Anonymous
Anonymous asked in Science & MathematicsChemistry · 10 years ago

Why does water expand when cooled?

When everything else shrinks when its cooled down?

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  • John
    Lv 6
    10 years ago
    Favorite Answer

    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.

  • 10 years ago

    I like John's answer the best. Reason? The hydrogen bonding stuff, which is the real answer to this behavior of water.

    IIRC, each water molecule has 4 hydrogen bonds in the liquid state whereas, in the solid state, each water molecule is only involved in 3 hydrogen bonds. However, I don't remember beyond that point.

    As to why water is most dense at 4 C and becomes less dense as it cools from 4 to zero, I suspect the water molecules are beginning to fall into an arrangement that, which still liquid, is beginning to look like the solid state. However, that is only speculation on my part, I have no certain knowledge of what happens from 4 to zero.

    Maybe someday!

    Source(s): ChemTeam
  • 10 years ago

    Water expands when it is heated as well (becomes less dense and therefore occupies more space). It contracts when cooled until it begins to freeze becoming a solid. It only expands when it changes state. It expands because when it crystallizes the water molecules occupy the most ideal arrangement for hydrogen bonds to occur therefore increasing the area that the volume of water occupies.

  • Anonymous
    10 years ago

    Water is at it's minimum volume at just above freezing. As you warm it, it expands to the boiling point, at which it releases steam which increases the volume greatly.

    Going the other direction - just above freezing - as it freezes, it expends as ice crystals form, it will continue to expand until it is completely frozen solid, then as it is cooled further, it will decrease in size, like other materials. The fact that ice floats is prof that it is less dense than water. An iceberg in the ocean has about 10% of itself above the surface, the rest is under water. Ever notice that ice cubes in an ice cube tray have little "peaks" on them? This is because the ice cannot expand on the sides because it is restricted by the tray, so the only way it can "grow" is up.

    Source(s): Common knowledge, almost 70 years of life.
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  • 10 years ago

    http://ca.answers.yahoo.com/question/index;_ylt=Am...

    Note that contrary to Mr/Ms CHEM TEAM there are less H bonds in water than in ice.

  • Anonymous
    10 years ago

    because when water gets frozen it creates crystals and those crystal grow

  • 6 years ago

    frogs burping

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