Q.State Boyle's law.

Boyle's law states that, at constant temperature, the volume occupied by a fixed mass of gas is inversely proportional to its pressure:

Robert Boyle established this using a J-shaped mercury tube: with mercury levels equal in both arms, the trapped gas is at 1 atm; adding mercury to compress the gas to half its original volume raises the height difference between the mercury columns to 760 mm, corresponding to a doubled pressure of 2 atm -- pressure and volume move in exactly opposite proportion.

Molecular picture. Gas pressure comes from molecules bombarding the container walls. Compress a fixed amount of gas to half its volume and the number of molecules per unit volume doubles, so twice as many collide with a given patch of wall per second -- the pressure doubles as a direct, mechanical consequence of the crowding.

Consequence: pressure and density. Since $V=m/d$ for a fixed mass m, Boyle's law $P_1V_1=P_2V_2$ rearranges to $\dfrac{P_1}{d_1}=\dfrac{P_2}{d_2}$ -- at constant temperature, the density of a fixed mass of gas is directly proportional to its pressure.

Everyday consequences. Aeroplane cabins must be pressurised because the low atmospheric pressure at cruising altitude would otherwise be dangerously thin to breathe. A weather balloon released at ground level swells larger and larger as it climbs, because the surrounding atmospheric pressure keeps falling with altitude while the gas inside must expand (at roughly constant temperature and amount) to match it -- exactly the inverse P-V relationship Boyle's law describes. Divers are warned never to hold their breath while ascending, because the air in their lungs, compressed by the water pressure at depth, will expand as that surrounding pressure drops on the way up.