May 2023

Understanding the Mpemba Effect: Why Hot Water Freezes Faster

May 12, 2023
Understanding The Mpemba Effect: Why Hot Water Freezes Faster

For centuries past, brilliant minds like Descartes and Aristotle have long suspected that hot water freezes faster than cold water somehow, as contrary as it may seem to all conventional wisdom. However, there exists no scientific consensus that this theory holds.

That is until 1963, when a fluke accident rekindled the idea and was proven once and for all. The Mpemba effect, named after the Tanzanian physics student Erasto Mpemba who discovered it while making ice cream at his school, confirms what the two scientists mentioned earlier had suspected: that hot water reaches water freezing point more swiftly than cold water. However, was he correct about his findings?

The Mpemba Effect

This physics concept suggests that if both hot and cold water are put in the same freezing environment, the former will freeze much faster. Mpemba noted this during their ice-cream-making activity when he placed a near-boiling mixture of milk and sugar (which is mostly water) in a freezer, and it frosted over before the other room-temperature mixtures that were cooled along with it.

He extrapolated from his observation that if equal volumes of water with differing temperatures, one being 100℃ and the other 35℃, are placed in the same beakers or containers and put into a freezer, the 100℃ water would harden into ice much faster. Mpemba’s water postulation and ice cream observation aligned him with numerous scientists over the centuries who also had a suspicion about this unusual property of water.

Understanding the Freezing Process

Water goes through a phase change when it freezes and turns to ice, essentially transitioning from a liquid state into a solid one. Traditionally, physicists declare a substance’s phase when it is at equilibrium, meaning the substance reaches a stable state and there is no significant amount of energy flowing from one region to another. Moreover, equilibrium means that the substance’s temperature and volume remain steady. In contrast, a substance that is not in equilibrium will have fluctuating energy levels and, potentially, state of matter as well.

For water to ice up and remain frozen, each of its water particles must reach equilibrium. An abundance of energy surging through nonequilibrium water will cause it to fluctuate between liquid and solid at low temperatures or gas and liquid at higher temperatures. The quicker these particles achieve equilibrium at a low-energy level, the sooner they can freeze.

So Why Does Hot Water Freeze Faster?

There is ongoing debate regarding the consistency of hot water freezing faster than cold water. As it turns out, there are certain conditions for this to happen. When a container of water is placed in freezing conditions, not all parts of the water reach equilibrium at the same time.

For instance, the water at the outskirts of the container gets colder swiftly and can freeze up while the water in the middle remains in its liquid state. And suppose you specifically put a container filled with hot water into a freezer, similar to what Mpemba did. In that case, it also releases steam that helps reduce the total water volume that needs to ice over.

In addition, cold and room-temperature water typically forms a layer of frost at its surface during the freezing process. This formation ironically causes temporary insulation of the water (similar to how ice igloos insulate the inhabitants against the freezing outside air) and delays the freezing process. This is in contrast to hot water, where it prevents a frost layer from forming, at least in its early stages of freezing up, and lets cold air penetrate deeper into the container.

All in all, these are some of the ways hot water can bring about a quick-freezing process compared to cold water. That said, remember that water needs to reach a state of equilibrium to freeze and stay frozen.

Conclusion

Despite the handful of studies proving the Mpemba effect, most notably the combined work of John Bechhoefer and Avinash Kumar in their 2020 study and Zhiyue Lu and Oren Raz’s “Nonequilibrium thermodynamics of the Markovian Mpemba effect and its inverse, it is not accepted as a proven scientific phenomenon. Nonetheless, centuries of observation and the works of the people mentioned before have convinced physicists that hot water can reach a freezing point faster than cold water under the right circumstances.

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