Why Hot Water Sometimes Freezes Faster Than Cold

Put a tray of hot water and a tray of cold water in your freezer. Common sense says the cold water should freeze first—it has less temperature to lose, after all. But sometimes, bizarrely, the hot water wins. This isn't a myth or a magic trick. It's called the Mpemba effect, and it's been puzzling scientists for decades.

A Tanzanian Student's Curious Discovery

The effect is named after Erasto Mpemba, a Tanzanian schoolboy who noticed something strange in 1963. While making ice cream in a cooking class, he observed that his hot mixture froze faster than a classmate's cold one. When he asked his teacher why, he was laughed at.

Years later, Mpemba posed the same question to a visiting physics professor, Dr. Denis Osborne. Rather than dismissing him, Osborne tested it—and confirmed the effect was real. They published their findings together in 1969.

Why Does This Happen?

Here's where things get complicated. Scientists have proposed multiple explanations, and the debate continues:

• Evaporation: Hot water evaporates faster, reducing the volume that needs to freeze
• Convection currents: Temperature differences in hot water create currents that speed up cooling
• Dissolved gases: Hot water holds less dissolved gas, which may affect freezing
• Supercooling: Cold water may supercool (drop below freezing without solidifying), while hot water freezes at the expected temperature
• Hydrogen bond behavior: A 2013 study suggested that hydrogen bonds in water store energy differently at higher temperatures

The frustrating truth? There's no single accepted answer. Different experiments under different conditions have supported different explanations.

The Catch: It Doesn't Always Work

The Mpemba effect isn't guaranteed. It depends heavily on conditions—the shape and material of the container, the initial temperatures, the freezer environment, and even the mineral content of the water. Some carefully controlled experiments have failed to reproduce it at all, leading some researchers to question whether it's a consistent phenomenon or a result of overlooked variables.

A 2016 study even suggested the effect might be explained entirely by random variation in when supercooled water finally freezes.

Ancient Observations

Mpemba wasn't actually the first to notice this. Aristotle wrote about it around 350 BCE, noting that water previously heated freezes faster. Francis Bacon and René Descartes also documented similar observations centuries later. The effect was simply forgotten by modern science until a curious student refused to accept dismissal.

Whether the Mpemba effect represents a genuine physical phenomenon or an artifact of experimental conditions, it remains a humbling reminder: even something as simple as freezing water can surprise us. Sometimes the "obvious" answer isn't the right one—and sometimes a schoolboy's question can stump the experts.