Why does H2O expand as it freezes?

Question

Most other things tend to compress as they reach their freezing point.

Answer 1

Water is one of the few substances that expand upon freezing. The expansion is due to the open structure of ice relative to that of liquid water. Each hydrogen atom on one H2O molecule is oriented toward a nonbonding pair of electrons on an adjacent H2O molecule. As a result, ice has an open, hexagonal arrangement of H2O molecules. So it is basically due to hydrogen bonding.

Answer 2

Water is the only liquid which expands when it freezes. Generally, all substances shrink as they cool. Water is no exception, for the most part. However, as water cools, it reaches its maximum density (has the smallest distance between molecules) at 34 degrees Fahrenheit. Between 34 degrees F and 32 degrees F, it actually expands slightly. At 32 degrees F, of course, ice (solid water) forms. There is a sharp expansion as the water freezes. You know this is true, that ice is lighter than water, because you can see ice cubes floating in a glass of water. The reason it expands is that the molecules of water re-arrange at the time of freezing to form the crystal structure of ice. This arrangement takes up a little more space than it did when it was still liquid, and thus,it is larger per unit of mass (less dense), and so the ice floats on the water.

Water is the only compound I know of that expands when it freezes. There may be some others I don't know about, but certainly it's a very rare property. As far as the reason for water having this unusual property, basically it has to do with the structure of the crystal that forms when the water becomes ice. You could use tinker toys to demonstrate this. When the water is liquid, the water molecules (represented by the wooden disks of the tinker toys) are close together, but can slide past each other and move around freely. When it freezes, bonds form which lock the molecules in place in a regular pattern, that stays nearly fixed (molecules can't move around). You can use the sticks of the tinker toys to represent the bonds, holding the disks in place a fixed distance (the length of the sticks) from each other. Whether the compound expands or contracts when it freezes depends on the spacing and structure of these bonds. In most compounds, the molecules end up closer together in this solid state than in the liquid state. But for water, the bonding is such that the molecules end up farther apart in the ice than in the water. By the way, the fact that water expands when it freezes means that ice is less dense than water. For this reason, it floats on the water (look at an ice cube in a glass of water). This property is very important for fish in the winter, because it means the ice forms on top of a pond and insulates the rest of the pond below, preventing it from freezing all the way through.

Answer 3

Water is a really neat molecule. It has one oxygen and two hydrogens, but the oxygen has to have the hydrogens on opposite sides, and at a slight angle. The net result is that the molecule is bent a little. If you were to draw it vertically on the page, it would look sort of like a parenthesis, with the negative oxygen in the center of the curve and the positive hydrogen at the ends.

When water freezes, the molecules stop moving around so much and start to "snuggle up" together. When they do approach each other slowly, they have to arrange themselves in particular ways in order to accomodate the charges (positive repels positive, negative repels negative, opposites attract) and some other forces. This means that they can't just line up like most other molecules. They must line up in a particular way which takes a lot of room.

The final effect is that the structure when they line up takes a lot of room, even more than when they are liquid.

When the molecules are warmer, they will bounce off of one another like little bumper cars. In that case the forces are still at work, but the energy the molecules has is stronger.

Incidentally, water really isn't even this simple. Rather than freezing all at once, water forms doubles, quadruples, etc at higher temperatures. These bounce around losely as if they were one big loosely held molecule. At 32F, they finally arrange themselves into a much larger crystal.