When an air is heated, the molecules will start to vibrate and bump into each other. This will cause an increase in the space around each molecule. Because each molecule uses more space for motion, the air expands and becomes less dense. Therefore, the air expands when heated.
As the molecules heat and move faster, they are moving apart. So air, like most other substances, expands when heated and contracts when cooled. Because there is more space between the molecules, the air is less dense than the surrounding matter and the hot air floats upward