Variables & Controls - ACT Science

In an experiment to test the effect of sugar concentration on yeast fermentation, different sugar concentrations were used while the type of yeast and temperature were kept constant. The amount of carbon dioxide produced was measured. What was the independent variable?
Answer: Sugar concentration The independent variable is sugar concentration (what was varied across different levels), the dependent variable is amount of carbon dioxide produced (what was measured), and controlled variables include type of yeast and temperature (kept constant). The independent variable is the factor deliberately manipulated by the experimenter to test its effects. Choice A correctly identifies sugar concentration as the independent variable since this was intentionally varied to study its effect on fermentation.
A nutrition student tested whether the concentration of vitamin C solution affects how quickly it reduces a blue dye (DCPIP) to colorless. She prepared vitamin C solutions at 0.10%, 0.20%, 0.30%, and 0.40% (mass/volume). For each trial, she added 5.0 mL of a vitamin C solution to 5.0 mL of DCPIP in the same type of test tube, gently swirled for 5 seconds, and measured the time (s) until the mixture became colorless. The dye volume, total liquid volume, swirl time, and room lighting were kept constant. To isolate the effect of vitamin C concentration, the experimenter controlled:
Answer: Volume of DCPIP used in each trial The independent variable is vitamin C concentration (0.10%, 0.20%, 0.30%, 0.40% - what was changed), the dependent variable is time to become colorless (what was measured), and controlled variables include DCPIP volume, vitamin C solution volume, total volume, swirl time, and lighting (all kept constant). The student kept DCPIP volume at 5.0 mL for all trials to isolate concentration effects. Choice C correctly identifies DCPIP volume as a controlled variable because it was held constant. Choice B (vitamin C concentration) is the independent variable being manipulated, not a controlled factor.
In a study on how different cooking oils affect food crispiness, the type of oil was varied while keeping cooking temperature and time constant. The crispiness of the food was assessed. Which variable was held constant?
Answer: Cooking temperature The independent variable is type of oil (what was varied), the dependent variable is food crispiness (what was assessed), and controlled variables include cooking temperature and cooking time (kept constant). Controlled variables are factors that researchers deliberately keep the same to isolate the effect of the independent variable. Choice B correctly identifies cooking temperature as a controlled variable since it was kept constant along with cooking time. Choice A represents the independent variable (what was varied), not a controlled variable.
A chemist studied how stirring speed affects the time required for 5.0 g of sugar to dissolve in 100 mL of water. She used a magnetic stir plate and tested four stirring speeds: 0 rpm, 200 rpm, 400 rpm, and 600 rpm. The water temperature was kept constant at 25°C using a water bath, and the same beaker type and sugar brand were used each time. For each trial, the chemist started a stopwatch when sugar was added and stopped it when no visible crystals remained. Which of the following is the independent variable?
Answer: Stirring speed (rpm) In this experiment, the independent variable is the stirring speed (0 rpm, 200 rpm, 400 rpm, and 600 rpm), the dependent variable is the time for the sugar to dissolve, and controlled variables include the mass of sugar (5.0 g), water volume (100 mL), water temperature (25°C), beaker type, and sugar brand. The independent variable is the factor the experimenter changes on purpose, the dependent variable is what is observed and recorded as the result, and controlled variables are maintained the same in all trials to prevent them from affecting the outcome. Choice C correctly identifies stirring speed as the independent variable because it was deliberately varied to study its effect on dissolution time. Choice A is a distractor as it represents a controlled variable, the water temperature kept at 25°C, not the manipulated factor.
Scientists aimed to determine how the concentration of salt in water affects its boiling point. They varied the salt concentration from 0% to 20% while maintaining a constant atmospheric pressure and used the same pot for each trial. The boiling point was recorded. The experimenter manipulated [blank] and measured [blank].
Answer: Salt concentration; Boiling point The independent variable is salt concentration (varied from 0% to 20%), the dependent variable is boiling point (what was recorded), and controlled variables include atmospheric pressure and pot type (kept constant). The independent variable is what the experimenter deliberately manipulates, while the dependent variable is the response that's measured. Choice D correctly identifies that researchers manipulated salt concentration and measured boiling point. The other choices incorrectly pair variables or identify controlled variables as manipulated ones.
