they move faster
Gases are compressible, meaning that when put under high pressure, the particles are forced closer to one another. This decreases the amount of empty space and reduces the volume of the gas. Gas volume is also affected by temperature. When a gas is heated, its molecules move faster and the gas expands. Because of the variation in gas volume due to pressure and temperature changes, the comparison of gas volumes must be done at one standard temperature and pressure. Standard temperature and pressure STP is defined as 0C 273.15 K and 1 atm pressure. The molar volume of a gas is the volume of one mole of a gas at STP. At STP, one mole 6.02 10 23 representative particles of any gas occupies a volume of 22.4 L Figure below .