Archimedes principle and melting sea
ice

We can use Archimedes principle to understand
why there's no change in water level when ice that is floating in
the water melts.

We'll imagine adding 20 grams of
ice to a glass of water. Archimedes law states that an
object
immersed in a fluid will experience an upward buoyant force equal
to
the weight of the fluid displaced. It's this upward force
that
causes the ice to float.

Ice has a density of about 0.9 g/cm^{3}, a little
bit less than the density of water (1 g/cm^{3}). The 20
grams of ice has a volume of about 22.2 cm^{3}. Once the
ice is
added to the water it will displace 20 cm^{3} of water (most of the ice is under water 20 cm^{3}or the 22.2 cm^{3}, only a little bit sticks
out above the water). This displaced water weighs 20 grams
and provides just the upward force needed to support 20 grams of
ice. Adding the ice also brought the water level right up to
the edge of the glass.

Once all the ice melts it will still have a mass of 20
grams. But it's volume will now be 20 cm^{3} instead of
22.2
cm^{3}. So the 20 cm^{3} of ice water will take
the space that was occupied by the 20 cm^{3}
of submerged ice. The displaced water is now supporting 20
grams
of ice water instead of 20 grams of ice. The ice cubes have
all
melted and the water level is the same as it was at the start of
the
demonstration.

This is a fairly long winded explanation. See if you can
boil it down to just a sentence or two.