History
The observation that hot water freezes faster appears in writings by Aristotle, Francis Bacon and Descartes (Jeng, 2006). It returned to modern attention through Erasto Mpemba, a Tanzanian secondary school student. In 1963 he noticed that an ice cream mixture he put in the freezer without cooling it first froze sooner, and in 1969 he and the university lecturer D. G. Osborne published the paper "Cool?" in Physics Education.
Proposed causes
- Evaporation: hot water loses mass and heat through the latent heat of evaporation.
- Convection: strong convection in hot water keeps the surface warm, so heat may be lost faster through the top surface. Mpemba and Osborne offered this as a tentative explanation.
- Dissolved gases: heated or boiled water holds less dissolved gas, which may change how it freezes.
- Supercooling: water passes through a supercooled state below 0°C, and the degree of supercooling may depend on the starting temperature, shifting when freezing begins.
- Environmental factors: differences in experimental conditions, such as a hot container melting frost on the freezer floor and improving thermal contact.
The ways heat moves are covered in heat transfer.
The reproducibility debate
Jeng (2006) concluded that the effect has been observed in several experiments, but results vary greatly with conditions and with the definition of "freezing" (onset of freezing, complete freezing and so on). Burridge and Linden (2016), through controlled experiments and a review of earlier studies, reported finding no meaningful evidence that initially hotter water reaches 0°C sooner. The Mpemba effect is therefore often used as a case study in the importance of precise hypotheses, measurement conditions and replication in science.
Recent research
For systems other than water, studies in non-equilibrium statistical mechanics have proposed Mpemba-like effects, in which a system starting farther from equilibrium relaxes to it faster, and have observed them in experiments such as those with colloidal particles. This does not violate the laws of thermodynamics; it means that the relaxation path depends on the initial state.
References
- Mpemba, E. B., & Osborne, D. G. (1969). Cool? Physics Education, 4(3), 172-175.
- Jeng, M. (2006). The Mpemba effect: When can hot water freeze faster than cold? American Journal of Physics, 74(6), 514-522.
- Burridge, H. C., & Linden, P. F. (2016). Questioning the Mpemba effect: Hot water does not cool more quickly than cold. Scientific Reports, 6, 37665.