Boyle's Law : V is proportional to 1/P (T, n constant) P₁V₁ = P₂V₂

 Demonstration : soda can with water is heated and fills with steam. When it cools, the pressure inside becomes less than that outside. The can is crushed because the pressure outside is greater than the pressure inside.

Picture diagram to appear here soon

Demonstration: Boyle's Law apparatus. Keeping the temperature constant. When the volume in a syringe or piston is decreased, the pressure increases. Why?

If the system's number of molecules does not change, then the pressure is greater when the volume decreases because there are more collisions per unit area in a given unit of time with the walls of the container and there are more collision per unit volume in a given unit of time with the other gas molecules. Since the temperature is kept constant, the molecules are moving with the same average speed (comparing before and after a decrease in volume).

At a constant temperature, the pressure of a gas is inversely proportional to the volume occupied by the gas.