The Mpemba Effect: Why Hot Water Sometimes Freezes Faster Than Cold

In 1963, a thirteen-year-old student named Erasto Mpemba was making ice cream in his cooking class at a school in Tanganyika (now Tanzania). In a rush to secure a spot in the freezer before they filled up, he put his ice cream mixture in while it was still hot, without waiting for it to cool. When he came back, he made a curious observation: his hot mixture had frozen before the cold mixtures of students who had prepared theirs correctly. When he asked his teacher to explain, he was told he was mistaken. When he later asked a visiting physics professor from the University of Dar es Salaam, the professor was intrigued enough to investigate.

The result was a 1969 paper co-authored by Mpemba and physicist Denis Osborne that formally introduced the observation to modern science. The phenomenon now bears his name: the Mpemba Effect.

The Schoolboy Who Challenged Science

What makes Mpemba's story particularly compelling is not just the observation but the institutional resistance he faced. His teacher dismissed the result. His classmates reportedly mocked him. The assumption that hot water cannot possibly freeze faster than cold water — because it must first cool down to reach the temperature the cold water was already at, and in that time the cold water should have a head start — seems so obviously logical that questioning it feels almost embarrassing.

But Mpemba was persistent, and Osborne's subsequent experiments confirmed that under certain conditions, the effect was real. Identical containers of water at different starting temperatures — one hot, one cold — placed in a freezer under identical conditions could result in the hot water freezing first. The observation was reproducible. The challenge was explaining it.

That explanation has proven remarkably elusive. Over the half-century since Mpemba and Osborne's paper, multiple competing theories have been proposed, tested, partially supported, and partially refuted. The Mpemba Effect remains one of the most studied and least resolved puzzles in everyday physics.

The Theories Explained

The challenge in explaining the Mpemba Effect begins with the observation itself: the effect is not always reproducible. It depends on the exact containers used, the composition of the water, the temperature of the freezer, the position of the containers, and a host of other variables that are difficult to control simultaneously. This inconsistency has led some researchers to question whether a single unified explanation exists, or whether the "effect" is actually several different phenomena that can each, under certain conditions, allow hot water to outpace cold water in freezing.

With that caveat in mind, the most prominent theoretical explanations include the following.

Evaporation

The simplest explanation is evaporation. Hot water evaporates faster than cold water. When you place a hot container of water in a freezer, significant evaporation occurs before the water cools to room temperature. This evaporation reduces the total mass of water that needs to be frozen. Less water, less time to freeze. The effect can be substantial: a container of hot water might lose 10–15% of its mass to evaporation before it cools, effectively giving it a head start over the cold container that retains its full mass.

This explanation is appealing in its simplicity, and in open containers it likely plays a significant role. However, it does not fully account for observations where the effect occurs in sealed or covered containers, where evaporation is minimal.

Dissolved Gases

Water contains dissolved gases — primarily nitrogen, oxygen, and carbon dioxide — and hot water holds less dissolved gas than cold water. When water is heated, these gases are driven off and escape into the atmosphere. Cold tap water, by contrast, can be quite gas-rich.

The theory is that dissolved gases interfere with the formation of ice crystals by disrupting the regular lattice structure that water molecules need to adopt as they freeze. Gas-poor hot water, having released its dissolved gases during heating, may begin forming ice crystals more readily than gas-rich cold water, giving it a kinetic advantage in the early stages of freezing.

Convection

When hot water cools in a freezer, it develops strong convective currents. The water at the surface and edges of the container cools first, becoming denser and sinking, while warmer water from the bottom rises to replace it. This continuous circulation keeps the water well-mixed and brings warm water into contact with the cold container walls more effectively, potentially accelerating heat transfer.

Cold water, by contrast, establishes weaker convective currents as the temperature difference between the water and the freezer is smaller. The result is that heat may be drawn out of hot water more rapidly per unit of time, at least in the early stages of cooling, than out of cold water.

The 2016 Singapore Study

In 2016, researchers at the Nanyang Technological University in Singapore proposed a molecular-level explanation that attracted significant attention. Their hypothesis centers on the behavior of hydrogen bonds in water. In warm water, the hydrogen bonds between water molecules are stretched longer than their natural length because the thermal energy keeps the molecules further apart. When water cools, these hydrogen bonds contract, releasing energy. The researchers proposed that this energy release from contracting hydrogen bonds provides an additional cooling mechanism that is not available to cold water — whose hydrogen bonds are already at their natural length and cannot contract further.

The Singapore study was notable for providing a quantitative model consistent with the measured energy differences, but it has not yet been independently verified to the satisfaction of the broader scientific community, and the debate continues.

Does It Always Work

The honest answer is no. The Mpemba Effect is not a reliable guarantee that your hot glass of water will freeze faster than a cold one. It depends on too many variables to make a simple rule. Some researchers have attempted large-scale systematic experiments and found conditions under which the effect reliably appears, and other conditions under which cold water always wins.

What the Mpemba Effect demonstrates is that physical intuition — the feeling that something "obviously" must be true — is not the same as physical reality. A system as seemingly simple as water in a freezer turns out to contain enough complexity to defy straightforward prediction. The schoolboy who noticed something strange in his cooking class and refused to accept "you're wrong" as an answer gave science a genuinely interesting puzzle. Half a century later, we are still working out the solution.