Name: Emily
Status: student
Grade: 6-8
Location: ME
Country: USA
Question:
When water freezes, why do its molecules move apart?
Replies:
Hi Emily,
The main reason for this is that the force that holds molecules in the liquid
state is different from the forces that hold molecules together in the solid
state, and it so happens that in the case of water, the force holding the
water molecules together in the liquid state is actually stronger than that
in the solid state.
In the liquid state water is held together by a force called hydrogen bonding -
this is an unusually strong attractive force between molecules and particularly
strong in water. Hydrogen bonding happens when one of the hydrogens of the
water is transferred to another water molecule -which in turn has one of its
hydrogen atoms transfer to a different water molecules . . . and so on. This
very fast transfers act to bridge the water molecules together so that they
get very close to each other.
In the solid state, water is held together by the fact that as heat is given
off (as water freezes), the water molecules have to "fit" together - like a
jigsaw puzzle, and only certain orientations are low enough in energy or stable.
This fitting together limits the way the water molecules can get close to each
other and as it turned out, is not as close as when water does hydrogen bonding.
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