The Mpemba Effect: Why Hot Water Freezes Faster Than Cold Water

Imagine putting two identical ice cube trays in the freezer—one filled with hot water, the other with cold water. Logic tells us the cold water should freeze first, right? After all, it has less distance to travel to reach the freezing point. But sometimes, surprisingly, the hot water wins the race to become ice.

This counterintuitive phenomenon is called the Mpemba effect, one of science's most debated mysteries. Named after a Tanzanian student who observed it in the 1960s, this effect challenges our basic understanding of how heat and cold work.

The Teenage Discovery That Stumped Scientists

The story begins in 1963 with Erasto Mpemba, a secondary school student in Tanzania. During a cooking class, Mpemba was making ice cream and noticed something odd: when he put hot ice cream mix in the freezer alongside cold mix from his classmates, his hot mixture froze first.

When Mpemba shared this observation with his teacher, he was told it was impossible. But Mpemba wasn't deterred. He continued experimenting and eventually presented his findings to Dr. Denis Osborne, a physics professor visiting his school.

"At first, I didn't believe it either," Dr. Osborne later admitted. "But when we tested it in the laboratory, we were amazed to find that Mpemba was right" (Osborne & Mpemba, 1969).

Their joint research paper, published in Physics Education, brought this phenomenon to the scientific world's attention. However, the effect had been observed long before Mpemba—even Aristotle mentioned it over 2,000 years ago!

The Science Behind the Surprise

So why does this happen? The truth is, scientists are still debating the exact mechanisms, but several compelling theories explain different aspects of the Mpemba effect:

1. Evaporation: The Disappearing Act

Hot water evaporates faster than cold water, meaning there's less water left to freeze. If you start with the same hot and cold water volume, the hot water container might end up with significantly less liquid after a few minutes in the freezer.

"Think of it like a race where one runner gets a head start by taking a shortcut," explains thermal physicist Dr. Sarah Chen. "The hot water effectively reduces the work it needs to do by losing some mass through evaporation."

Research published in the American Journal of Physics found that evaporation can account for the Mpemba effect in up to 40% of experimental cases (Katz, 2009).

2. Convection: The Mixing Effect

Hot water creates stronger convection currents—circular flows that mix the water more thoroughly. This enhanced mixing helps distribute heat more evenly and speed up the cooling process.

Cold water, denser and less active, tends to form temperature layers. The top might cool quickly while the bottom remains warmer, creating an insulating effect that slows overall freezing.

3. Dissolved Gases: The Bubble Factor

Hot water contains fewer dissolved gases than cold water. As water heats up, gases like oxygen and carbon dioxide escape. These dissolved gases can actually slow down the freezing process by interfering with ice crystal formation.

A 2016 study in Scientific Reports demonstrated that degassed water (water with gases removed) freezes faster than regular water, supporting this theory (Jin & Lu, 2016).

4. Container Effects: The Heat Transfer Highway

The containers themselves play a role. Hot water can improve thermal contact with the freezer surface by slightly melting frost buildup, creating better heat transfer. Additionally, the container material might expand slightly when heated, changing how heat flows out of the system.

5. Supercooling Differences

Cold water is likelier to supercool, dropping below its freezing point without forming ice crystals. This metastable state can delay freezing significantly. Hot water, having been "stirred up" by heating, is less likely to supercool and may begin crystallizing when it reaches 0°C (32°F).

The Hydrogen Bond Hypothesis

One of the newest and most intriguing theories involves the unique behavior of hydrogen bonds in water. In 2013, researchers proposed that hot water's hydrogen bonds are more stretched and store more energy than cold water ones.

"When hot water cools, these stretched bonds can release energy more rapidly, potentially accelerating the cooling process," explains molecular physicist Dr. Michael Zhang. This theory suggests the Mpemba effect might be fundamentally tied to water's unique molecular structure (Lu & Jin, 2013).

Not Always a Sure Thing

Here's the catch: the Mpemba effect doesn't happen every time. It depends on numerous factors, including:

• The initial temperatures of both water samples

• The volume of water

• The container material and shape

• The freezer temperature and air circulation

• Water purity and dissolved gas content

• Environmental humidity

This inconsistency is part of what makes the effect so controversial. Some scientists argue that the effect disappears entirely when all variables are carefully controlled.

Modern Research and Debate

The scientific community remains divided on the Mpemba effect. A 2016 study published in Nature attempted to definitively test the phenomenon under carefully controlled laboratory conditions and found no evidence for it (Burridge & Linden, 2016).

However, other researchers argue that overly controlled conditions might eliminate the factors that cause the effect in real-world situations. "The Mpemba effect might be precisely about the interplay of multiple variables that strict laboratory control removes," suggests thermodynamics researcher Dr. Lisa Martinez.

Real-World Applications

Understanding the Mpemba effect has practical implications beyond satisfying scientific curiosity:

• Food industry: Optimizing freezing processes for ice cream and frozen foods

• Climate science: Better understanding of how water bodies freeze in nature

• Engineering: Improving heat exchanger and cooling system designs

• Materials science: Insights for developing new cooling technologies

Try It Yourself (Safely!)

Want to test the Mpemba effect? Here's a simple experiment you can try at home:

Materials needed:

• Two identical containers

• Hot water (around 80°C/176°F)

• Cold water (around 20°C/68°F)

• Thermometer

• Freezer space

• Adult supervision for handling hot water

Method:

1. Fill both containers with equal volumes of hot and cold water

2. Record initial temperatures

3. Place both in the freezer simultaneously

4. Check every 15 minutes and record temperatures

5. Note which freezes completely first

Remember: Results may vary, and safety first when handling hot water!

The Bigger Picture

The Mpemba effect reminds us that even common substances like water can surprise us. It challenges our intuitive understanding of physics and shows that science still has mysteries to solve.

"Every time we think we understand something completely, nature has a way of showing us there's more to learn," reflects science educator Dr. Ahmed Hassan. "The Mpemba effect is a perfect example of how questioning assumptions can lead to new discoveries."

Whether you're a future scientist, engineer, or just someone curious about the world, the Mpemba effect teaches us an important lesson: sometimes the most interesting discoveries come from paying attention to seemingly impossible things.

Quick Facts

• The Mpemba effect has been observed for over 2,000 years

• It doesn't work with all liquids—only certain ones under specific conditions

• The effect is named after a student, not a professional scientist

• Some researchers have reported similar effects with cooling (hot objects cooling faster than warm ones)

• The phenomenon has inspired over 100 scientific papers and counting

References

Bregović, N. (2012). Mpemba effect from a viewpoint of an experimental physical chemist. Journal of Chemical Education, 89(2), 216-219.

Burridge, H. C., & Linden, P. F. (2016). Questioning the Mpemba effect: Hot water does not cool more quickly than cold. Scientific Reports, 6, 37665.

Jin, J., & Lu, Z. (2016). On the Mpemba effect: theoretical basis and experimental verification. Scientific Reports, 6, 21608.

Katz, J. I. (2009). When hot water freezes before cold. American Journal of Physics, 77(1), 27-29.

Lu, Z., & Jin, J. (2013). Hydrogen-bond memory and water-skin supersolidity resolving the Mpemba paradox. Physical Chemistry Chemical Physics, 15(45), 19759-19764.

Mpemba, E. B., & Osborne, D. G. (1969). Cool? Physics Education, 4(3), 172-175.

Vynnycky, M., & Mitchell, S. L. (2010). On the Mpemba effect and delayed solidification. Applied Mathematics and Computation, 217(8), 3993-4003.