The force of ice
Water is a vital part of our world. It can exist in three forms - solid
(ice), liquid (water) and gas (steam). The effects of forces in all
three forms are very evident in everyday situations. In this unit the
force of ice is examined.
Ice
When water freezes, a strange thing happens that has a major influence
on everyday life. When liquids freeze to a solid they usually contract,
that is they get smaller. In the case of water, the molecules arrange
themselves in such a way that ice at 0°C takes up MORE space than
the liquid at 0°C.
If a plastic container of water is frozen, the ice expands and the
plastic bulges outwards. If not enough space is provided, the container
will split under the force of the expanding ice. If the container is
glass, you know what would happen then! Yes, the force of the molecules
rearranging themselves within the ice will crack the glass - an enormous
force is exerted. A similar thing would happen with the water in your
car radiator or with water pipes which freeze with serious results.
In geology, water freezing in cracks in rocks expands the cracks
and can even split rocks. This is an important type of weathering in
highland areas where night and day temperatures vary above and below
0°C.
Some plants are also affected by the force of freezing water. The cells
contain water and, as the water freezes, it expands, breaking the cell
walls. That's why some plants, such as tomatoes, cannot survive when
the temperature goes below freezing. Other plants have more flexible
cell walls and these are said to be frost hardy.
Freeze drying
As has been discussed, water expands when it freezes and this can be
destructive. A process called freeze drying enables plants or animals
to be preserved without damage to the cell walls. In this process, the
plant or animal is cooled below freezing point, but in a low pressure
vacuum so that water molecules are removed from the cells before they
can form ice in the cells and do damage. Frozen vegetables are treated
in a similar way, as is instant coffee.
Icebergs
Because water expands when it freezes, ice is less dense than liquid
water, so it floats. (In fact the density of ice is about 0.9168 compared
to 0.9998 for water at the same temperature). A spectacular example
of this is an iceberg. They are often very heavy and can do damage to
ships if they are hit.
Ice and pressure
The melting point of ice is lowered under pressure. This has an important
application in ice skating. An ice skating blade has a very small surface
area so the weight of the skater exerts a high pressure. This means
that the ice melts under the blade, the skater glides on water, then
the ice refreezes behind the skater as the pressure is reduced.
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