A Glass of Ice Water
Ice floats because it's less dense than liquid water.
Water and ice are a perfect combination on a hot day. And if you look at the top of a glass full of ice water, you’ll notice the ice floats on top. But why?
First let’s answer a question: Isn’t ice harder then water? The answer —of course—is yes! But when it comes to determining materials that float and those that won’t, we’re more interested in their density.
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Density refers to the arrangement of a substance’s molecules. The more tightly packed the molecules, the denser the object. And in the case of water, liquid water has more densely packed molecules than ice.
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In liquid water, H20 molecules can be easily moved around and are in a random pattern; this allows them to be more tightly packed. But once the water gets to 32 degrees or lower, the random distribution of molecules starts to move into a rigid, hexagonal shape, forming a kind of lattice-like structure. This moves the molecules further apart, making ice less dense. That’s why a bottle filled to the brim with water will warp the container after it’s frozen.
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A snowflake magnified under a microscope. (Credit: Michael Peres)