Volume 95 Issue 1 | p. 8 | Concentrates
Issue Date: January 2, 2017

Why warm water freezes faster than cold water

Fewer hydrogen bonds in warm water make it easier to form ice lattice
Department: Science & Technology
Keywords: computational chemistry, Theoretical chemistry, Chemical bonding, Physical chemistry, water, Mpemba
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In warm water, weak hydrogen bonds break (top, red squiggles), leaving fragments that easily reorganize into an ice lattice (bottom), a new study says.
Credit: J. Chem. Theory Comput.
This reaction scheme shows how hydrogen bonding phenomena are involved in water freezing.
 
In warm water, weak hydrogen bonds break (top, red squiggles), leaving fragments that easily reorganize into an ice lattice (bottom), a new study says.
Credit: J. Chem. Theory Comput.

Nearly 50 years ago, Erasto B. Mpemba and Denis G. Osborne reported that if samples of water at 90 °C and 25 °C are cooled, the one starting at 90 °C begins freezing first. Many explanations for the “Mpemba effect” have been proposed, including ones based on evaporation, temperature gradients, impurities, and dissolved gases. A new computational study suggests that the effect arises from the liquid’s hydrogen bond network (J. Chem. Theory Comput. 2016, DOI: 10.1021/acs.jctc.6b00735). Southern Methodist University’s Dieter Cremer and colleagues investigated clusters of 50 and 1,000 water molecules, characterizing the types and strengths of the clusters’ 350 and more than 1 million hydrogen bonds, respectively. In (H2O)1,000, raising the temperature from 10 °C to 90 °C led to fewer hydrogen bonds, as weaker, predominately electrostatic bonds broke. That left behind cluster fragments with strong hydrogen bonds with more covalent character and proportionately more “dangling” or terminal hydrogen bonds. That hydrogen bond combination enables the fragments to easily reorganize and form the hexagonal lattice of ice.

 
Chemical & Engineering News
ISSN 0009-2347
Copyright © American Chemical Society
Comments
Joseph Dumais (Fri Jan 27 17:29:23 EST 2017)
An interesting and fairly in depth discussion of this can be found in the following link. The title to this C&E News article seems to be an overstatement.

https://www.scientificamerican.com/article/is-it-true-that-hot-water/

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