Why does water expand when it cools?
|By Kathy Wollard|
|Why does water expand when it cools? asks B.M. Vignesh Babu, of
Water is peculiar. When most substances change from liquid to solid form, they shrink together, become denser, their molecules packed most closely together.
But when water changes from a sloshy liquid to solid ice, it expands, becomes less dense. Which is why ice floats to the top of your Coke, rather than sinking like a stone to the bottom.
At normal atmospheric pressure, molecules usually behave in predictable ways as their temperature changes. Molecules fly apart into a gas when heated, condense into a flowing liquid when cooled, and shrink into a frozen solid when chilled still further. The changes in state parallel changes in energy: from high energy to medium energy to barely jiggling.
Boiling water expands into a gas (steam) and wafts off into the kitchen. But we also see water expand when chilled in the freezer. An ice cube tray filled to the rim the night before overflows with big cubes of ice in the morning.
Water starts out behaving normally. As its temperature drops, water obediently shrinks together--until it reaches 4 degrees Celsius (39 degrees F.). Then, amazingly, water reverses course, its volume slowly increasing as it chills. When water finally freezes, at 0 C (32 F.), it expands dramatically.
Scientists say water's quirky behavior is caused by the shape of its molecule and by how its molecules bond to one another.
Each water molecule is two hydrogen atoms bonded to one oxygen atom (H2O). Because of how the atoms share electrons, a water molecule is slightly positively charged at the hydrogen atoms, and slightly negatively charged at the oxygen atom. The molecule's charged ends attract the oppositely charged ends of other water molecules ("hydrogen bonding").
In liquid water, as molecules slip-slide past each other, bonds form, break, and re-form. But by the time water has cooled to 4 C., the molecules' energy has dropped enough that they are very near one another. So each H2O molecule forms more stable hydrogen bonds, with up to four fellow molecules.
By 0 C. (32 F.), the H2O molecules are snappily lined up in a frozen crystal lattice, an open hexagonal (six-sided) shape. Unlike in liquid water, the molecules in ice are held rigidly apart. That means more empty space between molecules, so frozen water occupies more room.
Result: Put 10 cups of water in the freezer, take out nearly 11 cups of ice!
Other real-world results: Water pipes freeze and burst in unheated houses. Water collects in roadway cracks in winter, turns to ice, and enlarges crevices into gaping potholes.
If frozen water weren't less dense than liquid, there would be no floating icebergs to sight off the bow of a ship. There would be no skating on ice-covered ponds, while fish and other life shelter in insulated water below. If water froze from the bottom up, much of Earth's water would solidify in winter, and life might be impossible.
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