ACT Science: Identify Controlled Variables Like a Lab Scientist

Every experiment has three types of variables: (1) Independent variable: the one the scientist changes deliberately (the input). (2) Dependent variable: what the scientist measures as a result (the output). (3) Controlled variables: everything else held constant so the independent variable is the only cause of change. Example: A scientist tests how temperature affects enzyme activity. Independent variable: temperature (changed by scientist). Dependent variable: enzyme activity (measured by scientist). Controlled variables: pH, enzyme concentration, substrate type (all kept the same). Identifying these three instantly answers 80% of experiment design questions on ACT Science.

Another example: A study investigates whether fertilizer increases plant height. Independent variable: fertilizer amount. Dependent variable: plant height (measured). Controlled variables: soil type, water amount, sunlight, pot size (all identical across all plants). If the scientist accidentally used different soil for different plants, that introduces a confounding variable and ruins the experiment. Recognizing what must be controlled is the core skill.

Pitfall 1: Confounding variables. If you change temperature AND pH together, you cannot tell which caused the result. Pitfall 2: No control group. A study with no baseline (no-treatment group) cannot measure the effect of treatment. Pitfall 3: Changing multiple independent variables at once. Each experiment should test one variable; testing multiple at once creates ambiguity. Pitfall 4: Insufficient sample size. One plant cannot represent all plants; you need replicates to ensure results are not due to chance. Pitfall 5: Biased measurement. If the scientist measures temperature with a thermometer that is off by 2 degrees, results are skewed. ACT Science questions often ask: "What would weaken this experiment?" The answer is usually one of these five pitfalls.

Example: A passage describes testing whether salt increases boiling point. The scientist boils water in a kettle, adds salt, and measures boiling point. Problem: temperature is not controlled (room temperature may vary). Better: boil water in identical containers at identical starting temperatures, add different salt amounts, measure boiling point. Controlled variables: container type, starting temperature, pressure (seal the containers).

Experiment 1: A biologist grows plant seeds under three different light conditions (red, blue, white) and measures stem height after 30 days. Identify independent, dependent, and controlled variables. Experiment 2: A physicist drops balls of different masses from the same height and measures the time to hit the ground. Identify the variables. Experiment 3: A chemist tests how pH affects the speed of a chemical reaction by varying pH from 3 to 11 and measuring reaction time in seconds. Identify the variables. For each, (1) write the independent variable, (2) write the dependent variable, (3) list three controlled variables that must remain constant. Do this drill twice this week until variable identification becomes automatic.

Answers: Exp. 1: Independent=light color, Dependent=stem height, Controlled=soil type, water amount, temperature, pot size. Exp. 2: Independent=mass, Dependent=time to ground, Controlled=height, surface (ground), air resistance (outdoors or in vacuum). Exp. 3: Independent=pH, Dependent=reaction time, Controlled=temperature, reactant concentrations, volume.

Questions about experimental design appear on every ACT Science section and test one core skill: can you identify what is being changed, what is being measured, and what must stay the same? Mastering this three-part framework answers design questions instantly, earning you 2-3 points per test with zero guessing.

This week, identify variables in every experiment passage you encounter. By test day, this skill will be so automatic that you answer design questions faster than you read them.