Chapter 18
Solids
Crystal Structure
• The
crystalline structure of many solids results from the
repeating orderly arrangement of molecules or atoms to form a
geometric shape
• How the atoms or molecules align themselves depends on the
type of molecule or atom
Na+
Cl-
Snowflake
H2O
Crystal Structures Made from Carbon:
Fullerine
Diamond
Carbon
Nanotube
Graphite
Density
Density – how tightly material is
packed together.
Density =
mass
volume
Density is not mass or volume
alone. It is a ratio between the
amount of mass per unit volume.
Water has a density of 1 gram/cm3
Specific Gravity
A standard measure of density is specific gravity which is the
ratio of the mass (or weight) of a substance to the mass (or
weight) of an equal volume of water.
Hydrometer – used for
measuring the specific
gravity of liquids
Practice Questions:
1. Which has the greater density – 1 kg of water or 10 kg of
water?
Neither. They both have the same density
2. Which has the greater density – 5 kg of lead or 10 kg of
aluminum?
Lead
3. Which has the greater density – 1 g of uranium or the
planet earth?
Uranium
4. The density of gold is 19.3 g/cm3. What is its specific
gravity?
19.3 g/cm3
Elasticity
Elasticity is the property of a body by which it experiences a
change in shape when a deforming force acts on it, and by
which it returns to its original shape when the deforming
force is removed.
Materials that do not resume their original shape after being
distorted are said to be inelastic.
By hanging a weight on a spring, we are applying a force
to the spring. It is found that the stretch is directly
proportional to the applied force:
This relationship between force and stretch of a spring is
called Hooke’s Law
F~x
Elastic Limit
Elastic Limit is the distance that a
material can be stretch at which
point it will not return to its original
shape
Practice Questions:
1. A certain tree brance is found to obey Hooke’s law. When
a 20-kg load is hung from the end of it, the branch sags a
distance of 10 cm. If instead, a 40-kg load is hung from
the same place, by how much will the branch sag? How
about if a 60-kg load were hung from the same place?
(Assume that none of these loads makes the branch sag
beyond its elastic limit.
20 cm for a 40 kg weight
30 cm for a 60 kg weight
2. If a force of 10 N stretches a certain spring 4 cm, how
much stretch will occur for an applied force of 15 N?
6 cm