Density
The
concentration
of matter in
an object is
known as
density.
Computing Density
RT =
Pg. 1
Density = mass (g)
volume (cm3)
DETERMINE MASS:
Use a triple beam balance to
determine the mass of the object.
DETERMINE VOLUME:
One way to determine volume is to use
a graduated cylinder to determine
the displacement of water
achieved by placing the object in a
predetermined volume of water.
Computing Density
DETERMINE VOLUME:
Another way to
determine volume is
to measure the
height, length and
width of a solid that
is cubic or
rectangular in design.
The formula shown
will determine the
volume of the object.
Sample Density problem #1
A student determines
the mass of a rock
sample to be 156.3 g.
The volume of the
same rock sample is
51.3 cm3. What is the
density of the rock
sample?
Density = mass (g)
volume (cm3)
Density = 156.3 g
51.3 cm3
Density = 3.0 g/cm3
Sample Density problem #2
Given the information
provided, determine
the density of the toy?
Mass of Toy
Dinosaur
= 10 g
Volume of
Toy
Dinosaur
=?
Density = mass (g)
volume (cm3)
Density = 10 g = 12.5 g/cm3
0.8 cm3
Determining Relative Density
The densities of
objects in gases
and fluids can be
determined by
observing the
“flotation” of
the object in the
fluid or gas
Density of
helium gas =
0.2 g/cm3
Density of
air= 1.2 g/cm3
Objects with lower densities than the
fluid or gas they are in will float above
objects with higher densities.
Determining Relative Density
Liquid water has a
density of 1.0 g/mL.
Determine the
relative densities of
the objects in the
glasses of water.
The object has
a relative
density less
than water
because it
floats.
The object has
a relative
density more
than water
because it
sinks.
Physical Changes That Affect Density
Changes in temperature and pressure can change the densities of substances ….
especially gases.
If temperature of a gas
increases while pressure is
constant…
-- molecules move faster
-- molecules will move farther
apart (expansion)
-- less mass per unit volume
DENSITY WILL DECREASE
Example: Air
that is heated
is less dense
than air that is
cooled.
Therefore,
heated air will
rise above
cooled air.
Physical Changes That Affect Density
Changes in temperature and pressure can change the densities of substances ….
especially gases.
If temperature of a gas
decreases while pressure is
constant…
-- molecules move slower
-- molecules will move closer
together (compression)
-- larger mass per unit volume
DENSITY WILL INCREASE
Example: Air
that is cooled
is more dense
than air that is
heated.
Therefore,
cooled air will
sink below
heated air.
Physical Changes That Affect Density
Changes in temperature and pressure can change the densities of substances ….
especially gases.
If the pressure on a gas
increases while the
temperature is constant…
-- molecules will move closer
together (compression)
-- larger mass per unit volume
DENSITY WILL INCREASE
Pressure
Pressure
Phases of Matter and Density
Matter on Earth exists in
three phases: solid,
liquid, gas.
Liquid Iron =
7.2 g/cm3
Liquid Water
= 1.0 g/cm3
Solid Iron =
7.8 g/cm3
Ice = 0.97
g/cm3
Most substances will
increases in density as
the substance changes
phase from gas to liquid
and from liquid to solid.
Water’s highest density
exists in its liquid phase
(3.98 C).
Density vs. Object Size and Shape
The density of an
object will NOT
change should
an object
change its size
and/or shape.
Quartz crystal:
Mass = 10.9 g
Density of quartz
2.6 g/cm3
Quartz globe:
Mass = 88..9 g