rjs (04/13/83)
At various times in my life, I have been told by people, in a matter of fact way, that warm water will freeze in an ice cube tray (in the freezer) faster than cold water. They always claim that experiments have been done which show this (but of course never supply references). Normally I just toss this off as being an old wive's tale, however, recently a person with a Chemistry degree also made this claim, making vague references to convection currents in the water. If you ignore convection currents, the warm water would take some time x to cool to the temperature of cold water, and then time y to freeze, but cold to start with water would just take time y to freeze. It also seems to me that with convection currents doing their thing, the time to go from warm to cold will still be much larger than the time it takes friction to drag those currents to a halt once cold water temp. has been reached. What gives? Is there really evidence for such a counterintuitive phenomenon, or am I the victim of a massive practical joke? Robert Snyder floyd!rjs
leichter (04/13/83)
This topic - warm water freezing faster than cold - has been discussed before, both here and in Scientific American's "Amateur Scientist" column a couple of years ago. Conclusions: 1. It really does work for appropriate choices of water temperatures; 2. The principle cause is evaporation; much of the warm water evaporates, taking a great deal of heat with it, while at the same time reducing the volume of water left to freeze. Thus, you quickly end up with water at the same temperature as the cold water is at, but there is less of it. Since further cooling occurs at the surfaces, and in most containers the available surface area is the same for full and partially full containers, a smaller volume of water will freeze faster. BTW, you CAN'T ignore convection currents, either; they probably contribute, although not as much. -- Jerry decvax!yale-comix!leichter leichter@yale
rjs (04/15/83)
I received a number of letters regarding my inquiry as to whether or not warm water freezes faster than cold water. Many of them refered to a Scientific American Amature Scientist article on the subject, so I looked back through my old issues and found it in the September 1977 issue. This was the special issue on microelectronics which may explain why so many people knew about it. This article does make the claim that hot water freezes faster than cold. It also says that the explaination is still the subject of controversy. The possible reasons offered are better circulation of hot water, hot water contains less dissolved gas, and that hot water loses a substantial amount of mass to evaporation, so you are actually freezing less water. The author then goes on to present the results of his experiments, which show a definite drop off in freezing time after a certain temperature which depends on the conditions (amount of water, size of container, type of freezer, covered or uncovered container). In any case, looking at his data, it still seems that cold tap water (usually < 20 deg. C.) than water that is warmer (but still water i.e. < 100 deg. C.). The turnover point on his data occurs in the 60-80 deg. C. range. My conclusion is that you are still better off using cold water as you don't have to pay to heat the water then cool off that heat. Also, you don't want to run the risk of getting rid of all the dissolved gasses or the ice won't float (just kidding). Thanks to all who answered. Robert Snyder floyd!rjs
gwyn@Brl-Vld.ARPA (04/15/83)
From: Doug Gwyn (VLD/VMB) <gwyn@Brl-Vld.ARPA> I do know that putting hot water into the ice cube trays to freeze gives much clearer ice cubes, since very little air is trapped inside.
kk9w (04/18/83)
I think the joke is on you. The same thing can be said about cold water heating up faster than warm water. If you agree with the statement, then to prove you wrong, the person who started the thing says that the total heat up time is what we are talking about, but if you disagree with the statement then what is being talked about is the rate of heating up (dQ/dt). Either way the thing is kind of funny. Dave Andersen pur-ee!kk9w