Density compares the masses of different
How do the masses compare with each other if you have
the same sized sample of each?
Scientists take a cubic centimeter
as their measure. This is roughly the size of a sugar cube.
Here you see some cubes with their densities.
You would need 3 cubic centimeters of
water to get the same mass as 1 cubic centimeter of rock.
You would need 8 cubic centimeters of
water to get the same mass as 1 cubic centimeter of iron.
Anything with a density of less than
1 (wood, styrofoam) will float in water.
Melted rock would float on melted iron.
This is why the core of the earth is made of iron, and why the
continents, made mostly of rock, are on the crust.
The earth as a planet has a density
of 5.52. This is because of the iron core. If the earth were
just made of rock, its density would be close to 3.
Density is related to volume. Whatever fits into a
cubic centimeter can be given a density. For example:
Egg whites in a bowl have a certain density.
If you beat the egg whites into fluffy peaks, the density of
the beaten egg whites per cubic centimeter will be less than
a cubic centimeter of the liquid egg whites in the bowl. The
mass of the actual egg whites does not change, however.
Sometimes rock coming out of a volcano is full of gas bubbles.
A volcanic rock called pumice has so many bubbles in it that
it will actually float on water. The density of a cubic centimeter
of pumice is less than the density of a cubic centimeter of bubble-free
When you put air into your tires, you are increasing the density
of the air in your tires to a density more than that of the air
When you let hair-setting mousse out of a spray can, you can
see the volume increasing. The density of the contents of the
spray can becomes less when they are released, but the mass of
the contents remains the same.
Out in space we see that stars can have the same masses but
At the ends of their lives, some stars shrink down into a sphere
about the size of a basket ball! The matter in them is very,
Planets do not all have the same densities. They can be made
of different materials: for example, the earth is made of rock
and minerals, but Jupiter and Saturn are made of gases such as
hydrogen and helium. Planets that are the same sizes may not
have the same densities. If two planets are the same size, the
denser planet will have a stronger gravitational pull. You would
weigh more on the denser planet.
1998,2003, 2004. Elizabeth Anne Viau. All rights
reserved. This material may be used by individuals for instructional
purposes but not sold. Please inform the author if you use it