixing cornstarch and water chemical?), (2) Identification of variables—what you'll change (independent: cornstarch presence), what you'll measure or observe (dependent: temperature, gas, separability), and what you'll keep constant (controlled: amounts, containers), (3) A safe, feasible procedure with clear steps that produce observable evidence, (4) A plan for what evidence to collect—which observations or measurements will answer your question. This systematic approach ensures your investigation actually tests what you want to know! The setup attempts separation and checks for reaction signs against a water control. Choice C provides complete investigation design with clear variables, appropriate controls, feasible procedure, and evidence collection plan that addresses the testable question. Choices A, B, and D rely on feel, unrelated additions, or no comparison. The investigation design recipe: (1) STATE THE QUESTION: 'Does mixing cause a new substance?' (2) IDENTIFY VARIABLES: Independent (cornstarch), Dependent (changes, recovery), Controlled (masses, volumes). (3) OUTLINE PROCEDURE: Mix, observe, filter. (4) EVIDENCE PLAN: Compare to original. Fair testing: control sample confirms physical mixture—nice job!

Question 11

A student mixes clear calcium chloride solution with clear sodium carbonate solution and observes that the mixture turns cloudy. They want to investigate whether the cloudiness indicates a chemical change (formation of a new substance) and how concentration affects the amount of solid produced. Which investigation design best tests this question with appropriate variables, controls, and evidence collection?

  1. Mix the solutions and, if it looks cloudy, conclude a chemical change occurred; do not record concentrations or amounts because the result is obvious.
  2. Change both solutions' concentrations and volumes at the same time and measure temperature only, concluding more temperature change means more precipitate.
  3. Keep the volume of calcium chloride constant (e.g., 25 mL) and vary only the sodium carbonate concentration across several trials (independent variable: concentration). Mix in identical cups for the same time. Collect the solid by filtration, dry it, and measure mass of precipitate (dependent variable). Control temperature, mixing time, and total volume. Include a control trial using water instead of sodium carbonate. (correct answer)
  4. Use a metal detector to measure how much solid formed, because more solid should produce a stronger signal.
Explanation: This question tests your ability to design scientific investigations that test whether chemical changes occur, including identifying variables, planning appropriate observations and measurements, and ensuring fair testing with controls. Designing an investigation to test for chemical change requires four key elements: (1) A clear testable question (Does concentration affect precipitate formation in this reaction?), (2) Identification of variables—what you'll change (independent: sodium carbonate concentration), what you'll measure or observe (dependent: precipitate mass), and what you'll keep constant (controlled: volumes, time, temperature), (3) A safe, feasible procedure with clear steps that produce observable evidence, (4) A plan for what evidence to collect—which observations or measurements will answer your question. This systematic approach ensures your investigation actually tests what you want to know! The design varies one concentration while quantifying the new solid formed. Choice C provides complete investigation design with clear variables, appropriate controls, feasible procedure, and evidence collection plan that addresses the testable question. Choices A, B, and D lack recordings, change multiple variables, or use unsuitable tools. The investigation design recipe: (1) STATE THE QUESTION: 'How does concentration affect chemical precipitate?' (2) IDENTIFY VARIABLES: Independent (concentration), Dependent (mass), Controlled (volumes, mixing). (3) OUTLINE PROCEDURE: Mix, filter, dry. (4) EVIDENCE PLAN: Weigh precipitate accurately. Fair testing: water control confirms reaction specificity—fantastic approach!

Question 12

A student claims that dissolving table salt (NaCl) in water is a chemical change because the salt "disappears." Design an investigation to test this claim using observable evidence. Which design best distinguishes a physical change from a chemical reaction?

