The Mpemba Effect
When hot water freezes faster than cold
The Paradox
Fill two identical containers with water—one boiling hot, one room temperature. Place them in a freezer. Which freezes first?
Intuition says the cold water should freeze first. But under certain conditions, the hot water wins. This bizarre phenomenon has puzzled scientists for over 2,000 years.
This counterintuitive observation is called the Mpemba Effect, named after Erasto Mpemba, a Tanzanian student who rediscovered it in 1963 while making ice cream. But the effect was known to Aristotle, Francis Bacon, and René Descartes long before.
❄️ Freezer Experiment ❄️
Watch hot water race cold water to freeze
The Story of Erasto Mpemba
In 1963, Erasto Mpemba was a Form 3 student at Magamba Secondary School in Tanzania. During a cookery class, students were making ice cream by boiling milk with sugar, letting it cool, then freezing it.
Mpemba, impatient and worried about running out of freezer space, put his still-hot mixture directly into the freezer. To everyone's surprise—including his teacher's—his ice cream froze before his classmates' cooled mixtures.
When Mpemba asked his physics teacher why, he was told he must have made a mistake. But Mpemba persisted, eventually asking visiting physicist Dr. Denis Osborne from University College in Dar es Salaam. Osborne took the question seriously, conducted experiments with his technician, and confirmed the effect. They published their findings together in 1969.
Proposed Explanations
Despite decades of research, no single explanation fully accounts for the Mpemba Effect. Several mechanisms likely contribute:
Historical Timeline
The Scientific Controversy
In 2016, researchers at Imperial College London published a paper titled "Questioning the Mpemba effect: hot water does not cool more quickly than cold." They argued that many observations of the effect are artifacts of poor experimental design—particularly thermometer placement, which varies at different heights in water.
The controversy highlights a fundamental challenge: the Mpemba Effect is highly sensitive to experimental conditions. Container material, water purity, freezer temperature, air currents, and dozens of other variables affect the outcome.
Some researchers argue the effect is real but only appears under specific conditions. Others say it's an illusion created by uncontrolled variables. Still others believe multiple mechanisms work together, making it appear sometimes but not always.
What's certain is this: after 2,000+ years of observations and decades of modern research, we still don't have a complete explanation. That's what makes it such a compelling paradox.
Try It Yourself
Want to test the Mpemba Effect at home? Here are some tips:
- Use identical containers (metal works better than plastic)
- Use distilled water to minimize dissolved mineral differences
- Start with temperatures around 35°C (cold) and 100°C (hot)
- Place containers at the same height and position in the freezer
- Check frequently but consistently (opening the door affects temperature)
- Repeat multiple times—the effect doesn't always appear!
Many home experimenters report seeing the effect, while others don't. That inconsistency is part of what makes this paradox so fascinating—and so controversial.