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Chemistry · Learn by Concept

Chemistry Help: Justify Whether Reaction Occurred

Review real example questions for Justify Whether Reaction Occurred in Chemistry.

Question 1 / 10

0 of 10 answered

A student adds 5.0 g of table salt (NaCl) to 100 mL of water in a beaker and stirs for 2 minutes.

Observations/data:

  • Salt crystals disappear; the liquid remains clear.
  • Temperature changes from 21.0°C to 19.8°C.
  • No bubbling, no odor, and no solid forms later.
  • The student evaporates the water overnight and recovers white crystals that look like the original salt.

Based on the evidence, did a chemical reaction occur? Choose the best CER-style justification.

Select an answer to continue

All questions

Question 1

A student adds 5.0 g of table salt (NaCl) to 100 mL of water in a beaker and stirs for 2 minutes.

Observations/data:

  • Salt crystals disappear; the liquid remains clear.
  • Temperature changes from 21.0°C to 19.8°C.
  • No bubbling, no odor, and no solid forms later.
  • The student evaporates the water overnight and recovers white crystals that look like the original salt.

Based on the evidence, did a chemical reaction occur? Choose the best CER-style justification.

  1. Claim: A chemical reaction occurred. Evidence: The temperature decreased. Reasoning: A temperature change always means new substances formed.
  2. Claim: No chemical reaction occurred. Evidence: The salt disappeared and the temperature decreased. Reasoning: Disappearing means the salt was destroyed and turned into something else.
  3. Claim: No chemical reaction occurred. Evidence: The solution stayed clear with no gas or new solid, and the original salt was recovered by evaporating the water. Reasoning: Recovering the original substance and observing only dissolving (a reversible physical change) indicates no new substances formed. (correct answer)
  4. Claim: A chemical reaction occurred. Evidence: The salt crystals were gone after stirring. Reasoning: If you can’t see the solid anymore, it must have reacted into a new substance.

Explanation: This question tests your ability to construct scientific justifications using the Claim-Evidence-Reasoning (CER) framework to determine whether a chemical reaction occurred, distinguishing strong evidence from weak or ambiguous observations. A complete scientific justification has three parts: (1) CLAIM: a clear statement of your conclusion (a chemical reaction occurred, or it did not), (2) EVIDENCE: specific observations or data from the scenario (temperature increased by 15°C, white solid formed, bubbles appeared), and (3) REASONING: explanation of WHY that evidence supports your claim using chemistry principles (temperature increase indicates exothermic reaction with energy release from bond formation, precipitate indicates new insoluble substance formed, bubbles indicate gas produced as reaction product). All three components are necessary—evidence alone doesn't justify without reasoning, and reasoning without evidence is just speculation! Here, the CER draws from the salt disappearing, a small temperature decrease, no new formations, and recovery of the salt via evaporation, constructing a justification for no reaction as it's a reversible physical change. Choice C provides complete justification with a valid claim of no chemical reaction, evidence including the clear solution and salt recovery, and sound reasoning that this indicates dissolving without new substances. Distractors like A and D fail by misinterpreting the disappearance as destruction or reaction without considering reversibility, while B incorrectly claims destruction. Building scientific justifications—the CER checklist: (1) CLAIM: State your conclusion clearly: "A chemical reaction occurred" or "No chemical reaction occurred, only physical change." Be definitive based on evidence. (2) EVIDENCE: List 2-3 specific observations or data points from the scenario: "Solution temperature increased from 20°C to 35°C, color changed from clear to yellow, and white solid formed." Use actual numbers and observations, not vague statements. (3) REASONING: For EACH piece of evidence, explain what it indicates: "Temperature increase indicates energy released from chemical bonds forming (exothermic reaction). Color change indicates new substance with different light absorption properties. Solid formation indicates precipitate—new insoluble substance created." Connect evidence to new substance formation! Justification strength evaluation: STRONG justifications cite multiple chemical indicators (gas + precipitate + temperature change) and explain why each indicates reaction. WEAK justifications cite one ambiguous observation (just got warm) without ruling out alternatives. INSUFFICIENT justifications lack reasoning (lists observations without explaining what they mean). The strongest justifications also acknowledge and address potential alternative explanations: "While dissolving can release heat, the combination of temperature increase AND precipitate formation AND color change that can't be explained by mixing strongly supports chemical reaction rather than simple dissolution." This shows critical thinking!

