8/20/2013
What is Density?
Density
Why Do Some Things Float While
Others Sink?
Another Way of Looking at It. . .
• Density is often referred to as “mass per unit
volume”
• Mass is how much matter the object has
• Unit volume is how much space it takes up
• Mass units are
– Grams for very small objects
– Kilograms for larger objects
• Density is how heavy something is for its
size
• Density is mass divided by volume or
Density = Mass
=
Volume
D = Density m = mass
• Volume units include
– Cubic centimeters for small objects
– Cubic meters for larger objects
– Milliliters for small amounts of liquids
– Liters for larger amounts.
• The most common unit is grams per cubic
centimeter
g/cm3
• Remember . . 1 cm3 = 1 mL
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 .
m
V
v = volume
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!!!
•
•
•
•
•
Densities of some materials:
Water
1 g/mL
Cork
0.24 g/mL
Lead
11.35 g/mL
Gasoline
0.67 g/mL
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Density of Water = 1 g/ml
Measuring density in the lab
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!!!!
• 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.
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.
• Concentration and density are directly related.
• This means that a plot of concentration vs.
density should produce a straight line!
Example #1
Example #2
• A block with dimensions 2.9cm x 3.5cm x 10.0cm has
a mass of 615.0grams. What is it’s density?
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?
• 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
•
Water Displacement gives us volume. 21.6mls –
12.7mls = 8.9mls
•
D = 33.42g/8.9mls = 3.8 g/ml
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Example #3
1. Diamonds have a density of 3.51grams/cm3.
1 carat is defined as 0.200grams of diamond.
If I have a 5.0 carat diamond, how much
volume should it occupy?
•
•
5.0 c
0.200g 1cm3 = 0.28 cm3
1c
3.51g
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