This is a great question! To answer it, we first
need to think about phases of matter: Gas, liquid,
and solid. Matter is made up of particles called
atoms and molecules. If the particles are
around very fast and they are spread far apart the
matter is in the gas phase. If the
closer together and moving slowly around each
other the matter is in a liquid phase.
the particles are locked in position and not
hardly moving at all the matter is in a solid
Water is the name of the liquid phase of
H2O molecules, and ice is the
The second thing we need to think
about is density, which tells us how tightly
matter particles are packed together. Tightly
packed matter is more dense than loosely packed
matter. Also, less dense matter floats on top of
more dense matter. So when you place ice cubes in
your soft drink (which is mostly water) and your
ice cubes float, you know that ice is less dense
than water. This is a very special property of
water. Almost every other type of matter gets
dense when it changes phase from a solid to a
liquid, but water gets more dense when
from solid (ice) to liquid. Why?
ice is a crystal, which means it has a regular
pattern with spaces in between molecules. The
spaces in the crystal are larger than the spaces
between molecules in the liquid. More space = less
dense, so ice is less dense than water.
So now we
understand why ice floats on top of water, but how
does this work on a lake? Imagine that it is the
beginning of winter, and the temperature has just
gotten below freezing. Air changes temperature
faster than water -- this is why a swimming pool
seems much warmer in the evening. The air cools
down at night, but the pool water remains almost
as hot as it was during the day. So although the
air is freezing cold, the water isn't freezing
yet. The water at the very top of the lake is in
direct contact with the cold air, so it freezes
first. And because ice is less dense than water,
the sheet of ice doesn't sink. If the temperature
stays very cold, the sheet of ice gets thicker,
but the ice also acts as a barrier, or insulator,
between the cold air and the warmer water
underneath. The sheet of ice helps the rest of the
water from becoming ice!
The amazing fact
that water freezes on the surface of a lake but
not below it allows plants and fish to live
through the winter in northern lakes and ponds.
Imagine a lake without fish! So it is a good thing
that H20 has the very strange property
solid phase is less dense than its liquid phase.
Water is an unusual substance.When water
it gets less dense. That is why ice cubes float in
a glass of water. It also takes a lot of energy
removal to freeze water, and the air is usually
much colder than the earth at the bottom of the
lake. So the cold comes from the air above the
lake, and starts freezing the water from the
surface. As the ice is formed, it floats and stays
at the surface of the lake until a solid sheet of
ice is formed. This ice prevents the cold air from
reaching the water beneath it - the ice acts like
a blanket for the water in the lake and helps keep
it warmerthan it otherwise would be. So a lot of
things cause the lake to freeze only on top. Of
course, if it stayed cold enough for long enough,
the lake would freeze solid.
This is one of the interesting properties of
water. Ice is lighter than water, even though it
is a solid form of water, so ice floats. If the
lake were made of alcohol, rather than water, it
would freeze first at the bottom, since solid
alcohol is heavier than solid ice.
There are two reasons why.
(1) The air is the
source of the cold, and the earth below retains
warmth better. Thus, the surface is frozen, while
underneath it is heated (or kept from freezing)
(2)Ice is less dense than liquid
water, and when a solid crust of ice is on a lake,
the water underneath is under pressure. The
freezing point drops with increasing pressure
for a substance, the liquid of which is denser
than the solid.
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