Explanation:

When the temperature of a fixed amount of gas at constant volume decreases, its pressure decreases. This is explained by the ideal gas law, which states that the pressure of a gas is directly proportional to its temperature, assuming constant volume and amount of gas. The ideal gas law is represented by the equation:

PV = nRT

Where:
- P is the pressure of the gas
- V is the volume of the gas
- n is the number of moles of gas
- R is the ideal gas constant
- T is the temperature of the gas

When the temperature of a gas decreases while keeping the volume constant, according to the ideal gas law, the pressure of the gas must also decrease. This can be understood by examining the relationship between temperature and pressure in the ideal gas law equation.

Key Points:

- The ideal gas law states that the pressure of a gas is directly proportional to its temperature, assuming constant volume and amount of gas.
- When the temperature of a gas decreases while the volume is kept constant, the pressure of the gas also decreases.
- This is because the decrease in temperature leads to a decrease in the average kinetic energy of the gas particles.
- With lower kinetic energy, the gas particles collide with the walls of the container less frequently and with less force, resulting in a decrease in pressure.
- Conversely, when the temperature of a gas increases, the pressure of the gas also increases, as the gas particles have higher kinetic energy and collide with the walls of the container more frequently and with greater force.

Conclusion:

In summary, when the temperature of a fixed amount of gas at constant volume decreases, its pressure decreases. This is due to the decrease in the average kinetic energy of the gas particles, resulting in fewer and weaker collisions with the walls of the container.