  1. Stir salt into water and, if the solution becomes clear, conclude a chemical change occurred because the solid is gone.
  2. Dissolve a measured mass of salt (e.g., 5.0 g) in a measured volume of water (50 mL), then evaporate the water from the solution in an evaporating dish and measure the mass of the recovered solid. Keep container type and heating time consistent across trials. Evidence against chemical change would be recovering the same substance with similar mass and appearance as the original salt. (correct answer)
  3. Heat dry salt strongly until it glows, then decide whether dissolving salt is chemical based on whether heating changed it.
  4. Add vinegar to salt water; if bubbles form, conclude dissolving salt was a chemical change.
Explanation: This question tests your ability to design scientific investigations that test whether chemical changes occur, including identifying variables, planning appropriate observations and measurements, and ensuring fair testing with controls. Designing an investigation to test for chemical change requires four key elements: (1) A clear testable question (Is dissolving salt a chemical change?), (2) Identification of variables—what you'll change (independent: dissolving then evaporating), what you'll measure or observe (dependent: recovered mass, appearance), and what you'll keep constant (controlled: initial mass, volume, container), (3) A safe, feasible procedure with clear steps that produce observable evidence, (4) A plan for what evidence to collect—which observations or measurements will answer your question. This systematic approach ensures your investigation actually tests what you want to know! The design recovers the original salt via evaporation to show reversibility (physical change). Choice B provides complete investigation design with clear variables, appropriate controls, feasible procedure, and evidence collection plan that addresses the testable question. Choices A, C, and D misinterpret dissolution, use irrelevant heating, or add unrelated vinegar. The investigation design recipe: (1) STATE THE QUESTION: 'Can salt be recovered unchanged?' (2) IDENTIFY VARIABLES: Independent (dissolving/evaporating), Dependent (mass recovery), Controlled (amounts, heating). (3) OUTLINE PROCEDURE: Dissolve, evaporate, measure. (4) EVIDENCE PLAN: Compare masses and appearances. Fair testing: consistent conditions prove no new substance formed—you've got this!

Question 13

A cafeteria manager wants to know whether mixing vinegar (acetic acid solution) with baking soda (sodium bicarbonate) causes a chemical change, because students sometimes combine them during cleanup. Design an investigation to test whether a chemical reaction occurs, using safe, high-school-lab equipment and clear variables. Which plan is best?

  1. Mix any amounts of vinegar and baking soda in an open beaker and decide it is a chemical change if it "looks different" than before.
  2. Smell the mixture closely to see if a new odor forms; if it smells different, declare a chemical change occurred.
  3. Combine 25 mL vinegar with 2.0 g baking soda in a flask fitted with a balloon. Independent variable: presence/absence of baking soda (compare vinegar alone vs vinegar + baking soda); dependent variables: balloon inflation (gas production) and temperature change (initial vs final with thermometer); controlled variables: vinegar volume, starting temperature, container type, timing. Run at least 3 trials and record observations. (correct answer)
  4. Heat baking soda until it glows red, then add vinegar and record the color; do not include a control because reactions are obvious.
Explanation: This question tests your ability to design scientific investigations that test whether chemical changes occur, including identifying variables, planning appropriate observations and measurements, and ensuring fair testing with controls. Designing an investigation to test for chemical change requires four key elements: (1) A clear testable question (Does mixing A and B cause a chemical reaction?), (2) Identification of variables—what you'll change (independent: substance type, temperature, concentration), what you'll measure or observe (dependent: temperature change, gas production, color change), and what you'll keep constant (controlled: volumes, time, equipment), (3) A safe, feasible procedure with clear steps that produce observable evidence, (4) A plan for what evidence to collect—which observations or measurements will answer your question. This systematic approach ensures your investigation actually tests what you want to know! Here, the design should involve mixing vinegar and baking soda with a control (vinegar alone), using a balloon to capture gas, measuring temperature, with variables like presence of baking soda (independent), balloon inflation and temperature change (dependent), and controls like volumes and temperature, including multiple trials for reliability. Choice C provides complete investigation design with clear variables, appropriate controls, feasible procedure, and evidence collection plan that addresses the testable question. Choices A, B, and D fail because A lacks variables and measurements, B relies on unsafe smelling without controls, and D introduces unrelated heating without a proper comparison. The investigation design recipe: (1) STATE THE QUESTION clearly: What are you testing? Be specific—'Does mixing vinegar and baking soda produce new substances?' not just 'What happens?' (2) IDENTIFY VARIABLES: Independent variable (what you'll change—make it ONE thing to change so you know what caused effects), Dependent variable (what evidence you'll collect—temperature? color? gas? be specific), Controlled variables (list 3-5 things you'll keep exactly the same—amounts, time, temperature, equipment). (3) OUTLINE PROCEDURE: Simple steps that safely produce the evidence you need. Usually: mix or treat substances, observe during and after, record specific measurements or observations. (4) EVIDENCE PLAN: Exactly what will you measure (temperature with thermometer before and after) or observe (color change—describe initial and final colors; gas production—count bubbles or note vigorous fizzing). The design is complete when someone else could follow it and get the same results! Fair testing through controls: imagine you're testing whether temperature affects reaction between vinegar and baking soda. If you use different amounts of vinegar at different temperatures, you won't know if changes come from temperature or amount—two variables changed! Fair test: same volumes (50mL vinegar, 5g baking soda) at different temperatures (10°C, 25°C, 40°C), measure fizzing time as dependent variable. Now temperature is the ONLY thing different, so any differences in fizzing time must come from temperature. Controls make results interpretable—without them, you can't draw conclusions!

