About Mpemba effect:

• It is named after Tanzanian student Erasto Mpemba, who brought attention to this counterintuitive phenomenon in 1969, making for curious observations.
• The effect is that hot water can freeze faster than cold water in similar conditions.
• While Aristotle, Francis Bacon, and René Descartes had noticed the effect centuries earlier, the Mpemba effect caught scientists’ attention only more recently.
• Different Experiments
  • Researchers have conducted numerous experiments to determine the causes of this confusing phenomenon, but a consensus conclusion remains wanting.
  • One cause, they have posited, is microbubbles left suspended in water that has been heated by boiling.
  • These cavities promote convection and transfer heat faster as the water cools.
  • Evaporation: as warmer water evaporates more, it also takes away some heat (evaporation is inherently endothermic, which is how sweat cools your skin). Both convection and accelerated heat transfer are enhanced in warmer water because such water is less dense.
  • Yet another factor could be the presence of frost in cold water. Frost is an insulator and could slow the loss of heat.
  • Scientists have also considered whether compounds in water like calcium carbonate could be precipitated by boiling, and then dissolve, thus increasing the water’s freezing point.