Scientists tested how different colors of light affect algae growth. Light color was varied while keeping light intensity and water temperature constant. Algae biomass was measured. In this experiment, the dependent variable is:
Answer: Algae biomass The independent variable is light color (what was varied), the dependent variable is algae biomass (what was measured), and controlled variables include light intensity and water temperature (kept constant). The dependent variable is the outcome or response that researchers measure to determine the effect of the independent variable. Choice B correctly identifies algae biomass as the dependent variable since it's what scientists measured to assess algae growth. Choice A represents the independent variable (what was varied), not the dependent variable.
A student investigated how salt concentration affects the boiling point of water. She prepared four 200 mL samples of distilled water in identical beakers and added NaCl to make concentrations of 0%, 2%, 4%, and 6% (mass/volume). Each beaker was heated on the same hot plate set to the same power setting, and the water was stirred at a constant rate with the same magnetic stir bar. For each trial, the student recorded the temperature when vigorous boiling was first sustained for 30 s using the same calibrated thermometer. Which of the following is the independent variable?
Answer: Salt concentration of the water The independent variable is salt concentration (what's being deliberately changed), the dependent variable is boiling-point temperature (what's being measured as a response), and controlled variables include water volume, hot plate setting, stirring rate, and thermometer type (kept constant across trials). The experiment tests four different salt concentrations (0%, 2%, 4%, 6%) to see their effect on boiling point. Choice B correctly identifies salt concentration as the independent variable since this is what the researcher systematically manipulated. The boiling-point temperature (Choice D) is actually the dependent variable being measured in response to the salt concentration changes.
A student tested the hypothesis that increasing the number of paper towel layers improves water absorption. She cut paper towels from the same brand into identical 10 cm × 10 cm squares and stacked them to make pads with 1 layer, 2 layers, 3 layers, or 4 layers. Each pad was placed in a shallow tray containing 100 mL of water for exactly 30 seconds. After removal, the pad was allowed to drip for 5 seconds, then weighed on the same balance. The tray size, water volume, soak time, and towel brand were kept constant. Which of the following is the independent variable?
Answer: Number of paper towel layers in the pad The independent variable is number of paper towel layers (1, 2, 3, or 4 layers - what was systematically changed), the dependent variable is mass after soaking (what was measured), and controlled variables include towel size, water volume, soak time, drip time, and towel brand (all kept constant). The student varied the number of layers to test absorption capacity. Choice B correctly identifies number of layers as the independent variable because it's the factor being manipulated. Choice A (mass after soaking) is the dependent variable being measured as the outcome.
To study how sound frequency affects a tuning fork's vibration duration, a student used 4 tuning forks labeled 256 Hz, 320 Hz, 384 Hz, and 512 Hz. Each fork was struck with the same rubber mallet using the same force technique and then held by the handle above a foam pad to reduce external vibrations. Using a stopwatch, the student measured how long (seconds) the fork remained audible to the same listener in a quiet room. The room, mallet, and listening distance were kept the same. Which variable was held constant?
Answer: Rubber mallet used to strike the forks The independent variable is tuning fork frequency (256Hz, 320Hz, 384Hz, 512Hz - what was changed), the dependent variable is vibration duration (what was measured), and controlled variables include the rubber mallet, striking technique, foam pad setup, room conditions, and listening distance (all kept constant). To ensure consistent striking, the same rubber mallet was used for all trials. Choice C correctly identifies the rubber mallet as a controlled variable because it was kept the same throughout. Choice A (frequency) is the independent variable being tested, not a controlled factor.
A student tested how the type of metal affects corrosion rate in saltwater. Equal-sized strips of iron, copper, aluminum, and zinc were cleaned the same way and then placed into separate beakers containing 200 mL of 3.5% saltwater. All beakers were kept at the same temperature and left undisturbed for 7 days. Corrosion was measured by the mass lost (g) from each strip after drying. Which of the following is the independent variable?
