The Mpemba effect is the counterintuitive observation that, under certain conditions, hot water can freeze faster than colder water; the phenomenon has been reported since antiquity but remains debated and not fully understood.
History and observation
Observers from Aristotle to modern investigators have noted cases where warmer water appears to reach the freezing point sooner than cooler samples, and the effect was popularised in the 1960s by Erasto Mpemba, who reported the behaviour while making ice cream and later worked with scientists to document it experimentally.
What science says
Laboratory studies have produced mixed results: some careful experiments reproduce a faster freezing of initially hotter water under specific setups while others find no effect, and physicists remain divided because the outcome depends strongly on experimental details and how \"freezing\" is defined.
Possible mechanisms
Proposed explanations include faster evaporation from the hot sample (reducing mass to freeze), differences in convection and temperature gradients that change cooling pathways, dissolved gas content and supercooling behaviour, and subtle nonequilibrium thermodynamic effects that alter how the systems approach thermal equilibrium.
Experimental caveats
Because small changes in containers, initial temperatures, freezer airflow and definitions of freezing can flip results, reproducibility is challenging; recent theoretical and empirical work seeks more precise definitions and frameworks to predict when a Mpemba‑like effect will occur across different substances and models.
Takeaway
The Mpemba effect is a real and intriguing scientific puzzle: hot water can sometimes freeze faster than cold water, but it is not a universal rule and arises from a combination of physical processes that researchers are still disentangling with experiments and theory.