What freezes faster? Cold or warm water?

This question has been tormenting me since I was a student in Banff Elementary School and a teacher asked us what would happen if you put two identical containers outside in the snow. The first container held hot water, the second cold. Which would freeze faster?

We all offered our opinions and explanations why the cold water would obviously freeze faster. The teacher, though, wouldn’t provide any sort of answer or further information, but instead said nebulous things like, “The obvious answer isn’t always the correct one.” By the tone of voice, we were all made to feel a bit stupid, but the teacher didn’t enlighten us, and then a weekend came and went and then we were on to other problems.

Recently, this came up again when The Belmont Rooster commented on a blog post that hot water freezes faster than cold (this in the context of me putting out hot water for the critters during a recent chilly snap) and this then led to a series of very informative comments and a couple of great links from my favourite Iowans of Jar Blog and Catbird Quilt Studios fame.

Thinking that maybe I was not the only one to wonder about this matter, here is my summary of what I think I now understand about the whole hot versus cold water issue.

First, though there have been a number of experiments done demonstrating that in fact, under certain conditions warm water will freeze faster than cold water, there is no consensus as to the exact mechanism by which this happens. Which might explain why my teacher, way back when, was unable to give a clear explanation of why we were all idiotically guessing the wrong, but seemingly obvious answer.

The effect (and this is intriguing to someone who writes books for kids) is known as the Mpemba effect after a high school student in Tanzania by that name noticed how fast hot fluids froze back in 1963. Not that this was the first time the phenomenon had been observed. Aristotle noted that people who wanted to fast-freeze water first stood it in the sun for a bit to warm it up… Others (Bacon, Marliani, Descartes) also observed the same thing, but somehow, this was relegated to urban legend status until Mpemba was in a hurry to make ice cream at his high school. A scarcity of fridge space meant he rushed to put his hot milk and sugar mix into the freezer before the milk had cooled. When he noticed that the ice cream seemed to freeze faster, he asked his physics teacher what was going on and was basically told he was a fool and to stop making up his own brand of physics.

[I am not making this up, btw – the long version of events is described here. Thanks to Iowa Jim for the link…]

Several theories have been put forth and many experiments done, but it seems water is a magical and complicated substance and reasons for why this happens are slow to be revealed. If the only thing that mattered was how warm it was, then the answer to the speed of freezing question would be pretty straightforward. Cooler water would freeze faster because you wouldn’t have to cool it down first. But, it turns out you need to take into account things like the way properties of water change as it changes temperature.

We all know that water changes shape, volume, and state when it gets cold, for example. Over-filing a jar with liquid, screwing on a lid, and then freezing it leads to trouble when the ice expands and breaks the glass. Trying to drink solid water in the form of ice is obviously not so easy. At the other end of the temperature scale, the stuff disappears and floats off into the atmosphere when you boil it, turning into some sort of misty haze that then magically condenses and turns back into drinkable liquid when it hits something cool.

These changes in property are dramatic and obvious – so why we don’t immediately think of warm water perhaps behaving differently to cool water is actually kind of interesting. The amount of gas dissolved in warm water is different to that in cold water. Convection currents occur as water cools (same idea as warm air rising) and some think that this causes the water to cool more efficiently. Then there is the matter of supercooling (when water gets colder than zero degrees C but doesn’t turn into ice) and whether this might occur at different rates for cold versus warm water.

There are also questions of the size, shape, and type of container, evaporation, and where the container is sitting (pour a lot of hot water into a bucket of chicken water sitting on the ground and it would melt the layer of frost or snow underneath, which could impact how fast the contents freeze).

The end result of all this is that nobody knows for sure why hot water freezes faster than cold, but often it does. Perhaps the most interesting part of this story to me is that some kid in Tanzania refused to let go of what he knew to be true even though all the adults who were supposed to know better (his physics teacher, for one) told him he was a fool. Perhaps I will add another theme word to my list for the coming year: PERSISTENCE.