ACT Experimental Design Practice Questions with Answers

Twenty-five percent of the ACT Science section specifically evaluates your ability to understand, evaluate, and design scientific investigations. This makes ACT Experimental Design a critical skill for students aiming for a top-tier score. Unlike biology or chemistry tests that require rote memorization, this portion of the exam focuses on the logic of the scientific method: how variables interact, why certain controls are necessary, and how modifications to a procedure might alter the results.

Concept Explanation

ACT Experimental Design is the study of how scientific trials are structured to ensure that the data collected is valid, reliable, and directly addresses a specific hypothesis. To excel in this area, you must identify three primary types of variables: independent, dependent, and controlled. The independent variable is the factor the scientist intentionally changes, while the dependent variable is the outcome being measured. Controlled variables, or constants, are factors kept the same to ensure a fair test. According to the Scientific American, rigorous experimental design is the backbone of all credible peer-reviewed research.

Standard questions on the ACT will ask you to identify the purpose of a specific step in a protocol or predict how a change in the setup would affect the data. You may also encounter "control groups," which are samples that do not receive the experimental treatment and serve as a baseline for comparison. Understanding the relationship between these elements is essential for success in ACT Prep. When analyzing a passage, always ask: "What is being changed?", "What is being measured?", and "What stayed the same?"

Solved Examples

1. Identifying Variables: A student wants to see if the temperature of water affects how fast salt dissolves. She adds 10g of salt to three beakers containing 100mL of water at $20^\circ \text{C}$, $40^\circ \text{C}$, and $60^\circ \text{C}$. She stirs each for 30 seconds and measures the mass of undissolved salt.
   1. Identify the independent variable: The temperature of the water (the factor being changed).
   2. Identify the dependent variable: The mass of undissolved salt (the factor being measured).
   3. Identify the constants: The amount of water (100mL), the amount of salt (10g), and the stirring time (30 seconds).
2. Understanding Control Groups: In a study testing a new fertilizer's effect on tomato plant height, Group A receives the fertilizer while Group B receives only plain water. Both groups are kept in the same greenhouse.
   1. What is the purpose of Group B? It serves as the control group to provide a baseline for how tomato plants grow without the fertilizer.
   2. Why must they be in the same greenhouse? To ensure that light and temperature (controlled variables) do not influence the results differently between the two groups.
3. Predicting Experimental Changes: A scientist measures the pressure of a gas in a sealed container as the volume is decreased. The data shows that as volume decreases, pressure increases. If the scientist repeated the experiment with a larger initial amount of gas, what would likely happen to the pressure readings?
   1. Analyze the relationship: Pressure and volume are inversely related.
   2. Apply the change: Adding more gas particles increases the frequency of collisions.
   3. Conclusion: The pressure readings at each volume would be higher than in the first experiment.

Practice Questions

1. A researcher investigates the effect of different pH levels on the activity of a specific enzyme. Five test tubes are prepared with the same enzyme concentration and substrate amount, but each has a different pH. After 10 minutes, the amount of product formed is measured. Which of the following is the independent variable in this experiment?

2. In an experiment designed to test the durability of a new car tire, four tires are placed on a machine that simulates road wear for 500 hours. A fifth tire of a standard brand is also tested under the same conditions. What role does the fifth tire play in this design?

3. A student hypothesizes that sound travels faster through solids than through liquids. To test this, the student measures the time it takes for sound to travel 5 meters through a steel rod and 5 meters through a tank of water. Which of the following must be kept constant to ensure a valid comparison?

4. An experiment measures the boiling point of water after adding varying amounts of sodium chloride (NaCl). If the researcher wants to improve the precision of the results, which of the following actions should be taken?

5. A study finds that plants grown under blue light grow taller than plants grown under green light. A critic suggests the results are invalid because the blue light was 50% brighter than the green light. This criticism targets which aspect of the experimental design?

6. Scientists are testing a new pesticide on a field of corn. They divide the field into two equal plots: Plot X receives the pesticide, and Plot Y receives a spray of plain water. If Plot Y were removed from the study, why would the results be less reliable?

7. A student measures the rate of a chemical reaction at $25^\circ \text{C}$, $35^\circ \text{C}$, and $45^\circ \text{C}$. He uses a 1.0M concentration of reactants for each trial. If he wanted to test the effect of concentration instead of temperature, how should he modify the next set of trials?

8. To determine the effect of caffeine on heart rate, 50 volunteers are given 200mg of caffeine and their heart rates are measured. Which of the following additions would most strengthen this experimental design?

Answers & Explanations

1. The pH level. The independent variable is the factor that the researcher deliberately changes or manipulates to observe its effect. In this case, the pH is varied across the five test tubes.
2. It acts as a control group. By testing a standard brand tire under the same conditions, the researchers can compare the performance of the new tire against an existing, known baseline. This is similar to how you might analyze ACT Reading Practice passages to find evidence for a claim.
3. The temperature of the substances. Temperature affects the speed of sound in both solids and liquids. If the steel rod were much hotter than the water, the results would be skewed by two variables instead of just the state of matter.
4. Perform multiple trials for each amount of NaCl and average the results. Increasing the number of trials reduces the impact of random measurement errors and provides a more reliable average, which is a key concept in ACT Subject Verb Agreement-style logical consistency.
5. Controlled variables. For the experiment to be valid, the intensity of the light should have been kept constant. Because the intensity varied, it is impossible to know if the growth was due to the color of the light or the brightness.
6. There would be no baseline to determine if changes in Plot X were caused by the pesticide or by natural environmental factors. Without Plot Y, scientists couldn't prove the corn wouldn't have grown that way regardless of the pesticide.
7. Keep the temperature constant at $25^\circ \text{C}$ and vary the concentration (e.g., 0.5M, 1.0M, 1.5M). To isolate the effect of a new variable, the previous independent variable (temperature) must become a constant.
8. Including a second group of 50 volunteers who receive a placebo (decaffeinated drink). This accounts for the "placebo effect" and ensures that any change in heart rate is actually due to the caffeine rather than the act of drinking something or the volunteers' expectations.

1. Which of the following best describes a "constant" in an experiment?

• AThe variable that is changed by the scientist
• BThe variable that is measured as the result
• CA factor that is kept the same across all trials
• DThe group that does not receive the experimental treatment

Frequently Asked Questions

What is the difference between an independent and dependent variable?

The independent variable is the factor you change to see what happens, while the dependent variable is the result you observe or measure. Think of it as cause (independent) and effect (dependent).

Why is a control group necessary in ACT Science passages?

A control group provides a baseline that allows researchers to see what would happen under normal conditions without the experimental intervention. It proves that the results were caused by the variable being tested rather than random chance.

How can I identify the experimental design in a complex passage?

Look for the "Methods" or "Study" section and identify the differences between the various groups or trials. If you're struggling with logical flow, practicing with ACT Transition Practice can help you follow the sequence of steps in a scientific protocol.

What are "confounding variables"?

Confounding variables are uncontrolled factors that might influence the results of an experiment, making it difficult to determine the true cause of an effect. Good experimental design seeks to eliminate these by keeping them constant.

How does the ACT test my knowledge of experimental design?

The ACT rarely asks for definitions; instead, it asks you to apply these concepts to specific scenarios, such as predicting what would happen if a step was skipped or why a certain tool was used. You can use tools like an AI Question Generator to practice these application-based scenarios.

What should I do if an experiment has more than one study?

Compare the studies to see what changed between them. Usually, Study 2 will build on Study 1 by changing a different variable or using a more precise measurement tool to refine the initial findings.