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Variables

Key Concepts

  • In an experiment, a Chemist causes a change and records the effect of that change.

  • The quantities that can be changed are called variables.

  • There are three types of variables:

          (a) independent variables

          (b) dependent variables*

          (c) constant variables*

  • Independent variable is the quantity that the Chemist causes to change.

  • Dependent variable is the quantity that responds to the change the Chemist has caused.

  • Constant variables are all the other quantities in the experiment that could change, but which the Chemist is keeping at constant values in order to ensure the experiment is a fair test.

Variables

Variables are all the quantities that could change in an experiment.

Typical variables in a chemistry experiment include:

  • nature of a substance

  • concentration

  • volume

  • mass

  • temperature

  • pressure

  • stirring rate

  • particle size

  • time

  • current

  • voltage

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Independent Variables

The independent variable is the variable that the Chemist decides to change. When the Chemist changes this variable, he/she will then observe the effect of this change on one of the other variables.

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Dependent Variables

The dependent variable is the variable that is observed to respond when the Chemist changes the independent variable.

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Constant Variables

After the Chemist has decided which variable will be the independent variable, and which variable is the dependent variable in the experiment, all the other variables must be held constant. All these other variables that are held constant are called constant variables.

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Examples

The aim of an experiment can often tell us which variable is the independent variable and which variable is the dependent variable.
Any other variable must be a constant variable.

Example: To determine the effect of variable x on variable y

  • variable x is the independent variable (the variable chosen to be changed)

  • variable y is the dependent variable (the variable chosen to be observed, the variable that responds to the change of the independent variable)

Aim of the experimentIndependent VariableDependent VariableConstant Variables
To determine the effect of temperature on the mass of sodium chloride dissolving in water. Temperature Mass of sodium chloride (a) volume of water
(b) purity of sodium chloride
(c) purity of water
(d) pressure
(e) stirring rate
(f) size of sodium chloride particles
(g) time

To determine the effect of the type of salt on water solubility. Various salts Amount of salt (a) volume of water
(b) temperature of water
(c) purity of salts
(d) purity of water
(e) pressure
(f) stirring rate
(g) size of salt particles
(h) time

To determine the effect of water volume on the solubility of sodium chloride. Volume of water Amount of sodium chloride (a) temperature of water
(b) purity of salts
(c) purity of water
(d) pressure
(e) stirring rate
(f) size of salt particles
(g) time

To determine the effect of stirring rate on the solubility of sodium chloride. Stirring rate Amount of sodium chloride (a) volume of water
(b) temperature of water
(c) purity of sodium chloride
(d) purity of water
(e) pressure
(f) size of sodium chloride particles
(g) time

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*Sometimes independent variables are referred to as controlled variables, because the experimenter controls this variable.
Unfortunately, constant variables can also be referred to as controlled variables, because the experimenter is also controlling these variables.
In order to prevent confusion, this discussion will refer only to independent variables and constant variables, not to controlled variables.
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