it was filmed in winter,
the presenter took a mug of boiling water, threw the water into
the air where it froze immediately, the point was that if you did the
same thing with cold water it would not freeze, this phenomena is known as
the Mpemba
effect, Aristotle first noticed that
hot water freezes faster than cold water but chemists have always struggled to
explain the paradox, until now, Xi Zhang at the Nanyang Technological
University in Singapore says that the Mpemba paradox is the result of the unique
properties of the different bonds that hold water together,
so what’s so odd about the
bonds in water? a single water molecule consists of a relatively large oxygen
atom joined to two smaller hydrogen atoms by standard covalent bonds, but put
water molecules together and hydrogen bonds also begin to play an important role,
these occur when a hydrogen in one molecule comes close the oxygen in another
and bonds to it, chemists have long known that they are important,
for
example, water’s boiling point is much higher than other liquids of similar
molecules because hydrogen bonds hold it together, but in recent years,
chemists have become increasingly aware of more subtle roles that hydrogen
bonds can play, for example, water molecules inside narrow capillaries form
into chains held together by hydrogen bonds, this plays an important role in
trees and plants where water evaporation across a leaf membrane effectively
pulls a chain of water molecules up from the roots, so now I know
why boiling hot water freezes faster than cold water, (I think).
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