Question 14

A student mixes hydrogen peroxide solution (3%) with a small amount of yeast and claims, "It's just bubbling because the yeast is fizzy—no chemical change." Your class must design an investigation to determine whether a chemical change occurs and to collect evidence. Which plan is best and safe for a high school lab?

  1. Mix peroxide and yeast in any container and watch; if it bubbles, it is chemical, and no further evidence is needed.
  2. Mix peroxide and yeast in a sealed container and shake hard; if the container expands, stop and conclude it is chemical.
  3. Set up two cups: (1) 50 mL of 3% hydrogen peroxide + measured yeast (e.g., 1.0 g), (2) 50 mL hydrogen peroxide alone as a control. Independent variable: presence of yeast; dependent variables: rate/volume of gas produced (foam height or gas captured in a balloon) and temperature change (thermometer). Controlled variables: peroxide concentration/volume, container size, starting temperature, timing. Repeat trials and record data. (correct answer)
  4. Test whether yeast is alive under a microscope; if it is alive, then bubbling must be physical, not chemical.
Explanation: This question tests your ability to design scientific investigations that test whether chemical changes occur, including identifying variables, planning appropriate observations and measurements, and ensuring fair testing with controls. Designing an investigation to test for chemical change requires four key elements: (1) A clear testable question (Does mixing A and B cause a chemical reaction?), (2) Identification of variables—what you'll change (independent: substance type, temperature, concentration), what you'll measure or observe (dependent: temperature change, gas production, color change), and what you'll keep constant (controlled: volumes, time, equipment), (3) A safe, feasible procedure with clear steps that produce observable evidence, (4) A plan for what evidence to collect—which observations or measurements will answer your question. This systematic approach ensures your investigation actually tests what you want to know! The plan compares peroxide with yeast to peroxide alone, with variables like yeast presence (independent), gas volume and temperature (dependent), and controls like volumes and temperature, using repeats for data. Choice C provides complete investigation design with clear variables, appropriate controls, feasible procedure, and evidence collection plan that addresses the testable question. Choices A, B, and D fail because A lacks controls and measurements, B uses unsafe shaking without proper evidence, and D misinterprets biological aspects irrelevant to chemical change. The investigation design recipe: (1) STATE THE QUESTION clearly: What are you testing? Be specific—'Does yeast cause a chemical reaction in peroxide?' not just 'What happens?' (2) IDENTIFY VARIABLES: Independent variable (what you'll change—make it ONE thing to change so you know what caused effects), Dependent variable (what evidence you'll collect—temperature? color? gas? be specific), Controlled variables (list 3-5 things you'll keep exactly the same—amounts, time, temperature, equipment). (3) OUTLINE PROCEDURE: Simple steps that safely produce the evidence you need. Usually: mix or treat substances, observe during and after, record specific measurements or observations. (4) EVIDENCE PLAN: Exactly what will you measure (temperature with thermometer before and after) or observe (color change—describe initial and final colors; gas production—count bubbles or note vigorous fizzing). The design is complete when someone else could follow it and get the same results! Fair testing through controls: imagine you're testing whether temperature affects reaction between vinegar and baking soda. If you use different amounts of vinegar at different temperatures, you won't know if changes come from temperature or amount—two variables changed! Fair test: same volumes (50mL vinegar, 5g baking soda) at different temperatures (10°C, 25°C, 40°C), measure fizzing time as dependent variable. Now temperature is the ONLY thing different, so any differences in fizzing time must come from temperature. Controls make results interpretable—without them, you can't draw conclusions!

