Mpemba effect Facts for Kids

The Mpemba effect is the observation that sometimes, a container of hot liquid will freeze more quickly than an identical container of cold liquid, even when they are in the same freezer.

The effect is named after Erasto B. Mpemba, a student from Tanzania who noticed this happening with ice cream mix in the 1960s. Scientists are still studying this strange effect and don't all agree on why it happens or even how often it happens. It has been seen not just in water and ice cream, but also in other special materials.

The Story of Erasto Mpemba
A Puzzle Through History
Why Might Hot Water Freeze Faster?
Scientists Still Studying the Puzzle
See also

The Story of Erasto Mpemba

The discovery of this effect is a great story about curiosity. In 1963, a Tanzanian student named Erasto Mpemba was in a cooking class at his school. The students were making ice cream. They had to boil a milk mixture, let it cool, and then put it in the freezer.

Mpemba was in a hurry and worried he wouldn't get a spot in the freezer. So, he put his hot ice cream mixture in the freezer without waiting for it to cool down. To his surprise, his hot mixture froze into ice cream even before the mixtures of the other students, who had followed the rules and cooled theirs down first!

Later, at a different school, a scientist named Dr. Denis Osborne came to give a talk about physics. After the talk, Mpemba asked him a question: "If you take two similar containers with equal volumes of water, one at 35 °C (95 °F) and the other at 100 °C (212 °F), and put them into a freezer, the one that started at 100 °C (212 °F) freezes first. Why?"

Dr. Osborne was surprised by the question but decided to test it back in his laboratory. He found that the schoolboy was right! The hot water did sometimes freeze faster. Dr. Osborne and Mpemba wrote a scientific paper about their findings together in 1969.

A Puzzle Through History

Erasto Mpemba wasn't the first person to notice this strange effect. Great thinkers from long ago also wrote about it.

The ancient Greek scientist Aristotle wrote that water that has been warmed first helps it to freeze quickly.
In the 1600s, famous thinkers like Francis Bacon and René Descartes also described how warm water could freeze faster than cold water.
In the 1700s, a Scottish scientist named Joseph Black studied the difference between water that had been boiled and water that had not. He found that the boiled water froze faster.

Even though people had noticed this for centuries, it was Mpemba's question that made modern scientists take a closer look at this cool puzzle.

Why Might Hot Water Freeze Faster?

Scientists have come up with several possible explanations for the Mpemba effect. It's likely that not just one, but a combination of these reasons is responsible.

Evaporation

When you have a container of hot water, some of it turns into steam and escapes into the air. This is called evaporation. This means the hot water container ends up with slightly less water to freeze than the cold one. With less mass, it can cool down and freeze more quickly.

Dissolved Gases

Tap water has gases like oxygen and carbon dioxide dissolved in it. When you heat water, these gases are forced out. Cold water has more of these dissolved gases than hot water. Some scientists think these gases might change how water freezes, making it harder for the colder water to turn to ice.

Convection Currents

Convection is how heat moves around in a liquid. In a container of water, warmer water is less dense and rises, while cooler, denser water sinks. This creates a current that helps the water cool down. In the container of hot water, this difference in temperature is much bigger, so the convection currents are stronger. These powerful currents might help the hot water lose its heat to the cold air in the freezer much faster.

Supercooling

Sometimes, water can get colder than its freezing point (0 °C or 32 °F) without actually turning into ice. This is called supercooling. Cold water is more likely to become supercooled than hot water. If the cold water supercools to a very low temperature, like -10 °C, it will take longer to finally start forming ice crystals compared to the hot water, which might start freezing at a higher temperature, like -2 °C.

Frosty Surfaces

Have you ever noticed a layer of frost inside a freezer? This frost can act like a little blanket, insulating things from the cold. A container of hot water might melt the frost underneath it, creating a patch of bare, cold metal. This direct contact with the freezer shelf allows heat to be pulled away from the container much more efficiently, helping it cool and freeze faster.

Scientists Still Studying the Puzzle

Even though the Mpemba effect has been known for a long time, it is very tricky to study. Many experiments have tried to test it, and they get different results. Sometimes the effect happens, and sometimes it doesn't.

This is because many small things can change the outcome. The shape of the container, the type of water used (tap water vs. pure water), and the kind of freezer can all make a difference. Because it's hard to get the same result every time, some scientists have argued that the effect isn't real.

However, recent studies continue to show that the effect is real under the right conditions. In 2024, one experiment using small drops of water and a special camera found that hot drops consistently froze faster than cold ones. Scientists are also exploring if similar "hotter is faster" effects can happen in other areas of science, from tiny particles to complex quantum systems. The simple question from a curious student continues to inspire new scientific research today.

Mpemba effect facts for kids

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