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A debate going on in the world of armchair physicists has been running for years and just won't seem to die. It concerns the apparently simple idea of whether hot or cold water will freeze faster.

At first thought, the answer is obvious-cold water will freeze faster. Need anything more be said? It seems so because people started claiming that their ice cubes would form quicker if they used hot water in the trays.

The big problem in the whole debate is that most people were not arguing from the same position. They were not conducting properly controlled experiments and in the world of science, there is no point arguing a comparison with intricate details when the starting points are different.

Following are a few factors that could influence the result and need to be controlled in a proper experiment. After we discuss them, we'll present the verdict.

1. Evaporation

We need to ensure that the same quantity of water is being frozen for both hot and cold cases. However, hot water will evaporate faster than cold so if both are put in the freezer at the same time, more of the hot water could evaporate to end up as ice crystals on the sides of the freezer. This would mean that a smaller volume of hot water was actually being frozen and could reduce the time taken for it to freeze. Whether the difference is enough to overtake the cold water depends on the specific details.

2. Thermal Conduction  التوصيل  الحراري

Heat can be removed from the water in a number of ways. One is through evaporation as mentioned above. Another way is through thermal conduction. If you have a freezer that hasn't been defrosted in a while, you will notice the build up of ice around the edges, base and ceiling. This ice acts as a fairly effective insulating layer-that is why igloos built of ice blocks can provide quite warm interiors!

For heat to be removed by conduction, the heat essentially needs to flow through solid materials to the cooling mechanisms of the freezer behind its walls. An ice cube tray full of hot water has the potential to melt through some of the ice thereby reducing the amount of insulation. This means that heat can be extracted more effectively from the hot water tray. Whether this is enough to mean the hot water freezes first again depends on the details.

Next page More Factors and the Verdict >Page 1, 2