Ice

Everybody knows it, everybody has had it in his or her hands, but nevertheless many people don’t know anything about the special properties of this solid form of water. In order to be able to answer a lot of questions, you have to take a close look on the structure of ice first.

What does an ice crystal look like?

Well, many crystalline forms of ice are known, but which of these crystals is actually formed depends on the temperature and the pressure under which the ice is formed. The most common type of ice on the earth is called ice type I. The other types are mostly artificially produced in a lab and therefore they are not very important. Ice I is the kind that cools our drinks in summer as an ice cube or obstructs our roads in winter as black ice. Its structure is shown in the picture below:

The red atoms represent the oxygen, and the white, smaller ones are hydrogen. In the picture you can also see that the respective H₂O-molecules are linked through additional bonds. These bonds are responsible for the solid nature of ice. At the same time you can derive further characteristics of ice by looking at the crystal lattice. For example, the characteristic that ice is able to float. You can easily spot that there is a lot of space between the molecules that are linked in the ice, which is totally different to its liquid state.

This fact is called the anomaly of water.

Water is heaviest at 4 degrees Celsius, i.e. the molecules are packed tightest at that point. Ice, on the other hand, expands and requires more space. This is why closed water bottles or water pipes burst as soon as the ice inside freezes and consequently expands. Therefore, ice is lighter than liquid water and floats.

This property is essential for aquatic organisms. The light ice sheet floats on the surface of a lake, enabling us humans to go skating while at the same time preserving the habitats underneath.

What percentage of an iceberg is beneath the water’s surface?

Or asked differently: How big is the density difference between water and ice? This question can be answered with a simple experiment.

Determination of the Density of Ice

Material: water, a big beaker (800 ml), a medium-sized beaker (400 ml), an ice cube with a known volume, graduated flask.
Procedure: Brim the small beaker with water and put it in the big beaker. Next, slowly suspend the ice cube in the beaker. The overflowing water will be collected in the big beaker. Now carefully remove the smaller beaker including the ice cube and pour the overflown water into the graduated flask. Through the amount of overflow water you can determine the volume of the submerged part of the ice cube and deduce the density.