Does Hot Water Freeze Faster Than Cold? The Mpemba Effect Explained

It sounds like a riddle: put a cup of hot water and a cup of cold water in the freezer. Which turns to ice first? Common sense says the cold one. But for decades a surprising claim has circulated that sometimes the hot water wins. It even has a name: the Mpemba effect. Is it true?

A schoolboy’s ice cream

In 1963, a secondary school student in Tanzania named Erasto Mpemba was making ice cream in a cookery class. Short of time, he put his mix into the freezer while still hot, instead of letting it cool first. To his surprise, it froze before his classmates’ cooler mixes.

His teachers did not believe him. But when the physicist Denis Osborne visited his school, Mpemba asked him about it. Osborne was intrigued and ran experiments, and in 1969 the two published a paper reporting that hot water sometimes froze sooner than cold.

The idea was not new. Aristotle wrote about something similar more than 2,000 years ago, and Francis Bacon and René Descartes discussed it in the 1600s. But Mpemba’s story made it famous.

Why it could happen

Scientists have suggested several reasons why hot water might sometimes freeze first:

• Evaporation: hot water evaporates faster, so there is less water left to freeze, and evaporation itself removes heat.
• Dissolved gases: heated water holds less dissolved gas, which might change how it freezes.
• Convection: hot water has stronger internal currents, which could carry heat to the surface faster.
• Supercooling: water can cool below 0 °C and stay liquid for a while before ice forms. If cold water supercools more than hot water, it might start freezing later.
• The freezer itself: a hot container might melt frost under it and make better contact with the shelf, cooling faster.

What careful experiments found

The problem is that the effect is very hard to reproduce. Results depend on the size and shape of the container, the type of freezer, where you place the thermometer, and even what counts as “frozen”: the first ice crystal, or a solid block?

In 2016, physicists Henry Burridge and Paul Linden at the University of Cambridge carried out careful measurements under controlled conditions. They found no evidence that hot water cools to 0 °C faster than cold water. Small differences in where the temperature was measured could easily create apparent “Mpemba effects”.

Other researchers have reported effects in special systems, and physicists have shown that similar strange cooling can happen in some carefully designed materials. But for an ordinary cup of water in a home freezer, the best answer is: the cold water normally freezes first.

A lesson in science

Mpemba’s story is still valuable. A student noticed something odd, asked questions even when teachers laughed, and started a debate that has lasted half a century. Testing it properly showed how many hidden factors can affect a simple experiment, and why scientists must control conditions carefully before drawing conclusions.

In short

• The Mpemba effect is the claim that hot water can freeze faster than cold.
• It is named after Erasto Mpemba, who noticed it with ice cream in 1963.
• Proposed reasons include evaporation, dissolved gases, convection and supercooling.
• Results depend heavily on the container, freezer and measurement.
• A careful 2016 study found no evidence that hot water cools faster.
• In a normal freezer, cold water usually freezes first.