How long does it take for water to freeze?
- 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.
Let's 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 let's move on to the second part. Let's 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, let's 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
This isn’t a question, but in fact a correction to your answer, Sara. It has been known for centuries that hot water freezes faster than cold water. Both Aristotle and Francis Bean believed this to be true. It has been proven fact in several different experiments by many different scientists. You gave a good answer, and certainly helped out Dara, but I just wanted to make sure she got all her facts straight.
- Mackenzie (age 11)
Midland, MI, USA
Mackenzie- Thanks for your note. You're right to remind us that the common-sense result isn't always right. Sometimes the hot water freezes faster.
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.http://math.ucr.edu/home/baez/physics/General/hot_water.html
) 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 08/21/06)
Follow-Up #2: what freezes first
Hot water does not freeze faster than cold water. That idea is ridiculous. It can be misconstrued as such when you look at it without a mind towards physics and chemistry. When a liquid is cooled, it may pass the freezing point and not appear to freeze. This is due simply to the fact that the molecules need additional energy or a solid to begin the crystallization process. If you put boiling water in the freezer along with cold water, the cold water would reach the freezing point of 0 C much more quickly, but it does not make the transition to ice until a force acts upon it. If you rap the glass you will see the liquid freeze in front of your eyes. The boiling water would not even have reached the previous temperature of the cold water. In conclusion, with both liquids under constant pressure, there is no way for hot or boiling water to "freeze" first.
- Cody (age 17)
Your argument makes a great deal of sense, but subtle complications in the real world can give strange results. Here's an example. Let's say that you set the water in a glass in a freezer with a lot of frost. The hot water may melt the frost, and then make excellent thermal contact with the cold shelf. The cold water may sit on top of the frost in poor thermal contact. Then the initially hot water could cool more quickly and end up freezing first. In some cases it's necessary to actually do experiments to find out how things act.
(published on 02/26/10)
Follow-Up #3: how long does it take for cocoa to freeze?
Can someone answer my question??
If you put 200ml-500ml of *cold* cocoa in the any normal freezer, how long does it takes 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 03/04/10)
Follow-Up #4: Time for water to freeze?
Can someone provide an answer on how long a specif volume of water will take to freeze at a typical residential freezer temp, i.e. 8 ounces of water starting at 38 degrees farenheight takes "X" minutes to freeze, or "x" minutes per ounce at "x" degrees?
- Peter Thompson (age 29)
Minneapolis, Minnesota, USA
No. This sort of problem is unanswerable because it depends on too many variables that we don't know the values of. At best one can give scaling laws, for example twice the heat transfer rate will halve the freezing time or twice the volume, at the same transfer rate, will double the time. The freezing time depends on: the air circulation rate, the surface and shape of the container, the amount of contact area, etc. The best way to answer this question is to do some experiments and vary some of the parameters. Eventually you will get some empirical idea of what's going on.
(published on 06/20/10)
Follow-Up #5: time and rate happeneth to all
Rate and time are being confused. RATE is DISTANCE/TIME. 100C to 0C / x = 100/x && 5C to 0C / x = 5/x. if x is 5 minutes then 100/5=20 > 5/5=1. 20>1 = FASTER RATE.Though related they are not the same thing. Hot water freezes at a faster RATE than Cold Water. The equal sized/surface/area/pressure/temp/volume of the container of water closest to 0C will freeze in a shorter period of time than the hotter container of water. Hot water is often used because it lacks the dissolved air which makes the ice look cloudy. Ice sculptures can be crystal clear like glass if the hot water is super cooled to freeze the water before much air gets caught in the water. Common Chemistry experiments are done by taking temperature readings of a solution while stirring it to maximize the solutions contact with the surface of the beaker which is submerged into a ice/salt bath. The ice/salt bath on the exterior provides an subzero environment to freeze the solution on the inside. A Computer is usually doing the sampling automatically and graphing the results in a 2d graph showing the temp reading on the y axis and the time frame on the x axis. typically the graph is logarithmic. you see the temperature of the solution plummet quickly, but the amount of energy required to break the phase barrier is exponential. So you see water go from 100C to 1C in a short period of time, then it takes a much longer period of time compared to the 100C to 1C to go from 1C to 0C. Thus, the misunderstanding is that hot water freezes 'faster' than cold water. The same can be said about boiling water. the amount of energy introduced to the system to achieve boiling point is exponentially larger than the quantity to heat it up to 99C from 1C.
observe the graphs where f(x) = y,
f(x) = log(x)
f(x) = x^2 or f(x) = x*x [pronounced x squared)
then turn each one 90 degrees to see they're the same thing on a different axis.
