Mass vs volume
Thought question
 Which
is heavier a pound of lead
or a pound of feathers?
Most people say lead…
They are thinking about if you have the
same size piece of lead and feathers
obviously the lead will be heavier.
 However, a pound of lead is much
smaller in size than a pound of feathers
 Why?

The answer is density
Lead atoms are packed tightly together
 Feathers are not

Density is…
The ratio of the mass of an object to its
volume
 Density = mass/volume
 A 10.0 cm3 piece of lead has a mass of
114 g.
 How would you find density?
 Density of lead is 11.4 g/cm3

Density
As mass increases, density increases
 As volume increases, density decreases
 Depends only on substance not on size

A block of maple wood with a volume of 405
cubic centimeters and a density of 0.67
g/cm3 is sawed in half. The density of the two
smaller blocks is now -A one-fourth the original density
B one-half the original density
C two times the original density
D the same as the original density
Densities of common materials


Can you find any
themes?
Water is 1 g/cm3
Density Trends
Solids usually most dense
 Gases usually least dense
 Exception is water

 Ice is less dense than liquid water
 Water expands as it freezes and creates air
pockets
 Volume increases and mass remains
constant, therefore density decreases
Floating is related to density
A substance that is less dense will float
in a substance that is more dense
 Example…

 Helium density 0.166 g/L Air density 1.2 g/L
 Water density 1 g/cm3
 Oil density 0.922 g/cm3
 Ice density 0.917 g/cm3
 Steel density approx. 8 g/cm3
What about ships?

The reason it is so easy is that a good
portion of the interior of any boat is air
(unlike a cube of steel, which is solid
steel throughout). The average density
of a boat -- the combination of the
steel and the air -- is very light
compared to the average density of
water. So very little of the boat actually
has to submerge into the water before it
has displaced the weight of the boat.
Calculations
What is the density of a piece of wood
that has a mass of 25.0 grams and
a volume of 29.4 cm3?
 D=m/v
 m=25.0 g
 v=29.4 cm3
 25.0 / 29.4
 D = 0.850 g/cm3

A piece of wood that measures 3.0 cm
by 6.0 cm by 4.0 cm has a mass of 80.0
grams. What is the density of the
wood? Would the piece of wood float in
water?
 D = 1.1 g/cm3

The density of aluminum is 2.70 g/mL. If
the mass of a piece of aluminum is 244
grams, what is the volume of the
aluminum?
 Use the triangle
 Cover up what you are looking for
 It will tell you math to do
 V=m/D
 V = 90.4 mL