Question 15

A student claims that mixing two clear solutions—calcium chloride solution and sodium carbonate solution—might or might not be a chemical change because "they're both just salts in water." The class needs to design an experiment to confirm whether a chemical reaction occurs. Which investigation design is best?

  1. Mix the two solutions and immediately pour them down the sink; if it looked cloudy for a moment, assume it was chemical.
  2. Mix equal volumes (e.g., 10 mL + 10 mL) of calcium chloride solution and sodium carbonate solution in a clear beaker. Independent variable: whether solutions are mixed (mixed vs each solution alone as controls); dependent variables: formation of a solid precipitate (cloudiness/solid that can be filtered) and mass of dried precipitate; controlled variables: solution concentrations, volumes, temperature, mixing time. Repeat trials and record observations. (correct answer)
  3. Measure only the conductivity of each solution before mixing; if both conduct electricity, no reaction can occur.
  4. Change two variables at once (use different volumes and different concentrations each time) until a solid appears; then conclude a chemical change always happens.
Explanation: This question tests your ability to design scientific investigations that test whether chemical changes occur, including identifying variables, planning appropriate observations and measurements, and ensuring fair testing with controls. Designing an investigation to test for chemical change requires four key elements: (1) A clear testable question (Does mixing A and B cause a chemical reaction?), (2) Identification of variables—what you'll change (independent: substance type, temperature, concentration), what you'll measure or observe (dependent: temperature change, gas production, color change), and what you'll keep constant (controlled: volumes, time, equipment), (3) A safe, feasible procedure with clear steps that produce observable evidence, (4) A plan for what evidence to collect—which observations or measurements will answer your question. This systematic approach ensures your investigation actually tests what you want to know! The investigation mixes the solutions with controls of each alone, using variables like mixing (independent), precipitate formation and mass (dependent), and controls like volumes and concentrations, with repeats. Choice B provides complete investigation design with clear variables, appropriate controls, feasible procedure, and evidence collection plan that addresses the testable question. Choices A, C, and D fail because A discards evidence unsafely, C measures irrelevant conductivity, and D changes multiple variables without controls. The investigation design recipe: (1) STATE THE QUESTION clearly: What are you testing? Be specific—'Does mixing these solutions produce a precipitate?' not just 'What happens?' (2) IDENTIFY VARIABLES: Independent variable (what you'll change—make it ONE thing to change so you know what caused effects), Dependent variable (what evidence you'll collect—temperature? color? gas? be specific), Controlled variables (list 3-5 things you'll keep exactly the same—amounts, time, temperature, equipment). (3) OUTLINE PROCEDURE: Simple steps that safely produce the evidence you need. Usually: mix or treat substances, observe during and after, record specific measurements or observations. (4) EVIDENCE PLAN: Exactly what will you measure (temperature with thermometer before and after) or observe (color change—describe initial and final colors; gas production—count bubbles or note vigorous fizzing). The design is complete when someone else could follow it and get the same results! Fair testing through controls: imagine you're testing whether temperature affects reaction between vinegar and baking soda. If you use different amounts of vinegar at different temperatures, you won't know if changes come from temperature or amount—two variables changed! Fair test: same volumes (50mL vinegar, 5g baking soda) at different temperatures (10°C, 25°C, 40°C), measure fizzing time as dependent variable. Now temperature is the ONLY thing different, so any differences in fizzing time must come from temperature. Controls make results interpretable—without them, you can't draw conclusions!

Question 16

A maker-space uses "instant cold packs" for minor injuries. Someone suggests making a reusable version by mixing ammonium chloride with water in a sealed bag, but they are unsure whether the process is a chemical change or just dissolving. Which investigation design best tests whether mixing ammonium chloride with water causes a chemical change, using observable evidence and a control?

