It’s a controversial topic. Some scientists agree to this and, many don’t. Mpemba effect is the appearance of hot water freezing faster than cold water. This effect has been named after Erasto Mpemba, a teenager who discovered this effect while making ice cream for a school project.
His teacher had told him to heat cream and sugar and cool the mixture before freezing it. But he was behind schedule so he put the hot mixture in the freezer. When he asked his friends how much time their ice cream had taken to freeze, he was shocked!! Because his ice cream froze faster.
He repeated his experiment with water and found the same results. Mpemba asked why this was happening to Dr. Denis Osborne, a physicist who had visited his school. Osborne experimented with laboratory conditions and got similar results. In 1969, they published a paper on this effect together.
But since then, no experiment has yielded similar results. But scientists who believe that the effect is true to have proposed a few hypotheses.
Evaporation
When water is heated, water molecules are evaporated. This leads to a decrease in the volume of hot water. But cold water does not undergo any evaporation and, the volume remains the same. So when both the liquids come to the same temperature, the volume of previously cold water will be more than that of the previously hot water. Since more water will require more time to freeze, the cold water appears to freeze slower than the hot water.
Frost melting
Frost is a thermal insulator. It is observed that around a cold beaker, the frost does not melt, but around a hot one, it does. Thus, creating a conductive layer of liquid which increases its cooling rate.
Convection
Movement in a liquid in which the warmer parts move up and the colder parts move down is called convection. Since the temperature of hot water is more, the convection rate will be high, causing it to cool down faster. The convection rate does not decrease even after the temperature of the liquid has been reduced. Thus, causing the hot water to freeze faster.
A theory also predicts an inverse Mpemba effect, that colder things can heat up faster than the warmer ones.