Two objects of the same volume and mass have the same density. If two objects have the same volume the lighter one is less dense. If two objects have the same weight the larger one is less dense. In general a less dense object or substance will float above a denser one. The density of a substance changes in response to heat.
Heat reduces density by increasing volume but leaving mass unchanged.
For this reason hot water is less dense than cool water. The density of a solution is greater than the density of the solvent. For example light is bent when passing from air to water or glass. The swirling lines seen when dissolving salt or sugar in water are due to local variations in density of the solution. Similar lines are seen in water being heated or cooled, and in the air on a hot sunny day.