If the volume of a fixed amount of gas at constant temperature decreases, its pressure increases.

This is due to Boyle's Law, which states that the pressure of a gas is inversely proportional to its volume at a constant temperature.

Here's why:

* More collisions: When the volume decreases, the gas molecules are confined to a smaller space. This means they collide with the walls of the container more frequently.

* Increased force: The increased collisions lead to a higher force exerted on the walls of the container.

* Higher pressure: Pressure is defined as force per unit area. Since the force increases with the more frequent collisions, the pressure also increases.

In mathematical terms, Boyle's Law can be expressed as:

P₁V₁ = P₂V₂

where:

* P₁ = Initial pressure

* V₁ = Initial volume

* P₂ = Final pressure

* V₂ = Final volume

So, if the volume decreases (V₂ < V₁), the pressure must increase (P₂ > P₁) to keep the product constant.