Does hot water freeze faster than cold water?

Cecil's Columns/Staff Reports
1

http://www.straightdope.com/classics/a2_098b

There’s the link. Reading this column stirred a memory in my mind (as opposed to other people’s minds) of hearing about this phenomenon in a 1st-semester chemistry class in college. The explanation was much more succint and matter-of-fact than the really-hot-water-freezes-faster-than-cold-water deal, but I can’t recall how it went.

Have any current takers of Chemistry 150 (the one that’s not Chem for Dummies) come across an explantion recently?

2

I don’t know if I can phrase this correctly or not, but I am going to try.

The cooling rate of the water depends on the temperature difference between the water and the environment. The greater the difference, the higher the cooling rate. Hot water has a much higher temperature difference (from the freezer temperature) than cold water and hence cools at a higher rate. This does not mean that hot water will freeze before cold water. It means hot water will lose more heat than cold water in the same amount of time. It also means that the cooling rate slows as the water cools.

Think of it this way: Your warm water cools, and at some point it is as cold as the cold water you might otherwise have used. At this point, the once warm water will now take as long to freeze as the once cold water would.

I think this whole discussion has come about because of a misunderstanding about cooling rates and time to freeze.

I wish I could provide a cite to back this up, but I am at work and don’t have my physics book handy.

3

Mort–

Thanks for the rap on cooling rate vs. what-freezes-faster. It is eminently common-sensical. I am guessing that the explanation I heard, which I could have sworn came out on the hot water freezes faster side, has something to do with the chemistry of the water molecule itself, as well as, probably, changes in state.

Tim

4

I’ve heard this one up here in Canada too. I heard a guy trying to impress a lady with his knowledge of science say that an ice resurfacing machine (a.k.a. Zamboni) uses hot water for this reason. Although reasonably fast, hot water is used to slow down the rate of freezing for a more even ice surface. The same guy is also an epicure, he once said that the best caviare comes from the beluga WHALE. I laughed and laughed…

5

Hot Water Freezes Faster Than Cold!

To summarize the link (although you really should read it), several things are happening. 1: Normal heat loss. This is what was described in the earlier replies. 2: convection. This causes the hot water to lose heat at a MUCH faster rate than if it were still. 3: evaporation. The mass of the water is decreasing, since some (probably a lot, depending on how hot it is) is leaving the container in gaseous form.

So if you start with a cup full of hot water and the same amount of cold, the hot will freeze faster. But that’s because you’re actually creating less ice, due to evaporation.

Again, read the article (and these other threads). It’s well-written and changed my mind on this question.

6

Hey, thanks, man!

7

Good link Joe!

One thing that is mentioned more or less in passing there, is the factor which I believe is the most important factor in most home ice-making experiments: shelf-to-tray conduction!

In the case of an ice tray in a freezer, the most important means of heat transportation is through conduction. I tried to explain it in an earlier thread, but it is described even better in Joes link above:

I think it’s time Cecil updated his column.

8

One other factor at work is the inverse relationship between temperature and volume; i.e water which is near boiling point, or superheated, is considerably less dense than cold water. So, if we have a “control” tray (A) and a “hot” tray (B), then tray B loses temperature at a quicker rate than tray A(see earlier posts). As this rate is a function of the temperature difference between the water and the freezer’s internal temp, it decelerates over time. (i.e. after 1 time unit, the water temperature has decreased, therefore the temp difference has decreased, therefore the cooling rate has decreased.) When the water in tray B reaches the original temp of tray A, it is now cooling at the rate at which tray A started cooling. Except, having started out at the same volume but a lower density, it has less mass of water to cool, and consequently an ice cube forms faster in tray B.

Right?

9

tc wrote:

[quote]
In the case of an ice tray in a freezer, the most important means of heat transportation is through conduction.

[quote]

This might have been true when I was a little kid in the 1960s, and freezers would get a nice thick coating of frost on all their surfaces. However, in the past 25 or 30 years, all freezers I’ve seen are frost-free. They don’t even bother to advertise that they’re frost-free anymore, because no one makes frost-ful freezers.

I recall my family having to defrost our freezer occasionally. We’d have to pull out all the food and warm up the freezer insides, mop up all the water, then put the food back in. It was a half-day long job. Thank god (or Kenmore engineers) for modern conveniences.

10

Another good point. Again, it looks like it all boils down to (groan sorry, pun was not intentional) is that freezing hot water is faster because you’re freezing less water. Although some other factors are at work that make me think even a higher mass of hot water might freeze at the same time or even sooner than cold. (less dissolved gases, convection, evaporation [takes huge amounts of heat with it, not just mass], supercooling, etc)