Hot water can freeze faster than cold water -- the Mpemba effect
Under certain conditions, hot water freezes faster than cold -- a phenomenon known since antiquity.
Scientific Explanation
Imagine filling two identical containers — one with hot water and one with cold water — and placing both in a freezer. Intuitively, you would expect the cold water to freeze first. Yet under certain conditions, the opposite happens: the hot water freezes sooner. This surprising phenomenon is known as the Mpemba effect, named after Tanzanian student Erasto Mpemba, who observed it while making ice cream in 1963.
Aristotle, Francis Bacon, and Rene Descartes had all described similar observations centuries earlier. Despite this long history, the exact cause remains debated. Several mechanisms have been proposed:
- Evaporative cooling: Hot water evaporates more, reducing its volume and allowing the remaining water to cool faster.
- Convection: Stronger convection currents in hot water lead to more efficient heat transfer.
- Dissolved gases: Hot water contains fewer dissolved gases, which may influence crystallization.
- Hydrogen bond dynamics: Recent research suggests that the arrangement and strength of hydrogen bonds in warm water may accelerate the cooling process.
Most likely, several of these factors act together, and the specific conditions — such as the container shape, the volume of water, and the ambient temperature — determine whether the effect occurs.
Step by Step
The Coordinate System
We compare the temperature of two water samples over time. The x-axis shows elapsed time in minutes, and the y-axis shows temperature in degrees Celsius.
Cold Water Cools
A sample starting at 25 degrees Celsius cools exponentially. The blue curve shows a typical cooling profile -- rapid at first, then gradually levelling off as the temperature difference with the environment shrinks.
Hot Water Cools
Now a second sample starts at 90 degrees Celsius. The cyan curve shows it cools much more steeply, because the larger temperature difference drives a stronger heat flow to the surroundings.
The Crossing
At a certain moment the two curves cross. The hot water has caught up with the cold -- despite starting from a much higher temperature.
The Surprise
The hot water reaches the freezing point first! Evaporative cooling, stronger convection, and altered hydrogen bond dynamics work together. The Mpemba effect contradicts our intuition -- yet it has been confirmed experimentally.
Everyday Relevance
The Mpemba effect is one of the most captivating examples of how water defies our expectations. In cold climates, people sometimes exploit this effect by throwing hot water into the air, where it instantly transforms into a cloud of vapor and ice crystals at very low temperatures — a spectacular sight on frigid winter days.
The effect also plays a role in home ice-making: some ice cream recipes recommend warming the mixture before placing it in the freezer. And on a larger scale, operators of ice skating rinks have reported that warm water sometimes produces a smoother ice surface faster than cold water.