How Long for Water to Freeze?
Most recent answer: 10/22/2007
- Dara (age 12)
The answer to your question really depends on three things: how much water you have, how cold it is to start out, and how cold the things around it are. Water actually freezes when it gets to 32 degrees Fahrenheit (0 degrees Celsius), but the time it takes to get there may be different.
Lets start with the first. If you take two glasses, and fill one with a tiny bit of water, and the other about halfway, then put them both in the freezer, the one with less water will freeze first (you can try this at home, but I recommend using plastic cups and not glass ones).
Now lets move on to the second part. Lets say you have two glasses, and you fill one with really cold water that has been in the refrigerator, and the other with really hot water from the sink. If you put both of them in the freezer, the one that started out colder will freeze first.
For the third part, lets imagine that you have two glasses with the same amount of water in them, and the water is at the same temperature. Imagine putting one outside on a really really cold day in Georgia, and having a friend in Alaska put one outside on the same day. Since it would be so much colder in Alaska, the glass of water there would freeze before yours.
So, if you took a tiny bit of really cold water in a glass, and put it outside on a cold day in Alaska, it would freeze a lot faster than a big glass of hot water outside on a cold day in Georgia.
Hope this helps!
(published on 10/22/2007)
Follow-Up #1: freezing and authority
- Mackenzie (age 11)
Midland, MI, USA
I would be a little more cautious than you about authority, however. Aristotle made many mistakes, even on simple questions like how many teeth women have. Francis Bacon (is that who you mean?) also wasn't much of a scientist.
My own attempts to repeat this experiment have flopped so far. I always forget to look at the glasses until they're both frozen. You might try it yourself, using metal cups so they don't break, and being sure to put the same amount of water in each. Also, try it a few times switching the positions of the hot and cold cups, since freezers don't cool evenly.
The most careful discussions of the subject (e.g.) suggest that which water freezes fastest depends on several factors, including whether it is allowed to evaporate, how well heat is conducted into it, how much gas is dissolved in it, etc.
I think Tamara has a good discussion on this site, under the name 'Mpemba effect', named after a high school student who had the courage to believe his observations rather than his teachers.
(published on 10/22/2007)
Follow-Up #2: what freezes first
- Cody (age 17)
(published on 02/09/2010)
Follow-Up #3: how long does it take for cocoa to freeze?
- Kavi (age 19)
The actual time depends on may factors: thermal contact and quantity of cocoa, beginning temperatures of the freezer and coca, composition of the cocoa, etc. The best answer is for you to do the experiment and find out for yourself. Try it several times varying one or another parameter. Keep a record and see if you can figure out a trend.
Let us know what you find.
(published on 03/04/2010)
Follow-Up #4: Time for water to freeze?
- Peter Thompson (age 29)
Minneapolis, Minnesota, USA
(published on 06/13/2010)
Follow-Up #5: time and rate happeneth to all
- WALTER (over 9000 years old)
Anchorage, Alaska, USA
Here's some reasons why hot water could actually freeze sooner, surprising as that is.
1. There's a burst of initial evaporation. That leaves less water in the hot container, so that less latent heat needs to get dumped to the surroundings. If that's the reason, there would of course be evidence: less final ice.
2. There's less dissolved gas. That means that the freezing point is higher. If the surroundings are just a little below 0°C, the time it takes to freeze is very sensitive to even small changes in the freezing point. For example, if there are enough solutes to lower the freezing point below the surrounding temperature, the time becomes infinite.
So the freezing time has a complicated dependence on the temperature of the surroundings, the way in which heat is conducted in, the relative humidity of the surroundings, the amount of dissolved gas..... As a result, you can occasionally get the weird situation is which the hotter water freezes sooner, at least according to some experimental reports.
(published on 12/08/2010)
Follow-Up #6: how long for water to freeze?
- E Wickliff (age 40s)
The original Q&A on this goes back to before my time on the site, so I'm not sure why an approximate answer wasn't given. It's not hard to imagine a reason, though. It's rare for the turn-around time on this site to be less than a day. Often it's weeks. Sometimes years. The time to freeze a cup of water in a home freezer is less than a day, so just doing it gets the answer quicker than asking us, and avoids having to rely on authority.
