Name: ______________________________________
Classifying Mixtures
Introduction
It is easy to see that some materials are mixtures because you can easily see what
they are composed of. For example, soil or dirt is a mixture of various substances,
including small rocks and decomposed animal and plant matter. You can see this by
picking up a handful of soil and look at it closely. Milk, on the other hand, does not
appear to be a mixture, but it is. Milk is composed mainly of fats, proteins, milk sugar,
and water. Both milk and soil are examples of heterogeneous mixtures because their
composition is not uniform.
Salt (sodium chloride) and water form a homogeneous mixture. The sodium and
chloride ions become distributed evenly throughout the water molecules, and the mixture
appears uniform.
Mixtures can be separated into 3 categories: solutions, suspensions and colloids.
A solution is a homogeneous mixture of two or more substances, a suspension is a
heterogeneous mixture containing particles that will settle out of solution, and a colloid is
a heterogeneous mixture that contains particles that do not settle out of solution. Many
colloids appear homogeneous because the individual particles cannot be seen. However,
the particles are large enough to scatter light. You may have noticed that a car headlight
bright beam is far more noticeable in fog than on a clear night. This effect is called “The
Tyndall Effect,” and it occurs when light is scattered by colloidal particles dispersed in a
transparent medium. The Table below summarizes the differences between these 3 types
of mixtures.
Properties of Solutions, Colloids, and Suspensions
Solution
Colloid
Suspension
Homogeneous
Particle: atoms, ions,
molecules
Does not separate
Cannot be separated with
filtration
Does not scatter light
Heterogeneous
(Looks Homogeneous)
Particles: large molecules
Does not separate
Cannot be separated with
filtration
Scatters light (Tyndall
effect)
Heterogeneous
Particles: large particles or
aggregates
Separates with standing
Can be separated with
filtration
May scatter light, but it is
not transparent
In this laboratory exercise you will make various kinds of mixtures in water and
determine whether each mixture can be classified as a solution, a suspension or a colloid.
Key Question
How is matter classified?
1
Fundamentals – Classifying Mixtures
Pre-Lab Vocabulary
Define each word below using this handout, a chemistry book in the lab or a dictionary.
Mixture: _______________________________________________________________
________________________________________________________________________
________________________________________________________________________
Heterogeneous: _________________________________________________________
________________________________________________________________________
Homogeneous: __________________________________________________________
_______________________________________________________________________
Solution: _______________________________________________________________
_______________________________________________________________________
Suspension: _____________________________________________________________
________________________________________________________________________
Colloid: ________________________________________________________________
_______________________________________________________________________
Tyndall Effect: __________________________________________________________
________________________________________________________________________
Transparent: ____________________________________________________________
________________________________________________________________________
Materials
8 large test tubes
250 mL beaker
glass stir rod
test tube rack
flashlight
marker to write on glass
cooking oil
2
gelatin
red food coloring
borax detergent
liquid starch
sugar
milk
dirt
Fundamentals – Classifying Mixtures
Procedure
1. Using the marker, label your test tubes with the following labels: milk, gelatin, sugar,
dirt, starch, oil, food coloring, and borax.
2. Put approximately 100 mL of water in your 250 mL beaker, and place on your hot
plate to bring to a boil.
3. Use the Table below to make each water mixture to study: Once each mixture is
assembled, make sure it is stirred and mixed thoroughly.
Mixture
1. milk
2. gelatin
3. sugar
4. dirt
5. starch
6. oil
7. food coloring
8. borax
mL of Water
5.0
2.5
10.0
10.0
5.0
8.0
10.0
10.0
Solute to Add to Water Other additions
5.0 mL of milk
0.12 g gelatin
Add 7.5 mL of hot water
0.48 g sugar
0.20 g dirt
5.0 mL of liquid starch
2.0 mL of cooking oil
2 drops food coloring
0.08 g borax detergent
4. Look at the milk, dirt, cooking oil, gelatin and starch mixtures under a microscope
and draw a picture of what you see in the microscope in your Data Table.
5. Use a flashlight and shine it through each mixture in a dark box or darkened room. A
solution will not scatter light, a colloid will scatter light (you will be able to see the
flashlight beam travel through the mixture) and a suspension may scatter light, but the
light beam will not be transparent. Record your observations for each mixture in the
Data Table.
6. Using your observations and the properties of each type of mixture summarized in the
Table on page 1 of this handout, classify each mixture as a solution, colloid or
suspension in your Data Table.
Question
1. How did you decide if a mixture was a colloid or a suspension?
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
3
Fundamentals – Classifying Mixtures
Name: ______________________________________
Data Table
Mixture
Homogeneous or
Heterogeneous?
Does it scatter
light?
Solution,
Suspension or
Colloid?
1. milk
2. gelatin
3. sugar
4. dirt
5. starch
6. oil
7. food coloring
8. borax
1
Fundamentals – Classifying Mixtures