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article imageWhich freezes faster: Hot water or cold water?

By Tim Sandle     Jan 9, 2017 in Science
A debate has been raging in the world of physics over whether the long-held theorem that cold water freezes faster than hot water remains correct, and that, instead, hot water freezes faster. We assess the issues.
The issue is actually whether hot water will freeze faster than cold water under certain conditions. For this to occur, a phenomenon called the Mpemba effect needs to be occurring. This is named after Erasto Mpemba, who noted during the 1960s that ice cream solidified faster when put into the freezer steaming hot. This has been defined by scientist Monwhea Jeng as:
“There exists a set of initial parameters, and a pair of temperatures, such that given two bodies of water identical in these parameters, and differing only in initial uniform temperatures, the hot one will freeze sooner.”
While the Mpemba effect appears to have been observed there has been disagreement among physicists as to what this means and why it happens. Theories for the Mpemba effect include:
Faster evaporation of hot water, which reduces the volume left to freeze,
Formation of a frost layer on cold water, insulating it,
Different concentrations of solutes such as carbon dioxide, which is driven off when the water is heated.
The video below shows one scientist's attempted to recreate the effect:
The new idea is that hot water can sometimes freeze faster than cold water due to the properties of the chemical bonds that link up neighboring water molecules.
A new research paper argues that that hydrogen bonds, which are the links between hydrogen atoms and the oxygen atoms of neighboring water molecules, are the reason why the effect can sometimes occur. This is based on hydrogen bonds changing in warming water. With cold water, both weak and strong bonds exist; however, at a higher temperature strong bonds predominate.
The theory continues with as water is heated weaker bonds break causing molecules to form into fragments that can realign to form the crystalline structure of ice. This becomes the starting point of the freezing process. For cold water to begin this process, the weaker bonds need to be broken first and this takes a longer period of time.
The new research presented is of interest but it has not convinced all scientists according to the website Science. This is either due to the new research not being convincing or because the Mpemba effect remains disputed. Those disputing the effect say they cannot replicate the study whereby hot water freezes faster than cold water.
The research is published in the Journal of Chemical Theory and Computation. The paper is titled “Different Ways of Hydrogen Bonding in Water - Why Does Warm Water Freeze Faster than Cold Water?”
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