The volume decreases when the pressure is increased or when the temperature is decreased.
According to Charles's law, the volume of a given mass of gas is directly proportional to its absolute temperature at constant pressure. This means that, if the temperature of the given mass of gas is decreased, its volume decreases accordingly and vice versa.
According to Boyle's law, the volume of a given mass of gas is inversely proportional to its pressure. Hence, when the pressure of a gas is increased, its volume decreases accordingly.
Summarily, the volume decreases when the pressure is increased or when the temperature is decreased.
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