Final answer:
Relative humidity is a measure of the current amount of water vapor in the air relative to the maximum it could hold at that temperature. When temperature changes, but water vapor density remains constant, relative humidity inversely changes with temperature. An increase in temperature leads to a decrease in relative humidity, and vice versa.
Step-by-step explanation:
The question pertains to relative humidity, which is a measure of the amount of water vapor present in air relative to the amount needed for saturation at the same temperature. When the temperature increases from 20.0°C to 30.0°C and the water vapor density remains constant, the relative humidity will decrease. This is because the capacity for the air to hold water vapor increases with temperature. Hence, with a constant amount of water vapor, the air is less saturated at higher temperatures, leading to a lower relative humidity.
For example, if the relative humidity is 80% at 30.0°C and the air cools to 25.0°C without changing the water vapor density, the relative humidity will increase. This is due to the reduction in the air's capacity to hold water vapor at the lower temperature. If the relative humidity becomes unrealistically high (over 100%), this indicates that condensation would occur, resulting in dew or fog, and the initial premise of constant water vapor density may not be correct as condensation removes water vapor from the air.