By the use of basic laws of thermodynamics, a system that contains a high amount of heat should require a long period of time to lose that energy and reach a state where it is solid and cold. To many people it is unexpected that a liquid which is hot can change its state or reach a temperature where it freezes before a sample that was cooler at the start. In science this event is called the Mpemba Effect. It is a topic that scientists discuss frequently because they do not agree on the exact physical limits of the process or how it occurs. There is also no agreement on if the results can be repeated when people use large amounts of water.
A Question Asked After Class
As the record of this event shows, the study of the effect started in the early 1960s. At that time a student in Tanzania named Erasto Mpemba saw that a hot mixture for ice cream became solid more quickly than mixtures that were not as hot. When he told his teachers, they did not believe him, but he later got the chance to raise the problem with a physicist named Denis Osborne, who was visiting his school to give a guest lecture. Intrigued by the question, Osborne performed tests on water in a laboratory. And in 1969 they wrote a paper together so that scientists around the world could learn about the observation.
The Study That Poured Cold Water On It
In the past researchers have found it difficult to explain this effect. For a long time many accepted the effect without question, until a major study from 2016 by Burridge & Linden challenged its validity in pure water. As they applied strict experimental controls to remove variations in temperature and errors in measurement, they observed that the macroscopic effect did not occur. But the phenomenon is still supported by empirical data and theoretical models.
One Third of the Time, It Happens
To investigate further, Janni, Bolero Ampudia and Dahlberg published a study in 2026 through the Royal Society of Chemistry. By evaluating the effect across different experimental setups, they tested deionized and tap water in various containers. They showed that the anomaly is stochastic, appearing in about one third of the total trials, with changes in air currents and the way liquids cool below their freezing points driving the variation. And the effect is larger in tap water because dissolved ions change how the surfaces of the liquid behave - those ions create the greatest differences in temperature when the effect happens.
Beyond Water, Into the Quantum
Scientists have used theoretical physics to show that this phenomenon exists in systems other than water. They have observed non equilibrium relaxation behavior that is similar to the Mpemba effect in granular fluids, magnetic alloys and specific Markovian models. On a very small scale, Joshi et al. published a significant discovery in Physical Review Letters in 2024. By using a trapped ion quantum simulator, they were able to confirm that a Quantum Mpemba Effect exists.
Escaping the Trap
In those frameworks, researchers demonstrate that systems that start at a high energy state can reach configurations that are far from equilibrium. By this method the system follows specific pathways to relax and completes the cooling process faster. It avoids the thermal "metastable traps" that instead slow down systems which begin at lower temperatures.
Many Forces, Acting at Once
When the effect is present in fluids, it exists because multiple physical processes act at the same time. For instance the mass is smaller because the liquid evaporates at a high rate. And dissolved gases or solutes leave the fluid while it is hot. There are also strong convection currents that move heat to the surface quickly. And the way ice begins to form, or nucleate, around small particles in the liquid can also vary between samples.
Logic Was Never Enough
To determine the result, one must consider the specific shape and the substance of the container. It is also necessary to account for what the fluid contains and what happened to it previously. On the outside, environmental boundary conditions like the movement of air and how steady the temperature is in the machine are important. The Mpemba story is a significant example that basic logic does not describe the complex ways that non equilibrium thermodynamics works. As a result there is a requirement for experiments that are exact and can be done again. With this approach, researchers use careful analysis instead of relying on what they expect to happen.