Hot water can sometimes freeze faster than cold water, but it doesn’t always happen. This counterintuitive phenomenon is known as the Mpemba effect, and it has puzzled scientists for decades. Whether it occurs depends on specific conditions like the container shape, water volume, and the temperature gap between the water and the freezer. Under everyday circumstances, cold water will usually freeze first.
What the Mpemba Effect Is
In the early 1960s, a Tanzanian high school student named Erasto Mpemba noticed that a hot ice cream mixture froze faster than a cold one. His observation eventually made it into a published scientific paper, and the phenomenon took his name. But the idea itself is ancient. Aristotle noted that hot water seemed to freeze more quickly than cold, and the observation has surfaced repeatedly throughout history.
The Mpemba effect specifically refers to situations where water starting at a higher temperature reaches a frozen state before water starting at a lower temperature, with all other conditions kept the same. It sounds impossible because hotter water has more thermal energy to lose. Yet under the right circumstances, it happens, and scientists have proposed several overlapping explanations for why.
Evaporation Reduces What Needs to Freeze
The most straightforward explanation involves evaporation. Hot water evaporates faster than cold water, which does two useful things at once: it carries away heat (cooling the water more aggressively), and it reduces the total mass of water left in the container. Less water means less material that needs to reach freezing temperature and solidify. Calculations published by G.S. Kell in 1969 showed that if water cooled solely through evaporation and stayed at a uniform temperature throughout, warmer water would indeed freeze first.
There’s a catch, though. When Mpemba and physicist Denis Osborne ran their original experiment, they measured how much water was actually lost to evaporation. It was substantially less than Kell’s calculations predicted. So evaporation contributes to the effect, but it doesn’t fully explain it on its own.
Dissolved Gases Change How Water Freezes
Tap water contains dissolved gases like oxygen and carbon dioxide. When you heat water, those gases escape, the same reason a pot of water on the stove develops tiny bubbles long before it boils. This matters because dissolved gases affect how ice crystals form.
For water to freeze, ice crystals need starting points called nucleation sites. Dissolved gas molecules get pushed out of the forming ice structure because they’re too large to fit into the crystal lattice. As freezing progresses, gas concentrations build up at the boundary between ice and liquid, creating bubbles that disrupt heat flow and slow the process down. Water that has been heated and lost most of its dissolved gas freezes more cleanly. Heat flows through it more uniformly and stably, without gas pockets acting as insulation. Experiments have confirmed that the Mpemba effect shows up more reliably in water with reduced dissolved gas content.
How Hydrogen Bonds Store and Release Energy
Water molecules are connected by hydrogen bonds, which behave in unusual ways compared to bonds in most other liquids. When you heat water, some bonds stretch while others compress, like a pair of bungee cords being deformed in opposite directions. This stores energy in a way that’s unique to water’s molecular structure.
When that heated water then cools, the compressed and stretched bonds snap back, releasing their stored energy faster than bonds that were never deformed as much. Think of it like pulling back a rubber band further: the more you stretch it, the more forcefully it snaps. Researchers have described this as “hydrogen-bond anomalous relaxation,” and it means that water with a history of being heated can shed its thermal energy at a faster rate during cooling than water that started cold and never had those bonds as tightly wound.
Why Scientists Still Disagree
Despite these explanations, the Mpemba effect remains genuinely controversial. A 2016 study published in Scientific Reports attempted to reproduce the effect under carefully controlled laboratory conditions and found no evidence to support it. The researchers noted, “somewhat sadly,” that they could not observe any meaningful version of the phenomenon. Other experimenters have reported difficulty reproducing results consistently, with different cooling curves appearing even for samples that started at identical temperatures. Small, hard-to-control factors, sometimes called “micro-physical processes,” seem to cause wide variation between trials.
Part of the problem is defining what “freezes” means. Water doesn’t instantly become solid at 0°C. It typically supercools a few degrees below that point before ice crystals begin forming, and then it takes additional time for the entire volume to solidify. Researchers who measure the moment freezing begins sometimes get different results than those who measure when the water is completely solid. One study found no evidence of the Mpemba effect when looking at the time solidification started, but “hinted at a freezing time inversion” when measuring how long it took for the ice to grow to a certain thickness. Which definition you use can determine whether you see the effect at all.
Conditions That Make It More Likely
When the Mpemba effect does show up, it requires specific conditions. A 2022 study that called itself the first systematic investigation of the effect with water found that several factors must align: the initial temperature of the hot water, the temperature gap between the hot and cold samples, the shape of the container, the volume of water, and the temperature of the cooling environment all influence whether the effect appears. Change any one of these variables, and the result can flip.
This is why you can’t reliably reproduce it at home just by putting a tray of hot water next to a tray of cold water in your freezer. The container geometry, the placement in the freezer, air circulation, and the amount of water all matter. In most household freezer situations, the cold water will win. The Mpemba effect is real enough to be observed and studied, but narrow enough in its conditions that it’s not a dependable shortcut for making ice cubes faster.
What This Means for Making Ice at Home
If your goal is simply to get ice as fast as possible, cold water is still the safer bet. The conditions required for the Mpemba effect to kick in are too specific and unpredictable for a home freezer. However, starting with water that has been boiled and then cooled to room temperature may give you a small advantage. Boiled water has fewer dissolved gases, which can lead to clearer ice cubes and slightly more efficient freezing. The practical difference in time is small, but if you’ve ever wondered why some bartenders and cocktail enthusiasts boil their water before freezing it, reduced gas content is the reason. It produces denser, more transparent ice that also melts more slowly in a drink.