Explanation:
When the temperature of a fixed amount of gas at constant volume decreases, its pressure decreases. This can be understood through the gas laws, specifically the ideal gas law.

Gas Laws:
The ideal gas law states that for a given amount of gas at a constant volume, the pressure is directly proportional to the temperature. It can be mathematically represented as:

P ∝ T

Where P is the pressure and T is the temperature.

Effect of Decreasing Temperature:
When the temperature of a gas decreases, the kinetic energy of the gas molecules decreases. This means that the gas molecules have less energy to move around and collide with the walls of the container. As a result, there is a decrease in the frequency and force of the collisions, leading to a decrease in the pressure exerted by the gas.

Particle Motion:
At higher temperatures, the gas molecules move faster and collide with the walls of the container more frequently and with greater force. This results in an increase in pressure. On the other hand, at lower temperatures, the gas molecules move slower and collide with the walls of the container less frequently and with less force. Hence, the pressure decreases.

Relationship between Temperature and Pressure:
The relationship between temperature and pressure can be explained using the kinetic theory of gases. According to this theory, the pressure of a gas is directly proportional to the average kinetic energy of its molecules. The average kinetic energy is directly related to the temperature of the gas.

Conclusion:
In conclusion, as the temperature of a fixed amount of gas at constant volume decreases, its pressure decreases. This can be explained by the decrease in the kinetic energy and frequency of collisions of the gas molecules with the walls of the container.