The Mpemba effect: Hot water freezes faster than cold water

As an African physicist, I have always wondered why there were no laws, theorems, equations named after African scientists. Living in a world where Schrodinger equation, Bose-Einstein statistics, Pythagorean theorem, and Newton’s laws are norms, and being African, I have always felt left out… It is as if my forefathers were not interested in science, or that modern day Africans were not as bright as Raman (Physics Nobel 1930) or Chandrasekhar (Physics Nobel 1983). Well… I was amazingly surprised when I stumbled upon ‘the Mpemba effect‘: made in Africa, by an African high schooler in 1963 (he later published his findings in 1969). Today, people apply his law without even thinking about where it comes from.

Erasto B. Mpemba, hailing from Tanzania, was still in high school when he came across this phenomenon when freezing hot milk; he noticed that it would freeze before the cold one. After repeating his experiments several times with water and milk, the Mpemba effect was born.  The heat transfer is bigger between the warm water and the freezer, than between the cold water and the freezer, leading to a faster process. Several mechanisms can be used to explain this effect, such as: – Evaporation (endothermic process); – Convection (faster heat transfers); frost formation (the colder water will tend to freeze from the top, reducing further heat loss by radiation and air convection, while the warmer water will tend to freeze from the bottom and sides because of water convection); supercooling; and – dissolved gases (Hot water can hold less dissolved gas than cold water, and large amounts of gas escape upon boiling.  So the initially warmer water may have less dissolved gas than the initially cooler water). More information on the ‘Mpemba effect’ can be found on Mpemba and Osborne, “Cool”, Physics Education vol. 4, pgs 172–5 (1969) and Can hot water freeze faster than cold water?

As of 2002 Erasto B. Mpemba is retired from being Principal Game Officer for the African Forestry and Wildlife Commission. This might not give rise to a Nobel prize, but it is enough to inspire others.