If molecules slow down and get closer as they get colder, then why does water expand when it freezes?

Question

It cant be unique to water, because milk does it too.

Answer 1

An interesting question, which relates the peculiar shape & properties of the water molecule.

In both liquids and solids, the particles are about as close together as their movement will allow. This is why both liquids and solids are incompressible. Generally, when you heat up a liquid or solid, its particles, which are now moving slightly faster, will jostle each other a little farther apart. On the scale we can see, this results in expansion.

In a gas, the kinetic (motion) energy of the particles overcomes the forces which attract particles to each other, so the particles actually spread out a great deal. Because of the space between gas particles, a gas can be compressed.

What makes water act a little differently? First, there is its shape. The water molecule consists of two fairly small Hydrogen atoms, connected about 120 degrees apart to a larger Oxygen atom. So the water molecule looks sort of like a microscopic boomerang with a really fat middle. Elbow macaroni would make a reasonable model, for our purposes.

. . (Oxygen - slightly negative; likes electrons)
O
/ \
H H (Hydrogens - slightly positive; e- drawn to O)
The next important factor is Polarity. The water molecule is polar, which means it has areas of positive and negative charge (sort of like static cling). The oxygen atom tends to hog electrons, so it is slightly negative and the hydrogens are slightly positive. Consequently, the hydrogens of one water molecule tend to stick to the (oppositely charged) oxygen of the next.
At temperatures where water is a liquid, this has little effect on how the molecules arrange themselves. Think of taking a bowlfull of those macaroni and shaking them around - they tend to pack in pretty well.

However, when water begins to freeze, the effects of polarity are considerable. Hydrogens link up with neigboring oxygen, to form an orderly crystal structure. Generally, this structure is hexagonal, which is why snowflakes have six sides. Think of taking all that macaroni and gluing it together, so that the end of one noodle attaches to the middle of the next. Of course, I don't recommend actually _doing_ this unless you have a really bored Cub Scout troop on your hands.

Once you have glued all of your (hopefully, imaginary) noodles together, you'll notice that they aren't packed as closely together as when they were loose. Water is they same way, which is why it expands, about 10% on average (which, incidentally, is why we see the top tenth of an iceberg).

Answer 2

Well, N8ball88 seems to have answered your main question very thoroughly. The crystalline structure of ice causes water to become less dense as it freezes, so it takes up more volume. As a side note, this also gives water a unique phase diagram, where the solid-liquid boundary line has a negative slope rather than positive. This is due to the fact that, since water is more dense than ice, putting higher pressure makes the liquid phase more and more stable compared to the solid phase.

Also, while I can see that your main question is answered fully, no one has mentioned your details about milk. Yes, this property is a unique behavior for water. No other known substance does this. The reason that milk also exhibits this behavior is that milk is in fact a solution of many molecules, mostly fat globules in a water-based fluid. Therefore, since milk contains water, it would have many of the same properties as water.

[Haha... "bass-ackwards"...]

Answer 3

When water forms a crystalline matrix, its crystal lattice takes up slightly more space than unordered liquid. The hexagonal crystal lattice that is formed by water ice is defined by the tetrahedral bond angle of the water molecule, and, most importantly, hydrogen bonding. Without hydrogen bonding, water molecules would simply collapse and form a more dense solid whose lattice unit was based more upon close-packed geometry. Hydrogen bonding aligns the molecules so that they freeze in the hexagonal pattern.

Water is highly unusual in this regard as very few compounds form a less dense crystalline lattice (including polymorphs). Consider the ramifications of this characteristic! If water ice was more dense than liquid water, all of the oceans and lakes of the world would be solid ice, with only a thin layer of liquid water on top melted by the sun. I don't believe that life as we know it would exist on this planet.

The Internet link below has some really nice pictures of water lattices and a pretty good explanation of the crystal lattice.

Answer 4

Water molecules are oxygen and hydrogen bound together by covalent bonding. This makes for a strong bond. The ends of the molecules are Hydrogen atoms, capable of hydrogen bonds. When molecules slow down, the hydrogen atoms get close together enough to make a hydrogen bond, creating a six-sided structure. when it is heated, the structures break up, due to the molecular movement, and it becomes a liquid once again.

Answer 5

If you mean an increase in volume by 'expand', then your wrong; since the volume of water does not change upon cooling. It is the density which varies & that is your concern as far as the molecules. If 100cc of water is cooled below 0 deg centigrade it will turn into ice & when heated it will re-enter the liquid state & retain 100cc. This shows that none of the volume is lost.
Hence, density varies with temperature with respect to water. However the anamolous expansion of water is a rare case. In this case water does not turn into ice between the temperatures 0 - 4 deg centigrade.

Answer 6

It looks like you've gotten at least one thorough answer. But I wanted to share this article with you.

It seems that as much as we think we know about water, there is much that we don't know. This article surprised me, because water seems such a simple little molecule, just three atoms. And life on Earth depends on the special properties of water.

Answer 7

Its to do with the crystal bonding shape the molecules adopt when they freeze. Molecules take up more space in this lattice than when they are freely moving around in liquid form.

Answer 8

The Hydrogen inermolecular bonds. When its molecules are not forced into a rigid shape, the Hydrogen bonds pull the molecules closer together. It is densest at 4C

Answer 9

The water crystals in ice are in a ring that takes up more space. Most liquids do get smaller when they get colder.

Answer 10

That is got to do with bonding in ice and water. In ice, intermolecular forces are weaker and bonds are longer. This is because of this anomalous property, water expands on cooling.