_____ air is less dense than _____ air.

Hot air is less dense than cold air due to the behavior of air molecules when heated. When air is heated, the molecules gain energy and move apart from each other, resulting in a decrease in density. This phenomenon is a fundamental physical property of gases. As hot air expands and its molecules become more spaced out, it rises because it is lighter compared to the denser, cooler air surrounding it.

This principle is crucial in various applications such as meteorology, HVAC systems, and understanding natural convection processes. The effectiveness of this principle can be observed in everyday scenarios like how hot air balloons rise or how warm air circulates in a room.