Mpemba effect: warmer water can freeze faster than colder water

A slightly different version of the effect well-known to all Canadians:

https://www.youtube.com/watch?v=pvgefs7J1rY

https://www.youtube.com/watch?v=ZGjwe-BCfms

To get the water to freeze into snow mid-air, you have to start with boiling water. Cold water won't freeze in time. I've even heard of research involving using boiling water in snow-making machines.

I've always disliked the explanations I've seen for the Mpemba effect. They seem to be based on poorly defined terms, odd assumptions, and uncontrolled variables. The (poorly) linked 2010 paper by Brownridge is the first description that I'm inclined to trust: https://www.binghamton.edu/physics/docs/Preprint%20and%20Sup...

Impressively, he describes a controlled duplicable setup in which he's able to reliably freeze (using an exact definition) identical quantities of water in identical containers under identical conditions such that the hot water freezes before the cold water. Incredibly, he was sometimes even able to observe this when starting with 100C water vs 0C water!

As you'd guess, there is a little bit of a trick to it, but it's one that not nearly as obnoxious as some of the earlier explanations: no evaporation resulting in a smaller volume or remaining water, no spontaneous ice spikes with greater surface area, and no differential melting of the thick layer of frost on the freezer shelf required.

Rather than giving a spoiler, I'll let the paper describe the exact technique he used. OK, a hint from the paper to encourage you to actually read it: cold water will indeed always reach 0C before identically handled hot water, but contrary to popular belief reaching 0C is merely a requirement for freezing rather than a guarantee.

I googled around and found this paper to be far more compelling than any of Wikipedia's vague explanations: http://arxiv.org/abs/1310.6514

I'm putting hot and room temperature water in my freezer...no way this will work.

The Royal Society for Chemistry ran a competition to come up with an explanation: http://www.rsc.org/learn-chemistry/resource/res00001018/the-...

Seriously you should use warm water to fill your if cube trays. Even if it doesn't freeze faster you get better ice cubes from having less dissolved gas.

They mention the difficulty of determining when the water is frozen, since water at zero degrees C can be either liquid or solid. It seems easy to solve by checking for an increase in volume or pressure, since we know that ice expands into more volume than liquid water.

nothing misterious really.. it only works where hot water turns more or less to steam ie very high surface area and thus can cool faster thus overtake cooler water droplets that have significantly less surface area.

After reading this article, I did some research on this topic and compiled some of the best resources I found here - http://knowledgemaps.org/learning-journey/1118/understanding...

Reminds me of an argument my brother and I used to have.

You pour two cups of tea. You add an exact amount of cold milk to one but not the other then wait five minutes. Then you pour the same amount of milk in the second cup. You stir both equally.

Which is colder?

This was presented as problem #15 (Cooling water) at IYPT 2000

As far as I remember, nobody confirmed effect experimentally at that time.

I think I know why.

Good temperatures for showing the effect are 5 and 35 C.

5 C is near 4 C, the point at which water has maximum density.

Best explanation I saw was from Beakman's World, a 90's Tv show.

Sorry can't find a YouTube video.