What to do
Blow up the balloons so they match in size and tie them shut. Mark one balloon with an 'H' (for hot) and the other with a 'C' (for cold).
Put the 'C' balloon in the freezer.
Put the 'H' balloon in the box and close the lid.
Gently blow warm air from the hairdryer in through the small flaps at the top of the box.
After about 10 minutes, compare the balloons.
What is happening?
When the air inside the box is warmed, it makes the particles in the air move faster as they have more energy.
As this requires more space, the balloon will increase in size.
The opposite happens to the balloon in the freezer. Here the air is cooled, the particles slow down, they need less space, so the balloon deflates.
The mass of air, or the number of particles, remains constant inside the balloons. So this activity shows that the warm air requires more space, the particles are more spread out and the air is therefore less dense than cool air. The particles in the cool air are more tightly packed so the cool air is more dense than the warm air.
■ The House of Science Tauranga is a charitable trust aiming to raise scientific literacy in the local community. It runs a range of programmes and provides science resources for local schools. See www.houseofscience.org.nz