Question 2

A student conducts two trials.

Trial 1: 3.0 g of sugar is stirred into 50 mL of water. The sugar disappears, the solution stays clear, and the temperature changes from 21.0°C to 20.2°C.

Trial 2: 3.0 g of sugar is heated strongly in a dry test tube. The sugar turns brown, then black, and a noticeable burnt smell appears. A dark solid remains stuck to the tube.

For Trial 2, did a chemical reaction occur? Choose the best CER justification.

  1. Claim: No chemical reaction occurred. Evidence: Sugar started as a solid and ended as a solid. Reasoning: If the state of matter is the same, no new substance formed.
  2. Claim: A chemical reaction occurred. Evidence: The sugar changed color (white to brown/black) and produced a new odor (burnt smell) and a different-looking residue. Reasoning: Persistent color change and production of new odor indicate new substances formed (decomposition products), which is evidence of a chemical reaction. (correct answer)
  3. Claim: No chemical reaction occurred. Evidence: Heat was applied. Reasoning: Heating only changes temperature and cannot cause chemical changes, so this must be physical.
  4. Claim: A chemical reaction occurred. Evidence: Trial 1 showed a temperature drop. Reasoning: Since sugar changed temperature when dissolved, it must always react chemically when heated too.

Explanation: This question tests your ability to construct scientific justifications using the Claim-Evidence-Reasoning (CER) framework to determine whether a chemical reaction occurred, distinguishing strong evidence from weak or ambiguous observations. A complete scientific justification has three parts: (1) CLAIM: a clear statement of your conclusion (a chemical reaction occurred, or it did not), (2) EVIDENCE: specific observations or data from the scenario (temperature increased by 15°C, white solid formed, bubbles appeared), and (3) REASONING: explanation of WHY that evidence supports your claim using chemistry principles (temperature increase indicates exothermic reaction with energy release from bond formation, precipitate indicates new insoluble substance formed, bubbles indicate gas produced as reaction product). All three components are necessary—evidence alone doesn't justify without reasoning, and reasoning without evidence is just speculation! Heating sugar leading to color change, burnt odor, and dark residue suggests thermal decomposition, with a good CER linking these persistent changes to new decomposition products. Choice B provides complete justification with a valid claim of a reaction, relevant evidence like color shift and odor, and sound chemical reasoning on new substances. Choices A, C, and D fail by ignoring state consistency, claiming heat prevents reactions, or irrelevantly referencing Trial 1. Keep building your scientific justifications using the CER checklist: clearly state your claim, list specific evidence with details, and explain each piece with chemistry principles—remember, strong justifications address alternatives, such as contrasting this with Trial 1's physical dissolution to highlight irreversible changes, refining your critical thinking!

Question 3

A student heats 15 g of a white solid labeled “wax” in a small metal dish.

Observations and data:

  • The wax melts into a clear liquid at about 60∘C60^\circ\text{C}60∘C
  • No bubbling or smoke is observed
  • The mass is 15.0 g before heating and 15.0 g after cooling back to room temperature
  • After cooling, the liquid becomes a solid again with the same appearance as before

Which CER justification best supports whether a chemical reaction occurred?

  1. Claim: A chemical reaction occurred. Evidence: The wax changed from solid to liquid. Reasoning: Any change of state means the substance became a new substance.
  2. Claim: No chemical reaction occurred. Evidence: The change was reversible (solid → liquid → solid), with no gas, no new color, and no mass change. Reasoning: Melting/freezing are physical changes that do not create new substances; the evidence supports a physical change only. (correct answer)
  3. Claim: A chemical reaction occurred. Evidence: Heat was added. Reasoning: Adding heat forces atoms to rearrange into new molecules.
  4. Claim: No chemical reaction occurred. Evidence: The wax became a liquid. Reasoning: Liquids cannot be products of chemical reactions.

