The Mpemba Effect

30.08.2018 |

Episode #7 of the course Brain-twisting paradoxes by John Robin


I hope you’re eager to dig back in today. So, picking up from where we left off, how hot is your cold water?

That’s the spirit behind today’s paradox. To explore it, let’s imagine something together.


Making Your Own Frozen Coffee

You probably have a hot cup of coffee as you’re reading this lesson. Great. Now (this is optional!), consider putting it in the freezer.

At the same time, imagine you put a glass of cold water in as well. Set a timer, and every ten minutes, check on them. Which one would you expect to freeze first?

You’ll find that the hot one freezes first.

This contradictory result is called the Mpemba effect, named after Erasto Mpemba, a Tanzanian high school student who discovered this in 1963. Scientists soon verified this wasn’t just an accident by conducting experiments.

This effect, though, has been speculated on for a long time, all the way back to Aristotle some 2,300 years ago. Other prominent philosophers like Francis Bacon (1521-1626) and Rene Descartes (1596-1650) also speculated on it. Even so, despite research and all our current innovation, scientists have not quite pinned down an exact reason why this occurs.


Your Coffee Might Not Have Frozen First, and Here’s Why

The Mpemba effect has been verified using the exact same substance. For example, water (with nothing added to it): One sample is hot, the other is cold. When both are placed in the same cooling environment, the hot one will freeze first.

Your coffee has small particles in it from when the water extracts oils and substance from the coffee grounds. This will change the melting point of the water. If you want to test this effect out in its purest form then, fill two glasses with water:

• one of them cold from the tap

• one of them very hot from the tap


Here’s What’s Happening to Make the Hot Water Freeze Quicker

According to the current theories, a couple things contribute to hot water’s speedier freezing rate:

• The hot water is still evaporating shortly after it’s placed in the freezer, reducing the number of molecules and therefore, the volume. With fewer molecules to freeze, the overall energy of the liquid water molecules (called kinetic energy) escapes to the outside freezer environment quicker, converting liquid to a solid slightly faster than in the cold water sample.

• Cold water being cold when it enters means it will form a frost layer in its initial freezing process. This frost layer insulates the rest of the water, since some heat must flow from those unfrozen molecules below into the frost, thus slowing its freezing process a little.

• The hot water molecules have more kinetic energy than cold. This means gas molecules that tend to mix with water, like carbon dioxide, mix less in the hot water. Mixed-in gas molecules like carbon dioxide lower the melting temperature slightly, so fewer gas molecules (in the hot water) mean its melting point is slightly higher, and it freezes a bit sooner than the cold water.

Notice that in all these explanations, I use “a little” and “slightly.” This is because, despite scientists having theories to explain this effect, there is still no consensus on exactly what happens. This is also due to the fact that the parameters are very fine. For example:
If your hot water is 99.9°C (0.01°C below its boiling point) and your cold water is 0.01°C (0.01°C above its melting point), the effect will not work. Your water must be cold, as in chilled below room temperature (~3-6°C), and your hot water must be hot, but not boiling too much (~70-90°C). The shape of the containers matter too, so if you put your coffee cup in alongside a glass, the different shapes could lead to the more naturally expected results.

The main takeaway from today’s lesson is that when you hear someone say, “Hot water freezes faster than cold,” you can correct them: “Only if the conditions are right, and even then, it’s complicated.”

I hope you’ve enjoyed this foray into yet another beautiful side of how strange our world can be. Tomorrow, we’ll turn back to money once more, this time to the concept of value itself and how that can be paradoxical.


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Why Water Freezes Faster After Heating


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