Sunday, December 22, 2013

How Fast Does Water Freeze?

Recently there have been some comments on Facebook and elsewhere explaining why hot water freezes more rapidly than cold water. Since hot water does not freeze more rapidly than cold water, these comments allow us to think about the nature of heat, and the use of a scientific theory. 

Heat and temperature are related but different phenomena. Heat refers to the quantity of energy of a particular kind in a lump of matter of a particular size. Temperature talks about the distribution of heat in a lump of matter regardless of size. That is, a large cup of coffee may have the same temperature as a small cup of coffee, but the large cup has more heat than the small one.  

In my high school physics class, our teacher explained that the amount of heat generated by an engine was a constant – regardless of where the heat went. One of the students asked, “Then how come drag racers put wide tires on the back? Wouldn’t the heat be the same whether the tires were large or small?” The class was stumped for a moment. The answer is that since the amount of heat is the same, distributing it through a broad tire would cause less of a rise in temperature than distributing it through a small tire. Since the total amount of heat was the same, putting it into a larger volume of matter would cause less of a temperature change – the bigger tire wouldn’t melt.

The same argument applies to the freezing phenomenon. The classic conundrum states that if you put a cup of hot water next to a cup of cold water in a freezer, they both freeze at the same time. This means that the hot water froze faster than the cold water.

This is an example of a flawed theory. A scientific theory offers an explanation of an observed phenomenon that can be disproved. In the case of the freezing water, the idea that the hot water freezes more quickly can be disproved simply by placing the hot water in the freezer, and seeing how long it takes to freeze; then putting the cold water in the freezer, and seeing how long it takes to freeze. While the hot water is cooling, at some point it will have the same temperature as the cold water. From that point, the question is how rapidly will two identical amounts of water, at the same temperature, take to freeze. The water’s history doesn’t matter. When the hot water and the cold water are in the same freezer, the hot water warms up the cold water while the freezer removes heat from all its contents.

A theory that cannot be disproved is not a scientific theory. The theory of evolution, like the theory of gravity, explains an observed set of phenomena and allows for predictions that can be either validated or disproved. Dismissing “a mere theory” because it is only a theory is fine. Dismissing a scientific theory as “a mere theory” is thoughtless.