If you fill up a balloon with a small amount of air, then set it in direct sunlight, you will see that the balloon expands. This expansion is due to an increase in volume, which results from

Increase in kinetic energy of the gases

Explanation:

The direct sunlight will heat up the molecules making up the gases in the balloon.

Kinetic energy resulting from the heating of these particles will make them become randomized. They then begin to move more haphazardly colliding vigorously with themselves and the wall of balloon.

According to Charles's law "the volume of a fixed mass of gas is directly proportional to its temperature provided pressure remains constant" .

The sunlight heats the balloon and causes a rise in temperature. This will make the gas volume to increase and the balloon expands.

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