Explanation: This question tests your ability to construct scientific justifications using the Claim-Evidence-Reasoning (CER) framework to determine whether a chemical reaction occurred, distinguishing strong evidence from weak or ambiguous observations. A complete scientific justification has three parts: (1) CLAIM: a clear statement of your conclusion (a chemical reaction occurred, or it did not), (2) EVIDENCE: specific observations or data from the scenario (wax melted at 60°C, no bubbling/smoke, mass unchanged at 15.0 g, reversible solid→liquid→solid), and (3) REASONING: explanation of WHY that evidence supports your claim using chemistry principles (reversibility with same properties indicates physical change, no mass change or new products formed, melting/freezing are phase changes not creating new substances). All three components are necessary—evidence alone doesn't justify without reasoning, and reasoning without evidence is just speculation! Choice B provides a complete justification: the claim correctly states no reaction occurred, the evidence cites key observations (reversible change, no gas, no color change, no mass change), and the reasoning properly explains that melting/freezing are physical changes that don't create new substances. Choice B provides complete justification with valid claim, relevant evidence from the stimulus, and sound chemical reasoning connecting evidence to the conclusion that no reaction occurred. Choice A incorrectly claims any state change means new substance formation, while choices C and D provide flawed reasoning (heat enables but doesn't force reactions; liquids certainly can be reaction products). Building scientific justifications—the CER checklist: State your conclusion clearly, list specific observations including the crucial reversibility test (solid→liquid→solid with same appearance), and explain what each means (reversibility to original form = same substance throughout = physical change only). The strongest justifications distinguish between physical changes (melting, freezing, dissolving) where substances retain their chemical identity, and chemical changes where new substances with different properties form permanently!

Question 4

A student mixes 10 mL of vinegar with a small spoonful of baking soda in a flask. Observations:

  • Immediate vigorous bubbling and foaming occurs.
  • The flask feels cooler; temperature drops from 23.5∘C23.5^\circ\text{C}23.5∘C to 19.0∘C19.0^\circ\text{C}19.0∘C.
  • After bubbling stops, the mixture is still liquid and looks slightly cloudy.
  • The total mass of the closed flask system stays the same (measured with a balloon sealing the top): 85.20 g before, 85.20 g after.

Did a chemical reaction occur? Choose the best CER justification.

  1. Claim: A chemical reaction did not occur. Evidence: The mass stayed the same. Reasoning: If mass is conserved, no new substances can form.
  2. Claim: A chemical reaction occurred. Evidence: Vigorous bubbling/foaming and a temperature decrease occurred. Reasoning: Bubble formation indicates gas production (a new substance), and the temperature change indicates energy change associated with bond rearrangement; mass staying constant in a closed system is expected even during reactions. (correct answer)
  3. Claim: A chemical reaction occurred. Evidence: The mixture became slightly cloudy. Reasoning: Cloudiness alone proves a new substance formed.
  4. Claim: A chemical reaction did not occur. Evidence: The mixture remained mostly liquid. Reasoning: Chemical reactions must always form a solid, so this was only mixing.

Explanation: This question tests your ability to construct scientific justifications using the Claim-Evidence-Reasoning (CER) framework to determine whether a chemical reaction occurred, distinguishing strong evidence from weak or ambiguous observations. A complete scientific justification has three parts: (1) CLAIM: a clear statement of your conclusion (a chemical reaction occurred, or it did not), (2) EVIDENCE: specific observations or data from the scenario (vigorous bubbling/foaming, temperature dropped from 23.5°C to 19.0°C, mass stayed constant at 85.20g in closed system), and (3) REASONING: explanation of WHY that evidence supports your claim using chemistry principles (bubbling indicates CO₂ gas production from acid-base reaction, temperature decrease shows endothermic reaction absorbing energy, mass conservation in closed system is expected per law of conservation of mass). All three components are necessary—evidence alone doesn't justify without reasoning, and reasoning without evidence is just speculation! Choice B provides a complete justification: the claim correctly identifies a reaction occurred, the evidence cites both gas production and significant temperature change, and the reasoning properly explains that bubbles indicate new gaseous product formation while addressing that mass conservation in a closed system is expected even during reactions (nothing escapes). Choice A incorrectly thinks mass conservation prevents reactions, Choice C relies on cloudiness alone without stronger evidence, and Choice D incorrectly claims reactions must form solids. Building scientific justifications—the CER checklist: The vinegar-baking soda reaction (HC₂H₃O₂ + NaHCO₃ → NaC₂H₃O₂ + H₂O + CO₂) provides multiple strong indicators! Gas production is one of the clearest signs of chemical reaction, and the 4.5°C temperature drop indicates an endothermic process where energy is absorbed to break bonds. The mass staying constant demonstrates the law of conservation of mass—atoms rearrange but total mass is preserved!

