- University of Virginia
- Physics Department
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Properties of
Mixtures and Solutions
A Physical Science Activity
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2003 Virginia SOLs
Objectives
Students will
- identify and separate the components in mixtures;
- determine the iron content in cereal;
- determine the concentrations of components of a mixture;
- use the following skills: making and applying observations, organizing and
analyzing information, and measuring and recording data.
Motivation for
Learning
Discrepant Event
Materials
- 4 g salt
- Two beakers (at least 500 mL)
- 4 g calcium carbonate or flour or talcum powder
- stirring rod
- flashlight
- water
Procedure
- To illustrate the difference between a solution and a
suspension, add about 4 grams of salt to 500 mL of water. Also,
prepare a solution of calcium carbonate (4.0 grams) and 500 mL of
water.
- Make certain that each mixture is stirred well. Ask the
students to list any differences or similarities between the two
mixtures.
- Darken the room and shine a flashlight through each beaker.
The beaker with calcium carbonate will appear cloudy. Point out
that this method is one way to differentiate between a solution
and a suspension.
The particles in a suspension are larger and more unevenly distributed than
those of a solution. When light passes through the suspension, the larger suspended
particles scatter the light in all directions producing a cloudy appearance.
This phenomenon is known as the Tyndall Effect. Most students have experienced
the same effect in a movie theater, when light from the projector becomes visible
through the dust in the air.
Background
Information
Mixtures are combinations of substances that are not combined
chemically. The amounts of substances in a mixture can vary. Students
should understand that mixtures can change in appearance but this
does not mean that any substances have changed in their chemical
composition. Mixtures can be combinations of elements or compounds.
Most substances found in nature are mixtures. A pure element or a
pure compound is rarely found. Mixtures can be in any of the four
phases of matter- or they can be in combinations of different phases.
Air is a mixture of gases, milk is a mixture of solids and liquids,
alloys are mixtures of solids. Mixtures that do not appear to be
distributed the same throughout are said to be heterogeneous, and
those that are the same throughout are called homogeneous.
Solutions are common types of homogeneous mixtures. Sugar and water form a
solution when mixed. The sugar becomes evenly distributed throughout the solution,
so that one portion is not sweeter than another. The dissolved portion of the
solution is called the solute (sugar) and the dissolving portion is the solvent
(water). If more sugar is added to the solution, the entire solution becomes
sweeter and we say that it is more concentrated. In solution, sugar and water
have not lost their properties, only combined them. Pouring a solution, like
sugar and water, through filter paper will not separate the mixture, the sugar
particles are too small. The best method is distillation; the water evaporates
and the sugar is left behind.
All materials are not soluble. A suspension is a mixture in which
the solute particles are larger than molecules or ions. Particles
this size cannot adhere to the molecules of the solvent and will
eventually settle out. The particles in a suspension are in the range
of 10-2000 angstroms in diameter (1A = .00000001 cm). The particles
in a solution are usually much less than 10 A. As a result, in
addition to settling, the particles in a suspension scatter light
when it is passed through, giving it a cloudy appearance (Tyndall
Effect).
Student
Activity
To print out the Student Copy only,
click
here.
Materials
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- Triple beam or electronic balance
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- Box of Total breakfast Cereal
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Teacher Preparation
All materials should be available for each group of students
(workstation). The sand, salt, iron filings and cooking oil can be in
cups or small beakers. Go over the background information with the
students. Show them examples of mixtures, solutions, and suspensions,
and go over any new vocabulary words.
Explain that in the following activity, each group has to make two mixtures
using three ingredients each from the list of ingredients in the procedure.
One of the mixtures must contain water; the other does not. After making the
mixtures, each group must come up with a procedure for separating the ingredients.
In part #2, the students have to grind Total Cereal into a powder
and determine the percentage of iron in the mixture. Have a cup of
cereal, magnet, scrap paper, empty plastic cup and balance available
at each station.
Procedure - Part 1:
Making and Separating Solutions
- Use three ingredients from the following list to include in your first mixture:
sand, salt, water, iron filings and cooking oil. Enter the name and physical
description of the ingredients that you chose in the data sheet.
- Mass out 2.0 grams of each solid and 15 mL of water if needed.
- Mix the ingredients in the beaker and stir with the stirring
rod. Record the description of the mixture in the table.
- Write out a procedure for separating the mixture in the data
sheet. Follow your procedure and separate the mixture. Save all
the parts of the mixture and show them to your teacher.
