Why does hot air rise and cold air sink? ?

Explanation:

In hot air, molecules have more kinetic energy and due to this they move from one place to another. As a result, there occurs more number of collisions between the molecules. Due to this molecules of hot air tend to rise.

Whereas when air is cold then its molecules are closer to each other. As a result, they do not have enough kinetic energy and they do not move from their initial position.

Hence, cold air sinks because of lack of movement in its molecules.

the old alchemists made significant contributions to science even though there starting premises were off a bit. their conclusions after failure would steer them into better directions for investigation. science is no more than close observation and the initial mistakes result in rethinking and retrying experiments.