Question 5

A student places 10 mL of hydrogen peroxide solution into a flask, then adds a small amount of yeast and gently swirls.

Observations and measurements:

  • Within 10 seconds, many bubbles form and foam rises.
  • The flask becomes warm; temperature increases from 21.0°C to 32.0°C.
  • A glowing splint held near the mouth of the flask relights.
  • After 5 minutes, bubbling slows.
  • No solid product remains other than the yeast.

Did a chemical reaction occur? Choose the best CER justification.

  1. Claim: No chemical reaction occurred. Evidence: No new solid formed. Reasoning: If there is no precipitate, then no reaction happened.
  2. Claim: A chemical reaction occurred. Evidence: Foaming/bubbling occurred, temperature rose by 11°C, and a glowing splint relit near the flask. Reasoning: Bubbling indicates gas production, the splint test indicates a new gas with different properties formed, and the temperature rise indicates energy change from bond rearrangement—together strong evidence of a chemical reaction. (correct answer)
  3. Claim: A chemical reaction occurred. Evidence: The yeast was added. Reasoning: Yeast is alive, so it always causes chemical reactions no matter what happens.
  4. Claim: No chemical reaction occurred. Evidence: Bubbling stopped after 5 minutes. Reasoning: If it stops, it must have been only trapped air escaping.

Explanation: This question tests your ability to construct scientific justifications using the Claim-Evidence-Reasoning (CER) framework to determine whether a chemical reaction occurred, distinguishing strong evidence from weak or ambiguous observations. A complete scientific justification has three parts: (1) CLAIM: a clear statement of your conclusion (a chemical reaction occurred, or it did not), (2) EVIDENCE: specific observations or data from the scenario (temperature increased by 15°C, white solid formed, bubbles appeared), and (3) REASONING: explanation of WHY that evidence supports your claim using chemistry principles (temperature increase indicates exothermic reaction with energy release from bond formation, precipitate indicates new insoluble substance formed, bubbles indicate gas produced as reaction product). All three components are necessary—evidence alone doesn't justify without reasoning, and reasoning without evidence is just speculation! The CER for this decomposition uses bubbling, temperature rise, and splint relight to confirm gas production (oxygen), indicating a reaction. Choice B provides complete justification with a valid claim of a reaction, evidence of foaming, temperature, and splint test, and sound reasoning on gas formation and energy change. A ignores gas evidence; C assumes yeast always reacts without specifics; D dismisses bubbling as trapped air despite the test—excellent job using tests to strengthen your CER! Building scientific justifications—the CER checklist: (1) CLAIM: State your conclusion clearly: "A chemical reaction occurred" or "No chemical reaction occurred, only physical change." Be definitive based on evidence. (2) EVIDENCE: List 2-3 specific observations or data points from the scenario: "Solution temperature increased from 20°C to 35°C, color changed from clear to yellow, and white solid formed." Use actual numbers and observations, not vague statements. (3) REASONING: For EACH piece of evidence, explain what it indicates: "Temperature increase indicates energy released from chemical bonds forming (exothermic reaction). Color change indicates new substance with different light absorption properties. Solid formation indicates precipitate—new insoluble substance created." Connect evidence to new substance formation! Justification strength evaluation: STRONG justifications cite multiple chemical indicators (gas + precipitate + temperature change) and explain why each indicates reaction. WEAK justifications cite one ambiguous observation (just got warm) without ruling out alternatives. INSUFFICIENT justifications lack reasoning (lists observations without explaining what they mean). The strongest justifications also acknowledge and address potential alternative explanations: "While dissolving can release heat, the combination of temperature increase AND precipitate formation AND color change that can't be explained by mixing strongly supports chemical reaction rather than simple dissolution." This shows critical thinking!

Question 6

A student drops a piece of magnesium ribbon into a test tube containing hydrochloric acid. Immediately, many bubbles form on the metal, the test tube becomes warm, and the magnesium ribbon gets smaller until it is gone. After the reaction, the solution is clear.

