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 can be described as the
concentration of mass in an area of
space
OR
is described as the mass of a given
substance per unit volume
• The more concentrated the mass is in an
object, the greater is its density.
Density
• Since 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.
Determining the Volume
• In order to figure the density we need
to determine the volume:
• For regularly shaped objects we use
direct measurement:
• Block I
• Mass = 79.4 grams
• Volume=29.8 cubic cm.
• D = 79.4 g ÷ 29.8 cm3 = 2.66443
= 2.66 g/ cm3
• Block II
• 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 1
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?
Determining Volume for an
Irregular object
• Done by displacement
• We did this in the Scientific
Measurement lab earlier in the quarter.
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.
• Example: Calculate the volume of a 100. g ice
cube with a density of 0.92 g /mL.
• Solution: The density translated as a
conversion factor is:
0.92 g = 1 mL
100. g x 1.0 mL = 108.7 mL = 110 mL
0.92 g
• 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
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.
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?
Density - Review
• The concentration of
mass in an area of
space
OR
• The mass of a given
substance per unit
volume
• The more
concentrated the
mass is in an object,
the greater is its
density.
mass
density =
volume