According to Boyle's Law, when the volume of a gas decreases at constant temperature and the number of particles remains the same, the pressure of the gas increases. This inverse relationship between volume and pressure is expressed mathematically as:

$$P_1 V_1 = P_2 V_2$$

where:

- \(P_1\) and \(P_2\) represent the initial and final pressures of the gas, respectively.

- \(V_1\) and \(V_2\) represent the initial and final volumes of the gas, respectively.

As the volume \(V_2\) decreases while temperature and the number of particles are held constant, the pressure \(P_2\) must increase to maintain the equality of the equation. In simpler terms, as the gas is compressed into a smaller volume, its particles become more concentrated, leading to a higher frequency of collisions with the container walls. This increased collision frequency results in greater force exerted on the walls, leading to an increase in gas pressure.

In summary, reducing the volume of a gas at a constant temperature and particle count causes an increase in its pressure.