- Repeat the procedure in steps 1-4 for your second mixture.
Data Sheet
To print out the Data Sheet only (Parts 1
and 2), click here.
- Mixture 1
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Description of
Mixture
Procedure for Separating
Mixture #1
1._____________________________________________________________________
______________________________________________________________________
2.
____________________________________________________________________
______________________________________________________________________
3.
____________________________________________________________________
______________________________________________________________________
4.
____________________________________________________________________
______________________________________________________________________
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Mixture
2
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Description of
Mixture
Procedure for Separating Mixture
#2
1._____________________________________________________________________
______________________________________________________________________
2.
____________________________________________________________________
______________________________________________________________________
3.
____________________________________________________________________
______________________________________________________________________
4.
____________________________________________________________________
______________________________________________________________________
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Procedure - Part 2: Determining the Percentage of Parts of a
Mixture
- Place an empty cup on the electronic balance, press "tare" to
remove the mass of the cup.
- Fill a 100 mL beaker with the breakfast cereal. Transfer the
cereal, a little at a time, to a mortar and grind it to a powder.
Transfer the ground cereal to the cup. When you are finished
grinding all the cereal from the beaker and transferring it to the
cup, find the mass. Enter the results in the table.
- Place the magnet into the ground cereal and stir it around
several times to collect the reduced iron. Remove the magnet and
rub off the particles that stuck to it onto a piece of scrap
paper. Repeat this procedure several times to insure you have
collected as much iron as you can. Remove any items that are
clearly not magnetic.
- Mass the cup and contents again and record the mass in the
table.
- Subtract the mass of the cup and contents after the iron was
removed from the original mass of the cup and contents to find the
mass of the iron that was removed. Enter the value in the table.
- Find the percentage of iron in the cereal by dividing the mass
of the iron (#5) by the original mass of the cereal (#2) and then
multiply by 100.
Data Table
To
print out the Data Sheet only (Parts 1 and 2), click
here.
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Original Mass of Cereal (g)
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Mass of Cereal After Iron is Removed (g)
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Mass of Iron (g)
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Percentage of Iron Contained in Cereal
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Extensions
1. A good extension or even an assessment would be to give each group an unknown
mixture or solution and have them determine the ingredients. You could use either
the substances given here or others.
2. Students can investigate how salt depresses the freezing point of water.
They can design their own experiment or follow the procedure below.
- Fill a 250 mL beaker 2/3 full of crushed ice. Add water to
fill up the spaces between the ice. Stir well. Measure and record
the temperature.
- Add 5 grams of salt (NaCl) to the ice water mixture and stir
well to dissolve the salt. Measure and record the temperature.
- Predict what will happen when a second 5 grams of salt is
added to the solution. Add the salt (total 10 grams). Stir to
dissolve and record the temperature.
- Add another 5 grams (total 15). Stir and dissolve. Measure and
record the temperature again.
- Keep adding salt, 5 grams at a time, dissolving and recording
the temperature each time.
- Make a data chart and graph Grams of Salt vs. Temperature.
Students with special needs
Click here for information
on laboratories with students with special needs.
Assessment
To print a copy of the assessment only, click
here.
- How do the properties of a mixture differ from the properties of the components
of the mixture?
- In terms of physical properties, what is there about mixtures that makes
it possible to separate out the components?
- Describe in detail the steps you would take to find the percentage of salt
in a sample of salt water.
Answers to Assessment
- The chemical properties of the components of a mixture do not change within
a mixture. The properties of the whole mixture vary greatly depending on the
amount of each component and the specific property being investigated.
- The components of a mixture are not chemically combined; therefore, the
individual chemical and physical properties of the two components remain individual
and unique, allowing for easy separation based on these properties. Separating
a mixture can be likened to separating two different colored marbles from
a jar.
- Step one: Find the initial volume and mass of the sample. Use a graduated
cylinder and a balance (be sure to scratch the mass of the cylinder).
Step two: Heat the solution gently in a flask with a stopper that has
tubing running through it and into a separate flask. Continue until all water
is gone and only salt remains.
Step three: Put the remaining wet salt onto a Buchner funnel with pre-massed
filter paper over top the second collection flask. Vacuum filter the sample
of salt.
Step four: Weigh the filter paper and subtract the actual mass of the
paper for the mass of the salt. Find the volume of the separated water. Use
the density to find the mass (the two values should be equal).
Step five: Calculate the percentage of salt to water by dividing the
mass of salt by the mass of water and multiplying by 100.
