A cup of water is put into a freezer and cools to the solid phase within an hour. The water remains at that temperature for six months. After six months, the cup is retrieved from the freezer. The cup is empty. What happened?

The frozen water sublimed, moving directly from a solid state to gaseous state. This process although slow, over a time period of 6 months, will eventually result in all the frozen water vaporizing.

Explanation:

The process whereby a solid changes from solid to gaseous state without passing through the liquid phase is known as sublimation. Any solid that turns into a vapor without going through the liquid phase can be said to sublime or "sublimate."

Frozen water or Ice sublimes by turning directly into water vapor without first transitioning into a liquid. Though the process occurs slowly at the temperatures and pressures within household freezers and is not readily observable, over a long period as in the example given, it can then be observed.

Some other examples of ice to water vapor transition include;

1. A wet sweater hung on a line in freezing temperatures. Although it may take a few days, the wet clothing article will eventually dry out.

2. Snow changing to water vapor. On very cold days, when it's too cold for snow to melt, it will after a long while sublime into the air as water vapor.

3. Glaciers undergo ablation which is a form of sublimation. Ablation is the process whereby the snow and ice of the glacier slowly begins to evaporate or turn into vapor.