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Attempted Mpemba Effect [Uncertain Principles]
One of my favorite modern tales of scientific discovery is the Mpemba Effect, named after Erasto Mpemba, a schoolboy in Tanzania who noticed while making ice cream that hot mix put in the freezer solidified faster than cold. This counter-intuitive result has been replicated a bunch of times, and physicists and chemists continue to debate the reason for it.
It was bitterly cold this weekend, dipping down into the negative Fahrenheit, which wasn’t great for, you know, leaving the house, but did provide an opportunity to test the Mpemba Effect. Because when the icy winds of winter blow, that’s a great time to do SCIENCE!…
Materials for an attempt to observe the Mpemba Effect.
One of the tricky things about the Mpemba Effect is quantifying what it means for hot liquid to freeze faster than cold, but I thought of a way to (attempt to) handle that, which was another reason for running the test. So I grabbed a couple of water glasses out of the cabinet, and filled one with 200g of filtered tap water at 18C, and the other with filtered tap water heated in the microwave, at 81C. The water glasses had a mass of about 350g; there’s a +/-5g error on all those masses, because the cheap kitchen scale we have at home only reads to the nearest 5g.
Then, I took the two glasses and put them out on the patio table:
Starting condition for a science experiment.
(Actually, I put them like this for the photo, then separated them a little more, just to make sure that there wasn’t any thermal contact between the samples…)
I left them outside for about two hours, which was long enough for them to partially freeze, but not completely:
Frozen water from the Mpemba Effect experiment.
I weighed the two glasses again, and found that the hot water had lost some mass– the total mass was about 540g, so 10g down from the start. This was presumably due to evaporation– you can just see a cloud of steam rising from it in the outdoor photo. The cold water didn’t lose any mass, continuing to register at 550g, though again: cheap kitchen scale.
To get at the amount of freezing, I took a steak knife and chopped a hole in the top layer of ice in each glass, then poured out the liquid water from underneath. Then I weighed the glasses again: the cold water glass plus solid ice clocked in at 455g, and the hot water glass with ice at 420g.
To put that in terms of a freezing fraction, the cold water had 105g of ice out of 200 g of water, or 53% ice, and the hot water had 70g of ice in 190g of water, so 37% ice.
So, as an attempt to observe the Mpemba Effect, this was pretty much a complete failure. Alas. This isn’t entirely surprising, though, as the effect is notoriously finnicky. Which is why it’s been argued about in the literature for 40-odd years. I thought about repeating the trial with slightly different conditions– one of the things you see in a lot of the discussions of this is a comparison between water that was heated and allowed to cool before being put in the freezer and water that was put in the freezer while hot. The idea being that heating may drive off some impurities, changing the water a bit. I didn’t try that on Sunday because I was in a hurry to set it up before dinner and the kids’ bedtime, and we got snow on Monday that would’ve messed up the conditions. But it’s something I’ll keep in mind for the next time it’s unpleasantly cold…
Source: http://scienceblogs.com/principles/2016/02/16/attempted-mpemba-effect/
