What Is the Mpemba Effect?

When Hot Water Freezes Faster Than Cold Water

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Have you ever wondered whether hot water really can freeze more quickly than cold water and if so, how it works? If so, then you need to know about the Mpemba Effect.

Simply stated, the Mpemba Effect is the name given to the phenomenon when hot water freezes more quickly than cold water. Although the effect has been observed for centuries, it was not published as a scientific observation until 1968.

The Mpemba Effect is named for Erasto Mpemba, a Tanzanian schoolboy who claimed ice cream would freeze faster if it was heated before being frozen. Although his peers ridiculed him, Mpemba got the last laugh when his instructor performed an experiment, demonstrating the effect. Mpemba and headmaster Dr. Denis G. Osborne observed the time required for freezing to start took longest if the initial water temperature was 25 °C and took much less time if the starting temperature was 90 °C. 

Reasons Why the Mpemba Effect Happens

Scientists aren't completely certain why hot water sometimes freezes more quickly than cold water. The Mpemba Effect is not always seen -- often cold water freezes before hot water. The explanation for the effect likely has to do with impurities in the water, which serve as nucleation sites for freezing. Other factors may include:

  • an effect from the evaporation of hot water
  • increased convection in hot water
  • increased tendency of cold water to supercool compared with hot water
  • potential different amounts of dissolved gases in cold water compared with hot water
  • effect of frost formation -- hot water tends to freeze from the bottom while cold water tends to freeze from the top
  • thermal conductivity, causing the container of hot water to melt through insulating ice in the freezer, potentially exposing the container to a colder layer beneath the ice

Learn more about the freezing point of water.

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Helmenstine, Anne Marie, Ph.D. "What Is the Mpemba Effect?" ThoughtCo, Mar. 20, 2017, thoughtco.com/the-mpemba-effect-definition-609415. Helmenstine, Anne Marie, Ph.D. (2017, March 20). What Is the Mpemba Effect? Retrieved from https://www.thoughtco.com/the-mpemba-effect-definition-609415 Helmenstine, Anne Marie, Ph.D. "What Is the Mpemba Effect?" ThoughtCo. https://www.thoughtco.com/the-mpemba-effect-definition-609415 (accessed November 19, 2017).