Why does hot water freeze faster than cold water?

Evaporation is the large part of the answer. In evaporation, molecules of water escape into the air as water vapor. It takes energy from the molecules to make this lap, so even when water is heated to the boiling point, that is, 100 degrees, a little more energy is needed for molecules to leave the kettle as steam. Water does not have to be boiling in order to vaporize. There are so many trillion of molecules in just a small cupful that some of these will absorb energy from their neighbors simply by chance and escape into the air. As result, the remaining molecules become a little bit cooler.

In a pot of almost boiling water – say, 100 degrees – the molecules dance in a heated frenzy. They are livelier than those in a 30 or 50 degree cupful and are thus more apt to jump into the air. Within a few minutes, so many molecules will leave the water that it becomes cooler than the cupful that was cooler to begin with. This is why hotter water, if it is in a vessel with a wide opening, will freeze first. But if opening is narrow, vapor will have a hard time escaping. Evaporation will be inhibited and the cooler water will freeze first.

Many other factors also influence quicker freezing of hotter water. For example, hot water circulates more vigorously than cold. As a result, more of it is brought to the surface, thus speeding the loss of heat to the cool air. Also, the more water that is lost by evaporation, the less that is left behind to freeze.

More reading:
Steam (Wikipedia)
Evaporation (Wikipedia)
Water vapor (Wikipedia)

Related posts:

Leave a Reply

Your email address will not be published. Required fields are marked *