Gas is a physical state of matter in which interparticle attraction is weak and interparticle space is so large. Because of this, gas particles become completely free to move randomly in the entire available space. Boyle's law states that the volume of a given mass of a dry gas is inversely proportional to its pressure at a constant temperature. The gas becomes denser. Thus at a constant temperature, the density of a gas is directly proportional to its pressure. All gases show uniform behaviour under similar conditions of temperature and pressure irrespective of their chemical nature or colour or odour. ICSE Selina solutions for class 9 chemistry provided here explains the behaviour of gases under different temperatures and pressure. This solution page has questions regarding Charles law, Boyle's law, mathematical calculation involved in Charle's and Boyle's law, the relationship between Kelvin and Celsius scale of temperature and their conversions.