Data:

  • Temperature before: 20.5°C
  • Temperature after 1 minute: 32.0°C

Did a chemical reaction occur? Choose the best CER justification.

  1. Claim: A chemical reaction occurred. Evidence: Bubbles formed, the temperature increased from 20.5°C to 32.0°C, and the magnesium disappeared. Reasoning: Gas production and a significant temperature increase indicate new substances formed with an energy change, and the metal being consumed suggests it was converted into different substances—evidence of a chemical reaction. (correct answer)
  2. Claim: No chemical reaction occurred. Evidence: The solution was clear at the end. Reasoning: Clear solutions mean nothing changed chemically, so the magnesium only dissolved physically.
  3. Claim: No chemical reaction occurred. Evidence: The magnesium got smaller. Reasoning: Getting smaller is a physical change like breaking apart, not evidence of new substances.
  4. Claim: A chemical reaction occurred. Evidence: The test tube became warm. Reasoning: Warmth is the only evidence needed because all chemical reactions release heat.

Explanation: This question tests your ability to construct scientific justifications using the Claim-Evidence-Reasoning (CER) framework to determine whether a chemical reaction occurred, distinguishing strong evidence from weak or ambiguous observations. A complete scientific justification has three parts: (1) CLAIM: a clear statement of your conclusion (a chemical reaction occurred, or it did not), (2) EVIDENCE: specific observations or data from the scenario (temperature increased by 15°C, white solid formed, bubbles appeared), and (3) REASONING: explanation of WHY that evidence supports your claim using chemistry principles (temperature increase indicates exothermic reaction with energy release from bond formation, precipitate indicates new insoluble substance formed, bubbles indicate gas produced as reaction product). All three components are necessary—evidence alone doesn't justify without reasoning, and reasoning without evidence is just speculation! Bubbles, warming, and the disappearance of magnesium in acid point to a reaction producing gas and new compounds, with a strong CER connecting these to bond breaking and forming. Choice A provides complete justification with a valid claim of a reaction, relevant evidence like bubbles and 11.5°C temperature rise, and sound chemical reasoning on gas and energy changes. Choices B, C, and D fail by misinterpreting the clear solution or size reduction as physical, or oversimplifying heat as sole evidence. Keep building your scientific justifications using the CER checklist: clearly state your claim, list specific evidence with details, and explain each piece with chemistry principles—remember, strong justifications address alternatives, such as noting disappearance isn't just dissolution due to gas and heat, honing your critical thinking!

Question 7

A student places a piece of magnesium ribbon in a small crucible and heats it strongly.

Observations:

  • The magnesium glows with a bright white light while heating.
  • After heating, the shiny silver metal is gone and a white powder remains.
  • The crucible and contents have a mass increase of 0.30 g compared with before heating.
  • The white powder does not look like the original metal and cannot be reshaped into ribbon.

Based on the evidence, did a chemical reaction occur? Choose the best CER-style justification.

  1. Claim: No chemical reaction occurred. Evidence: A white substance is present after heating. Reasoning: Heating can bleach materials and change their appearance without forming new substances.
  2. Claim: A chemical reaction occurred. Evidence: Bright light was produced, the original metal changed into a different-looking white powder, and mass increased. Reasoning: These observations indicate energy release and formation of a new substance that incorporated something from the air, which supports a chemical reaction. (correct answer)
  3. Claim: No chemical reaction occurred. Evidence: The mass increased. Reasoning: Mass increase proves the sample absorbed heat, not that new substances formed.
  4. Claim: A chemical reaction occurred. Evidence: The crucible was heated strongly. Reasoning: Strong heating always causes chemical reactions, even if no new substances form.

