Doug's Sounding Board


The Mpemba Effect

I just found the experimental results from our testing of the Mpemba effect at my last job. I learned about the Mpemba Effect from Trivial Pursuit when the answer to “According to the MPemba Effect, what does hot water do faster than cold water?” was “Freeze”. Sure enough, after looking it up, hot water tends to freeze before cold water. I told my co-workers and we decided we needed to reproduce the effect.

First we put 4.6oz (not fl.oz.) of water in four 0.4oz styrofoam cups.

Cup 1 was tap cold water.
Cup 2 was room temperature water.
Cup 3 was tap hot water.
Cup 4 was boiling water.

Unfortunately I didn’t record how long we kept them in the freezer, but when we removed them we weighed the total water mass, the ice mass, and the water mass.

4.6oz water results
Cup Total water weight(oz) Ice weight(oz) Water weight(oz) % Frozen
Cold 4.6 1.8 2.8 39
Room temp 4.6 1.7 2.9 37
Hot 4.5 1.2 3.3 27
Boiling 4.4 1.4 3.0 32

10oz water results
Cup Total water weight(oz) Ice weight(oz) Water weight(oz) % Frozen
Cold 9.9 6.6 3.3 67
Room temp 9.9 5.2 4.7 53
Hot 9.6 5.5 4.1 57
Boiling 9.4 6.3 3.1 67

I recall the first cups weren’t left in long enough so we went for the 10oz cups. In the 10 oz cups it’s clear that through evaporation the boiling cup has gained surpassed the cold water cup, not in total ice mass, but in remaining water to freeze. It’s clear it will freeze first.

The reasons for the rapid freeze:

  1. Evaporation. It’s clear that the water in the hotter cups evaporates more quickly which remove energy from the cup but also reduces the mass requiring freezing.
  2. Convection. The hot water has more churning as parts cool at the top they get heated again by still warm water which gives off more energy. Also it keeps the warm water circulating more than the cold water so they warming effect goes on longer and can give off energy faster.

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