What is the Mpemba effect?

Answer

Experiments disagree, so scientists still argue about whether it is real.

Context

The effect is named for Erasto Mpemba, a Tanzanian schoolboy who in 1963 noticed that a hot ice-cream mixture froze before a cooled one. He later published with physicist Denis Osborne in 1969. Similar observations had been recorded much earlier, and writers going back to Aristotle are often credited with comments, though the details of those older accounts are debated.

Whether it is real is contested because it depends on definitions and conditions. 'Freeze faster' could mean the time until the first ice appears, until the water is fully solid or until it reaches 0 degrees Celsius, and each can behave differently. Container shape, the amount of dissolved gas, evaporation, convection, supercooling and the position of the thermometer all alter the result. Several careful experiments have failed to reproduce it, and others have reported it in certain setups.

Proposed explanations include faster evaporation from the warm sample reducing its mass, convection currents that keep its surface cooler, differences in dissolved gases, differences in frost on the container and variation in how much a sample supercools before ice forms. None has been accepted as a general explanation for water.

In 2020 physicists at Simon Fraser University published a Nature study in which a tiny glass bead in a controlled energy landscape cooled faster from a hotter start, so the effect was shown in a simple model system and could even be engineered. That supports the idea that such behavior is possible in physics, but it does not settle the question for a cup of water.