What is Density?
Density is defined as mass
per unit volume.
Density = Mass
Volume
Density describes how matter
is packed into space.
Density is a derived unit. It is
calculated from other
measurements.
Density is a physical constant.
• A physical constant  a physical
property that can be described
with a number and a unit.
• A physical constant is the same for
all samples of a substance
regardless of sample size or shape.
1 drop of pure water
has the same density
as 1 liter of pure
water.
Density does not
change with
sample size.
Physical Constants
properties such as
density to identify substances.
• We use physical
• Some other physical
constants include melting
point, boiling point, heat
capacity, heat of
vaporization, and heat of
fusion.
All samples of
pure gold have
the same density.
What are the units of density?
•The most common units of density
are grams per cubic centimeter
(g/cm3) or grams per milliliter (g/mL).
•By the way, 1 cm3 = 1 mL.
Density and Phase
• For most substances, the solid is
the densest phase and the gas is
the least dense.
SLG
• Water is Weird! Water is at its
densest at 4.0C, when it’s a liquid.
LSG
source
• The density of water is 1.0
g/mL (At 4.0°C).
• The relative density of any
substance can be
determined by adding it to
water.
0.80 g/mL
• If the substance floats, it’s
density is less than the
1.0 g/mL
density of water:
d < 1.0 g/mL
1.2 g/mL
• If the substance sinks, it’s
density is greater than the
density of water:
d > 1.0 g/mL
Density Column
Density of Gases
• Balloonx.mov
• Balloony.mov
• Flow of CO2
What is the density of …?
Substance
water
Density
(g/cm3)
1.00
Substance
seawater
Density
(g/cm3)
1.025
ice
.92
aluminum
2.70
gasoline
.70
oil
0.90
lead
11.30
gold
19.30
white pine .50
rubber
1.10
cork
.24
bone
1.80
iron
7.90
human fat
.92
mercury
13.60
helium
.000178
What is the density of some
selected elements?
source
Can you explain this?
Picture 1
Insert V. 2, CCA
What variables affect density?
• Temperature, T
• Pressure, P
Liquid or solid: Even large changes
in P & T  small changes in
density.
Gas: Even small changes in P & T
 large change in density.
How does Temperature affect Density?
• Most materials expand when they are
heated.
• Mass stays the same (it’s constant), but
volume increases. So the matter isn’t
packed as tightly.
• Density must decrease. (Hot air
balloons!)
When the
material
expands, the
density
decreases.
Temperatue & Density
As T , liquid
becomes less
dense. Spheres
sink.
As T , liquid
becomes more
dense. Spheres
rise.
Source:
http://www.cogifts.com/products/execgifts/galileo.jpg
How does Pressure affect Density?
• If you  the pressure on an object,
the volume .
• If the volume , the matter must
be packed more tightly & density
must .
When you
apply pressure
to an object,
the density
increases!
Remember: Water is weird!
Ice Bomb
Insert V. 2, CCA
Density Problems
• Algebra-type problems.
• The density equation has 3
variables.
• In any given problem, you are given
information about 2 of them and
you solve for the 3rd.
• You might have to rearrange the
equation.
Problem-solving Rubric
• Given: Identify the given info.
• Unknown: Identify the unknown.
• Equation: Identify the correct
equation and rearrange it as
necessary.
• Substitute: substitute numerical
values in for the letters.
• Solve: Do the math!
You can’t ignore
reciprocals!
1 can’t magically become V.
V
M
D=
V
or
V= M
D
M= DXV
Algebra Helper
• You might remember the density
triangle from earth science:
M
D
V
Density Problems
• An aluminum block 1.25 cm X
1.25 cm X 1.25 cm has a mass of
5.27 g. What is its density?
Find the volume!
2.70 g/cm3
Density Problems
• Mercury has a density of 13.60
g/ml. What is the mass of 25 ml of
mercury?
340 g
Density Problems
• Silver has a density of 10.5 g/cm3.
What is the volume of 210 g?
20
3
cm
Density Measurements
• Mass – Use a balance.
• Volume of regularly-shaped objects:
• direct measurement of length,
width, & height.
• Volume of irregularly-shaped
objects: use water displacement.
Mass is
measured
on a
balance.
Water displacement
is used to find the
volume of oddly
shaped objects.
Direct measurement
is used to find the
volume of simple
objects.