Module 1, Lesson 4
Grade 8 Science
35
Lesson 4
Density
Learning Outcomes
After completing this lesson you will be able to
•
mass
•
volume
+
density
•
regularly
•
calculate and compare densities of different substances
•
use the particle theory of matter to explain density and the
effect of heat on density
•
describe the relationship between density and buoyancy
shaped objects
• irregularly
shaped objects
Density
• displacement
When working with solids, liquids, or gases, it is important to
understand their properties. You've already investigated one
important property of liquids: viscosity. There is another
important property of matter that is essential to understand.
• particle theory
of matter
• hydrometer
• buoyant force
Imagine trying to compare the following pairs of substances.
1.
2.
3.
4.
a rock and a cork of approximately the same size
1 kg of feathers and 1 kg of lead
a hot air balloon before taking off and after taking off
a small piece of cedar (1 cm x 3 cm x 10 cm) and a large piece
of cedar (3 cm x 1 cm x 100 cm)
When thinking about these objects, you probably began making
comparisons of things such as mass (the amount of matter
present), volume (amount of space taken up), and changes to
volume that can happen when heat is added. With the cedar, you
observe that the mass and volume are different, yet you know
that they are the same material. How could you prove they were
the same? Let's leave this question for a moment.
The rock and a cork have the same volume, but different masses.
If you held one in each hand with your eyes closed, it would be
obvious that they were different substances, even though they
have the same volume. If you had to construct a container for the
1 kg of feathers and one for the 1 kg of lead, you would quickly
realize that the container for the feathers would have to be much
larger than the container for the lead. While their masses are
Module 1, Lesson 4
36
equal, their volumes would be drastically different. You should
remember from Grade 7 that heat causes particles to move faster
and become spread out, so the same number of particles would
take up more space (volume) as in the hot air balloon. With the
cedar, you would need to do some "tests" or other measurements
if you wanted to prove to someone they were the same material.
The measurement that scientists use to illustrate the important
relationship between volume and mass is called density. If you
have objects of the same volume, it is easy to compare densities.
You can determine the mass of the objects and compare the mass
for the given volumes. This is actually comparing densities! To
make things even easier to compare, scientists use a standard
volume for densities. The most common units are grams per
cubic centimetre or grams per millilitre. The formula for
calculating density is as follows:
density =
mass
volume
Determining Volume
You must first be able to calculate volume before you can
calculate density. Calculating the volume of regularly shaped
solids is straightforward, as in the example below.
Volume of a regularly shaped solid is calculated by
multiplying length times width times height.
Volume = length x width x height
Grade 8 Science
Module 1, Lesson 4
Grade 8 Science
37
Practice: Determining Volume
1. Calculate the volume of object A on the previous page. (The
dimensions have been provided.)
3
(length)
X
(width)
X (height)
cm
2. Calculate the volume of object B. You must first measure the
dimensions of object B.
3
(length)
X (width)
X (height)
cm
Grade 8 Science
Module 1, Lesson 4
38
The volume of an irregularly shaped object is a little more of
a challenge to determine. The easiest way to find the volume is
to use displacement of water. Place the solid in a graduated
cylinder or measuring cup, and subtract the water's initial
volume from the newly combined volume.
25 mL
volume = 25 mL - 15 mL = 10 mL
mass = 20 g
20mL
g _= 2 g/mL
mass __ 10
density = volume
For a liquid, the volume is determined simply by pouring it into
a graduated cylinder, measuring cup, or similar container with
measurements indicated.
Determining Mass
Determining the mass of solids is easy to do and it doesn't
matter whether they are regularly or irregularly shaped. Simply
use some type of balance (scale) to directly measure the mass.
Determining the mass of liquids is a little more complicated. It
involves finding the mass of the container first, then the mass of
the container and the liquid, and then finding the difference
between these two measurements.
Try the following:
mass of empty container = 15 g
mass of container with 100 mL of liquid = 25 g
What is the mass of the liquid alone?