Do you think that a liter of cold water weighs the same as a liter of hot water?
A liter of cold water will weigh more than a liter of hot water. Most liquids, not all, become heavier as they grow colder. The answer all boils down, nay cools down, to density. The colder the water is, the denser it is. Density will normally determine the weight of two liquids which have the same volume. It is quantified by mass divided by volume and here on our planet Earth, mass is the same as weight. In the case of water, aka H2O, the weight is measured by the number of H2O (2 parts hydrogen, 1 part oxygen) molecules present in a given volume.
So, assuming you have two bottles-each 1 liter-and you fill one with cold water and the other with hot, you will notice that the cold water bottle is a bit heavier than its counterpart. The reason is because molecules in hot water are spaced further apart and this means fewer molecules are present in any given volume, hence leading to less mass, or weight if you rather. Now if we could just figure out a way to heat ourselves up…