Why boiling water freezes faster

Theory: Convection currents in the warm water caused by large temperature differentials will cause it to cool more rapidly, and those convection currents continue even after the water has dropped to the same temperature as the cooler water, thus allowing it to overtake the cooler water in freezing.

Theory: Hot water evaporates. Less water left behind means less water to freeze. Theory: Hot water creates convection patterns in the air inside a freezer, which increases its cooling efficiency. Theory: Cold water freezes in a layer on top, creating insulation and preventing the rest from cooling very fast. The experimental problems are large because there are so many variable to control—aside form starting temperatures, there's also the shape of the freezer, the volume and shape of the container, the insulative properties of the container, the dissolved solids in the water, etc.

The new paper claims that there's actually a chemical explanation for the effect, and one that mathematically fits observed data—as far as I know, the first explanation to be able to do so. Water molecules consist of two hydrogen molecules attached to an oxygen molecule primarily through strong covalent bonds.

Normally, covalent bonds will soften and lengthen as they are heated. But in water, because of the unique properties of hydrogen bonds—the interaction between the hydrogen atoms in one water molecule and the oxygen molecule in a neighboring molecule—the opposite effect happens.

As a body of water absorbs energy, the hydrogen bonds will stretch causing individual water molecules to move apart from each other , but the covalent bonds within each molecule become shorter and stiffer—the same thing that happens when water freezes. So on an individual molecular level, heated water more closely resembles frozen water than the initial colder water did. More importantly, the rate at which the energy in these shrunken covalent bonds is released dependent exponentially on how much energy was initially stored.

A non-uniform temperature distribution in the water may also explain the Mpemba effect. Hot water rises to the top of a container before it escapes, displacing the cold water beneath it and creating a "hot top. These currents are a popular form of heat transfer in liquids and gases, occurring in the ocean and also in radiators that warm a chilly room. With the cooler water at the bottom, this uneven temperature distribution creates convection currents that accelerate the cooling process.

Even with more ground to cover to freeze, the temperature of the hotter water can drop at a faster rate than the cooler water. One of his jobs was to make the ice cream. He noticed that if he boiled the milk, the hot boiled milk froze before cold milk. No one believed him. Mpemba and Osborne published a set of experiments that are deceptively simple, conducted by high school students, where they boiled tap water, and then let it sit to cool to a given temperature, before putting the hot water into the freezer, then observed the time to the onset of freezing first solidification.

They timed how long it took. The fastest was with initial temperature 95degC and the hottest temperature tested. The slowest had initial temperature High school students for the last fifty years have tried these experiments.

Debate has raged about the Mpemba Effect, as there is no agreed explanation. In , the Royal Society of Chemistry held an international competition, with over entrants, for the best "solution". Please update your billing details here to continue enjoying your subscription. Your subscription will end shortly. Please update your billing details here to continue enjoying your access to the most informative and considered journalism in the UK.

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