hot and cold water

Why don't you perform the experiment and learn through hands on work?

The cold water will freeze first - think about it ......... there is less heat energy to take away.

The popular myth says that "hot water freezes faster" - which is not true, this statement is based on the fact that the hotter water's rate of cooling is faster than the cooler water's rate. This is because of the larger delta T. However, once the "hot" water becomes the same initial temperature of the cooler water, it's rate will decrease and be the same as the "cool" water was. In other words the time it takes the hot water to cool from 90 deg. C. to 70 deg. C. is shorter than the time it would take the cold water to cool from 30 deg. C. to 10 deg. C. (But both times are the same from 30 - 10 deg.)

I hope that makes sense, I may have butchered the explanation a little

This was done a while ago already.

Under most conditions the cold water will freeze first because less energy to be lost.

If there are fuzzy ice in the freeze compartment it will act as an insulator, the hot water will melt this away and facilitate better transfer resulting in faster cooling.

Faster evaporation in hot water may also help.but may not be enough.

The hot water will freeze first.

Do you have any information to substantiate such a terse statement?

Clearly some definitions are appropriate here: HOW HOT?

How much of the water must crystallize before it is considered frozen? How do you determine the moment when the freezing has taken place?

What purity of water are we talking about? Any dissolved substances (solid, liquid, or gas) will affect the freezing point. Most solutes (think salt) will lower the freezing temperature, so any heating process that removes solutes will make the purified water freeze at a higher temperature.

Boiling water expels air and other gasses. Presumably water with a lower gas content will have a higher thermal conductivity, so it is conceivable that boiled water that has subsequently been cooled to room temperature will freeze slightly faster than the same amount of unboiled water placed under the same conditions.

I did the experiment several times, several decades ago, but no longer remember the details, other than: The hot water freezing first is generally an urban myth.

Refer to previous soapbox rant about scientific illiteracy

I have done this experiment numerous times. If you reasonably control evaporation, and if you define freezing at being a solid at 0°C, and if you lightly stir the water such that there is no localized freezing to confuse you, cold water always freezes first in a freezer. Under certain carefully controlled circumstances, you can get hot water to freeze faster (for example, go outside on a day when it is -40°C and throw a pan of hot water and a pan of cold water into the air. The hot will freeze faster.)

There is an anomaly (some of you ChemEs jump in please - I don't know the proper word) at about 2 or 3°C in which there is a mixed-phase and that could confuse results.

Ive always heard many urban legions about this subject. The only ones that concern me are about the pipes freezing in your house. I've had this happen, and it was the hot water pipe that froze, (when it get's really cold, turn the heat up, not down). An interesting article, the Mpemba effect:

http://math.ucr.edu/home/baez/physics/General/hot_water.html

What we may be looking at in the Mpemba effect is based on how water freezes.

Temperature is really a measure of the molecular kinetic energy of a system. Each molecule moves around at a speed based upon its temperature. Molecules may move by translation, rotation around each bond (unless constrained), and vibration (unless constrained). There can be several rotational and vibrational modes in a molecule.

Ok, on to water. Water is known as good old H2O, but that is really something of a myth. A single oxygen connnected to two hydrogen and to nothing else would be a molecule of water vapor. In order to make water liquid, these molecules join together. At high temperatures, they form small clumps of wildly dancing, well, lets just call the coligomers, because they aren't really chemically bound, but are indeed together. (and I can't think of the right term anyway).

At any rate, these coligomers get larger and larger as the temperature decreases. Why? Well, the molecular dancing is getting less and less vigorous, and the coligomers are able to stay closer to one another, eventually sticking.

Now, somewhere above 70 F, we start to see some results. The viscosity of water starts to increase more rapidly as the temperature decreases. That is because larger and larger coligomers of water are in play.

When we hit about 4C or 39F, whichever comes first, the energy is low enough that the coligomers are as close as possible. They still have enough kinetic energy to bounce close to one another but not attach. They can also use this kinetic energy to make energetically unfavorable approaches, such as on O to and O, or an H to an H.

