It might seem logical that cold water—which is closer to the 0 degrees Celsius needed to turn water to ice—would freeze faster than hot water. However, it’s not always true.
The “Mpemba effect,” named after Tanzanian high school student Erasto Mpemba, was first observed in 1963. It occurs when two bodies of water with different temperatures are exposed to the same subzero surroundings and the hotter water freezes first.
According to Live Science, evaporation is the strongest candidate to explain the Mpemba effect. The idea is that the overall mass of hot water placed in an open container decreases as it cools. So, less water takes less time to freeze. However, Live Science notes, this doesn’t always work, especially when using closed containers that prevent evaporated water from escaping.
Another explanation for hot water freezing more quickly is there might be less dissolved gas in the warmer water, which can reduce its ability to conduct heat, allowing it to cool faster. However, according to Live Science, Polish physicists in the 1980s were unable to conclusively demonstrate this relationship.
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