3. Hot water can sometimes freeze quicker than cold water – Observed by scientists as far back as 400 BCE, this uncanny phenomenon is called the Mpemba effect. Several theories attempt to explain it. Concentrations of solutes (which evaporate from hot water could play a part, or it could be that frost forming on cold water insulates it from further cooling. Convection is also a likely culprit. Inside the freezer, the water touching the container walls cools quicker than water in the centre.
This creates convection currents as warmer, less dense water rises. These currents are much stronger in heated water, where the temperature gradient is more extreme, helping it cool faster. More recent research has implicated that supercooling – the phenomenon in which water doesn’t always freeze at 0 degrees Celsius, but continues to cool by several degrees before ice appears – may play a role too. Some believe that initially cold water supercools more than hot water, although why that may be remains to be confirmed.
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