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• A variable is a factor that can be changed in an experiment.
• Identifying control variables, independent and dependent variables is important in making experiments fair.
• Knowing about variables can help you make scientific predictions and test them.

Watch this video of a number of experiments being carried out.

Scientists ask questions to find out more about the world, like ‘how can we get more energy from the sun?’ and ‘how can we cure diseases?’. To answer these questions scientists do experiments. During experiments, factors that can change are called variables.

A variable is anything that can change and be measured. Two important types of variables are:

• Independent variables – the variable that is being changed during the experiment

• Dependent variables – the variable being tested or measured during the experiment

In an experiment, the effect of changing just one variable on another is tested - testing how the independent variable affects the dependent variable. For this reason, other variables must be controlled so that they don't affect the independent variable. These variables are control variables.

Making a scientific prediction involves saying what might happen.

Knowing which variables to control is important when designing experiments to find out if a prediction is right or wrong.

Identifying control variables makes sure that only the independent variable affects the dependent variable. This will ensure results from the experiment are valid.

A variable is a factor that can be changed or measured. It is important to be able to identify variables in experiments. Below are some examples.

1. Plant growth and water

Adding different amounts of water to a plant could affect its growth. To investigate this, plant some seeds and water each plant with different amount over time.

• The independent variable is the volume of water given to each plant.

• The dependent variable is how high the plant grows.

• Control variables include the size of pots, the type of soil and the position in a room.

2. Dropping a ball from different heights

Dropping a ball from different heights could affect how high it bounces.

• The independent variable is the height of the drop.

• The dependent variable is how high the ball bounces.

• Control variables include the type of ball, the surface that it is dropped onto and the size of the ball.

3. Testing reaction times

Reaction times can be investigated by dropping a ruler and seeing how quickly someone can grab it.

• The independent variable is the person who is trying to catch the ruler.

• The dependent variable is how far the ruler dropped.

• Control variables include the position of the ruler above the hand and the distance between the finger and thumb.

4. Speed and angle of a ramp

How the angle of a ramp affects the speed of a car can be investigated by timing how long the car takes to travel a certain distance.

• The independent variable is the angle of the ramp.

• The dependent variable is the speed the car travels.

• Control variables include the distance that the car travels, the surface it travels over and the car.

Choose three different types of biscuit and follow the four steps below to carry out the experiment:

1. Dunk each one in turn into a cup of tea.
2. Time how long it takes for each biscuit to break in half.
3. The one that takes the longest is the best for dunking.
4. Now identify the dependent, independent and control variables in this experiment.

• Tennis balls at Wimbledon are stored at 20°C so that they all bounce in a similar way. The temperature is a variable being controlled.
• The balls can also become scuffed, which affects how they bounce - this is another variable.