7.1 - Variables & Controls
What a variables question looks like
A typical question describes an experiment, then asks for the variable changed, the variable measured, and one variable kept constant.
A worked example of the question type
An investigation into how the thickness of insulation affects how quickly hot water cools: beakers with the same volume of hot water are wrapped in different numbers of layers, and each temperature drop over ten minutes is recorded.
Credit goes to naming each exact variable specifically:
- The variable changed is the number of layers of insulation.
- The variable measured is the temperature drop over ten minutes.
- A controlled variable must be named specifically - such as the volume of water, the starting temperature, or the type of beaker.
A typical question
A student investigated how the thickness of insulation affects how quickly hot water cools. Beakers holding the same volume of hot water were wrapped in different numbers of layers of insulating material, and the temperature drop of each over ten minutes was recorded.
(i) Name the variable that was changed.
(ii) Name the variable that was measured.
(iii) Suggest one variable that should be kept constant.
The three kinds of variable
Every experiment has one variable you change, one you measure, and several you hold still.
The three kinds side by side
| Variable | What it is | How to spot it | Example (insulation and cooling) |
|---|---|---|---|
| The variable you change | The one thing you deliberately alter | It's what the question says was varied - it goes in the first column of the results table and along the bottom of the graph | Number of layers of insulation |
| The variable you measure | What you measure to see the effect of that change | It's what you read off an instrument - it goes up the side of the graph | Temperature drop over ten minutes |
| Controlled variables (kept the same) | The things you keep the same so the test stays fair | Everything else that could change the result if you let it | Volume of water, starting temperature, type of beaker, room temperature |
Reading the aim
- Read the aim as "how does X affect Y": the X is the variable you change and the Y is the variable you measure, every time.
- "How does the thickness of insulation affect the temperature drop" hands you both.
Controls and a fair test
Controlled variables and validity
- Controlled variables are the things you keep the same - such as the volume of water, the starting temperature, or the type of beaker.
- Keeping them the same is what makes the investigation a fair test, and that's what gives a valid result.
- If only the independent variable changes and everything else stays constant, any difference in the result must be down to that one variable.
- Name a controlled variable in full: "the volume of water" and "the starting temperature" score; "the conditions" does not, because it's too vague.
Validity is not reliability
- When a question asks why the other variables are kept the same, the answer is about validity - it keeps the test fair.
- Answering "to make it reliable" is wrong, and markers don't accept it.
- Reliability is about getting consistent values in repeats, achieved by repeating readings and taking a mean.
A control experiment
- A control experiment is an extra set-up with the factor you're testing left out, run alongside the others for comparison.
- In the insulation experiment, the control is a beaker with no insulation at all. The water in it still cools, and comparing it with the wrapped beakers shows how much difference the insulation actually made.
Common exam questions
Question 1
A student investigated how the length of a wire affects its resistance. Identify the variable that was changed and the variable that was measured.
The variable changed is the length of the wire. The variable measured is the resistance.
Question 2
Suggest one variable that would have to be controlled to ensure the results are valid.
Any named factor kept the same, such as the thickness of the wire, the material the wire is made from, or the supply voltage. Keeping these the same makes it a fair test, so any change in resistance can be put down to the length.
Question 3
A student compared how fast water cooled in an insulated beaker with an uninsulated one. Give one reason for including the uninsulated beaker.
It acts as a control, giving something to compare against, so it shows that any slowing of the cooling was caused by the insulation and not by something else in the set-up.
Question 4
Describe how the reliability of these results could be improved.
Repeat the measurement at each thickness of insulation and take a mean, so that consistent values across the repeats show the result is reliable.
Question 5
Why is "to make it reliable" wrong when asked why the other variables were kept the same?
Because keeping the other variables the same is what makes the test fair, so it is about validity, not reliability; reliability comes from repeating the measurements and averaging.
Question 6
A student says the resistance should be kept constant in the wire experiment. What is wrong with this?
The resistance is the variable being measured, so it is the thing expected to change. What should be kept constant is the thickness of the wire, the material, and the supply voltage.