My guess would be maybe three hours to freeze if you really wanted it frozen or maybe half an hour to freeze if you were just trying to cool it and the glass would break if you forgot to take it out in time. But children shouldn't be exposed to that kind of superstition.
(published on 04/07/2011)
Follow-Up #7: philosophy of freezing water
- Peter Ryan (age 31)
Vancouver, BC, Canada
The main thing I hope students would take from this is that each of these speculations can be tested. If the theory is that some of the hot water evaporated, you can check to see if the amount of ice it makes is noticeably reduced. If the theory is that the dissolved gases in the cold water lowers its freezing point significantly, you could try using cold water that had been degassed, maybe with a vacuum pump or other means. And so forth- each idea should be checkable.
This brings us back to the simple question of about how long it takes to freeze water in a home freezer. Why would anyone ask us that when it's so easy and quick to check directly?
(published on 09/09/2011)
Follow-Up #8: testing freezing times
- Tina (age 12)
Wesley Chapel, FL, USA
So if you were looking for the strange effect of the hot water freezing sooner, you might test with the temperature just a little below 0°C, say -5°C. That's because dissolved gases (more likely to be in cold water) lower the freezing point, and that's very noticeable if the freezer temperature is close to the freezing point. If you were just trying to see what happens, without any particular favorite result, you could do that at any temperature, with the results maybe depending on temperature.
(published on 10/06/2011)
Follow-Up #9: time to freeze water
- nnugles (age eh?)
(published on 08/01/2012)
Follow-Up #10: How long to freeze water?
- Xander (age 6)
Cave Creek, AZ, US
(published on 02/17/2013)
Follow-Up #11: wisdom of babes
- Peter W Lindner (age 63)
I hope he sees your thanks.
(published on 08/30/2013)
Follow-Up #12: experimenting on freezing water
- aoife (age ...)
I can't follow what you're comparing here, but at any rate it seems that you want to compare freezing times for several samples.
Unless you can get some sort of automatic data logger you should check pretty often, maybe every 15 minutes or 1/2 hour. I grew up on experiments like that where we had to keep an eye on things almost continuously.
Maybe now you could do something clever, like conduct the experiment in a styrofoam cooler with a viewing port. Cooling maybe could be supplied by some dry ice/alcohol slurry. Each sample could have a thermometer in it. You could take pictures with a digital camera, maybe even with a timer. To avoid fogging, the viewing port could just be a long styrofoam tube with open ends, also serving to vent the CO2 from the evaporating dry ice. A little LED light could be turned on inside when you wanted to view or take a picture.
Whatever you end up doing, a crucial part will be to write it up clearly so people can know why you did it and what you did. What are you trying to find out?
(published on 09/09/2013)
Follow-Up #13: looking at freezing water
- aoife (age 14)
Sure, opening the freezer every 15 minutes will change the freezing time. It will change the time a little for both the water that started out warm and the water that started out cold. This is just like using a slightly different freezer. What's wrong with that? It's not as if you started out knowing exactly which freezer with exactly which properties was the one you wanted to do the test in.
The first thing to do here is to write up carefully what you're trying to find out and why. Without that there's not much point in worrying about the details of what to do.
And who is the "it" that's telling you what to do?
(published on 09/16/2013)
Follow-Up #14: Teach a man to fish
- Mark (age 40s)
It's a little ironical to get your suggestion now. Our group recently had a discussion about how we'd like to do more to help people figure out their own answers and test answers that they might hear, rather than just passively write down 'facts' from the Web. You know that old saying about teaching a man to fish.
(published on 11/24/2013)
Follow-Up #15: philosophy of teaching
- Robert (age 62)
We're happy to pass those thoughts on.
(published on 01/25/2014)
Follow-Up #16: Does warm water freeze faster?
- Marty Shows (age 71)
From what we hear, which freezes faster depends on detailed conditions. The reasons are discussed in the earlier part of the thread. The reason that you suggested though, is not one of them. More stored heat by itself would mean that it would take longer to get rid of the heat. It's other factors, such as less dissolved air, that can make the warm water freeze first.
