What happens to a gas when the temperature decreases?It expands.It rises.It condenses.It becomes less dense.

When the temperature of a gas decreases, several physical changes occur due to the properties of gases and the behavior of their molecules.

1. Kinetic Energy Reduction: Gas molecules have kinetic energy proportional to their temperature. When the temperature decreases, the kinetic energy of the gas molecules decreases, which means they move more slowly.

2. Volume Changes: According to Charles's Law, when the temperature of a gas decreases at constant pressure, its volume also decreases. Conversely, if the gas is allowed to expand (for example, if it’s placed in a larger container), the pressure will also decrease, but the gas will still attempt to occupy more space if possible.

3. Condensation: If the temperature decreases significantly, and especially if the gas is near its dew point, the gas may condense into a liquid. This is particularly relevant for gases like water vapor when temperature drops below a certain threshold, leading to liquid water formation.

4. Density Increases: As gas molecules come closer together when temperature decreases, the density of the gas increases; thus, it becomes heavier per unit volume.

Among the given options, the most appropriate effect of a decrease in temperature in relation to gases would be "It condenses." This indicates that the gas transitions into a liquid phase due to the decrease in temperature.