The Mpemba Effect: Why Hot Water Can Sometimes Freeze Faster Than Cold — and Why Scientists Still Argue About It

Imagine filling two identical containers with water. One contains cold water, while the other contains water that has just been heated. Logic suggests the colder sample should freeze first.

After all, the hot water has farther to go before reaching the freezing point.

Yet under certain conditions, experiments have shown something surprising: the hot water freezes before the cold water.

This puzzling observation is known as the Mpemba Effect, one of the most famous unsolved questions in physics. Scientists have studied it for decades, proposed numerous explanations, and conducted countless experiments. Even today, there is no universal agreement about exactly why it happens—or whether it always happens at all. DOI

The mystery is particularly fascinating because it involves water, perhaps the most familiar substance on Earth. Despite being everywhere, water continues to surprise researchers with behaviors that seem to break intuition.

A Mystery Older Than Modern Science

The idea that hot water can freeze faster than cold water is not new.

Ancient Greek philosopher Aristotle wrote about observations resembling the phenomenon more than 2,000 years ago. Similar claims later appeared in the writings of Francis Bacon and René Descartes. Yet nobody could explain it convincingly. Royal Society of Chemistry

The mystery resurfaced dramatically in the 1960s thanks to a Tanzanian student named Erasto Mpemba.

While making ice cream at school, Mpemba noticed that a hot mixture placed in the freezer sometimes solidified faster than a cooler mixture. When he questioned teachers, he was initially dismissed. Later, however, physicist Dennis Osborne investigated the claim, and together they published a scientific paper describing the phenomenon.

The effect eventually took Mpemba’s name. Royal Society of Chemistry

What Exactly Is the Mpemba Effect?

At first glance, the claim seems impossible.

Water freezes at 0°C (32°F). If one sample begins at 80°C and another at 30°C, the hotter sample must pass through every temperature the colder sample experiences before reaching the freezing point.

So how could it ever get there first?

That question is exactly why scientists remain intrigued.

The key detail is that freezing is more complicated than simply reaching 0°C. Many factors influence how quickly water loses heat and transforms into ice, including evaporation, dissolved gases, convection currents, container shape, impurities, and a phenomenon known as supercooling. DOI

Under certain combinations of these factors, hot water can occasionally gain an unexpected advantage.

Evaporation: Losing Water, Losing Time

One of the oldest explanations involves evaporation.

Hot water evaporates faster than cold water. As water molecules escape into the air, the remaining liquid loses both mass and heat.

If enough water evaporates, the hot sample may end up containing less water than the cold sample. With less liquid remaining, there is less material that must freeze. DOI

Imagine racing two runners to a finish line. If one runner suddenly finds the track shortened halfway through the race, the outcome may change.

Evaporation certainly contributes in some experiments, but researchers generally agree it cannot explain every reported case of the Mpemba Effect.

The Role of Convection Currents

Another explanation involves the way heat moves through water.

Hot water develops stronger convection currents. Warm water rises while cooler water sinks, creating circulation patterns that distribute heat differently throughout the container.

These currents can increase the rate at which heat escapes into the surrounding environment. In some freezer setups, stronger convection may help hot water cool more efficiently than expected. DOI

A 2026 experimental investigation found that variations in air convection around cooling samples may be one of the major contributors to observed Mpemba-like behavior. DOI

The Strange World of Supercooling

Perhaps the most intriguing explanation involves supercooling.

Most people assume water freezes immediately when it reaches 0°C. In reality, very pure water can remain liquid well below that temperature.

Scientists call this supercooling.

A container of water might cool to -5°C or even lower while still remaining liquid. Then, suddenly, ice crystals form and freezing begins rapidly. arXiv

Researchers have found that heating water can sometimes alter the impurities, dissolved gases, or microscopic structures that influence supercooling.

In some experiments, previously heated water begins freezing at a higher temperature than colder water, giving it a surprising advantage despite starting warmer. arXiv

This explanation remains one of the strongest candidates for at least some versions of the Mpemba Effect.

Dissolved Gases and Impurities

Water is rarely pure.

It contains dissolved oxygen, carbon dioxide, minerals, and countless microscopic particles.

Heating changes that chemistry.

As water warms, dissolved gases escape. Minerals may precipitate out. Tiny bubbles form and disappear. These changes can affect how ice crystals nucleate and grow. DOI

Some researchers believe these microscopic differences help explain why heated water sometimes behaves differently when returned to cold conditions.

The challenge is that no two experiments are perfectly identical. Even small variations can influence results.

Why Scientists Still Disagree

Here’s where the story becomes fascinating.

Some experiments have successfully observed the Mpemba Effect.

Others have failed to reproduce it.

