Boyle's law of gases. Animation showing the relationship between the pressure and volume of a fixed amount of gas where the temperature is kept constant. As the container holding the gas expands, this increase in volume lead to a decrease in pressure. This is Boyle's Law which states that the gas volume is inversely proportional to the pressure at a given temperature. The decrease in pressure is because the molecules have the same amount of energy but more space to move around, meaning they come into contact with the walls of the container less frequently.