Air is a mixture of many gases. The molecules of gases present in air have weak inter-molecular forces of attraction. Due to this reason these molecules lie far apart from each other i.e. there are large vacant spaces present between these molecules. So, these molecules are free to move in any direction.

When we heat air, the kinetic energy of these molecules increase and they begin to move farther apart from each other. In this way volume of air increases and we say that air has expanded. On the other hand, when air is cooled down, the kinetic energy of its molecules decreases.

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