As the temperature drops below 4C, there is less energy available, and molecules must start to obey the electrical charges more. The coligomers realign themselves even more, which takes a little time, and a bit of energy to accomodate O interacting with H, and vice verse.

If there is gas in the water, then the water will form a cage around the gas, called a hydrate. If there is sufficient gas, then this hydrate will exist at about 4C, and can store quite a great deal of gas at a very high pressure. It will look like a round odd ice, and will burn.

I digressed again.

Now for the Mpemba effect. My guess is that if water is cooled quickly, then the thermal energy is lost faster than the molecules can form the intermediate coligomers. Thus, more energy is still in slower moving small clumps than in large clumps. The smaller clumps are able to align themselves more quickly and easily into the proper conformation for freezing than the longer, larger clumps.

In order to get the hot water to freeze faster, you would need the right conditions of cold sink temperature and capacity, hot water temperature, and definition of when something is considered frozen.

We could run a few experiments to check this by looking at the viscosity of water as it is rapidly cooled (not an easy experiment, but possible), and see if it differs from that of water that is slowly cooled. I would expect that the rapidly cooled water would have a lower viscisosity if this explanation is right.

Last year this issur was discussed by the cr4. I guess next year may somebody still will put out here to discuss. for the issue has been last tens years.

That Mpemba has already graduated and has on any new message about it. so dont take it seriously. it might a fun.

Chinese middle school students did this experiment for nearly a month and result in a fact, its impossible. unless the water mixes some substance in it.

haha many peple was victims in this hoax.

Hot.

It's impossible to KEEP hot water in a freezer. If it is in a thermos-type container, it will take longer to cool, but it won't be kept hot. Rephrase the question!

This is actually a legend that has survived the test of time because, it seems, no one remembers the original situation; that was the freezing of hot vs cold water in the old, original freezers inside a refrigerator in the early days and it comes down to a question of "Heat Transfer Rate" associated with setting an ice-cube tray of hot water vs cold water on the frost/ice covered coils of such a refrigerator unit. If done, the hot water melts the frosted ice and the cold water does not; they used aluminum trays I suspect. Heat transfer through the melted water on the coils is greater than through ice as I recall. If one puts cold water vs hot water in a plastic trays or in aluminum trays you might also see differences in heat transfer rate too.

There are too many 'unknowns' needed.

I guess 1 m³ of hot water 1 mm deep on a 1,000 m² tray would freeze faster that 1 m cube of cold water

.......or if both containers were exactly the same size and shape, then hot water would freeze first if put in the freezer long before the cold water.

...silly I know.... but assumptions have to be made....

A variation of the question.

Two glasses of water are put in the freezer, each identical in shape and size except the 'hot' one is 50^c and the 'cold' one is 25^c.

Apart from which freezes first, this time however, at the start, a more dense 'heavy' blue marble is dropped into the 25^c cold water, and a much less dense 'lighter' red marble into the 50^c hot water.

Which marble sinks to the bottom first.

Now repeat the test but waiting until the temperature of the hot glass has dropped by 30^C before dropping the marbles in.

Now which marble sinks to the bottom first ?

Don't know and don't care...BUT if anyone try's to put a hot ice cube in my whiskey I'll thump them

I believe the original "urban legend" was simply some practical observations using freezer trays, which would have been relatively shallow and probably made of aluminium. In this case, the melting of any ice beneath the trays would have resulted at the least in a larger contact area under the 'hot' tray and probably also direct contact with the coils. Under those circumstances, the water could clearly cool faster and under some conditions freeze earlier.

If these were not the conditions, then only direct confirmatory experiments will determine whether or not any other explanation is required, but not at the expense of ruining anyone's whisky! I can't help feeling that detailed theoretical explanations of the apparent impossibilty of the alleged observations are a bit superfluous.