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Why Is Water Less Dense Than Ice?

Ice Floats on Water

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Ice is less dense than liquid water, so ice cubes float.

Ice is less dense than liquid water, so ice cubes float.

Anthony Bradshaw, Getty Images
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Water is unusual in that its maximum density occurs as a liquid, rather than as a solid. This means ice floats on water. Density is the mass per unit volume of a material. For all substances, density changes with temperature. The mass of material does not change, but the volume or space that it occupies either increases or decreases with temperature. The vibration of molecules increases as temperature rises and they absorb more energy. For most substances, this increases the space between molecules, making warmer liquids less dense than cooler solids. However, this effect is offset in water by hydrogen bonding. In liquid water, hydrogen bonds connect each water molecule to approximately 3.4 other water molecules. When water freezes into ice, it crystallizes into a rigid lattice that increases the space between molecules, with each molecule hydrogen bonded to 4 other molecules.

Learn More

Why Ice Floats
Can Melting Ice Overflow a Glass of Water?
Does Ice Melt Faster in Water or Air?
Make a Density Column
Density Calculations

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