Borosil bottle or good quality bottle, a bowl of hot water, a bowl of ice-cold water, balloon
1. Attach the rubber balloon to the neck of the bottle and carefully put the bottle in a bowl of hot water.
2. Hold it in place and soon you will observe that the balloon has puffed up a bit.
3. Now remove the same bottle and put it in ice-cold water.
4. You will notice that the balloon is getting deflated and is pulled a bit inside.
So how did this happen? We used the same water but why does the balloon act differently.
There is air present all around us and like solids and liquids, it too heats up and cools down. The air molecules expand on heating and contract on cooling. The basic principle involved is that when hot air rises it develops low pressure in the area and the surrounding cooler air is relatively at the higher pressure.
When the air inside the empty bottle was heated up because of hot water, it expanded resulting in the puffing up of the balloon while when the air was cooled due to cold water the balloon not only deflated but was also seen to contract a bit. So hot air expands and cold air contracts.
By Vedanth Kapoor, Class III, Mount Carmel School, New Delhi