Boyle's Law

Gas is made of tiny particles bouncing around at high speed. When you compress the gas (reduce volume), the same number of particles are squeezed into a smaller space — they collide with the walls more frequently, creating higher pressure. When you expand the gas, particles have more room, hit the walls less often, and pressure drops.

Boyle's Law: pressure and volume are inversely proportional — double the pressure and you halve the volume. This only holds at constant temperature, because temperature controls how fast particles move. Watch the P × V value — it never changes, no matter where you move the slider!