  1. Mix ammonium chloride with water and decide it is chemical if the bag feels cold; do not compare to anything else.
  2. Measure temperature change when 5.0 g ammonium chloride dissolves in 50 mL water, and also measure temperature change when 5.0 g table salt dissolves in 50 mL water for comparison. Independent variable: solute type; dependent variables: temperature change and whether any gas/precipitate/new color appears; controlled variables: water volume, starting temperature, stirring time, container type. Use multiple trials and record observations. (correct answer)
  3. Boil ammonium chloride in water until the water is gone; if crystals form, that proves a chemical change happened.
  4. Use a spectrometer to identify new molecules formed in solution; if none are detected, it is physical.
Explanation: This question tests your ability to design scientific investigations that test whether chemical changes occur, including identifying variables, planning appropriate observations and measurements, and ensuring fair testing with controls. Designing an investigation to test for chemical change requires four key elements: (1) A clear testable question (Does mixing A and B cause a chemical reaction?), (2) Identification of variables—what you'll change (independent: substance type, temperature, concentration), what you'll measure or observe (dependent: temperature change, gas production, color change), and what you'll keep constant (controlled: volumes, time, equipment), (3) A safe, feasible procedure with clear steps that produce observable evidence, (4) A plan for what evidence to collect—which observations or measurements will answer your question. This systematic approach ensures your investigation actually tests what you want to know! This design compares ammonium chloride to table salt in water, with variables like solute type (independent), temperature change and other signs (dependent), and controls like masses and volumes, using trials for observations. Choice B provides complete investigation design with clear variables, appropriate controls, feasible procedure, and evidence collection plan that addresses the testable question. Choices A, C, and D fail because A lacks comparison, C uses boiling without relevance to chemical change, and D requires advanced equipment not suitable for high school. The investigation design recipe: (1) STATE THE QUESTION clearly: What are you testing? Be specific—'Is dissolving ammonium chloride a chemical change?' not just 'What happens?' (2) IDENTIFY VARIABLES: Independent variable (what you'll change—make it ONE thing to change so you know what caused effects), Dependent variable (what evidence you'll collect—temperature? color? gas? be specific), Controlled variables (list 3-5 things you'll keep exactly the same—amounts, time, temperature, equipment). (3) OUTLINE PROCEDURE: Simple steps that safely produce the evidence you need. Usually: mix or treat substances, observe during and after, record specific measurements or observations. (4) EVIDENCE PLAN: Exactly what will you measure (temperature with thermometer before and after) or observe (color change—describe initial and final colors; gas production—count bubbles or note vigorous fizzing). The design is complete when someone else could follow it and get the same results! Fair testing through controls: imagine you're testing whether temperature affects reaction between vinegar and baking soda. If you use different amounts of vinegar at different temperatures, you won't know if changes come from temperature or amount—two variables changed! Fair test: same volumes (50mL vinegar, 5g baking soda) at different temperatures (10°C, 25°C, 40°C), measure fizzing time as dependent variable. Now temperature is the ONLY thing different, so any differences in fizzing time must come from temperature. Controls make results interpretable—without them, you can't draw conclusions!

Question 17

A student mixes hydrogen peroxide solution (3% H2_2O2_2, the kind sold at pharmacies) with a small amount of yeast in a flask and observes foaming. They think the foam might just be trapped air from stirring (physical) rather than a chemical change. The teacher asks for an investigation design that can confirm whether a chemical reaction occurred. Which design best provides observable evidence?

Assume standard lab safety and small quantities.

  1. Testable question: "Does yeast cause H2_2O2_2 to chemically decompose into water and oxygen gas?" Independent variable: presence of yeast (H2_2O2_2 + yeast vs H2_2O2_2 alone control). Dependent variables: gas production measured by foam height or collected gas volume over a fixed time, and temperature change (°C). Controlled variables: H2_2O2_2 volume/concentration, yeast mass, container size, and timing. Run multiple trials and compare to the control to determine if gas production indicates chemical change. (correct answer)
  2. Shake the flask harder; if more foam appears, it proves the foam is chemical because shaking increases reactions.
  3. Smell the foam; if it smells like bread, that proves a chemical reaction happened.
  4. Use litmus paper on the foam; if the pH is neutral, conclude no chemical change occurred.
Explanation: This question tests your ability to design scientific investigations that test whether chemical changes occur, including identifying variables, planning appropriate observations and measurements, and ensuring fair testing with controls. Designing an investigation to test for chemical change requires four key elements: (1) A clear testable question (Does mixing A and B cause a chemical reaction?), (2) Identification of variables—what you'll change (independent: substance type, temperature, concentration), what you'll measure or observe (dependent: temperature change, gas production, color change), and what you'll keep constant (controlled: volumes, time, equipment), (3) A safe, feasible procedure with clear steps that produce observable evidence, (4) A plan for what evidence to collect—which observations or measurements will answer your question. This systematic approach ensures your investigation actually tests what you want to know! Compare hydrogen peroxide with yeast to a yeast-free control, controlling volumes and time, measuring foam or gas volume and temperature to confirm if decomposition is chemical. Choice A provides a complete investigation design with a testable question on decomposition, independent variable (yeast presence), dependent variables (gas production, temperature), controlled variables (volumes, mass, timing), multiple trials, and control comparison for chemical evidence. Choices B, C, and D are weak: B confuses shaking with reaction, C uses smell irrelevantly, and D misuses pH without addressing gas. Follow the recipe: question yeast's role specifically, change its presence only, measure foam height, control concentrations, outline safe mixing, and plan timed data—bravo! A control without yeast shows if foaming is from decomposition or just mixing; without it, you can't confirm—keep verifying, you're brilliant!

