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.
In warm water, weak hydrogen bonds
break (top,
red squiggles), leaving
fragments
that
easily reorganize into an
ice lattice
(bottom), a new study says.
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.
Apart from learning what the name of this effect is and why it occurs, you can now answer two unit 1 past paper questions with the knowledge that:
1. Hydrogen bonds are largely electrostatic in nature.
2. Each water molecule forms four hydrogen bonds (from diagram).
