Density
Why Do Some Things Float While
Others Sink?
What is Density?
• Density is how heavy something is for its
size
• Density is mass divided by volume or
Density = Mass
=
Volume
Units:
m
V
grams/cm3 , grams/mL or kg/L
Mass vs. Volume Graph
•Mass and volume have a
direct relationship
•This means it will
produce a straight line
graph with a positive
slope
•As mass increases, so
does the volume.
•The Slope of the line is
the density and it remains
constant .
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Density is a Physical Property
•
It can be used to identify unknown
pure substances.
It also allows us to convert from mass
to volume or vice versa. It is a
conversion factor!!!
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Densities of some materials:
Water
1.0 g/mL
Cork
0.24 g/mL
Lead
11.35 g/mL
Gasoline
0.67 g/mL
Density of Water = 1 g/ml
MEMORIZE THIS!!!
• More dense materials sink in water
• Less dense materials will float or rise in
water.
• Liquids form layers if they have different
densities. Like oil and water.
– Water is more dense than oil, so it settles on
the bottom, and oil on top.
• Density varies with temperature
– Usually density decreases with an increase in
temperature.
– Ice is the exception!!!!
Density of Solutions
• The density of water will increase when it has
something dissolved in it.
• The concentration of the solution will
determine the density.
• The unknown concentration of a solution can
be predicted by its density.
Measuring density in the lab
• Take the mass on the balance
• Measure volume (2 ways to do this)
–Regular shaped objects (cubes,
cylinders) – measure the dimensions
and use volume formulas
• Cube Volume = (L)(W)(H)
• or Cylinder = πr2h
Volume Displacement
•For irregular shaped
objects
•The object will
displace an equal
volume of water to its
volume.
Example #1
• A block with dimensions 2.9cm x 3.5cm x 10.0cm has
a mass of 615.0grams. What is it’s density?
• Calculate volume of block using dimensions. Make
sure all given dimensions have the same unit!!!!
• V = (2.9cm)x(3.5cm)x(10.0cm) = 101.5 cm3
• D = 615.0g/101.5cm3 = 6.059 g/cm3
• **round at the end based on sig. Figs
• D = 6.1 g/cm3
Example #2
A metal rock has a mass of 33.42grams. It is placed
into a graduated cylinder filled with water and the
water level raise from 12.7mls to 21.6mls. What is
the density of the rock?
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Water Displacement gives us volume.
21.6mls – 12.7mls = 8.9mls
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D = 33.42g/8.9mls = 3.8 g/ml
Example #3
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Diamond has a density of 3.26 g/cm3. What
mass of diamond has a volume of 0.35 cm3?
Two ways to solve.
Using formula
D = m/v
3.26 g/cm3 = mass
0.35 cm3
Mass = (3.26g/cm3)(0.35cm3) = 1.141 g
Round to proper sig figs Mass = 1.1 g
Solving using Dimensional Analysis
We use density as a conversion factor.
0.35 cm3
3.26 g
1 cm3
=
1.1 g
Example #4
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Mercury has a density of 13.6 g/cm3. What
volume of mercury, in Liters, does a 76.2
gram sample of mercury occupy?
Formula D = m/v
13.6 g/cm3 = 76.2g
V
Cross multiply (13.6) V = 76.2
V = 76.2
= 5.60 cm3
13.6
5.6 cm3 1mL .001 L = 5.60 x 10-3 L
1cm3 1mL
Using Dimensional Analysis
• Again using Density as a conversion factor.
• We can solve this in one set up.
• 76.2 g
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1cm3
13.6g
1mL
1cm3
• = 5.60 x 10-3 Liters
0.001 L
1 mL