s8pe-20503-ca
12/7/05
1:56 PM
MAZER
Page 151
Physical Properties of Substances
Some physical properties are specific to a substance. It makes no difference how much of the substance you have; the property does not
change. Density is one such physical property. Remember, density is
mass divided by volume. In the example on page 150, a 200 g piece of
clay with a volume of 100 cm3 was broken in half to make two pieces,
both with a mass of 100 g and a volume of 50 cm3. Each piece, however, has a density of 2 g/cm3. If you found another piece of clay with
a different density, it is probably not the same type of clay.
Another physical property of a substance is color. If you have a red
cloth and tear off a piece, it still remains a red cloth. The mass and the
volume of the cloth change. The fact that the cloth is red, however, has
not changed.
Hardness also does not depend on the amount of the substance
present. Diamonds are one of the hardest substances on Earth.
Denting or scratching a diamond is very difficult. A smaller diamond
is as difficult to scratch as a larger diamond. This is why diamonds are
often used in saws designed for precision cutting.
reading tip
The density of solids is
usually measured in grams
per cubic centimeter
(g/cm3). The density of
liquids is usually measured
in grams per milliliter
(g/mL). Recall that
1 mL 1 cm3.
Physical properties such as density and hardness can help you identify substances. Mass and volume cannot. Knowing an object’s density,
for instance, allows you to eliminate possibilities. If it is a very high density, you would know that the substance is probably not feathers or cloth.
Physical Changes
You have read that a physical property is any property that can be
observed without changing the identity of the substance. What then
would be a physical change? A physical change is a change in any
physical property of a substance, not in the substance itself. Breaking
a piece of clay in half is a physical change because it changes only
the size and shape of the clay. Stretching a rubber band is a physical
change because the size of the rubber band changes. The color of the
rubber band sometimes can change as well when it is stretched.
However, the material that the rubber band is made of does not
change. The rubber band is still rubber.
VOCABULARY
Add a description wheel
diagram for physical
change in your notebook.
What happens when water changes from a liquid into water vapor
or ice? Is this a physical change? Remember to ask yourself what has
changed about the material. Ice is a solid and water is a liquid, but both
are the same substance—both are composed of H2O molecules. As you
will read in more detail in the next chapter, a change in a substance’s
state of matter is a physical change.
Check Your Reading
How is a physical change related to a substance’s
physical properties?
Chapter 5: Properties of Matter 151
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