22.1 - Variables & Controls

Naming variables in a practical question

• Almost every Cambridge practical paper asks you to name the variables in an experiment, or to give a variable that should be kept the same.
• Each part is usually worth one mark, and the mark goes to the exact variable, named precisely.
• These are among the most reliable marks on the paper - every experiment has these three kinds of variable, and the rules for telling them apart never change.

A typical question

A student investigated how the concentration of hydrochloric acid affects the rate of diffusion into cubes of agar. Cubes of the same size were placed in acid at five different concentrations, and the time for each cube to change colour was measured.

(i) Name the independent variable. [1]

(ii) Name the dependent variable. [1]

(iii) Suggest one variable that should be kept constant. [1]

The answers

1. The independent variable is the concentration of hydrochloric acid. [1]
2. The dependent variable is the time for the cube to change colour. [1]
3. A controlled variable, named specifically - such as the size of each cube, the volume of acid, or the temperature. [1]

The three kinds of variable

• Every experiment has one variable you change, one you measure, and several you keep the same.
• Getting the names straight is the whole of this skill.

The three variables side by side

Spotting the independent and dependent variables

• Read the aim as "how does X affect Y": X is the independent variable and Y is the dependent variable, every time.
• "How does acid concentration affect the time to change colour" hands you both.
• On a graph, the independent variable goes along the bottom and the dependent variable up the side.

Controls and controlled variables

The word "control" is used for two different things in an experiment - it's worth keeping them apart.

Controlled variables

• Controlled variables are the things you keep the same - e.g. the cube size, the volume of acid, the temperature.
• Holding them still is what makes the comparison valid: if only the acid concentration changes and everything else stays put, any difference in the time to change colour must be down to the concentration and nothing else.
• You may know this as a "fair test" from earlier science, but Cambridge doesn't use that phrase - it asks it as keeping variables constant, so use that language in an answer.

Control experiment

• A control experiment is different: an extra tube or sample set up with the factor you're testing left out, run alongside the real ones for comparison.
• Example: in an experiment on whether sucrose draws water into dialysis tubing, the control is a tube of water with no sucrose in it.
• If water moves into the sucrose tube but not the water-only tube, you can be sure the sucrose caused it.
• Cambridge marks the reason for the control in exactly those terms - it is there to ensure that it was the sucrose that was causing the effect.

Naming a controlled variable

• Name it in full: "temperature" and "the volume of acid" score marks; "the conditions" or a bare "time" does not, because the examiner can't tell which time or which condition you mean.
• If time matters, say "the time each cube was left in the acid".

Common exam questions

Question 1

An experiment measures how the temperature of the water affects the number of gas bubbles a piece of pondweed gives off. What are the independent and dependent variables?

The independent variable is the temperature of the water (the thing changed), and the dependent variable is the number of bubbles given off (the thing measured).

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Question 2

A student is asked to name a variable that should be kept constant and writes "time". Why might this not score?

Because it is too vague to say which time is meant; a controlled variable must be named specifically, such as "the time the pondweed was left at each temperature".

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Question 3

In an experiment on whether an enzyme is needed to break down starch, one tube contains boiled enzyme that no longer works. What is this tube called, and what is it for?

It is a control, and it is there to show that any breakdown in the other tubes was caused by the working enzyme, not by something else in the mixture.

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Question 4

Why must all the variables except the independent one be kept the same?

So that the comparison is valid: if only the independent variable changes, any change in the dependent variable must be caused by it and nothing else.

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Question 5

A student says the dependent variable in a diffusion experiment is "the acid". Why is this wrong?

The acid concentration is the independent variable, the thing being changed; the dependent variable is what is measured to see the effect, which is the time for the cube to change colour.