Warm water freezes faster than cold water?

[bambbbam]

Has anyone else noticed this?

If I have an ice cube tray to refill, putting cold water in it and letting it freeze it takes longer than when I put in luke-warm water.

I've never been able to work out why? The warm water must pass through the cold stage 'on the way' to frozen, so why does it arrive sooner?

[mkhannah]

I remember that being "discovered" some time ago but I can't remember the reason myself. Something about going through the different states of gas to liquid to solid being quicker than liquid to solid but I'm probably wrong
It was done with boiling water into an ice cube tray from memory rather than luke warm but I'm sure the reasoning is the same...

[moeee]

I'm not clever.
And if this is true, then it explains why I have problems.

Its like you said, it must pass through the cold stage.
Only way I'm thinking is that the thermostat stays on longer and the machine works harder.

[Fernbay]

Yes, I heard of this years ago. Apparently it is true, but a larger about than a ice tray is required. More like a bucket....

Here's a bit more from my friend, Mr Google.

Since the time of Aristotle, some scientists have claimed that hot water freezes faster than cold. Philip Ball looks at current attempts to shed light on this puzzling phenomenon

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.
On thin ice
On thin ice

Indeed, Aristotle, Francis Bacon and René Descartes all claimed that hot water does freeze more quickly. Erasto Mpemba, a secondary-school student in Tanzania, may have been unaware of their claims, but it was something he also observed in 1963. To make ice cream for a school project, he was told to boil milk and then let it cool before putting it in the refrigerator. But, fearful of losing his place, Mpemba put his mixture in the fridge while it was still hot. He found that it froze before the other, cooled mixtures.
Just desserts
Just desserts

Others have since claimed to have observed this "Mpemba effect" in their own experiments. Nevertheless, many scientists find it hard to accept such a seemingly counterintuitive phenomenon. The problem is that the effect is frustratingly hard to reproduce - sometimes it appears, and sometimes not. In fact, no-one has agreed exactly how the experiments should be conducted in the first place. And even if the Mpemba effect is real - if hot water can sometimes freeze more quickly than cold - it is not clear whether the explanation would be trivial or illuminating.
Against the grain

Condensed-matter physicist Monwhea Jeng of Southern Illinois University in the US, who has researched the history of the Mpemba effect, believes that scientists are much more likely to react with disbelief than laypeople when they first hear about the phenomenon. That is because scientists know why it "cannot" be right, he says. Indeed, when Mpemba learned about Newton's law of cooling a few years after making his discovery and asked his teacher how this could be reconciled with his observations, his teacher replied, "All I can say is that is Mpemba's physics and not the universal physics."

Fortunately, Mpemba was not deterred by this cruel put-down, and he went on to carry out further experiments of his own. When local physics professor Denis Osborne of University College in Dar es Salaam visited the school, Mpemba seized the chance to ask for an explanation for his findings. Osborne had none, but he was less sceptical than Mpemba's teacher and wisely concluded that "it is dangerous to pass judgement on what can and cannot be". Osborne then asked a technician at his university to repeat the experiments, and the results seemed to show that Mpemba was right. In 1969 Osborne wrote about the work with Mpemba (then at the College of African Wildlife Management in Moshi) and published it in the journal Physics Education. Coincidentally, a physicist named George Kell at the National Research Council of Canada in Ottawa reported the same phenomenon that year in the American Journal of Physics.

These reports revealed that the Mpemba effect was already familiar in folk wisdom. Kell, hailing from a country with ample experience of freezing water, stated that "some say that a car should not be washed with hot water because the water will freeze on it more quickly than cold water will, or that a skating rink should be flooded with hot water because it will freeze more quickly". Mpemba, meanwhile, pointed out that Tanzanian ice-cream makers routinely froze their mixtures while they were hot, because that was a faster method. And when Mpemba's work was described in an article in New Scientist in 1969, it prompted a rush of anecdotes about food-freezing practices and hot-water pipes freezing while cold ones did not.

Those making such claims are in good company. In his Meteorologica from about 350 BC, Aristotle wrote that "if water has been previously heated, this contributes to the rapidity with which it freezes, for it cools more quickly". The idea was questioned by the great medieval champion of experimentation Roger Bacon, but his namesake Francis asserted in the 17th century that "water a little warmed is more easily frozen than that which is quite cold". Francis Bacon was deeply interested in freezing and refrigeration - he is said to have caught a fatal chill while conducting an experiment on preserving a chicken by stuffing it with snow. Around the same time, Descartes made careful observations of the freezing of water that enabled him to identify the liquid's unusual density maximum at 4 °C. These studies convinced him that "water which has been kept hot for a long time freezes faster than any other sort".

