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Control Experiment, by example
Imagine that your science project is
about the effect of heat on evaporation of water. In other words you
want to know if heat can cause evaporation of water. (Seems simple,
doesn't it?)
Your hypothesis is "If we heat up
water, it will evaporate."
As your experiment, you place some
water in a beaker and heat it up over an electric stove. All water
evaporates in about 5 minutes. You write your results and conclude that
heat does in fact cause evaporation of water.
Is this a reliable result?
Someone may argue that water would
probably evaporate by itself even if you would not apply any heat. As a
result, there is no clear relation between the heating and evaporation
of water.
This is when you need to repeat your
experiment; however, this time you will also set aside a second beaker
with the same amount of water and do nothing with that. You call it a
control experiment.
You heat up the first beaker until all
the water evaporates, then you look back at your control experiment and
see that all the water is still there. Now you have shown that water
would not evaporate by itself and conclude that heat has caused
evaporation.
Control experiment is a parallel
(simultaneous) experiment in which you do not change any thing in order
to show that changes in your independent variable are not caused by an
unknown phenomena, but they are a response to the changes in the
independent variable.
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