How does heat affect air when heated?

Answer 1

It rises

Because atmospheric air (as opposed to, say, compressed air in a Scuba tank) will expand upon heating, it will become less dense than the air around it. As long as the air around it is more dense, it will rise, due to buoyancy effects. As it rises, the pressure around it drops and it expands more or less isenthapically so that it cools as it rises and becomes more dense. When the density becomes the same as the air around it, it will stop rising. The whole time it is rising it will also be attempting to come to thermal equilibrium as well (i.e. come to the same temperature as the surrounding air).

Answer 2

Gases obey the Universal Gas Law: P * V / T = a constant, where P = gas pressure [Pa]; V = volume [m3]; T = Temperature [K].

Assume the gas molecules are inside a closed system (confined in a fixed volume, like a gas tank) --> V = a constant. Hence, P1 / T1 = P2 / T2, where the suffices 1 and 2 indicate the original and final conditions. When the gas tank wall is heated, T2 > T1. From the simplified Gas Law, P2 > P1. Pressure is just the amount of force per unit area that gas molecules exert on the tank wall. A higher pressure means that the gas molecules are hitting the wall harder -- they are very excited/agitated (more energetic) than before (moving at a higher speed).

In an open system (no walls), gas molecules of higher speed means expansion of volume. Warm air therefore rises due to lower density and cold air (higher density) refills the vacated volume. Temperature difference is the major cause of breeze.

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Answer 3

When air is heated it expands and becomes less dense; thus it will be replaced by more dense and colder air volume.