Chemistry

Why Ice Floats on Water



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Why does ice float on water? Solids usually sink in liquid. A rock, for instance, dropped into water, will go straight to the bottom. What prevents ice from displacing liquid water? Water is unique due to hydrogen bonding. The physical and chemical characteristics of water set it apart from other liquids. As water freezes it expands to take up more space. Other liquids contract when frozen. Rocks sink because their density is greater than the density of water. Density is expressed in mass per unit volume, meaning the amount of weight of an object compared with its volume. A rock will displace water in its plunge to the bottom. Ice is less dense than liquid water, so that water displaces ice. Ice floating on water is an interesting phenomenon to observe, but there's more to it. If ice sunk instead of floating, life on the planet might be very different.

Hydrogen bonds have a profound effect on biochemistry. The oxygen of the H2O molecule shares covalent bonds with two hydrogen atoms, but that's only part of the story. Oxygen has an overall negative character, while the two hydrogen atoms carry an overall positive charge, making the H2O molecule dipolar. The positive charges associated with hydrogen have a strong attraction to the negative charges on the oxygen atoms of neighboring H2O molecules. This is how water molecules stick together. Hydrogen bonds are much closer together in liquid water, and rearrange, or change partners, at a rapid rate. As water freezes, hydrogen bonds adjust their spatial position in order to balance positive and negative forces within the molecules. Water freezes into the form of a crystal lattice, or diamond shape, occupying a larger space than liquid water. This is why pipes sometimes freeze and burst in the winter.

How different would life on earth be if ice did not float? Instead of a frozen layer on top of a lake or stream in winter, bodies of water would freeze solid. Fish and other organisms would not survive. Ice crystals would form in the water of animal and plant cells, disrupting the cell cycle. Water is an extraordinary substance, and supports all life on the planet. Mother Nature thought it through carefully when she designed water, so that the liquid that cools and hydrates our bodies year round would not cause us to freeze to death from the inside out when winter comes. And for that reason, ice floats on water.

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