(published on 02/22/2014)
Follow-Up #17: experimental results on freezing hot and cold water
- Carl F (age 28)
Anchorage, AK, USA
Many thanks for these carefully reported results!
(published on 01/27/2015)
Follow-Up #18: pure heat conduction
- Mike Watson (age 23)
Vicotira, BC, Canada
Let's forget about the part about magic water that keeps its internal temperature uniform. That removes the only real physics of the problem, which is thermal diffusion. You've made our life easier by removing gravity, which causes convection, a much mesier process.
The water will gradually approach -20°C. If by "reach -20°C" you mean get very close, you have to wait several times the thernal diffusion time, the time it takes for heat to diffuse a distance of about the radius of the sphere.
So you need the thermal diffusion constant, which is the thermal conductivity divided by the heat capacity per unit volume. So that's (0.60 W/m-K)/ 4.2*106 J/m3-K =1.4*10-6 m2/sec. WIth your 1 cm radius, the typical diffusion time is about 100 sec. The exact solution for the time-dependent temperature can be obtained by decomposing the spatial dependence into a bunch of patterns each of which relaxes exponentially toward equilibrium. The pattern with the longest spatial wiggles has the slowest relaxation.
(published on 03/16/2015)
Follow-Up #19: freezing milk and water
- Hugh G. Rexcion (age 37)
In equilibirum, the water freezes at slightly higher temperature, because the sugars and salts, etc. in the milk slighty lower it's freezing temperature. (See .) Since in practice solutions can supercool (see ), i.e. stay liquid a bit below the equilibrium freezing point, I can't guarantee that the milk won't freeze first, because the fat droplets might reduce the tendency to supercool. We'd love to hear the results of the experiment.
(published on 03/23/2015)
Follow-Up #20: physics, banking, and figuring things out
- anna (age 42)
We really appreciate these comments!
(published on 06/25/2015)
Follow-Up #21: does cold water freeze faster?
- Jay (age 33)
NY, NY, USA
The site you refer to has nice discussions of the limitations of some common explanations of the effect whereby sometimes hot water freezes before cold water. Unfortunately the explanation they give, that the heat changes the molecular bonds between the water molecules so the resemble those of ice, is pure baloney, far more than the ideas they criticize. The relaxation time for those little local modes in the water is somewhere crudely in the neighborhood of 10-12 s, not exactly long enough to help the cooling water remember anything about its past.
The main explanation that they leave out is probably the best. Heating causes dissolved gases to leave the water. Dissolved gasses lower the freezing point. So the heated water has a slightly higher freezing point.
We discuss these issues in another thread.
(published on 08/14/2015)
Follow-Up #22: stop the van!
- Kyle (age 30)
Any special reason? What direction do you want to see the site go?
(published on 08/12/2016)
Follow-Up #23: hot or cold water freezes faster?
- Ricky (age 20)
We're basically in agreement that various complicated effects can cause hot water to freeze faster than cold under some circumstances. Perhaps the most common of those is found when the cold surroundings are just a bit below 0°C. Then the freezing rate is very sensitive to the exact freezing temperature, which depends on dissolved non-water molecules. Typically, hot water has fewer dissolved air molecules than cold water. That can make the initially hot water freeze before the initially cold water. Another reason can be that more of the hot water evaporates, leaving less behind. The smaller amount left can cool more quickly than a larger amount. These effects can both operate together.
You're pointing to another mechanism. That evaporated water gets into very cold air, causing it to condense and then freeze. In effect, the initial heat helped get the water out into better thermal contact with the air, in the form of vapor.
So yes, rates can be complicated.
(published on 01/07/2017)
Follow-Up #24: finding slush on the Web
- Alex E. (age 38)
Seoul, S. Korea
thanks for the info.
(published on 01/07/2018)
Follow-Up #25: ask us vs do experiment
- Gertie (age 50)
Denver, CO, USA
You make a good point in that many experiments are so hard to do that we all have to rely on getting information from others. This water-freezing-time experiment is really easy to do at home. The home answer will be the one people want. Our results will be different because we'll use a different freezer, different glass, etc.
(published on 04/20/2018)