Ice: merely solid water, but a wonder substance. Water itself is a miraculous substance, possessing a host of unusual properties, not the least of which is its ability to remain liquid over the range of temperatures and air pressures that occur on planet Earth.
The liquid state is rare, in the universe at large, and here on Earth, water is one of only a few substances that occurs naturally as a liquid. Elsewhere, if there is water at all, it is most likely in the solid form as ice.
Most substances can exist in one of three physical states: solid, liquid or gas. Substances like water that are composed of lightweight atoms typically have a small range of temperatures over which they can exist as a liquid. Take carbon dioxide, for example. At Earth temperatures, which range from around minus 60 degrees Fahrenheit up to 135, it exists only in the gaseous state. It can be squeezed and frozen into the solid state (“dry” ice) and will melt to a liquid at atmospheric pressure, minus 150. But at temperatures above 50, it simply cannot exist as a liquid at any pressure.
The water molecule is shaped like the head of Mickey Mouse (minus the nose), with a large oxygen atom for the head and hydrogen atoms for the two ears.
To be more precise, the spherical shape of the atom is an outer density boundary of electric field density. Because the shape of the molecule is not symmetrical, it causes the electric charges within the molecule to be unequally distributed. The result is that the water molecule is electrically polar, with a slight excess of negative charge on the oxygen end and a slight excess of positive charge on the hydrogen side.
This small charge imbalance produces weak attractions between adjacent water molecules, which is called hydrogen bonding. It makes the molecules “sticky.” Hydrogen bonds are responsible for all of the unusual properties of water, which include high specific and latent heat, high viscosity, high surface tension, high solvent ability, high dielectric constant and more.
Hydrogen bonding is also responsible for the properties of ice, the most unusual solid substance on Earth. The difference between the solid, liquid and gaseous states has to do with the orderliness of molecules. The molecules in solid substances like ice are locked into a rigid three-dimensional structure referred to as the crystal lattice.
At low temperatures the molecules do not have enough energy to overcome the attractive forces of the hydrogen bonds. In the gaseous state, molecules are unbonded and are free to move about, occasionally bumping into each other. As the temperature increases, the molecules vibrate faster. At the melting temperature of 32 degrees, the average speed of the vibrating molecules is fast enough that they have enough energy to break away from the crystal lattice.
Liquid water is a nebulous state between the solid and gaseous states. In liquid water the individual molecules are still joined, but not in the rigid crystal structure of ice. They are clumped together or connected in short chains. Hydrogen bonds are broken and reformed millions of times per second. At any given temperature above freezing, a certain number of molecules will be free from the crystal lattice. At higher temperatures there are more free molecules and fewer confined ones.
At the boiling temperature the average speed of the moving molecules is fast enough to escape the liquid. In the liquid state some free molecules are moving fast enough to escape the surface of the liquid. Thus, evaporation takes place, even below the boiling temperature. Even in the ice phase, there are enough fast-moving molecules for some of them to escape. Ice can sublimate to vapor without passing through the liquid state.
Richard Brill is a retired professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.