But were all these reports just the result of bad experimental technique? Surely it should be a simple matter to settle the issue once and for all by carrying out experiments? That turns out to be surprisingly difficult, not least because the statement "hot water freezes faster than cold" is ill-defined. In a recent paper, Jeng suggests a more precise wording (arXiv.org/abs/physics/0512262v1): "There exists a set of initial parameters, and a pair of temperatures, such that given two bodies of water identical in these parameters, and differing only in their temperatures, the hot one will freeze sooner."

There are many such parameters that could affect the rate of freezing, the most obvious including the volume and type of water used, the size and shape of the containers, and the temperature of the fridge. This presents a significant challenge for the experimentalist, who in principle would have to set up a vast multidimensional array of experiments involving containers with different sizes and shapes, while independently varying the masses and gas content of the water and the refrigeration method used, in order to test for the effect.

There is also the key problem of how to define the time of freezing. Does this refer to the moment when the first ice crystals appear or to the time when the entire body of liquid is frozen? "Both of these times can be very hard to observe, perhaps especially in a refrigerator," says ice specialist Charles Knight of the National Center for Atmospheric Research in Boulder, Colorado, US.
Jearl Walker
Jearl Walker
Looking for clarity

These complexities perhaps explain why the Mpemba effect remains a puzzle to this day. A number of scientists have investigated Mpemba's claim, but their results remain inconclusive. In 1977, for example, Jearl Walker reported in Scientific American that he had observed the time it took a beaker of water to cool to 0 °C from different initial temperatures under various conditions. These tests provided some clarification of the effect (see figure). But although Walker reported that he could reproduce most of his results, he still obtained large deviations in some of them. "I have not been able to resolve the controversy," he said.

However, despite the continuing uncertainties surrounding the effect, Pablo Debenedetti, a physicist at Princeton University and a specialist in phase transitions of water, is happy to believe Mpemba's account. "I do not see any reason to doubt observations showing that under some circumstances hot water can freeze faster than cold water," he says.

But what causes the effect? Both Debenedetti and Knight point out that there could be at least one obvious explanation for it. If the containers are left open, the hot water will evaporate more quickly and its volume will decline compared with that of the cold water. With a smaller volume, the cooling of the hot water could then overtake that of the cold. That should be easy to test, according to Debenedetti, because the evaporation rate is proportional to the area of the liquid surface. "This can be systematically controlled in experiments conducted in pairs of containers with different geometry," he says.

Another possibility is that the freezing process could be affected by dissolved gas. Hot water generally holds less dissolved gas than cold, which means that two samples that differ only in their initial temperature would not contain "identical" substances. Debenedetti points out that tiny bubbles of gas can provide nucleation sites where ice crystals start to form. In principle, this might be expected to make ice formation easier in cold water, contrary to the Mpemba effect. But Debenedetti says that the solubility of nonpolar gases such as nitrogen or methane do not necessarily vary smoothly with temperature, so there could be temperature ranges within which the hotter water contains more dissolved gas.

Experiments to pinpoint these influences would require the water to be thoroughly degassed. The effects of other dissolved impurities could be even harder to probe: for example, one could divide the water up into tiny droplets in an oil-water emulsion so that most of them are too small to contain any impurity particles.

Then there is the role of chance, since the nucleation of ice in freezing water depends on enough water molecules coming together to form the core of an ice crystal that can then grow indefinitely. The further the water is below freezing point, the more likely this is to happen. But because it can take some time for ice crystals to nucleate, water can often be "supercooled" such that it remains liquid well below freezing. Random impurities in the liquid, such as specks of dust, can, however, increase the rate of nucleation and suppress supercooling. "Keeping everything constant from experiment to experiment may not be possible without resorting to purposeful nucleation, and that might destroy the effect one is looking for," says Knight.

Knight adds that he was reminded of the part played by chance while conducting some recent experiments on ice formation. "I had to sit in a cold room at -15 °C and watch water freezing in ice-cube trays on a table top. This exercise emphasized that everything is variable. Some compartments started freezing in about 15 minutes, but many did not for an hour or more."
Further testing required

In 1995 German physicist David Auerbach at the Max Planck Institute for Fluid Dynamics in Göttingen looked at the role of supercooling in the Mpemba effect. But what he found only made things more complicated. He observed that hot water froze at a higher temperature than cold and therefore in a sense froze "first". However, the cold water took less time to reach its supercooled state and so seemed to freeze "faster". To add to the confusion, earlier researchers had reported the opposite: that initially hot water could be supercooled to lower temperatures than cold water. In 1948 Noah Dorsey of the US National Bureau of Standards argued that this is because heating expels impurity particles that acted as nucleation sites for ice. It has been claimed that this effect leads to hot-water pipes bursting more readily than cold, since deeper supercooling leads to ice fingers that advance right across the pipe and block the flow, while freezing nearer to 0 °C just produces a sheath of ice on the pipe surfaces with an open channel in the centre.

