If you take a fluid (i.e. air or water) and heat it, the portion that is heated usually expands. The same mass takes up more volume and as a consequence the heated portion becomes less dense than the portion that is<span><span> not heated.</span> </span>
Cooling a substance causes molecules to slow down and get<span> slightly closer together, occupying a smaller volume that results in an increase in </span>density<span>. Hot water is less dense and </span>will<span> float on room-temperature water. Cold water is more dense and </span>will<span>sink in room-temperature water.</span>