Question 18

Two unlabeled white powders were found in a classroom prep room: one is baking soda (NaHCO3_3) and the other is cornstarch. A teacher wants students to design a test to determine which one undergoes a chemical change when mixed with vinegar (acetic acid solution). Which investigation design best answers the question using observable evidence and fair-test controls?

  1. Taste a tiny amount of each powder; the one that tastes salty must react chemically with vinegar.
  2. Testable question: "Which powder produces gas when mixed with vinegar?" Independent variable: powder type (Powder 1 vs Powder 2). Dependent variables: amount/rate of bubbling (CO2_2 evidence) and temperature change (°C). Controlled variables: same mass of powder (e.g., 2.0 g), same vinegar volume (e.g., 20 mL), same container, same stirring and timing. Run multiple trials for each powder and compare observations/measurements to identify the reacting powder. (correct answer)
  3. Add different volumes of vinegar to each powder until one reacts; the powder that needs more vinegar is the one that chemically changes.
  4. Mix both powders together first, then add vinegar; if bubbles form, conclude both powders reacted chemically.
Explanation: This question tests your ability to design scientific investigations that test whether chemical changes occur, including identifying variables, planning appropriate observations and measurements, and ensuring fair testing with controls. Designing an investigation to test for chemical change requires four key elements: (1) A clear testable question (Does mixing A and B cause a chemical reaction?), (2) Identification of variables—what you'll change (independent: substance type, temperature, concentration), what you'll measure or observe (dependent: temperature change, gas production, color change), and what you'll keep constant (controlled: volumes, time, equipment), (3) A safe, feasible procedure with clear steps that produce observable evidence, (4) A plan for what evidence to collect—which observations or measurements will answer your question. This systematic approach ensures your investigation actually tests what you want to know! The setup should test each powder separately with vinegar, controlling masses and volumes, and measure bubbling and temperature to identify which reacts, likely baking soda producing gas. Choice B provides a complete investigation design with a testable question on gas production, independent variable (powder type), dependent variables (bubbling, temperature), controlled variables (masses, volumes, stirring), multiple trials, and comparison for chemical change. Choices A, C, and D fail as A unsafely suggests tasting, C varies volumes unfairly, and D mixes powders confounding results. Apply the recipe: clearly question which reacts, change powder type only, measure gas specifically, control amounts and timing, outline mixing steps, and plan observational evidence— you're rocking this! Fair testing by using identical vinegar amounts for each powder ensures differences are due to the powder itself; without controls, you can't tell—keep it up, your designs are spot on!

Question 19

A science club wants to make "invisible ink" messages using lemon juice that appear when gently warmed with a desk lamp. Some members argue this is only drying (physical change), while others think heating causes a chemical change in the lemon juice residue. The club needs evidence because they plan to demonstrate it at an elementary school and want an accurate explanation. Which investigation design best tests whether heating lemon juice on paper causes a chemical change?

Avoid open flames; use safe warming methods.

