8.1 - Variables & Controls
The three kinds of variable
Every experiment has one variable you change, one you measure, and several you keep the same.
| Variable | What it is | Example: insulation and cooling |
|---|---|---|
| Independent | The one thing you change on purpose | The number of layers of insulation |
| Dependent | The thing you measure to see the effect | The temperature drop in ten minutes |
| Control | The things you keep the same | Volume of water, starting temperature, the same beaker and lid |
Spotting which is which
- Read the aim as "how does X affect Y": X is the independent variable and Y is the dependent variable.
- On a graph, the independent variable goes along the x-axis (bottom) and the dependent variable goes along the y-axis (side).
Controls and valid results
- Control variables are the factors you keep the same, such as the volume of water and the temperature it starts at.
- Keeping them constant is what makes the results valid: if only the independent variable changes, any change in the result must be down to that variable and nothing else.
- When you name a control variable, name it in full: "the volume of water", not "the conditions".
Control experiment
- A control experiment is a separate set-up with the factor you're testing left out, run for comparison.
- For example, to show that insulation slows cooling, set up an identical beaker with no insulation at all. Comparing the two cooling rates shows the difference the insulation made.
Hazards, risks, and control measures
- A hazard is anything with the potential to cause harm, such as hot water or a live mains supply.
- A risk is the chance that a hazard actually causes harm in the procedure you're carrying out, and how serious that harm would be.
- The same hazard can carry a different risk depending on how it's handled: carrying a full beaker of boiling water across the room is riskier than moving it a few centimetres along the bench.
Common hazards, risks, and control measures
| Hazard | The risk it poses | A control measure |
|---|---|---|
| Hot water in a beaker | Scalds if it spills or splashes onto skin | Let it cool before moving it, and keep the beaker away from the bench edge |
| A wire overheating at high current | Burns if touched, and damaged insulation | Keep the current low, switch off between readings, and don't touch the wire while the circuit is on |
| A live mains supply | Electric shock | Use a low-voltage supply where possible, and never touch bare connections or handle plugs with wet hands |
| A radioactive source | Ionising radiation damaging cells | Handle with tongs, keep it pointing away from people, and return it to its lead-lined box straight after use |
| A falling mass | Crushing a foot or hand if dropped | Put a padded box or stool under the masses, and keep feet clear of the drop zone |
Using less of something, or a lower setting (such as a smaller mass, lower current, or lower supply voltage), is a control measure that reduces the risk without removing the hazard entirely.
TermDefinition
Independent variableThe variable that you choose to change in an experiment.
Dependent variableThe variable that you measure to see the effect of changing the independent variable.
Control variableA variable that is kept the same throughout an investigation so that only the independent variable can affect the results.
ValidDescribes a result from a fair test, where all other variables were controlled so that any effect is due to the variable being changed.
Control experimentA copy of the experiment without the factor being tested, set up alongside it to show that any effect is caused by that factor.
HazardAnything that has the potential to cause harm, such as equipment in an experiment or an obstacle on a road.
RiskThe chance that a hazard will actually cause harm, and how serious that harm would be.
Control measureAn action taken to reduce the risk of a hazard causing harm.
Key points to remember
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