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Although most substances are more dense when they are in a solid state, water is actually less dense when it is a solid. Therefore, ice will float above the liquid water below. This can be seen with a glass of ice water--the cubes are floating at the top. The molecules of water in ice expand. This allows life under a pond for example, to exist through the winter months because the temperature of the liquid water below the ice is above freezing or greater than 32 degrees Fahrenheit. At four degrees C or 39 degrees F, water reaches it maximum density. As it further cools, it becomes less dense due to strong, orientation dependent intermolecular interactions as seen in water molecules. Water of 4 degrees C accumulates at the bottom of a lake due to its maximum density and chunks of ice float, rather than sink to the bottom, due to its lighter density. This allows life to exist during the winter months beneath the ice.
Ice is less dense than liquid water
Water is a member of a very exclusive group of substances that are less dense as a solid than as a liquid. It is very important that ice floats and has numerous biological impacts; namely, all life on earth. Most substances will contract as they cool, their individual molecules slowing down and "staying put" until finally forming a solid. Water will also do this - up to a point. As the temperature of water drops, the molecules slow down and contract just like any other substance. But once it reaches 4° C, water will start to expand. The reason water does this lies in its hydrogen bonds.
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