Explain air expands on heat in …

When air is heated, its volume increases and it occupies a bigger space. Air expands on heating. 

Take a boiling tube. The empty boiling tube contains air. Take a balloon and stretch the mouth of balloon and fix it tightly over the neck of the boiling tube. Use a tape to make it tight. Pour some hot water in a beaker. Insert the boiling tube with the balloon in the hot water. After 2-3mins the balloon gets inflated. It is very clear that the air present in the tube gets heated, expands and its volume increases.

Take the tube out let it cool down to the room temperature. Take some ice-cold water in another beaker and place the tube with the balloon in cold water for 2–3 minutes. The balloon gets deflated because the air present inside the boiling tube contracts on cooling due to which its volume decreases. The hot air is lighter than the cold air. The air gets heated at a place it expands becomes lighter and rises up in the sky.