How did a love of frozen treats lead to one of the greatest mysteries in physics? Find out in our tale of the Mpemba Effect, a still-unexplained phenomenon where boiling liquids freeze faster than cool ones.
In the 1960s in Tanzania, a high school kid wanted ice cream enough to do something unscrupulous. Who among us can blame him? Most of io9's readers are adults, and it's a safe bet that a number of them are willing to do something unscrupulous right now to get ice cream, just because they're reading about it. Young Erasto Mpemba's ethical lapse was far more minor. His entire class was making ice cream from scratch. They had to boil the milk, let it cool, and then store it in the freezer so it could freeze. Mpemba watched his classmates filling the freezer with their cooled milk, while his was just-boiled and still steaming. He didn't want to risk missing out, so he put his milk in the freezer even though it was hot.
To his surprise, his teacher's disbelief, and the no-doubt smoldering resentment of the kids had who let their milk cool and missed out on ice cream as a result, Mpemba's milk froze fastest.
Mpemba remembered this phenomenon well into adulthood, studied past instances in which people observed that warm liquids froze faster than cold ones, and worked with other scientists to verify this. They found that under certain circumstances, the Mpemba Effect does work. Hot liquid, especially water, freezes faster than cold liquid when it is placed in a freezer.
Why should this be possible?
Because the problem is so complex, there hasn't been a definite explanation. There are many posited theories, however. Cool water, when placed in a freezer, insulates itself. A layer of ice forms at the top, preventing more heat from escaping. Hot water does not form the layer of ice as quickly, and so it cools faster. Hot water in a frosty freezer melts the ice underneath it. Soon, however, the ice reforms around the dish that the water is put in, and wicks away the extra heat. Since cool water doesn't melt the ice underneath it, it sits on craggy frost, instead of a fitted sleeve of ice. The surface area between it and the ice is reduced, and heat won't escape as quickly.
Only two theories, though, have passionately valiant defenders. These two theories are currently battling it out Rocky I, II, III, or V style.
The first theory features Io9's old friend, nucleation. Disgusting, yet unseen, pieces of dirt in water form sites on which ice crystals can accumulate. Different kinds of filth form crystals at different temperatures. The high-bidder piece of refuse, the one that forms crystals at the highest temperature, will determine what temperature water freezes at. James Brownridge, an independent researcher, believes that the nucleation temperature of water determines whether or not the Mpemba Effect will occur. If the impurities in one container of water are such that, it freezes at least five degrees higher than another container of water, then when the first container of water will freeze faster than the second if it is heated.
In other words, if you have two glasses of water, and you put them both in the freezer at room temperature, and the Wolverine Origins glass of water freezes at five degrees higher than the Thundercats glass of water; thaw them out, warm up the Wolverine glass to eighty degrees, and bet someone twenty bucks that it will freeze first.
But there's another contender for the title.
Apollo Creed to Brownridge's Rocky, or Mickey to Brownridges' Paulie, or perhaps the Robot Butler to Brownridge's Adrian – it's tough to make Rocky metaphors – is Jonathan Katz.
His theory rests on another property of water. Water doesn't just have invisible specks of grime floating in it. It also has little specs of grunge dissolved in it. Whether they are salt, or carbon dioxide, or magnesium, they merge with the water to form a briny muck that freezes lower and boils higher than water should. Heating water will shake free some of these substances, claims Katz, and will cause the Mpemba Effect to occur. Whether it occurs or not will depend on the composition of the water, the container, and how much the substance was heated.
The debate rages on. Perhaps someone reading this will finally put the question to rest.
More likely, though, they will go out for ice cream.
Top image by thepma.