Explanation: This question tests your ability to construct scientific justifications using the Claim-Evidence-Reasoning (CER) framework to determine whether a chemical reaction occurred, distinguishing strong evidence from weak or ambiguous observations. A complete scientific justification has three parts: (1) CLAIM: a clear statement of your conclusion (a chemical reaction occurred, or it did not), (2) EVIDENCE: specific observations or data from the scenario (temperature increased by 15°C, white solid formed, bubbles appeared), and (3) REASONING: explanation of WHY that evidence supports your claim using chemistry principles (temperature increase indicates exothermic reaction with energy release from bond formation, precipitate indicates new insoluble substance formed, bubbles indicate gas produced as reaction product). All three components are necessary—evidence alone doesn't justify without reasoning, and reasoning without evidence is just speculation! Using the bright light, transformation to white powder, and mass increase, the CER justifies a reaction through energy release and new substance formation incorporating air. Choice B provides complete justification with a valid claim of a chemical reaction, evidence of light and mass change, and sound reasoning about incorporation from air. Distractors like A and C misinterpret appearance or mass as non-chemical, while D overgeneralizes heating effects. Building scientific justifications—the CER checklist: (1) CLAIM: State your conclusion clearly: "A chemical reaction occurred" or "No chemical reaction occurred, only physical change." Be definitive based on evidence. (2) EVIDENCE: List 2-3 specific observations or data points from the scenario: "Solution temperature increased from 20°C to 35°C, color changed from clear to yellow, and white solid formed." Use actual numbers and observations, not vague statements. (3) REASONING: For EACH piece of evidence, explain what it indicates: "Temperature increase indicates energy released from chemical bonds forming (exothermic reaction). Color change indicates new substance with different light absorption properties. Solid formation indicates precipitate—new insoluble substance created." Connect evidence to new substance formation! Justification strength evaluation: STRONG justifications cite multiple chemical indicators (gas + precipitate + temperature change) and explain why each indicates reaction. WEAK justifications cite one ambiguous observation (just got warm) without ruling out alternatives. INSUFFICIENT justifications lack reasoning (lists observations without explaining what they mean). The strongest justifications also acknowledge and address potential alternative explanations: "While dissolving can release heat, the combination of temperature increase AND precipitate formation AND color change that can't be explained by mixing strongly supports chemical reaction rather than simple dissolution." This shows critical thinking!

Question 8

A student mixes 10 mL of clear vinegar with 10 mL of clear liquid soap solution in a cup.

Observations:

  • The mixture becomes cloudy and forms lots of foam when stirred.
  • No sustained bubbling occurs when the mixture sits (the foam slowly collapses over 5 minutes).
  • Temperature stays at 20.5°C before and after mixing.
  • When the mixture is left undisturbed for 30 minutes, it separates into two layers.

Based on the evidence, which CER-style justification is best supported?

  1. Claim: A chemical reaction occurred. Evidence: Foam formed. Reasoning: Foam is a gas product, so new substances must have formed.
  2. Claim: No chemical reaction occurred. Evidence: The mixture foamed only when stirred, temperature did not change, and it separated into layers over time. Reasoning: These observations can be explained by physical mixing and trapping air (a physical change) rather than formation of new substances. (correct answer)
  3. Claim: A chemical reaction occurred. Evidence: The mixture separated into layers. Reasoning: Layering proves new substances formed because reactions always make mixtures separate.
  4. Claim: A chemical reaction occurred. Evidence: The mixture turned cloudy. Reasoning: Cloudiness always means a precipitate formed, so a reaction definitely happened even though it later separated into layers.

Explanation: This question tests your ability to construct scientific justifications using the Claim-Evidence-Reasoning (CER) framework to determine whether a chemical reaction occurred, distinguishing strong evidence from weak or ambiguous observations. A complete scientific justification has three parts: (1) CLAIM: a clear statement of your conclusion (a chemical reaction occurred, or it did not), (2) EVIDENCE: specific observations or data from the scenario (temperature increased by 15°C, white solid formed, bubbles appeared), and (3) REASONING: explanation of WHY that evidence supports your claim using chemistry principles (temperature increase indicates exothermic reaction with energy release from bond formation, precipitate indicates new insoluble substance formed, bubbles indicate gas produced as reaction product). All three components are necessary—evidence alone doesn't justify without reasoning, and reasoning without evidence is just speculation! The CER is formed from foaming only when stirred, no temperature change, collapsing foam, and layer separation, supporting no reaction as physical mixing with air trapping. Choice B provides complete justification with a valid claim of no chemical reaction, evidence of stirring-dependent foam and separation, and sound reasoning about physical changes. Choices like A and D overinterpret foam or cloudiness as definitive chemical signs without considering alternatives, while C misattributes layering. Building scientific justifications—the CER checklist: (1) CLAIM: State your conclusion clearly: "A chemical reaction occurred" or "No chemical reaction occurred, only physical change." Be definitive based on evidence. (2) EVIDENCE: List 2-3 specific observations or data points from the scenario: "Solution temperature increased from 20°C to 35°C, color changed from clear to yellow, and white solid formed." Use actual numbers and observations, not vague statements. (3) REASONING: For EACH piece of evidence, explain what it indicates: "Temperature increase indicates energy released from chemical bonds forming (exothermic reaction). Color change indicates new substance with different light absorption properties. Solid formation indicates precipitate—new insoluble substance created." Connect evidence to new substance formation! Justification strength evaluation: STRONG justifications cite multiple chemical indicators (gas + precipitate + temperature change) and explain why each indicates reaction. WEAK justifications cite one ambiguous observation (just got warm) without ruling out alternatives. INSUFFICIENT justifications lack reasoning (lists observations without explaining what they mean). The strongest justifications also acknowledge and address potential alternative explanations: "While dissolving can release heat, the combination of temperature increase AND precipitate formation AND color change that can't be explained by mixing strongly supports chemical reaction rather than simple dissolution." This shows critical thinking!

