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Fundamental to the answer is the scientific principle of density. This is basically related to how far apart the molecules in a compound are from each other. In lead and rock, for example, they are very tightly packed. In aluminum and most plastics, they are more loosely packed.

Another key concept is that of molecule activity - at higher temperatures, molecules take up more space, as they are not entirely stationary; they actually move (or vibrate in a solid), and move or vibrate faster and harder when heated, taking up more space.

Thus, using these two principles, we find that water that is hot is less dense than water that is cold. In a fluid, the densest portion sinks to the bottom, as gravity has more pull on it. A cubic metre of cold water weighs more than a cubic metre of hot water, as the hot is less dense. This is the same reason that wood and oil floats on water - heating water just adjusts its density.

As for why ice floats, that is a very special and unique circumstance, and for a different question. In general, solids of a compound will sink in a fluid of that compound.

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13y ago

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