# Which freezes faster, hot water or cold water

Ask Marilyn ® by Marilyn vos Savant is a column in Parade Magazine, published by PARADE, 711 Third Avenue, New York, NY 10017, USA. According to Parade, Marilyn vos Savant is listed in the "Guinness Book of World Records Hall of Fame" for "Highest IQ."

## Question

I poked around your Web site looking for one error in physics that I am sure Marilyn made, but so far as I know, was not followed up in print.

This was several years ago at least. Someone wrote in with a question about whether hot or cold water freezes faster. Marilyn replied that it is logical that the cold water freezes faster since it has does not have to cool down as much to hit freezing. There was a considerable debate about this in Scientific American Amateur Scientist an even longer time ago, because the correct answer, that hot water (for equal masses of water) will freeze faster than cold water is certainly unexpected. The debate was not about whether or not this was true, but why. The Boltzman fourth power law for cooling was invoked, but cooling due to evaporation and mixing from a greater thermal differential were also included. Supposedly, folks who live in very cold climates wash their cars with cold water rather than hot (or maybe rinse off the road salt) during winters.

I also remember seeing this column, but no longer have a copy. If you have this column, please send it to me.

## Some Interesting Hypotheses

John Bailey <jbailey@ida.org> wrote to suggest the following hypothesis:

Perhaps hot water provided a partial melt of the frost below the tray (in an older model refrigerator) which ultimately provided a better heat sink to the water in the tray, allowing it to freeze faster.

Ed Foster <erfoster@ziplink.net> wrote to suggest an alternative possibility:

Far more cooling is needed to freeze 32 degree water than cooling the water from even a high temperature down to 32 degrees (heat of fusion and all that.) Perhaps enough of the hot water evaporates off while cooling to 32 degrees that the reduced amount remaining freezes before the water that started out cold. The calculations to show how much of the original hot water needs to evaporate should be easy to do and if I get inspired enough to do it I'll send the results. Doing an experiment and weighing the resultant ice might also prove informative.

Eliot W. Collins <ewcollins@att.com> wrote to suggest yet another possibility:

I believe that boiling water before attempting to freeze it, removes the air from the water and this allows the water to freeze more quickly.

## The Last Word

Sebastien Blondeel <blondeel@clipper.ens.fr> wrote to point out that many of the problems on the Marilyn is Wrong web pages are discussed in the Frequently Asked Questions for the usenet (Internet) newsgroups, in particular alt.folklore.urban, rec.puzzles, and sci.astro.

From the alt.folklore.urban FAQ, the answer is that under some conditions (e.g. in wooden buckets, where most of the cooling is caused by evaporation), hot water freezes faster. However, since the cooling is caused by evaporation of a significant amount of hot water, the hot water produces less ice.

Two references are cited:

• "Hot water freezes faster than cold water. Why does it do so?", Jearl Walker in The Amateur Scientist, Scientific American, Vol. 237, No. 3, pp 246-257; September, 1977.

• "The Freezing of Hot and Cold Water", G.S. Kell in American Journal of Physics, Vol. 37, No. 5, pp 564-565; May, 1969.

## Hot Water can also Freeze Faster in a Freezer

Thanks to Bert Koehler <koehler@equator.com> who wrote to point out that, according to New Scientist Magazine, hot water can, under some conditions, freeze faster than cold water in a freezer.

One of the 350 questions discussed in their archive is Is it true that hot water placed in a freezer freezes faster than cold water? And if so why does this happen?

## Yet another reference

Jud McCranie <jud.mccranie@mindspring.com> recommended this online reference from the Usenet Physics FAQ: Hot Water Freezes Faster than Cold!

## And Some More

Jeff Kaylin <jkaylin@aol.com> recommends Scientific American: Ask the Experts: Physics.

Bo Briggs <bobriggs@usa.net> recommends How Things Work: Water, Steam, and Ice. (This is a comprehensive web page that covers numerous questions.)

http://www.wiskit.com/marilyn/freezing.html last updated June 14, 2000 by herbw@wiskit.com