Question 9

A student mixes 50 mL of a blue solution with 50 mL of a yellow solution in a beaker.

Observations:

  • The mixture becomes green immediately.
  • The temperature remains 22.0°C before and after mixing.
  • No bubbles form.
  • No solid forms, even after sitting 10 minutes.
  • When the student separates the mixture using a simple paper chromatography strip, two distinct color bands (blue and yellow) appear.

Based on the evidence, is there sufficient evidence to conclude a chemical reaction occurred? Choose the best CER-style justification.

  1. Claim: A chemical reaction occurred. Evidence: The solution turned green. Reasoning: Any color change proves a new substance formed.
  2. Claim: No chemical reaction occurred. Evidence: The green color can be explained by mixing blue and yellow, there was no temperature change, no gas, and chromatography shows the original dyes are still present. Reasoning: The observations are consistent with physical mixing of pigments rather than formation of new substances. (correct answer)
  3. Claim: A chemical reaction occurred. Evidence: Two solutions were combined. Reasoning: Combining chemicals always produces new substances.
  4. Claim: No chemical reaction occurred. Evidence: No solid formed. Reasoning: Chemical reactions only happen if a solid precipitate forms.

Explanation: This question tests your ability to construct scientific justifications using the Claim-Evidence-Reasoning (CER) framework to determine whether a chemical reaction occurred, distinguishing strong evidence from weak or ambiguous observations. A complete scientific justification has three parts: (1) CLAIM: a clear statement of your conclusion (a chemical reaction occurred, or it did not), (2) EVIDENCE: specific observations or data from the scenario (temperature increased by 15°C, white solid formed, bubbles appeared), and (3) REASONING: explanation of WHY that evidence supports your claim using chemistry principles (temperature increase indicates exothermic reaction with energy release from bond formation, precipitate indicates new insoluble substance formed, bubbles indicate gas produced as reaction product). All three components are necessary—evidence alone doesn't justify without reasoning, and reasoning without evidence is just speculation! From the color change to green, no temperature shift, and chromatography separation into original colors, the CER justifies no reaction as it's physical mixing of dyes. Choice B provides complete justification with a valid claim of no chemical reaction, evidence including chromatography results, and sound reasoning that this shows unchanged substances. Distractors such as A and C overgeneralize color changes or mixing as always chemical, while D incorrectly requires precipitates for reactions. Building scientific justifications—the CER checklist: (1) CLAIM: State your conclusion clearly: "A chemical reaction occurred" or "No chemical reaction occurred, only physical change." Be definitive based on evidence. (2) EVIDENCE: List 2-3 specific observations or data points from the scenario: "Solution temperature increased from 20°C to 35°C, color changed from clear to yellow, and white solid formed." Use actual numbers and observations, not vague statements. (3) REASONING: For EACH piece of evidence, explain what it indicates: "Temperature increase indicates energy released from chemical bonds forming (exothermic reaction). Color change indicates new substance with different light absorption properties. Solid formation indicates precipitate—new insoluble substance created." Connect evidence to new substance formation! Justification strength evaluation: STRONG justifications cite multiple chemical indicators (gas + precipitate + temperature change) and explain why each indicates reaction. WEAK justifications cite one ambiguous observation (just got warm) without ruling out alternatives. INSUFFICIENT justifications lack reasoning (lists observations without explaining what they mean). The strongest justifications also acknowledge and address potential alternative explanations: "While dissolving can release heat, the combination of temperature increase AND precipitate formation AND color change that can't be explained by mixing strongly supports chemical reaction rather than simple dissolution." This shows critical thinking!

