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Alaska Science Forum
Does Hot Water Freeze Faster Than Cold?
by Larry Gedney
This article is provided as a public service by the Geophysical Institute, University of Alaska Fairbanks, in cooperation with the UAF research community. Larry Gedney is a seismologist at the Institute.
Every year, there are some questions repeated by readers regarding the same subject. The most common of these has been: would a glass of hot water or a glass of cold water freeze first, if left outside in subzero temperatures?
Although the issue has been discussed before in this column, the interest generated merits repetition of the subject.
Almost any scientist would scoff at the question and state that it is frivolous--I did myself. Obviously, so the reasoning goes, the glass of hot water has many more calories of heat to dispose of before it can become frozen, and therefore must take the longer time to freeze.
"Not So!" say the proponents of the other side. "How come my hot water pipes always freeze up before my cold water pipes?" And "How come when I put out glasses of hot and cold water when it's - 40°F, the hot water freezes first?"
I never put much credence into these claims until I started reading different accounts of the phenomenon. Most recently, the distinguished scientist and author, Isaac Asimov, in his Book of Facts (Red Dembner Enterprises Corp., 1981), flatly states in one of his "3000 facts," that "Water freezes faster if it is cooled rapidly from a relatively warm temperature than if it is cooled at the same rate from a lower temperature."
This may be true, but only under certain circumstance. Dr. T. Neil Davis (editor of this column from 1976 to 1981) performed his own experiments, and found that only when he used Styrofoam cups at temperatures near zero, did warm water freeze first.
Under almost any other circumstance, especially those which allowed heat to escape through the sides of the container, such as from a metal cup, the colder water froze first.
Insulation being taken into account, we can now attribute the hot water freezing first to the fact that, in hot water, circulation currents move faster, exposing more water to the air, and resultant evaporation (hence, cooling) occurs at a greater rate. Also, boiling the water before exposure to frigid temperatures removes the dissolved air which inhibits freezing in the colder water.
The stubbornness sometimes exhibited by scientists in accepting observations of laymen in favor of accepted scientific dogma sometimes leads to such comments such as those appearing in a recent Ann Landers column. An expert scientist was consulted and asked to reply on this same question--could warm water sometimes freeze before cold, given the same conditions. He said absolutely not. I would have said the same thing.
We were both wrong.