Answer: Type of metal strip used The independent variable is type of metal strip used (what's being systematically changed), the dependent variable is mass lost from each metal strip (what's being measured), and controlled variables include saltwater concentration, immersion time, strip size, cleaning method, and temperature (kept constant). The experiment tests four different metals (iron, copper, aluminum, zinc) to compare their corrosion rates. Choice C correctly identifies type of metal as the independent variable since this is what the student deliberately varied. Mass lost (Choice D) is actually the dependent variable being measured as the response to different metal types.
In an experiment to determine the effect of wind speed on evaporation rate, wind speed was varied while keeping ambient temperature and humidity constant. The rate of water evaporation was recorded. Which of the following is the independent variable?
Answer: Wind speed The independent variable is wind speed (what was varied), the dependent variable is evaporation rate (what was recorded), and controlled variables include ambient temperature and humidity (kept constant). The independent variable is the factor that researchers deliberately manipulate to observe its effects on the outcome. Choice D correctly identifies wind speed as the independent variable since it was the factor being varied to test its effect on evaporation. Choice B represents the dependent variable (what was measured), not the independent variable.
An environmental science student tested whether pH affects how quickly limestone dissolves. He placed identical 5.0 g limestone chips into 100 mL of solution at pH 3, pH 5, pH 7, and pH 9. Each solution was kept at $25^\circ\text{C}$ and stirred at the same rate. After 30 minutes, he removed, dried, and weighed the remaining limestone to determine mass lost. To isolate the effect of pH, the experimenter controlled:
Answer: Temperature of the solutions ( $25^\circ\text{C}$ ) In this experiment, the independent variable is the solution pH values, the dependent variable is the mass lost by limestone after 30 minutes, and controlled variables include the temperature of the solutions, limestone chip mass, solution volume, stirring rate, and time. The independent variable is changed on purpose to test its effect, the dependent variable is measured to see the response, and controlled variables are maintained constant to prevent confounding the results. Choice C correctly identifies the temperature of the solutions as controlled, kept at 25°C across all trials to isolate the effect of pH on dissolution rate. Choice D is the independent variable, as pH was manipulated (3, 5, 7, 9), not controlled.
In an experiment measuring how soil pH affects plant leaf color, the pH of the soil was altered while light exposure and water availability were kept constant. The color intensity of the leaves was measured. Which variable was held constant?
Answer: Light exposure The independent variable is soil pH (what was altered), the dependent variable is leaf color (measured as color intensity), and controlled variables include light exposure and water availability (kept constant). Controlled variables are factors that researchers deliberately keep the same to ensure that only the independent variable affects the dependent variable. Choice C correctly identifies light exposure as a controlled variable since it was kept constant along with water availability. Choice D represents the independent variable (what was altered), not a controlled variable.
A biology student tested how salinity affects brine shrimp survival. She prepared four tanks with salinities of 0 ppt, 10 ppt, 20 ppt, and 30 ppt using the same brand of salt and the same total water volume. Each tank received 25 shrimp of similar size. All tanks were kept under the same light cycle and at $24^\circ$ C. After 48 hours, she counted the number of living shrimp. Which of the following is the independent variable?
Answer: Salinity of the tank water (ppt) The independent variable is salinity of the tank water in ppt (what's being systematically changed), the dependent variable is number of living shrimp after 48 hours (what's being measured), and controlled variables include water temperature, light cycle, total water volume, and number of shrimp per tank (kept constant). The experiment tests four different salinities (0, 10, 20, 30 ppt) to determine their effect on shrimp survival. Choice C correctly identifies salinity as the independent variable since this is what the student deliberately manipulated. The number of living shrimp (Choice D) is actually the dependent variable being measured as a response.
Researchers explored how different soil types affect plant root depth. Soil type was varied while keeping water and light levels constant. Root depth was measured. The experimenter manipulated [blank] and measured [blank].
Answer: Soil type; Root depth The independent variable is soil type (what was varied), the dependent variable is root depth (what was measured), and controlled variables include water and light levels (kept constant). The independent variable is what researchers systematically change, while the dependent variable is the outcome they measure in response. Choice D correctly identifies that researchers manipulated different soil types and measured plant root depth. The other choices incorrectly pair variables or identify controlled variables as manipulated ones.