Maybe a more real world scenario is in order for clarification. Bob is standing at the beginning of a 100 meter dash. Walter is standing 1 meter away from the end of the 100 meter dash finish line. The whistle blows! Bob is off to a speedy start traveling at 10 meters per second. It takes Bob 12 seconds, accounting for the the time it takes to accelerate from 0 to 10 m/s, to finish the race. Walter, being 1 meter from the end took his sweet time traveling at at only 1 meter per second and crossed the finish line 11 seconds before Bob. Bob traveled at a FASTER RATE, but Walter still finished first. Who was faster? Bob was faster! Who finished in the least amount of time? Walter, because he finished in 1 second.
To properly calculate the time it would take to freeze a substance from point x to point y...
As many posts before said there is not enough information given. Glasses are typically cylindrical in shape. So lets start with the Surface Area of a Cylinder Equation: (2*pi*radius^2)+(2*pi*radius*height). The SUBSTANCE of the container is very important for heat transfer rates as well as the mass of the container.
- WALTER (over 9000 years old)
Anchorage, Alaska, USA
Walter- I should defer to your advanced age, but I see no evidence that anyone was confusing time and rate. These messy practical heat flow problems cannot reliably be reduced to your simple equations.
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 12/12/10)
Follow-Up #6: how long for water to freeze?
I don't know why you couldn't give a rough answer to the initial question. Would it take minutes or hours for a room temperature cup of tap water to freeze in a typical residential freezer? That was all the little girl was asking. You didn't answer her question, you told her to find out herself. Mythbusters answered the question about beer. Try to help, okay?
My guess would be about an hour.
UIUC Engineering Grad & Mother
- E Wickliff (age 40s)
I've made a guess as to which question your comment was intended to follow-up.
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 04/08/11)
Follow-Up #7: philosophy of freezing water
Like E. Wickliff pointed out, its really funny that no one really answered the girl's question.
But moving forward, here's a link to why hot water DOES freeze faster than cold...
Basically put, the hot water won't create an insulating layer of frost on the top and it contains less gas bubbles due to evaporation which in turn causes the hot water to cool more quickly as it evaporates more quickly than cold water. Faster cooling means it will eventually become colder than the initial temperature of the cold water"
And age is mostly irrelevant to knowledge.
- Peter Ryan (age 31)
Vancouver, BC, Canada
I looked at your link about hot water freezing faster. It basically speculated about the same reasons as we had discussed, plus adding another speculation about how the lack of bubbles in the hot water would promote supercooling. That's a little odd, since by definition that effect would make the freezing of the hot water slower.
They then add a speculation that since the cold water would start freezing sooner, it would develop an icy crust that would slow further freezing. However, they really have no argument as to why a similar crust wouldn't form on the hot water when it starts to freeze.
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 09/11/11)
Follow-Up #8: testing freezing times
Yes, we can be able to test it. But what is the correct temp for the freezer? And why would it be better?
- Tina (age 12)
Wesley Chapel, FL, USA
There's no particular "best temperature" for the freezer for this experiment.
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 10/07/11)
Follow-Up #9: time to freeze water
1 cup of cool water (in a glass tumbler) took about 3 hours to freeze in my freezer. ;)
- nnugles (age eh?)
(published on 08/01/12)
Follow-Up #10: How long to freeze water?
I did this for my science fair project and it took 80 minutes for 4 ounces of room temperature water in a thin disposable cup to freeze in my freezer.
- Xander (age 6)
Cave Creek, AZ, US
Xander- Thanks, some of our readers will like this information.
(published on 02/17/13)
Follow-up on this answer.