We use it to cool our drinks in the summer; we skate on it when it covers our lakes in the winter; and we slip and slide on it when it covers our roads. We all take ice for granted. However, it is actually a very unique material.

The one property that makes the solid form of water so unusual is that, unlike most materials, the solid occupies a greater volume of space then an equal amount of the liquid form (ice takes up about 1/9 more space than the same amount of liquid water). This means that the density of liquid water is greater than the density of ice. The density of water is at a maximum at 4 degrees C. Below this temperature the density decreases slightly until it sharply drops off at the freezing point (0 degrees C). The lower density of ice, relative to water, is why ice floats in a glass of water. The fact that ice occupies more space than the same amount of liquid water is the primary reason that freezing can do so much damage to roads in the winter. If there is a crack in the road, liquid water will fill it. When the water freezes it exerts tremendous forces on the road material that can actually break it apart.

Take a second to think about what the world would be like if ice was more dense than water. Although the ice that would sink to the bottom of our glass of soda might not be much of a problem, consider what would happen to our lakes and rivers. Let me speculate. Ice wouldn't stay on the top of a lake. As soon as it formed it would begin to sink and further cool the deep water. If the body of water was cold enough, ice would build up on the bottom. This "bottom ice" would be very insulated from the warm air above and might not melt, even in the middle of the summer! The "bottom ice" would probably kill most of the bottom dwellers in the lake or river. This would have a very bad effect on all other life in the lake that feeds on the bottom dwellers, or who use the bottom for protection from predators. Life in northern lakes and rivers would be very different than it is today. Some of these lakes might eventually freeze solid in the winter, and thaw only a few inches in the summer. Since large bodies of water act as tremendous heat sinks (they moderate our climate), sinking ice may cause huge changes in the climate. Allow me to go so far as to speculate that life on earth may not be possible if ice sank.

Another neat property of ice is that it becomes a liquid under pressure. This is why ice skates work so well. The blade on a skate is very small, so all your weight is applied to a small area of ice. This creates a lot of pressure on the ice which causes it to melt. The layer of water under the blades helps them glide over the ice. The water freezes again afterward.