Physical Science Institute
Summer 2013
Just Add Water
In the last unit we developed and used the kinetic molecular model and molecular attractions to explain and predict the
behavior of gases, liquids, and solids. This model was based on pure substances, a substance composed of only one type
of molecule. For a model to be useful it should be able to guide us in explaining further observations we may make. In
this activity you will investigate properties of different types of mixtures.
MATERIALS
(1) balance, top loading
(1) bottle magnesium sulfate (Epsom salt)
(1) bottle non-dairy coffee creamer
(1) bottle copper acetate
(1) bottle citric acid
(1) bottle sugar
(1) bottle ammonia solution
(1) bottle purified water
(1) bottle ethyl alcohol
(7) test tubes, 20x150mm
(1) test tube rack
(3) stoppers, rubber #2
(1) funnel
(5) sheets filter paper, 12.5 cm
(1) graduated cylinder, 100-mL
(2) weighing dishes
Part 1 – Aqueous Mixtures
PROCEDURE
1. Fill three test tubes about 1/2 full with water
2. To test tube #1, dissolve 0.5 grams of Epsom salt (magnesium sulfate). After the salt dissolves, add 5 drops of
ammonia solution using a disposable pipet. Stopper and mix. Continue to add drops of the ammonia solution until a
white precipitate forms.
3. To test tube #2, add a pea-sized amount of nondairy creamer. Stopper and mix.
4. To test tube #3, add a few small crystals of copper acetate. Stopper and mix
Perform the following tests on these mixtures:
Scattering light
5. Observe if the substances mixed in the water scatters light. This can be determined by whether the mixture is cloudy
(scatters light) or clear. Record your observations in the data table provided.
Filtering the mixture
6. Place a funnel on the mouth of an empty test tube.
7. Fold a piece of filter paper as demonstrated by your facilitator and place it in the funnel.
8. Shake the mixture in test tube #1 and pour half of it into the filter paper. Note whether the filtrate—the liquid coming
out of the filter—contains any of the substance(s) that mixed with the water.
9. Perform this filter test on the other two test tubes and record your results in the data table below.
The effect of gravity on the mixture
10. Leave what remains of the mixtures in the test tube undisturbed for at least 15 minutes while you start Part 2 of the
procedure. After 15 minutes, record your observations in the data table provided.
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Physical Science Institute
Summer 2013
DATA
Mixture
Test tube #1
Test tube #2
Test tube #3
Is the mixture cloudy
(scatters light)?
Can mixture be
separated by filtering?
Does the mixture settle
out over time?
Part 2 – Solutions
PROCEDURE
11. Place 10.0 mL of water in a test tube and 20.0 mL of water in another. Add 0.50 grams of copper(II)acetate to each
tube, then stopper and shake the test tubes thoroughly until all the solid dissolves.
Questions:
1. Is the blue color evenly dispersed—uniform—for each solution?
2. We can use how dark or light (the shade) the solution is as an indicator of the concentration of copper acetate
in solution. Which solution is more concentrated, i.e., has a darker shade?
The solutions produced in the procedure above are said to be unsaturated because more solute could be dissolved in
the water. A solution in which more solid solute will dissolve is said to be an unsaturated solution. A solution in which
no more solid can dissolve and has its maximum concentration of solute is called a saturated solution.
12. To make a saturated solution add an additional 1.3 g of copper acetate to each test tube. Stopper and mix
thoroughly for about 1 minute. Put the test tubes back in the rack and let them settle while you go on to procedure #13.
13. Add 10 mL of purified water to a test tube. To the test tube add about 0.5 gram of sugar. Mix well for about a
minute. Repeat this process substituting 0.5 gram of citric acid for the sugar. Note whether the 0.5 gram amount of each
solute produces a saturated or unsaturated solution when mixed with water.
14. Add 10 mL of ethyl alcohol to a test tube. To the same test tube add about 0.5 gram of sugar. Mix well for about a
minute. Repeat this process substituting 0.5 gram of citric acid for the sugar. Note whether the 0.5 gram amount of each
solute produces a saturated solution when mixed with ethyl alcohol.
Questions:
3. Which solute produced a saturated solution when 0.5 g gram was mixed with the water?
4. Which solute produced a saturated solution when 0.5 gram was mixed with the ethyl alcohol?
The concentration of a given substance in a saturated solution at a specific temperature is a characteristic property. This
is called the substance’s solubility; like density, solubility in different solvents can be used to distinguish between
substances.
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Physical Science Institute
Summer 2013
5. Which solute, citric acid or sugar, has a greater solubility in ethyl alcohol?
15. Now take a look at the saturated solutions of copper acetate you made in procedure #12.
Questions:
6. Which test tube contains the greatest mass of undissolved solid?
7. What limited the mass of copper acetate that would dissolve in the water?
8. Look at the shade of each solution above the undissolved solid. Are the concentrations noticeably different?
9. What visual cue can be used to determine if a solution is saturated?
10. Which of the diagrams in Fig 5.1 represents the concentration of copper acetate in solution when both 10 mL
and 20 mL of water were saturated with 1.8 grams (0.5 g + 1.3 g) of copper acetate?
Figure 5.1
Test tube w/ 10 mL
HO
Test tube w/ 20 mL H2O
A)
= water
= copper acetate
B)
C)
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Physical Science Institute
Summer 2013
11. Some people say that sugar “melts” when it is added to water. How would you describe the difference
between the processes of melting and dissolving?
12. Suppose that 200 mL of a saturated solution of copper acetate was left standing in an open beaker for two
weeks. During this time half of the water evaporated.
a. Would the mass of copper acetate dissolved in the solution change?
b. Would the concentration of the copper acetate solution change during the two weeks?
13. A saturated solution is one in which no more solute will dissolve. Our kinetic model of matter says that
molecules are in constant motion. How can a solute “stop dissolving” if the molecules of the solute and solvent
are in constant motion and always interacting with one another?
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