Still others conclude that the effect only appears under highly specific conditions. DOI

A 2016 study published in Scientific Reports argued that carefully controlled experiments showed no meaningful evidence that hot water cools faster than cold water under identical conditions. The researchers concluded that many claims arise from inconsistent definitions and experimental setups. DOI

Meanwhile, other studies continue to report genuine observations of the effect, suggesting that certain combinations of supercooling, evaporation, and heat transfer may indeed allow hot water to freeze first. DOI

The disagreement isn’t necessarily about whether observations occur.

It’s about understanding exactly why.

Water: The Most Familiar Strange Substance

The Mpemba Effect becomes even more interesting when viewed alongside water’s many unusual properties.

Scientists often describe water as one of the strangest common substances in nature.

Without its peculiar behavior, life on Earth might not exist.

Water Has More Than 70 Known Anomalies

Most liquids behave predictably as temperatures change.

Water often does not.

Researchers have documented dozens of physical properties that differ dramatically from what would be expected based on comparisons with similar molecules. Royal Society of Chemistry

These anomalies arise largely from hydrogen bonding, the powerful attraction between neighboring water molecules.

Those bonds create an ever-changing network that gives water many remarkable characteristics.

Ice Floats — And That’s Weird

One of water’s most famous anomalies is something we rarely think about.

Ice floats.

Most substances become denser when they freeze. Solids typically sink in their own liquids.

Water does the opposite.

When water freezes, molecules arrange themselves into an open crystalline structure that occupies more space than the liquid form. As a result, ice is less dense and floats on top. Royal Society of Chemistry

If ice sank, lakes and oceans would freeze from the bottom upward. Many aquatic ecosystems would collapse.

Life on Earth owes a great deal to this anomaly.

Water Is Densest at 4°C

Another strange property occurs before freezing.

As liquid water cools from room temperature, it becomes denser.

But at approximately 4°C, something unusual happens.

Further cooling causes water to expand again rather than contract. This creates a maximum density point at around 4°C. Royal Society of Chemistry

This anomaly helps lakes develop stable temperature layers and contributes to the seasonal turnover that distributes oxygen and nutrients.

Water Stores Huge Amounts of Heat

Compared with many substances of similar size, water has an exceptionally high heat capacity.

It can absorb large amounts of energy without changing temperature dramatically. Royal Society of Chemistry

This property stabilizes Earth’s climate.

Oceans act as giant thermal reservoirs, reducing extreme temperature swings between seasons and helping make the planet habitable.

Water Has an Unusually High Boiling Point

Water molecules are tiny.

Based solely on molecular weight, scientists would expect water to boil at a much lower temperature.

Instead, hydrogen bonding holds molecules together strongly, raising the boiling point to 100°C under normal atmospheric pressure. Royal Society of Chemistry

Without this anomaly, oceans would evaporate far more easily and Earth’s climate would look very different.

Surface Tension Makes Water Behave Like a Skin

Water’s surface tension is extraordinarily high compared with many liquids.

This is why droplets form beads instead of spreading immediately.

It’s also why some insects can walk on water and why capillary action helps plants transport water from roots to leaves. Royal Society of Chemistry

Again, hydrogen bonding is responsible.

Scientists Are Still Discovering New Things About Water

Despite centuries of study, water continues to surprise researchers.

Modern investigations explore how water behaves under extreme pressures, inside living cells, and in nanoscale environments.

Some scientists even suggest that certain anomalies may arise because liquid water can exist in multiple structural forms simultaneously, shifting between different molecular arrangements as conditions change.

Many questions remain open.

And that is remarkable for a substance covering roughly 71% of Earth’s surface.

What the Mpemba Effect Teaches Us About Science

The Mpemba Effect is more than a quirky freezer trick.

It highlights how science actually works.

A phenomenon can be observed repeatedly, yet remain poorly understood. Researchers can gather evidence, develop theories, challenge assumptions, and still debate the answer decades later.

The story also reminds us that valuable observations can come from anyone.

Erasto Mpemba was a student who noticed something unexpected and refused to ignore it. His curiosity transformed a classroom observation into a scientific mystery that continues to generate research today. Royal Society of Chemistry

Conclusion

The Mpemba Effect remains one of physics’ most intriguing puzzles. Under some circumstances, hot water appears capable of freezing before colder water, yet scientists continue debating exactly why it occurs. Evidence points toward a complex combination of evaporation, convection, dissolved gases, and supercooling rather than a single universal explanation. DOI

The mystery also highlights a broader truth about water itself.

Far from being ordinary, water is one of the strangest substances known to science. Its unusual density, heat capacity, surface tension, and freezing behavior help make life on Earth possible.

For something we drink every day, water still holds plenty of secrets. And somewhere in those secrets may lie the final answer to why, sometimes, hot water really does seem to beat cold water to the ice tray.