Chemistry

8.1 - Variables & Controls

2 min

note 1 of 2

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.

note 2 of 2

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".

key terms

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.

end of notes

Key points to remember

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".