Density
Chapter 3
Scientific Measurements
I CAN
• I CAN define DENSITY and explain how
it is calculated and determine the
volume both a regular object
(mathematically) and and irregular
object (Archimedes' Principle).
DENSITY
• Which weighs more? A pound of lead or a
pound of feathers?
• Neither…they each weigh a pound!
• Which takes up more space? Why?
• The feathers do because they are not as
tightly packed as the particle in the lead.
What is DENSITY?
• Density COMPARES the MASS of an
object to its VOLUME.
• Density is a UNIQUE PROPERTY of a
substance; it can be used to help
IDENTIFY UNKNOWN SUBSTANCE.
• DENSITY is defined as MASS PER UNIT
OF VOLUME.
Calculating Density
• The mathematical equation for finding
density is:
DENSITY = MASS
VOLUME
-any MASS and VOLUME unit can be used, but
commonly in science GRAMS are used for
mass and MILLILITERS (ml) and CUBIC
CENTIMETERS (cm3) are used for volume.
• mass and volume units cannot cancel
each other; the final unit for density is the
MASS UNIT over the VOLUME UNIT.
• For example
g/ml
or kg/l
• Pure water has a density of 1 g/ml.
• When we compare a substance’s density
to that of water, we refer to the value as
SPECIFIC GRAVITY.
• If an substance’s density is LESS THAN
that of water, it will FLOAT in water; if
MORE THAN water’s density, it will SINK!
Practice Problems
• What is the density of a substance if 25
grams has a volume of 5.0 ml?
• Density = Mass = 25 grams = 5g/ml
Volume 5.0 ml
• What is the density of a substance if 60
grams has a volume of 100 mL?
• Density = Mass = 60 grams = 0.60g/ml
Volume
100 ml
Mass and Volume
• Finding the MASS of an object is
easy….place it on a balance and get the
value.
• But what about VOLUME? Volume can be
found in two ways:
– A. Mathematically
– B. Archimedes Principle
Calculating Volume
• The volume of regular objects, such as
cubes or boxes is found by this formula:
– VOLUME = Length X Width X Height
– What would the units be?
– Since you are multiplying the same unit by
itself three times, the final unit is CUBED!
Sample Problem
• A student measure a cube of wood and
gets the following results:
• L= 5 cm
• W = 4 cm
• H = 3 cm
• What is the volume of the cube?
• V=LXWXH
• V = 5cm X 4 cm X 3 cm = 60 cm3
Archimedes’ Principle
• Archimedes’ Principle says that an object
will displace a volume of water equal to its
own volume.
• Used to determine the volume of oddly
shaped object, such as a stone.
Using Archimedes’ Principle
•
1.
•
•
record
•
Fill a graduated cylinder
with enough water to cover the
object.
2. Record the INITIAL water level.
3. Carefully add the object and
the FINAL water level.
4. The difference between the two
numbers is the object’s volume.
Practice Problem
• A student is asked to find the volume of a
44 g stone in lab. She fills a graduated
cylinder to the 50 ml mark. She carefully
adds the stone, then records the final
volume of water, which is now 62 ml.
• What is the stone’s volume?
• Volume = 62 ml – 50 ml = 12 ml
• What is the stone’s density?
• D = M / V = 44g / 12 mL = 3.7 g/mL
Other Calculations
• How can the density equation be used to
find other values, such as Mass or
Volume?
• D=M/V
V=M/D
M=VxD
Practice Problems
• A certain mineral has a D = 2.1 g/cm3
What is the MASS of a sample having a
V= 8cm3?
• A piece of wood has a D=2.3 g/mL
A piece of this wood having a M=22.6 g
would have a Volume of?