Density
• Is a physical property of matter
• Every pure substance has its own
density
• Is important for the identification of a
sample of matter
• Is an intensive property of matter
Density
• Density is defined as mass per unit volume.
• The relationship can be written
mathematically as:
mass
density =
volume
units:
solids [ g / cm3 ]
liquids[ g / mL]
gases [ g / L]
OR
m
d =
v
Comparison of Densities
Each cube represents a volume of 1.00 cm3. The
mass of each sample increases while the volume
remains constant.
Example:
• The sample of matter you have has a
mass of 3.51 g and occupies a volume of
1.30 cm3.
m
3.51 g
3
d=
=

2.70g
/
cm
v
1.30 cm 3
EXAMPLE:
You need to add 45.0 g of alcohol to a sample. It is difficult to
weigh an exact mass of a liquid. It is much easier to measure an
exact volume. Using the density of the alcohol (0.789 g/cm3), you
can determine what volume you would need to add to your sample so
that you would be adding only the 45.0 grams.
m
45.0g
v=

 57.034mL
d 0.789g / mL
= 57.0 mL in the correct number of sig figs
Determining the Volume
• In order to figure the density we need
to determine the volume:
• For regularly shaped objects we use
direct measurement:
Measuring Volume by
Displacement
• Used for irregular
shaped objects
• Select a graduated
cylinder large enough
to put the object in
• Add water and
accurately record the
initial volume.
• SLOWLY add the object
to be measured:
rubber stopper
rock
metal cylinder
• Read the new level of the
water
• The water will rise an
amount equal to the
volume of the object
What not to do
Here’s what the cylinder should
look like
Do the math
1 . Original reading of volume
2. Final volume of water
3. This will equal the volume of the object
•
•
22.5 mL - 17.4 mL = 5.1 mL
Therefore, the volume of the object by
displacement is 5.1 mL
EXAMPLE:
An irregularly shaped piece of quartz was place in a
graduated cylinder containing 20.45 mL of water. The
volume of the ater rose to 24.35 mL. What is the
volume of the water?
Final volume
Initial volume
= volume of solid
24.35 mL
- 20.45 mL
3.90 mL
Classroom Exercises:
Blue Block
• Mass = 79.4 grams
• Volume=29.8 cubic cm.
• D = 79.4 g ÷ 29.8 cm3 = 2.66443
= 2.66 g/ cm3
Green Block
• Mass= 25.4 grams
• Volume=29.8 cubic cm
• D = 25.4 g ÷ 29.8 cm3 = .85235
= .852 g/ cm3
Which block is more dense?
Cylinder
•
•
•
•
Radius = 3.5cm
Length = 7.25 cm
Mass = 32.3 g
What is the volume?
V = π X radius2 X height
π = 3.14
What is the density?
Sphere
• Diameter = 7.0cm
• Radius = 3.5cm
• Mass = 32.3 g
• What is the volume?
V = 4/3 π X radius3
π = 3.14
What is the density?
Density
• Can also be used as a means of separating
components of matter
• Ex: if 2 solids of a different density are
mixed together, the solid with the greater
density will gravitate to the bottom
• When a solid object is placed in a liquid that
has a great density, the solid object will float.
• We will test this hypothesis in lab.
Density Gradient
THE DENSITY CONCEPT
• The picture illustrates
a density gradient.
• Each item (solid or
liquid) is arranged in
the cylinder according
to its density.
• The samples are
arranged with the
greater densities
toward the bottom
and the lowest density
on the top.
The Concept of Density for Liquids
and Solids
Which kind of matter is more
dense?
Question for you- Why does ice float?
• Solid H2O (ice) is less dense than liquid
H2O (water).
• Rivers never
become completely
frozen because as
soon as water
freezes, it floats
up to the surface.
• Fish live in the
unfrozen, denser
liquid water below
the layer of ice.
• Comparison of:
Liquid water
Mass = 100 g
Volume = 100 mL
Density = 1.0 g/mL
Ice
Mass = 100 g
Volume = 110 mL
Density = 0.92 g/mL