A student tested the hypothesis that fertilizer amount affects bean plant growth. She planted one bean seed per pot in identical pots containing the same mass of potting soil. Each pot received one of four fertilizer amounts mixed into the soil: 0 g, 2 g, 4 g, or 6 g. All pots were placed on the same windowsill and rotated daily. Each pot received 50 mL of water every day at the same time. After 21 days, the student measured plant height (cm) from soil surface to the top leaf. Which factor was controlled in this experiment?
Answer: Daily water volume per pot In this experiment, the independent variable is the amount of fertilizer added, the dependent variable is the plant height after 21 days, and controlled variables include the daily water volume per pot, number of days grown, pot type, soil mass, and location. The independent variable is changed intentionally to study its influence, the dependent variable is the measured effect, and controlled variables are kept unchanged to avoid confounding the results. Choice C correctly identifies the daily water volume as a controlled variable because it was held constant at 50 mL per day for all pots to isolate the fertilizer's effect. Choice D is the independent variable, as fertilizer amounts were varied (0 g, 2 g, 4 g, 6 g), not controlled.
Which of the following did the students have to hold constant for this comparison to be fair?
Answer: The speed of the air in the wind tunnel Wind speed is set to 8 m/s for every model in Figure 1. If one model were tested in faster air, any difference in power could come from the wind rather than the blades. The number of blades is the variable being changed on purpose, and the power produced is what is being measured, so neither can be held constant.
A student investigated how salt concentration affects the boiling point of water. She prepared four 250 mL beakers, each containing 200 mL of distilled water. She added table salt to make solutions of 0%, 2%, 4%, and 6% salt by mass. Each beaker was heated on the same hot plate setting, starting at room temperature (22°C). The student stirred each solution at a constant rate with the same glass rod and recorded the temperature when the liquid reached a steady rolling boil for 30 seconds. She used the same thermometer for all trials and measured boiling point to the nearest 0.1°C. Which of the following is the independent variable?
Answer: Salt concentration of the solution In this experiment, the independent variable is the salt concentration of the solution, which was varied at 0%, 2%, 4%, and 6%; the dependent variable is the temperature at which boiling began, which was measured for each solution; and controlled variables include the volume of water (200 mL in each beaker), the hot plate model and setting, the starting temperature (22°C), stirring rate, and thermometer used. The independent variable is the factor deliberately manipulated by the experimenter to observe its effect, the dependent variable is the outcome measured to see the response, and controlled variables are kept constant across trials to ensure the results are due to the independent variable alone. Choice C correctly identifies the salt concentration as the independent variable because it was systematically changed while other factors were held steady. Choice D is a distractor as it represents the dependent variable, which is the measured boiling temperature, not the manipulated factor.
A student examined whether fertilizer amount changes the number of flowers produced by marigold plants. She grew identical seedlings in 12 identical pots with the same soil type. Pots were assigned to receive 0 g, 5 g, 10 g, or 15 g of fertilizer per week (3 pots per group). All plants received the same volume of water each day, were kept in the same greenhouse with the same light schedule, and were grown for 6 weeks. At the end, she counted the number of flowers on each plant. Which factor was controlled in this experiment?
Answer: Daily volume of water given to each plant The independent variable is fertilizer amount (0g, 5g, 10g, or 15g per week - what was changed), the dependent variable is number of flowers (what was measured), and controlled variables include water volume, soil type, pot size, light schedule, and growth duration (all kept constant). To isolate fertilizer's effect, the student kept water volume the same for all plants. Choice C correctly identifies daily water volume as a controlled variable because it was held constant across all treatment groups. Choice A (fertilizer amount) is the independent variable being tested, not a controlled factor.
Researchers studied how the concentration of a pollutant affects fish respiration rates. Pollutant levels were varied, while water temperature and fish species were kept constant. The respiration rate was recorded. The experimenter manipulated [blank] and measured [blank].
Answer: Pollutant concentration; Respiration rate The independent variable is pollutant concentration (what was varied), the dependent variable is respiration rate (what was recorded), and controlled variables include water temperature and fish species (kept constant). The independent variable is what researchers systematically manipulate, while the dependent variable is the response they measure. Choice A correctly identifies that researchers manipulated pollutant concentration and measured respiration rate. The other choices incorrectly pair variables or identify controlled variables as manipulated ones.

Start this quiz