Question

why hot air is lighter and less denser than cold air

Answer 1

Hot air rises because when you heat air (or any other gas for that matter), it expands. When the air expands, it becomes less dense than the air around it. The less dense hot air then floats in the more dense cold air much like wood floats on water because wood is less dense than water.

Answer 2

hiiiiiii friend!!!
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When anything is heated then its molecule starts vibrating and moves apart from eachother. Before the substance was heated the molecules were packed and when heat was given then the distance between the two molecules get increased. Example:When a solid (ice) is heated then it gets converted into liquid(water) hence its molecule acquires more space , it means its volume has increased thus density of the substance has decreased by the formula density = mass/volume. Similarly when water or air is heated then its molecule acquire more space volume gets increased and the density decreases. On the other hand cold water/air has molecules packed and thats why have lower volume and higher density
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i hope it helps u!!!!