Such contradictions continue to make the Mpemba effect as puzzling as ever. Knight is happy to leave it that way, because he thinks that attempts to clarify it would demand too much effort for little return. But Jeng is more positive. He says that despite the complexity of the effect, the experiments needed to study it can be carried out by undergraduates and high-school students - so long as they are planned carefully. As well as thinking about exactly how to heat the water and the kind of thermometer that should be used, experimenters should also consider precise details of the environment surrounding the container. "It can make a difference whether the water is in the middle of an empty freezer, or jammed between a frozen pizza and a frost-covered tub of ice cream," he says

A bucket of hot water will not freeze faster than a bucket of cold water. However, a bucket of water that has been heated or boiled, then allowed to cool to the same temperature as the bucket of cold water, may freeze faster. Heating or boiling drives out some of the air bubbles in water; because air bubbles cut down thermal conductivity, they can inhibit freezing. For the same reason, previously heated water forms denser ice than unheated water, which is why hot-water pipes tend to burst before cold-water pipes.

Some choose not to believe it, but this phenomenon can be demonstrated in your own freezer under the proper conditions.

If you have two pails filled with equal amounts of water, one hot and one cold, and you set them out in the snow on a cold winter's morning, what happens?
bucket of hot water bucket of cold water

What Should Happen

Any reasonable person would think that the hot pail would take longer than the cold pail to freeze. After all, the hot water needs extra time to reach the same temperature as the cold pail. When the hot pail finally does reach the same temperature as the cold pail initially was, the cold pail should already be frozen.



What Actually Happens

There are several things that help the hot pail freeze faster than the cold pail. Here are what is thought to be the most significant factors:
Layer of ice forms on the top of the cold water.

The hot water is more likely to be supercooled. This means that the hot water's temperature is more likely to cool to temperatures below zero degrees Celsius. In the cold non-supercooled water, ice crystals form and float to the top, forming a sheet of ice over the top of the water, creating an insulating layer between the cooler air and the water. This ice sheet also stops evaporation. In the hot water that has become supercooled (thus, no longer hot) the water, when it does freeze, freezes throughout, creating more or less of a slush before freezing solid.

Why is hot water more likely to be supercooled? Because hot water is less likely to contain tiny gas bubbles. Gas bubbles form from dissolved gasses as the water cools. When the hot water was heated, these dissolved gasses may have been driven out. In cold water, ice crystals use the tiny bubbles as starting points for formation (in physics, we call them nucleation points). But in the hot water, there are no bubbles, so there aren't as many starting points for the ice crystals.

Dissolved gasses also lower the freezing point. Since heated gas is less likely to contain dissolved gasses, it's more likely to freeze first.


Water in the hot water pail evaporates at a much faster rate than the cold water. This does two things.

First, the process of evaporation is endothermic, which means it takes energy for something to evaporate. As a molecule of water evaporates, it leaves the surface of the water and flies into the atmosphere. Thus, in simplified terms, the molecule converted heat energy into kinetic energy (energy of motion). Since the hot water evaporates quicker than the cold water, it loses heat energy quicker than the cold energy.

Second, since some of the hot water evaporates away, there is less water left to have to freeze.

The hot water pail will melt the surrounding snow. Later, as it begins to freeze, the snow around the pail will freeze back so that it more closely "touches" the pail. The cold water pail is then only sitting in fluffy airy snow, while the hot water pail is in a form fitting ice-crust. The ice-crust will obviously conduct the cold better that the airy snow.

Other factors, such as convection currents (the movement made as hot water rises while cool water sinks) may or may not play a role in this odd phenomenon.

[porkop]

maybe the freezer works harder to remove the extra heat from the water?

[trash]

With regards to ice cube trays, the physics will be a little different, but I have shown that hot water freezes faster when I was last in Siberia.
In the middle of winter, the temperatures in some places can get down to a nice -40deg at night. If you take a bucket of cold water and throw it into the air, most of it falls as water and then freezes in a few seconds as the droplets break up o hit the ground.

But when you throw the same volume of hot water, the droplets break up very fast and the whole lot freezes in mid air and floats away as snow.

So when demonstrated in seconds, it's easy to see hot water freezing quicker than cold water. This doesn't take into account volumes or the absolute temperatures or even how the water is thrown or wind conditions. It was just a causal experiment at the time,and yes I have repeated it on different occasions.

What I can say for the conditions is that the air is very very dry. This is a big factor.
The air is of course also very cold.

We have to remember that cooling is not a property of the material, but rather the ability of other matter around it being able to carry that energy away. In the case of water freezing there are two gases that can do this job. Air and water vapour (steam).

Air conductively and then convectively carries the heat away. When there isn't much of a breeze, this happen slowly. When of course there is a wind, it happens much quicker.
This is the effect behind "wind chill factor".

