PHYSICAL AND CHEMICAL CHANGES
SEPARATION OF A MIXTURE
Mixtures are physical combinations of two or more substances that are not
chemically combined. Even though they are mixed together, each component
of the mixture retains its own specific chemical identity. For example,
in mixing together table salt (sodium chloride, NaCl) and sand (silicon
dioxide, SiO2), the resulting mixture still has the same chemical
formulas as it did when the two compounds were separate.
In order to separate such a mixture, chemists must use the physical and
chemical properties of the substances in order to best determine how to
separate out and purify the materials. In today’s exercise, you will
separate a heterogeneous mixture of salt, sand, and chalk (calcium
carbonate, CaCO3). Salt is the only compound in this mixture that readily
dissolves in water, so washing the mixture with water will remove it from
the other two components. Addition of acetic acid (vinegar) will react
with the chalk to produce carbon dioxide gas (CO2) and calcium chloride
(CaCl2). CaCl2 is soluble in water and easily separated from the sand,
which is then left untouched by the acid.
Objectives
When you have completed this activity, you should be able to do the
following in order to recognize and differentiate between physical and
chemical changes:
Describe a method for separating mixtures.
Calculate the percentage of each component in the original mixture.
Describe the potential sources of error in the lab.
Materials
sample of mixture
evaporating dish
watch glass
balance
tripod
wire gauze
burner
crucible tongs
vinegar (acetic acid)
Procedure
CAUTION: Wear goggles and apron at all times.
Weigh a clean, dry ramekin or dish with a lid, to the nearest 0.01 g.
Record this value on Line 1 in the Data Table.
Weigh approximately 5.00 g of the salt, sand, and chalk mixture into the
ramekin. Reweigh the dish, mixture, and lid. Record this value on Line 2
in the Data Table.
Add 20.0 mL of water into the dish. Carefully stir, taking care not to
spill the contents.
After allowing the solids to settle, carefully decant as much water as
possible, without spilling the solids in the bottom.
Repeat Steps 3 and 4 three more times.
Place the lid on top of the ramekin as a cover, and place the assembly in
the oven set on low heat.
Carefully heat the dish until the water has evaporated.
Allow the dish and contents to cool, then weigh the dish and lid. Record
this value on Line 4 in the Data Table.
Slowly add 20.0 mL of acetic acid to the dish. It is recommended that an
eyedropper or pipette be used to add small amounts of acetic acid at a
time. Addition of acetic acid to the CaCO3-SiO2 mixture will cause the
mixture to bubble.
CAUTION: Acetic acid is mildly corrosive. If you get any on you, wash it
off immediately.
Allow the sand to settle and then carefully decant the solution.
Repeat Steps 9 and 10 two more times.
Add 10.0 mL of water to the dish. Carefully decant.
Heat the ramekin and lid until the sand is dry.
Allow the dish to cool, and then reweigh the dish, lid, and its contents.
Record this on Line 7 in the Data Table.
Data and Calculations
Mass of evaporating dish + watch glass
Mass of evaporating dish, watch
glass + sample
Mass of sample
Determining Percent of NaCl in mixture:
Mass of evaporating dish, watch glass, and solids
(after water washes)
Mass of NaCl
Percent of NaCl in unknown
Determining Percent of CaCO3 in mixture: Mass of dish, watch glass, and
SiO2
(final weighing)
Mass of CaCO3
Percent of CaCO3 in unknown
Determining Percent of SiO2 in mixture: Mass of SiO2
Percent of SiO2
in unknown
Analysis and Conclusion
Show your work on calculating the percentages of each component.
Do your percentages add up to 100.0%? If not, why?
List at least two sources of error in this lab.
Explain what the effect on the percentage of each component would be if
the student did not fully dry the evaporating dish after Step 7.
PRE-LAB: PHYSICAL AND CHEMICAL CHANGES: SEPARATION OF A MIXTURE
Read the entire laboratory investigation and the relevant pages of your
textbook. Then answer the questions below.
In this experiment, what property of NaCl is used to separate it from the
other two components? Is this a chemical or physical property?
In this experiment, what property of CaCO3 is used to separate it from
SiO2? Is this a chemical or physical property?
An unknown sample of a mixture of salt and sand weighs 7.52 g before
washing with water and 3.45 g after. What is the percent of sand in the
sample? (Show all work.)