Why does warm air rises?

Answer 1

Now the molecules of the heated air have room to move around more freely, and continue to do so as long as the heat gives them extra kinetic energy. The higher the hot air floats, the more room there is for its molecules to move about, and so it continually expands, its energy pushing it out and up. Here the air runs into a balancing problem due to the laws of thermodynamics. The energy from all of its molecules can be used to propel the molecules out at greater distances so it can expand more, or the energy can be used to keep the air hot, but it cannot do both at the same time.

Answer 2

Warm air is less dense than cold air. So, being lighter, warm air rises until it is cooled. Once cooled, the once warm air begins to sink.