Planning experiments and variables

In science, a variable is something that can be changed, measured or controlled in an experiment.

Scientists look at variables to understand how they affect things.

Independent variables

This is the variable that a scientist changes on purpose to see how it affects something else.

For example, if you want to see how water affects plant growth, you would change the amount of water a plant is given. Therefore, the amount of water is the independent variable.

Dependent variables

This is the variable that the scientist measures or observes.

In the plant example, the growth of the plant is the dependent variable.You would measure how tall the plant grows based on how much water it receives.

Controlled variables

These are the variables that the scientist keeps the same during the experiment to make sure that the test is fair.

In the plant experiment, controlled variables might include the type of plant, the amount of sunlight and the type of soil.

• Understanding relationships
  Variables help scientists understand how different things are connected.
• Fair tests
  By controlling variables, scientists can be sure that any changes they see in their experiments is because of the independent variable and not something else. In a fair test only the independent variable changes
• Learning
  Studying variables allow scientists to make discoveries and learn more about how the world works.

Once you have identified your variables, the next step is to form a question, which you hope to answer at the end of your investigation or experiment.

Your questions should mention the independent variable and dependent variable. You could use this sentence stem to help form your question:

What is the effect of X (independent variable) on Y (dependent variable) if I change X but keep everything else the same?

Examples:

• What is the effect of water on plant growth if I change the amount of water but keep everything else the same?
• What is the effect of exercise on heart rate if I change the duration of exercise but keep everything else the same?

Once you have decided on your question, the next step is to decide how you will go about collecting information and answering the question.

This is called the method. You will need to consider the following:

• What equipment do you need?
• What method will you follow?
• How will you measure and collect the data?
• Will the experiment take place over a long period of time?
• How will you make sure that the investigation is a fair test?

In the plant example, the method may look something like this:

Equipment
3 measuring cylinders
3 sunflower seeds
3 pots
Soil
Water

1. First, place each sunflower seed inside of the pot and cover with the same amount of soil.
2. Then, …

And so on…

Controlled variables – The variables that scientists keep the same during an experiment to make sure that a test is fair.

Dependant variables – The variable that a scientist measures or observes.

Experiment – A scientific procedure or test, often in a laboratory, that produces results.

Fair test – Keeping variables the same apart from the independent variable.

Independent variables – The variable that a scientist changes on purpose to see how it affects something else.

Method – How scientists go about collecting information and answering a question.

Scientists – Scientists are people who are experts in an area of science.

Variables – Something that can be changed, measured or controlled in an experiment.

Ask a responsible adult to help you with this activity.

You will need sugar or salt to dissolve, and water at different temperatures.

It is very important that you only use hot water if the adult is working with you, because spilling it might hurt you.

1. Put water into three glasses or cups:

   • cold (from the fridge or left outside overnight)
   • medium (half-filled from above and the other half from below)
   • warm (left on the radiator or in the sun)

2. Put one teaspoon of sugar or salt into each cup and stir it until it disappears.

3. Record the time this takes.