Why water expands on freezing?

Question

Most other substances contract on freezing and its pretty logical but i dont understand why does water expand? can anybody suggest a reason?

Answer 1

The reason why water expands upon freezing is due to the intermolecular forces of attraction which exist between the water molecules, in particular, Hydrogen Bonding.

Water (H2O) is a polar molecule, the Oxygen side gaining a partial negative charge and the Hydrogen side gaining a partial positive charge. The partially positive Hydrogen's in a water molecule are attracted to partially negatively charged Oxygen's in other water molecules.
It is this intermolecular attraction (called Hydrogen bonding) which gives water many of its unusual properties. As a matter of fact, without Hydrogen bonding, water would be a gas at room temperature, not the liquid we are all familiar with.

As water freezes, the molecules begin to slow down. As the molecules slow down they become less and less able to break free of these attractive forces and begin to align themselves in a position which takes the least energy. Due to the geometry of the water molecule, this least energy position forms a lattice that just happens to be larger than if the molecules were in the liquid state. The water molecules form ice crystals which are larger than if the molecules were moving fast enough to [partially] overcome the attractive forces.

So in other words, water expands upon freezing due to the Hydrogen bonding within the water molecule.

Answer 2

Yea, water is unlike most molecule in the sense that when it freezes and that the molecules arrange in a crystal structure, they occupy more space. In the liquid phase, the crystals do not exist, thus the molecules would be free to be in the position where they use the least space. Notice that the fact that the same amount of water have a larger volume in the ice form implies that water is denser than ice. If it were not that way, I would never float and the ocean would freeze. There would be no life and no yahoo answer.

Answer 3

It is important to know that water is at its maximum density at 4°C. As the water cools further, it gets LESS dense, and thus begins to expand.

In the process of crystallizing (freezing), the water molecules start to orient themselves, guided/pushed on by hydrogen bonding, into a crystalline lattice that has more void space, and thus is less dense than liquid water.

The fact that water expands IS unusual, but may be one of the reasons that earth can sustain life. If ice were more dense than water, the vast majority of water on the planet would be tied up as ice at the bottom of the oceans and deep lakes.

Answer 4

It's pretty simple and has to do with the fact that when water is in it's liquid form it contains weak hydrogen - oxygen bonds between molecules which actually compress it's liquid form, when it freezes these weak bonds are broken and (though the frozen form is a contraction of the liquid without these bonds) it is actually bigger than the material it replaces.

Answer 5

Many of the answers above are quite good, but I think the following link provides the best description. There are two things going on: the water contracts like other liquids as it cools, but it also arranges into a structure influenced by hydrogen bonding that causes it to expand.
The link has got pictures explaining what's going on.

Answer 6

The atoms get farther apart. They atoms in water are very active and "fluid" so they move around a lot, but when the water starts to freeze, the atoms slow down and move apart from one another because they can just bump into one another and move around like before. Water doesn't expand a whole lot, but when it does, it's just the atoms making more space for themselves.

Answer 7

Freezing involves the removal of heat from a liquid to form a solid without any chemical change in the molecular structure of the molecules. Liquid water consists of moving molecules possessing kinetic energy(an amount of heat) that pushes the water molecules apart held loosely together by the balancing forces existing between the weak intermolecular attractions between the outer electron-loving electron structure of oxygen atoms in each H-O-H molecule, the electron-loving electron structure of H atoms in other nearby H-O-H molecules and the even weaker electron attractions between the O atoms in different water molecules and the H-atoms in different water molecules . Reduction in KE of a liquid by heat removal, results in less motion and a shift in favor of the intermolecular attraction forces between atom that make water especially the attraction of the 2 H atoms of one water molecule to the 1 O atom of another molecule. The preferred attraction results in the organized alignment,orientation, or stacking of V- shaped water molecules to form a unique solid lattice, that ,unlike most of the solids we know of formed by freezing, are less dense than the original liquid that the solid came from.

Note:The angle between the V-shape of the water molecule is the bond angle formed by the 2 H atoms bonded to the O atom , and if my memory serves me, is about 105.6 degrees. The V-shape of water molecules is always maintained during any phase change.
file TWH 08012006

Answer 8

Hi. Water as a liquid has no stucture to it so the molecules are free to take as little space as possible. Ice has a structure (crystal) which does NOT allow for molecules to fill in the spaces between them. Look at a snowflake and you'll get the idea.

Answer 9

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!

Answer 10

oxygen expands