These simple activities allow children to investigate expansion and contraction through hands-on experiments at home.

Expansion and Contraction

There are lots of science experiments and investigations that kids can do in the home. These two experiments are designed to illustrate the principle of expansion and contraction of air through heating and cooling, using simple household objects.

Crushing Can Experiment

This classic experiment has long been a favourite in schools, both with students and teachers, but it is possible to do in the home with adult supervision.

You will need:

  • Empty soda can
  • Gas stove (could possibly be done on a camp fire or BBQ)
  • Bowl of water
  • Salad tongs / BBQ tongs

Method:

  1. Take and empty drinks can, and put about 1cm of water in the bottom.
  2. Using the tongs, hold the can over the stove until the water starts to boil, and you see steam coming out of the hole in the top of the can. This shows you that the air inside the can has heated up, and some of the air has been pushed out of the can as it has expanded.
  3. Once you see the steam, quickly turn the can upside down into the bowl of water, so that the water forms a seal across the hole in the top of the can.

Because the air inside the can had expanded, there were fewer air particles per cm3 inside the can than there are in the air around it. This means that the air pressure on the outside of the can is much greater than on the inside, so the walls of the can will be pushed inwards, and the can will instantly crush itself, sometimes with a loud POP!

Squashy Sports Bottle

A safer alternative to the Crushing Can experiment for younger children is to use a pastic water bottle (the ones with the sports tops work best as they are designed to flex as you drink) to demonstrate the same principle.

You will need:

  • Empty sports top water bottle
  • Refrigerator

Method:

  1. Place the empty bottle, with its lid open, in a hot sunny spot or on top of a radiator for about ten minutes until it is nice and warm.
  2. Close the lid so that the bottle is sealed, and place it in the refigerator for about ten minutes (you can check on it before this, but make sure you don’t take the lid off!)

As the air inside the bottle cools down it should cause the bottle to ‘squash’ like it does when someone sucks water out of it as they drink.

The Science of Expansion and Contraction

In both experiments, by heating the air inside the can or bottle, you have caused the air particles to move more quickly. This means that the particles bump into each other more often, so push each other apart. This causes the air to expand.

As the air expands, some of the particles will be pushed out of the can or bottle, leaving fewer particles on the inside.

As soon as the system is sealed, by putting on the lid in the case of the bottle experiment, no more particles can get in or out.

Cool air has less energy, meaning that the particles are moving more slowly, and not bumping into each other as much. This means that once the air inside cools down, it contracts to take up less space, and causes the container around it to ‘squash’ as well!