Is it true that hot water freezes more quickly than water at room temperature?

Question

"It sounds like the kind of question you would be dismayed to hear schoolchildren getting wrong: which takes less time to freeze, cold or hot water? Common sense and the laws of thermodynamics appear to insist that cold water must freeze first. For example, Newton's law of cooling states that the rate at which a body cools is proportional to the temperature difference between the object and its surroundings. But, in fact, it does seem as though hot water sometimes "overtakes" cold as it cools." THANK YOU GRAY MATTER!!!

PS: I tried to test it myself but the results were inconclusive.

Thank you very much too, campbelp2002, I had no idea the same question had been asked 3 days before mine!

Answer 1

Okay, let me throw a monkey wrench into previous posters' comments. Let's consider a pan of water brought to boiling and a pan of water at room temperature, if we measure them both equally in terms of volume (not by mass or weight), it's possible we could get a result of the boiling water freezing faster. Though I've never actually tried this, so bear with me in a thought experiment.

Let's say we take two equal pans and draw a line on the inside at the same level on both, now, in a separate pan we boil water. At rhe same time two people pour tap water up to the line in one pan and boiling water up to the line in the other. These are equal right?

Well, no not really. The boiling water will be more energetic, and likely a bit less dense, yes? So, we're actually putting less mass into the pan via boiling water than we are into the pan with room temperature or cold water, which is more dense. So, we're actually starting on an uneven playing field (the hot pan has less mass more than likely since you have a lower desity of stuff in the same space).

Now, if we put them both in the freezer at the same time, we have two problems. 1) the hot pan is less dense and likely has less actual mass/matter in it 2) the hot pan is more energetic.

So, what happens next. If my thought experiment is moving in the right direction, the hot pan will cool faster relative to the cold pan, and in essence catch up. But there's also less water to cool, and the water is cooling evaporatively due to steam. So it's got a jump start on the cold water.

So, I guess the question is whether the pan with less mass that starts with a higher gradient and evaporative cooling will catch up or overtake the pan with more mass.

So, an extra question I don't happen to know the answer to: will a pan with less matter in it at the same temperature cool more quickly or freeze more quickly than one with more mass at the same temperature?

My guess would be that the pan of the same "average temperature" but with a lower mass or number of particles will have a higher energy per particle than the one with a higher mass, thus a still slightly higher heat gradient, thus it will continue to cool faster? (but I don't know that for sure)

So, yeah, I'd vote for the hotter pan filled to the same line. 'cause I'm guessing that's the usual method of measure. Now if it was a hotter pan, but measured by weight rather than volume, I'd guess the results would be considerably different, yeah? You'd still get the initial evaporative cooling which would evvectively lower the mass overall, but would it be enough to overtake and cool faster than the other pan that has a temperature head start on the hotter pan? Dunno...

Likely, you'd also get different results based on the size and shape of the container. If it's a long wide shallow pan, three's much more surface area for evaporative cooling of the hot pan, so it might lose even more mass that way. If it's a tall skinny container, there will be more surface area for radiative heat loss but almost no surface area for evaporative cooling...

You'd also want to make sure that the area in question is cooled evenly and both pans are an equal distance away from the opening to the compressor/cooling unit. Otherwise one might be getting more of the cool air directly on it than the other... If one's right under it, it'll obviously cool faster than the one a foot away that has to wait for the cooler air to dissipate in its direction.

My 2c.

Answer 2

I think that people misunderstand the concept of the thermal gradient and its effect on heat loss. That's where this rumor came from.

From what I remember from thermodynamics in college, a hot water bottle should COOL (I.E.: lose its heat) faster initially when placed in a freezer than a room temperature bottle of water placed beside it -- because the bigger the temperature differential between the hot body (water bottle) and the cool body (the freezer), the quicker the heat is conducted away (I.E.: the greater the thermal gradient, the faster the heat loss.).

That shouldn't mean that the hot water freezes first, however. It simply means that the hot water will soon cool down to the same temperature as the room temperature bottle. But since the room temperature bottle is closer to the freezing point to begin with (It had a head start, since it started out with less total heat already -- remember both bottles are cooling at the same time, just at different rates), I think that the room temperature water bottle will freeze first. The hot water bottle will lose heat faster initially, because of the greater temperature difference, but as it cools, the rate at which it loses heat will slow down to the same rate of loss as the room temperature bottle (when it "catches up" with the other bottle's temperature.).

So, the answer to your question should be "no".

Here’s an analogy: if you have two tires full of air, but at different air pressures, and you punch a hole in each, the tire that has the most air (higher pressure) will lose air faster initially (it has more air), but it will also take longer for it to run out of air (because it has more air to begin with).

BTW some cool thermo (pun intended) experiments to show this are found at the link below:

Answer 3

I've heard people say this and their logic behind it is that the molecules of warm water are farther apart (distance-wise) than molecules of cooler water, and therefore, somehow, they think it's easier for the warmer water to freeze than for the cooler water.

I don't honestly know the answer, I've never tried it out myself. It's a safe enough experiement though, and if you're curious, you should take the advice from above and test it out yourself. Isn't that the beauty of science?

Answer 4

How long does it take the hot water to cool to room temperature in the freezer? Add this time to the time it takes room temp water to freeze.

Answer 5

No, it is not true.

I think the myth originates from Zamboni machines used on ice hockey rinks. The Zambonis recoat the ice with hot water.

This isn't because hot water freezes faster. It's because hot water will melt the chopped up ice below and both will freeze together as one nice smooth coat of ice.

Answer 6

hmm... no its not true. How on earth would you freeze faster 1 L of water at 60°C than 1 L of water at 20°C with the same amount of energy transfer?? It can't happen... something hot takes more energy to freeze, ergo more time than something at room temperature.

Answer 7

absolutely not. The water has to cool past the point of room temperature water to get to the frozen point. It is all about slowing the H2O molecules down...hot water molecules are moving rapidly and must slow down more than room temp water

Answer 8

HE*L no....who told you that? It take less time to freeze cooler water that warmer water. However, You mentioned " At Room Temp " Water, of course, will not freeze at room temp. unless the room is 31.5 degrees LOL

Trick Question? I want the 10 later, thanks for the 2 now!

Answer 9

http://physicsweb.org/articles/world/19/4/4

Answer 10

This question has been answered already. See the source below.