  1. Write messages with lemon juice and with water on identical paper strips. Warm both under the same lamp for the same time. If the lemon-juice writing darkens while the water control does not, conclude chemical change occurred without collecting any other evidence.
  2. Testable question: "Does gentle heating cause lemon-juice residue to chemically change (brown) compared to water?" Independent variable: liquid used to write (lemon juice vs water control). Dependent variable: degree of browning measured by a consistent color scale or photo comparison after set heating times. Controlled variables: paper type/size, amount of liquid applied, lamp distance, heating time intervals, and room conditions. Use multiple trials and record browning vs time to support whether a chemical change occurs. (correct answer)
  3. Heat the paper until it smokes; if it turns brown, that proves the lemon juice reacted.
  4. Only test lemon juice (no control) but use three different papers; if any paper browns, then lemon juice must undergo chemical change.
Explanation: This question tests your ability to design scientific investigations that test whether chemical changes occur, including identifying variables, planning appropriate observations and measurements, and ensuring fair testing with controls. Designing an investigation to test for chemical change requires four key elements: (1) A clear testable question (Does mixing A and B cause a chemical reaction?), (2) Identification of variables—what you'll change (independent: substance type, temperature, concentration), what you'll measure or observe (dependent: temperature change, gas production, color change), and what you'll keep constant (controlled: volumes, time, equipment), (3) A safe, feasible procedure with clear steps that produce observable evidence, (4) A plan for what evidence to collect—which observations or measurements will answer your question. This systematic approach ensures your investigation actually tests what you want to know! Use lemon juice versus water on paper, control heating time and amount, measure browning over time with a scale to see if heating causes chemical change in the juice. Choice B provides a complete investigation design with a testable question on browning, independent variable (liquid type), dependent variable (browning degree), controlled variables (paper, amount, heating), multiple trials, and timed recordings for chemical evidence. Choices A, C, and D fall short: A lacks detailed measurement, C over-heats unsafely, and D omits controls. Design recipe: state the heating question, vary liquid independently, measure color quantitatively, control warming safely, outline steps, and plan comparative evidence—superb! Controls like water ensure browning is from lemon juice chemistry, not just heat; no control means unclear causes— you're acing this!

Question 20

A local pool store claims that adding a small amount of "pH Down" (a sodium bisulfate product) to pool water causes a chemical change that lowers pH. A student argues that pH change alone doesn't prove a chemical reaction occurred. The class must design a simple lab investigation using dilute solutions to test whether adding sodium bisulfate to water produces evidence consistent with a chemical change. Which design is best?

  1. Add sodium bisulfate to water and measure pH once; if pH is lower, that automatically proves a chemical change occurred.
  2. Testable question: "Does dissolving sodium bisulfate in water produce ions that change pH compared with a water-only control?" Independent variable: solute added (sodium bisulfate vs no solute control; optionally compare to NaCl as a non-acidic dissolving solute). Dependent variables: pH change measured with pH paper/probe and temperature change (°C) after mixing for a fixed time. Controlled variables: water volume, solute mass, initial water temperature, mixing time, and container. Run multiple trials and compare to controls to support whether the observed changes are consistent with a chemical process (acid formation in water). (correct answer)
  3. Heat the sodium bisulfate until it melts, then add it to water; if it splatters, it reacted chemically.
  4. Measure only the mass of the beaker before and after adding sodium bisulfate; if mass stays the same, conclude no chemical change occurred.
Explanation: This question tests your ability to design scientific investigations that test whether chemical changes occur, including identifying variables, planning appropriate observations and measurements, and ensuring fair testing with controls. Designing an investigation to test for chemical change requires four key elements: (1) A clear testable question (Does mixing A and B cause a chemical reaction?), (2) Identification of variables—what you'll change (independent: substance type, temperature, concentration), what you'll measure or observe (dependent: temperature change, gas production, color change), and what you'll keep constant (controlled: volumes, time, equipment), (3) A safe, feasible procedure with clear steps that produce observable evidence, (4) A plan for what evidence to collect—which observations or measurements will answer your question. This systematic approach ensures your investigation actually tests what you want to know! Compare sodium bisulfate in water to controls like plain water or NaCl, measuring pH and temperature changes under controlled masses and times to evaluate if ion formation indicates chemical change. Choice B provides a complete investigation design with a testable question on pH ions, independent variable (solute type with controls), dependent variables (pH, temperature), controlled variables (volume, mass, time), multiple trials, and comparisons for chemical evidence. Choices A, C, and D are flawed: A assumes pH proves chemical without controls, C uses unsafe heating, and D ignores relevant changes. Design recipe: question dissolution's effect, vary solute, measure pH accurately, control conditions, outline mixing, and compare to controls—outstanding! Controls like no-solute water distinguish chemical pH drops from physical; without them, changes could be misattributed— you're excelling at this!