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: acid concentration and marble chips |
|---|---|---|
| Independent | The one thing you change on purpose | The concentration of the hydrochloric acid |
| Dependent | The thing you measure to see the effect | The volume of gas collected in a set time |
| Control | The things you keep the same | The mass and size of marble chips, volume of acid, temperature |
Variables on graphs
- On a graph, the independent variable goes on the x-axis (bottom) and the dependent variable goes on the y-axis (vertical).
- When an experiment's aim is written as "how does X affect Y", X is the independent variable and Y is the dependent variable.
Controls and valid results
- Control variables are the factors you keep the same so that only the independent variable affects the result.
- Controlling these variables is what makes the results valid.
A control experiment is different
- A control experiment is a separate set-up with the factor you're testing left out, run for comparison.
- Example: to confirm that the acid produces the gas when reacting with marble chips, set up an identical flask with the same marble chips in water instead of acid. No gas forms, which confirms the reaction is caused by the acid.
Naming control variables
Name each control variable specifically (e.g. "the volume of acid" or "the mass of marble chips"), rather than using vague terms like "the amount" or "the conditions".
Hazards, risks, and control measures
- A hazard is anything with the potential to cause harm, such as a corrosive chemical or a naked flame.
- A risk is the chance that a hazard will cause harm during a procedure, and how serious that harm would be.
- The same hazard can carry different levels of risk: handling a large volume of concentrated acid carries a higher risk of splashing than handling a single drop.
Hazard pictograms and control measures
| Hazard | What it means | Control measure |
|---|---|---|
| Corrosive | Destroys living tissue and metal on contact | Eye protection and gloves |
| Flammable | Catches fire easily | Keep away from naked flames; use a water bath to heat |
| Toxic | Can cause death or serious illness if inhaled, swallowed, or absorbed through skin | Use in a fume cupboard; wear gloves |
| Oxidising | Releases oxygen, making fires burn more fiercely | Keep away from flammable materials |
| Harmful or irritant | Causes irritation or mild harm to skin, eyes, or lungs | Eye protection and gloves; avoid inhaling vapours |
| Environmental hazard | Harmful to aquatic life or habitats | Dispose of in a dedicated waste container, not down the sink |
Using a lower concentration or a smaller volume of a hazardous substance is a control measure: it does not remove the hazard, but it reduces the risk.
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.
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.
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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