The Mpemba Effect

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The Mpemba Effect
The Mpemba effect is a rather unusual physical phenomenon where hot water freezes faster than cold water, under certain conditions. Intuitively, this doesn’t make any sense. How could water of higher temperature reach freezing temperature quicker than water of colder temperature? Water freezes when it’s molecules lose energy and slow down enough to stick together, forming ice. The molecules of water of high temperature have more energy the molecules in the colder water. So, provided the temperature of the freezer is uniform, it should take longer for the molecules in the higher temperature water to slow down enough to stick together. In certain circumstances, however, this is not the case. This effect, which was later named
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A paper published by Jonathan Katz in 2009 discusses the influence of solutes in the water. Katz looks at so called ‘hard’ water, water that contains solutes like calcium bicarbonate and magnesium bicarbonate [16]. When hard water is heated, these solutes precipitate out of the water. This is what causes boiler scale or “fur” on the inside of your kettles [17]. Hard water that is not heated still has these solutes obviously. As it freezes and ice crystals form, the remaining water can contain up to 50 times the normal amount of solute [18], since the ice contains no concentration of solute. This will lower the freezing point of the water: a useful analogy is salting the roads in winter to reduce the chance of ice forming. If the freezing point is lowered, it will take longer for the water to reach that temperature, and so the heated water, which the solutes have been removed, will freeze faster. The freezing points of hard water is lower than normal water anyways, due the freezing point depression of solutes [19]. In this scenario, we can ignore the effects of supercooling: hard water, or water with solutes will not be affected by supercooling enough that it would change the freezing rate. Katz’s theory suggests that an Mpemba Effect for hard water depends on solute concentrations whose solubility decreases with increasing temperature [20]. Using solutions of Sodium Chloride, one could easily test the viability of his idea, mixing salt with soft water and investigating if the Mpemba effect occurs. The evidence shows that people trying to investigate the Mpemba Effect could run into difficulty if they are using soft water, or water with low concentrations of solutes like calcium, magnesium and sodium. Katz’s work on the Mpemba effect is seen as one of the most complete and well documented studies on the

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