At constant temperature if volume of a gas is reduced, gas molecules will come close to each other but there is no change in the velocity of gas molecules. In small volume chances of molecular collision with the walls of container increases thus pressure of gas also increases.

i.e. V a 1/P

which is according to Boyle’s law.

If at constant pressure, temperature of a gas is reduced, velocity and K.E of gas molecules will also decrease . Due to decrease in K.E, force of attraction between gas molecules will become large, consequently volume of gas decreases.

i.e. V aT

which is according to Charles law.