Variables in Chemistry Experiments Tutorial

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⁽¹⁾

  (c) constant variables⁽¹⁾

• 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.⁽²⁾

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Types of 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
• electrical current
• voltage

For example, A student wants to design an experiment to see if more salt can be dissolved in hot water than in cold water.

What are some of the possible variables in this experiment?
That is, what are the quantities that could change in this experiment?

• Nature of the salt (What salt is it? Is it sodium chloride, or potassium chloride, etc?)
• Purity of the salt (if lots of impurities are present they might effect the results of the experiment)
• Purity of the water (once again, impurities in the water might effect the results of the experiment)
• Amount of the salt used (the amount of salt used may effect how long it takes to dissolve and how much dissolves)
• Amount of water used (the amount of water used may effect how much salt can be dissolved)
• Temperature (of the salt, of the water, of the salt water solution)
• Pressure (atmospheric pressure, pressure in a closed vessel etc)
• Rate at which the salt is added to the water
• Rate at which the salt is stirred into the water
• Size of salt particles (is the salt first ground to a fine powder, is it in cubes, etc ?)
• Time (how long are you going to observe the process for?)

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.

Example 1: A student wants to design an experiment to see if more salt can be dissolved in hot water than in cold water.

The independent variable, the variable that the student will change is the temperature of the water (cold water and hot water).

Example 2: A student wants to design an experiment to see if more sodium chloride can be dissolved in water than potassium chloride can be dissolved in water.

The independent variable, the variable that the student will change is the nature of the salt (sodium chloride or potassium chloride).

Example 3: A student wants to design an experiment to see if more salt can be dissolved in a large amount of water than in a small amount of water.

The independent variable, the variable that the student will change is the amount of water (small amount or large amount).

Dependent Variables

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

Example 1: A student wants to design an experiment to see if more salt can be dissolved in hot water than in cold water.

The independent variable, the variable that the student will change is the temperature of the water (cold water and hot water).

The dependent variable, the variable the student will observe, is the amount of salt dissolved.

Example 2: A student wants to design an experiment to see if more sodium chloride can be dissolved in water than potassium chloride can be dissolved in water.

The independent variable, the variable that the student will change is the nature of the salt (sodium chloride or potassium chloride).

The dependent variable, the variable the student will observe, is the amount of salt dissolved.

Example 3: A student wants to design an experiment to see if more salt can be dissolved in a large amount of water than in a small amount of water.

The independent variable, the variable that the student will change is the amount of water (small amount or large amount).

The dependent variable, the variable the student will observe, is the amount of salt dissolved.

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.

Example: A student wants to design an experiment to see if more salt can be dissolved in hot water than in cold water.

The independent variable, the variable that the student will change is the temperature of the water (cold water and hot water).

The dependent variable, the variable the student will observe, is the amount of salt dissolved.

The constant variables, the variables that must not change during the experiment, are all the other variables. Constant variables for this experiment include:

• The nature of the salt must be held constant, for example, only sodium chloride will be used for each experiment.
• The purity of the salt used must be held constant, for example, the sodium chloride will be taken from the same jar of reagent each time and a clean, dry, spatula will be used to transfer the sodium chloride to the water.
• The purity of the water used must be held constant, for example, collect distilled water in a clean, dry, bottle and use water only out of that bottle during the experiment.
• The amount of water used in each experiment must the same (held constant), for example, use 50.00 mL of water in each experiment.
• The pressure under which the experiment is conducted must be held constant, for example, conduct all the experiments in vessels open to the atmosphere so that the pressure is the same as atmospheric pressure in the laboratory (record this pressure).
• The stirring rate must be held constant, for example, if you don't stir the salt into the water to dissolve it the stirring rate will always be 0, but if you do want to stir the salt into the water you will need to use a mechanical stirrer and ensure the rate of stirring is always the same.
• The size of the sodium chloride particles must be the same in each experiment, for example, you could use 1 cm³ cubes of sodium chloride, or you could assume that all the particles that come out of the reagent jar are the same (you could place a spatula-full of the sodium chloride under a binocular microscope and see if the particles are all about the same size).
• The time you observe the experiments for should be held constant, and in this case, must be long enough for the dissolving process to be completed for all the experiments.

Worked Examples of Variables in Chemistry Experiments

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 of the "aim of an experiment": 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)