Water that is hotter has a higher vapour pressure and so molecules of water can acquire enough kinetic energy to leave the liquid and become a gas. As they do, they take the energy with them. If that energy is high enough, more water molecules can carry more energy away faster, which of course means quicker cooling.
A larger surface area increases the effect exponentially which is why it is so noticeable
with the bucket experiment.

With the ice cubes in the freezer, the surface area is restricted. The water in the ice cube tray has more energy to loose if it is hotter, which suggests it should take more time too cool. What may be happening is that the hotter water forms a surface crust quicker, giving the appearance that the whole cube is frozen quicker.

There was a question put on whirlpool a couple of months ago which is the inverse of this question. It was "What is the quickest way to make water boil ?"

Conventional thinkers debated a gas stove vs an electric stove and inductive stoves and microwave ovens. However, the quickest way to make the water boil is; to put it in a vacuum, it boils instantly !

I cheated of course. I didn't increase the temperature of the water, I actually decreased when the water started boiling. I reduced the pressure to make the water boil.
As it does, the steam takes energy away from the water cooling it.
If it is kept in the vacuum long enough the water temperature will drop enough that the water instantly freezes, while it is still boiling.

Once it is below this point, the temperature of the water/ice now decreases much slower as the ice has to sublime instead of boiling to cool further.

[dogsbreath]

I pondered this question some time ago. My thoughts were along the lines of the warm water forming a thermal gradient between the bottom and the top of the ice cube as it freezes.
With the cold water being consistantly cold it will start to form ice on all six sides simultaneously. Once a layer of ice starts forming it starts to insulate the remaining water at the centre of the cube.

[mate]

water molecules are 'tighter' at lower temperature. Thus warm water has a greater surface area per molecule and is easier for the cooling to take place.

[trash]

Here's an idea for you kiddies to experiment with.
Some hands on science.

Take an ice cube tray and fill it progressively with cold water and top off each section with some hot water, so that one end is warmer than the other.
Pop it in the freezer. For the first experiment, leave it overnight to freeze.
Next day take it out of the freezer and quickly inspect the surface of the ice cubes with a microscope or a magnifying glass and take not of the size of the crystals.
Small crystals indicate that it froze quickly, while larger crystals are an indication of slower freezing.

If you have the patience, repeating the experiment but checking the tray every 30 minutes, touch the cubes with a matchstick to see if they are frozen.
disturbing them may of course effect the results. So testing all of them is an idea if you can't visually inspect them and not disturb them at all.

[watchdog]

I remember this being put to my science teacher in high school. He said that it doesn't "freeze" quicker it "cools" quicker because of the thermal difference.

[z1gg33]

When you have a frosty windscreen, the AC will defrost it in seconds, whereas HOT takes a while. Why is that?

[Gerbertron]

When you have a frosty windscreen, the AC will defrost it in seconds, whereas HOT takes a while. Why is that?

I think it's because the ac has had mositure removed from the air it's blowing.

[dogknees]

One "reason" that doesn't seem to have been mentioned is the turbulence in the water. Hotter water is more turbulent and therefore cools more evenly as it's getting mixed. The cold water probably isn't moving/mixing as much and you get stratification and slower cooling.

[rob916]

Interesting stuff guys.

Here is one for you to try.
Get a coles plastic shopping bag, fill it up with water and then hold it over a small fire, apparently you can get the water to boil in the bag. I tried it for a short time and it actually doesn't melt because of the water inside it.

[z1gg33]

Interesting stuff guys.

Here is one for you to try.
Get a coles plastic shopping bag, fill it up with water and then hold it over a small fire, apparently you can get the water to boil in the bag. I tried it for a short time and it actually doesn't melt because of the water inside it.

yeah I have seen this, with a water bottle, full of water, placed in a fire, takes ages to melt, whereas, throw a plastic bottle into the flame, incinerates it instantly.

[rob916]

I am talking about a plastic bag, it doesn't melt if the flame is only touching the bit that contains water. I thought it would have melted straight away.

[trash]

yeah I have seen this, with a water bottle, full of water, placed in a fire, takes ages to melt, whereas, throw a plastic bottle into the flame, incinerates it instantly.

You lot are such pussies. Water ? I use petrol.

[z1gg33]

You lot are such pussies. Water ? I use petrol.

lol.....

edit: i avoid petrol after a nasty round of "fla me ball"... (tennis ball dipped in..2xtennis racquet's@ night)

[bambbbam]

Interesting stuff guys.

Here is one for you to try.
Get a coles plastic shopping bag,

Didn't work with my Woolies one.. maybe it's specific to the particular store?

[iwacelect]

back years ago as a scout, we were taught to make and boil water in a "billy" made from paper, and it worked, the water keeps the paper from buring by keeping it wet and the water wont go over 100deg c if it's still a liquid,

therefore the water will boil in a paper billy, ive done it.