Simply put, Charles law of gases states that a gas expands as the temperature increases. So if you fill a balloon in a cool room, then go out on a hot summer day, the balloon may burst because as the air inside gets warmer it expand. Conversely, if you fill a balloon outside on a hot day and go into a cool room, the balloon will get smaller. Temperature goes up, gas expands. Temperature goes down, gas contracts.

Charles' law (also known as the law of volumes) is an experimental gas law which describes how gases tend to expand when heated. It was first published by French natural philosopher Joseph Louis Gay-Lussac in 1802,1 although he credited the discovery to unpublished work from the 1780s by Jacques Charles. The law was independently discovered by British natural philosopher John Dalton by 1801, although Dalton's description was less thorough than Gay-Lussac's.2 The basic principles had already been described a century earlier by Guillaume Amontons.

Gay-Lussac was the first to demonstrate that the law applied generally to all gases, and also to the vapours of volatile liquids if the temperature was more than a few degrees above the boiling point.needed His statement of the law can be expressed mathematically as:

V_{100} - V_0 = kV_0\,

where V100 is the volume occupied by a given sample of gas at 100 °C; V0 is the volume occupied by the same sample of gas at 0 °C; and k is a constant which is the same for all gases at constant pressure. Gay-Lussac's value for k was 1⁄2.6666, remarkably close to the present-day value of 1⁄2.7315.

A modern statement of Charles's law is:

At constant pressure, the volume of a given mass of an ideal gas increases or decreases by the same factor as its temperature on the absolute temperature scale (i.e. the gas expands as the temperature increases)