Density When most liquids freeze, the molecules line up into a rigid structure, and move closer together as they lock into place, forming a solid. This means that a given number of molecules of the same mass take up less volume. The result is that the solid is more dense than the liquid. (The same mass divided by a smaller volume gives a larger density). As these liquids freeze, the newly formed solid, still immersed in the liquid, sinks to the bottom, because the solid is more dense than the liquid it's in. Water molecules don't behave this way, and we're here because they don't! Let's explain. When water freezes, the water molecules line up, and as they do, they move farther apart ! This makes solid water take up more room ... the same mass of molecules occupy a larger volume, so the density of ice is less than that of water. (The same mass divided by a larger volume gives a smaller density). As water freezes, the newly formed ice, still immersed in water, floats, because it is less dense than the water. Why are we here because of this fact? Well, three billion years or so ago, life first developed in the oceans. And life on earth ever since has depended on large quantities of water to circulate (the water cycle). If solid water sank to the bottom as it froze, lakes and the oceans would freeze solid from the bottom up; the warmth of summer wouldn't be sufficient to melt ice at the bottom of these bodies of water. Heat from the sun would melt the top layer, which would then also evaporate; this evaporation would steal heat from the water/ice below, keeping it cold. Lakes would stay mostly frozen year-round. So if ice were denser than water, there would have been very little liquid water left after it had frozen the first time! Ice floats. And there's more! The floating layer of ice insulates the water below from the cold air, slowing down the freezing process. And because of the high heat of fusion of water, once a layer of ice forms on a body of water, any water below the ice is actually warmed as more and more layers above it give off heat energy turning into a solid. So large lakes rarely freeze to the bottom. At 4°C, water that is being cooled begins to expand, and become less dense. The closer it gets to 0°C, the less dense it gets. So a frozen lake actually has a temperature gradient, with the denser (warmer) layers near the bottom. The temperature near the bottom stays fairly constant, allowing living organisms to survive the winter. Water Resources