What You Need:
- Empty bottle
- Pot of hot water
- Stretch the balloon over the mouth of an empty bottle.
- ¨Put the bottle in a pot of hot water, let it stand for a few minutes and watch what happens.
As the air inside the balloon heats up, it starts to expand and the balloon starts to stretch. The molecules begin to move faster and further apart from each other. Note that there is still the same amount of air inside the balloon and bottle - it has just expanded as it heats up. Warm air takes up more space than the same amount of cold air. It also weighs less than cold air. This principle is used in hot air balloons.