Question 10

A student adds a small scoop of a powdered drink mix to a cup of water and stirs.

Observations/data:

  • The powder disappears and the liquid becomes uniformly red.
  • A few tiny bubbles appear on the sides of the cup for about 10 seconds, then stop.
  • Temperature changes from 24.0°C to 23.6°C.
  • After 15 minutes, no solid settles out.
  • When a drop of the drink is placed on a watch glass and the water evaporates, a sticky red residue remains.

Based on the evidence, which CER-style justification is best supported?

  1. Claim: A chemical reaction occurred. Evidence: The liquid turned red. Reasoning: A color change always means a new substance formed, so it must be a reaction.
  2. Claim: No chemical reaction occurred. Evidence: The powder dissolved to make a colored solution, there was only a small temperature change, and evaporation left residue (the solute) behind. Reasoning: Dissolving and later recovering solute by evaporation are physical changes; the brief bubbles could be trapped air released during stirring rather than gas produced by a reaction. (correct answer)
  3. Claim: A chemical reaction occurred. Evidence: Tiny bubbles appeared. Reasoning: Any bubbles, even for a few seconds, prove a gas was produced by a chemical reaction.
  4. Claim: No chemical reaction occurred. Evidence: The temperature decreased by 0.4°C. Reasoning: Temperature decreases can only happen in physical changes, never in chemical reactions.

Explanation: This question tests your ability to construct scientific justifications using the Claim-Evidence-Reasoning (CER) framework to determine whether a chemical reaction occurred, distinguishing strong evidence from weak or ambiguous observations. A complete scientific justification has three parts: (1) CLAIM: a clear statement of your conclusion (a chemical reaction occurred, or it did not), (2) EVIDENCE: specific observations or data from the scenario (temperature increased by 15°C, white solid formed, bubbles appeared), and (3) REASONING: explanation of WHY that evidence supports your claim using chemistry principles (temperature increase indicates exothermic reaction with energy release from bond formation, precipitate indicates new insoluble substance formed, bubbles indicate gas produced as reaction product). All three components are necessary—evidence alone doesn't justify without reasoning, and reasoning without evidence is just speculation! The CER incorporates dissolving to red solution, brief bubbles, small temperature drop, and residue recovery, justifying no reaction as physical dissolving with possible air release. Choice B provides complete justification with a valid claim of no chemical reaction, evidence including evaporation recovery, and sound reasoning distinguishing from chemical indicators. Options like A and C overstate color or bubbles as definitive proof without alternatives, while D misattributes temperature decreases. Building scientific justifications—the CER checklist: (1) CLAIM: State your conclusion clearly: "A chemical reaction occurred" or "No chemical reaction occurred, only physical change." Be definitive based on evidence. (2) EVIDENCE: List 2-3 specific observations or data points from the scenario: "Solution temperature increased from 20°C to 35°C, color changed from clear to yellow, and white solid formed." Use actual numbers and observations, not vague statements. (3) REASONING: For EACH piece of evidence, explain what it indicates: "Temperature increase indicates energy released from chemical bonds forming (exothermic reaction). Color change indicates new substance with different light absorption properties. Solid formation indicates precipitate—new insoluble substance created." Connect evidence to new substance formation! Justification strength evaluation: STRONG justifications cite multiple chemical indicators (gas + precipitate + temperature change) and explain why each indicates reaction. WEAK justifications cite one ambiguous observation (just got warm) without ruling out alternatives. INSUFFICIENT justifications lack reasoning (lists observations without explaining what they mean). The strongest justifications also acknowledge and address potential alternative explanations: "While dissolving can release heat, the combination of temperature increase AND precipitate formation AND color change that can't be explained by mixing strongly supports chemical reaction rather than simple dissolution." This shows critical thinking!