The Mpemba Effect: Why Hot Water Can Freeze Faster Than Cold

It sounds like it shouldn't be true. If cold water is already closer to freezing, shouldn't it always freeze first? Yet in certain conditions, hot water beats it. Let's understand the science behind it. 

What Exactly Is the Mpemba Effect?

Simply put, it is the observation that hot water can, under certain conditions, freeze faster than cold water (starting at the same volume, in the same environment). It seems to contradict basic intuition, which is exactly what makes it so fascinating.

Why Does It Happen?

One explanation involves evaporation. Hot water evaporates more as it cools, reducing the total mass of water left to freeze. Less water means less time to freeze, which gives hot water a head start.

Another explanation involves dissolved gases. Cold water contains more dissolved oxygen and other gases than hot water, because heating drives gases out. These dissolved gases may affect how efficiently water transfers heat, slightly slowing down freezing in cold water.

A third explanation involves convection currents. Hot water develops strong internal circulation as its surface cools faster than its interior. This keeps mixing the water and accelerating heat loss. Cold water, being more uniform in temperature, doesn't develop these currents as strongly.

Does It Always Happen?

No, and this is important. The Mpemba effect is not universal or guaranteed. It depends heavily on specific conditions like the container, the temperature difference, the freezer environment, and even the water's mineral content. In many controlled experiments, cold water freezes first, as you'd expect. The effect appears inconsistently, which is part of why it's been so hard to pin down scientifically.