Have you ever wondered why do air-filled rubber tires often burst in the summer? During hot days of summer, the air in the tubes expands, bursting the tube. Another example to prove air expands on heating is that of hot air balloons. Have you ever thought about how the balloon operators manage to fill such huge balloons? Actually, burners are used to heat the air inside the balloon.

When the balloons are on the ground, the air inside is just as dense as that outside. Burners heat the air inside the balloon. You know that gaseous molecules are not strongly bonded to each other. Heating causes the air molecules to move faster as a result distance between them increases. Consequently, Air expands on heating.

Let's do an activity to understand how air expands on heating.

Take a boiling tube, a sello tube & two beakers, one containing hot water and another containing cold water. Stretch a balloon tightly over the neck of the test tube. You can use tape to make it tight. Now place the test tube into a hot water beaker.

Do you see any change in the shape of the balloon? Yes, it is getting inflated. It happens because the heat from the hot water heats up the air inside the test tube. This makes the air inside the test tube expand. The expanded air seeks more space & the balloon gets inflated.

Air contracts on cooling

Now, take the test tube out & place it with a balloon into a cold water beaker. Observe the change in the shape of the balloon. Why is the same balloon deflated when the tube is kept in cold water? It happens because air contracts on cooling. After placing the boiling tube into ice-cold water, the air inside the bottle gets cool & so contracts. A low-pressure area is formed inside the balloon & the balloon gets deflated.