The manipulated variable is what you change, the responding variable is what you measure, and the fixed variables are what you keep the same. Say those three things in one sentence and the answer falls out.
This lesson follows reading apparatus and measurement precision in measurement and chemical investigation.
What is the sentence test?
Fill in this sentence using the experiment: “I change ___, I measure ___, and I keep ___ the same.”
The blank after “change” is the manipulated variable. The blank after “measure” is the responding variable. The blank after “keep” lists the fixed variables.
Worked example: two experiments, two sets of variables
Experiment A. A student reacts 2 g of magnesium ribbon with 50 mL of hydrochloric acid of 0.5, 1.0 and 1.5 mol dm⁻³ and times how long the ribbon takes to disappear.
- Manipulated: concentration of the acid.
- Responding: time for the magnesium to disappear.
- Fixed: mass and size of magnesium, volume of acid, temperature.
Experiment B. The same student heats the acid to 30 °C, 40 °C and 50 °C, using 1.0 mol dm⁻³ acid each time.
- Manipulated: temperature of the acid.
- Responding: time for the magnesium to disappear.
- Fixed: mass and size of magnesium, volume and concentration of acid.
Notice that the responding variable is the same, but the fixed variables differ. In A the temperature is fixed, and in B the concentration is fixed.
The swap that loses marks
The usual slip is to write the responding variable as the manipulated one. “Time taken” is measured, so it cannot be the thing you change.
Another slip is to list “the experiment” or “the same apparatus” as a fixed variable. Name the physical quantity that could affect the result, such as volume of acid or mass of magnesium.
How to justify a fixed variable
If a question asks why a variable is fixed, write one sentence linking it to the result. For example, “the mass of magnesium is fixed because a larger mass would take longer to react and change the time, so the test would be unfair”.
This pattern works for any fixed variable. Name it, then state what would change in the result if it varied.
Check yourself
A student compares how fast a 4 g marble chip reacts with 0.5 mol dm⁻³ acid when the chips are whole, halved and crushed. She measures the volume of gas in the first 2 minutes. State the manipulated, responding and two fixed variables.
Answer
Manipulated variable: the size of the marble chips (whole, halved, crushed).
Responding variable: the volume of gas collected in the first 2 minutes.
Fixed variables: the mass of marble (4 g), the concentration and volume of acid, and the temperature. Any two earn the marks.
What to study next
Go on to writing observations separately from inferences. The same variables appear again in rate of reaction, where they drive the conclusions.
For a teacher who can set you new experiments to name, see online one-to-one Chemistry tuition.