When you fill two identical glasses — one with warm water and the other with cold — logic suggests that the cold water should freeze faster. However, the warm water often freezes first. This phenomenon is known as the Mpemba effect, and it has puzzled scientists for centuries.
The fact that hot water can freeze faster than cold water had already caught the attention of philosophers such as René Descartes and Francis Bacon. But for a long time, no serious study was conducted on this subject. Finally, a young Tanzanian student named Erasto Mpemba observed that a warm ice cream mix froze faster than a cold one. When he asked a visiting physicist, Dr. Denis Osborne, about this observation, Osborne initially did not believe it — but later verified the results. A few years later, they published a paper together and proved the existence of the Mpemba effect.
To understand why, consider a simple example. Suppose we have two identical glasses of water: one at 10°C and the other at 40°C. If the 10°C water takes 10 minutes to freeze, we might expect the 40°C water to take longer — first cooling down to 10°C, then freezing. At first glance, the Mpemba effect seems impossible. However, this is not true for every case.
A major theory explains the Mpemba effect through the behavior of water molecules. When water is heated, it becomes less dense and the hydrogen bonds stretch. This causes the covalent bonds to relax and shorten, which releases stored energy — technically a cooling process, allowing warmer water to cool down much faster under the right conditions.
Does the Mpemba effect actually exist? The reason it remains unexplained may be that it does not exist universally — a 2016 study found no consistent evidence to support it. However, Spanish researchers later discovered that under specific conditions, a hot sample can indeed cool faster than a cold one. They also identified a symmetrical phenomenon — where a cold sample warms up faster — which they termed the "inverse Mpemba effect." Science, as always, is more complex and more fascinating than it first appears.