The Mpemba Effect: Does Hot Water Really Freeze Faster Than Cold Water?

The Mpemba effect is a physics concept that postulates that when hot water and cold water are placed in the identical freezing environment, the hot water will freeze faster than the cold water.

Erasto Mpemba noted that when his class was making ice cream, he placed a near-boiling blend of sugar and milk (which is mostly water) into a freezer, and it froze before other mixtures which had been cooled to room temperature before freezing.

Mpemba's extrapolation from this observation was that when identical volumes of water — one at 212 degrees Fahrenheit (100 degrees Celsius) and the other at 95 degrees Fahrenheit (35 degrees Celsius) — were placed in identical beakers and put in a freezer, the 212 degree water would turn to ice faster. Mpemba's ice cream observation and water postulation aligned him with many centuries of scientists who had also suspected this unusual property of water.

When water freezes into ice, it undergoes a phase change; it turns from a liquid into a solid. Physicists traditionally declare the phase of a substance when it's at equilibrium. This means the substance is in a stable state, and significant amounts of energy are not flowing from one region to another. It also means that its volume and temperature remain steady. When a substance is not at equilibrium, its energy levels fluctuate, and so does (potentially) its state of matter.

For water to freeze and stay frozen, individual water particles have to reach equilibrium. If too much energy surges through nonequilibrium water, it will fluctuate between solid and liquid (at low temperatures) or liquid and gas (at higher temperatures). The sooner that water particles reach equilibrium at low energy levels, the sooner they can freeze.

Physicists are still debating whether hot water consistently freezes faster than cold water. When it does happen, certain conditions have to be met.

When a vessel of water is submerged in a freezing environment, different parts of the water reach equilibrium at different times. Water around the outskirts of the vessel gets colder faster, which means that it may freeze while water in the middle of the vessel stays liquid. And when you specifically place a vessel of hot water in a freezer (like the 212 degree boiling water described by Mpemba), it is also releasing steam from the top of the vessel, and this decreases the total volume of water that needs to freeze.

Furthermore, cold water (or even room temperature water) often develops a layer of frost on its surface as part of the freezing process. Ironically, this frost temporarily insulates the water (kind of like how an ice igloo insulates its inhabitants against cold air), which can slow down the overall freezing process. Hot water, at least in the early stages, blocks the formation of frost, which allows cold air to penetrate deeper into the vessel.

These are some of the ways that hot water can engender freezing faster than cold water can. But remember that for water to freeze and stay frozen, it must achieve a state of equilibrium.

If there's proof that the Mpemba effect is real and consistent, it comes from a 2020 study by John Bechhoefer and Avinash Kumar. Published in the journal Nature, the study subjected microscopic glass beads to what they called an "energy landscape" controlled by lasers. The researchers heated beads to different temperatures. They then observed which of the beads first reached a state of equilibrium within that energy landscape.

Bechhoefer and Kumar observed that microscopic beads that started at high temperatures reached equilibrium faster than those that started at lower temperatures. That's interesting enough, but how does reaching equilibrium relate to freezing?

The connection comes from prior work done by Zhiyue Lu of the University of North Carolina and Oren Raz of the Weizmann Institute of Science in Israel. Their paper, "Nonequilibrium thermodynamics of the Markovian Mpemba effect and its inverse," published in Proceedings of the National Academy of Sciences (PNAS) and described by Quanta Magazine, postulates that hotter systems of matter may be able to skip ahead in the process of reaching equilibrium, thus reaching a stable state faster than a colder system.

If relaxing toward equilibrium is a critical benchmark in the freezing process of water, then the combined work of Bechhoefer and Kumar along with Lu and Raz might prove the existence of a Mpemba effect.

The Mpemba effect is not uniformly accepted as a proven scientific phenomenon. However, centuries of observation, plus recent work by Bechhoefer, Kumar, Lu, and Raz have convinced many physicists that under the right circumstances, hot water really can reach a freezing point faster than cold water.

Some scientists, like Harry Burridge and Paul Linden, remain skeptical. They acknowledge that while some vessels of hot water can freeze faster than equal-sized vessels of cold water, even the slightest shift in conditions erases the effect. Burridge and Linden's own 2016 study, "Questioning the Mpemba effect: hot water does not cool more quickly than cold," found that any proof of a Mpemba effect depended on the size of a water vessel and the placement of a thermometer. In a separate study, researcher James Brownridge found that impurities in a vessel of water (such as those in Mpemba's ice cream concoction) will alter the liquid's freezing point. While acknowledging there are times when hot water freezes faster than cold water, these scientists argue the phenomenon does not uniformly apply in nature.

However other physicists, like Raúl Rica Alarcón of Spain’s University of Granada, believe these new datasets, such as those offered by Bechhoefer and Kumar, are significant. "My view is that the Mpemba effect can take place under some special circumstances," says Rica Alarcón, "but we are still trying to figure out what are the minimal conditions for this to happen."

Rica Alarcón notes that observances of the Mpemba Effect always involve drastic differences in temperature between a vessel of water and its surrounding environment. And, he adds, you can observe equally intriguing phenomena when you reverse the temperatures and place frozen ice into a hot environment.

The Mpemba Effect, says Rica Alarcón, "seems to be one of a large group of anomalous thermalization effects, which take place when a system is suddenly put in contact with a thermal bath at a different temperature." The Mpemba Effect describes a hot-to-cold phase change like "when you take a hot cup and you put it in the fridge or in the freezer." But cold-to-hot phase changes also invoke unusual results. "Interesting effects take place when you perform temperature quenches from cold to hot," says Rica Alarcón, "like when you put an ice cube into boiling water."

We know that many generations of scientists have observed hot water freeze with surprising speed. Rica Alarcón urges us to regard this process more holistically, and think about the Mpemba Effect as part of a broader phenomenon. "Thermalization," he explains, "can follow counterintuitive paths due to the fact that the processes take place out of equilibrium."