Separation of a Mixture
Introduction:
Mixtures are not unique to chemistry; you use and consume them
on a daily basis. The beverages you drink each morning, the fuel you use
in your automobile, and the ground you walk on are mixtures. Very few
materials that you encounter are pure. Any material made up of two or
more substances that are not chemically combined is a mixture.
The isolation of pure components of a mixture requires the
separation of one component from another. Techniques needed to do this
separation take advantage of the differences in physical properties of the
components. The techniques you will use in this lab include the
following:
1.
Sublimation. This involves heating a solid until it passes directly
from the solid phase into the gaseous phase. The reverse process,
when the vapor goes back to the solid phase is called deposition.
2.
Extraction. This uses a solvent to selectively dissolve one
component of the solid mixture. With this technique, a soluble
solid can be separated from an insoluble solid.
3.
Decantation. This separates a liquid from an insoluble solid by
carefully pouring the liquid from the solid without disturbing the
solid.
4.
Filtration. This separates a solid from a liquid through the use of a
porous material as a filter. Paper is a good filter. Filters allow the
liquid to, pass through but not the solid.
5.
Evaporation. This is the process of heating a mixture in order to
drive off a volatile liquid and make the remaining component dry.
The mixture that will be separated in this lab contains three
components: naphthalene, C10H8, common table salt, NaCl, and sand,
SiO2. The separation will be done according to the scheme shown on the
next page and involves three basic steps:
1.
2.
3.
Heating the mixture to sublime the naphthalene.
Dissolving the table salt with water to extract.
Evaporating water to recover dry NaCl and sand.
2
Mixture: naphthalene,
NaCl
sand
heat to
250ºC
naphthalene
sublimes
Residue remaining:
NaCl
sand
extract with
water
filter
NaCl
solution
Residue:
wet sand
evaporate
water
evaporate
water
NaCl
sand
All heating with Bunsen burners or hot
plates is to be done in the fume hoods!
Procedure:
Weigh a dry 150 mL beaker and record the weight on the report
sheet. Obtain about 3 grams of the mixture and using a mortar and pestle,
grind the mixture into a fine powder. Transfer about 2 grams of the mixture
into the weighed 150 mL beaker. Record the weight of the beaker and
mixture.
Place an evaporating dish on top of the 150 mL beaker containing the
mixture. Place the beaker with the evaporating dish on a wire gauze with
an iron ring and ring stand. Put some ice cubes into the evaporating dish
(don’t get any in the beaker). Carefully heat the beaker with a Bunsen
burner until vapors appear in the beaker. A solid should collect on the
underside of the evaporating dish. After about 10 minutes, remove the
Bunsen burner from under the beaker, then carefully remove the
evaporating dish and collect the solid by scraping it off with a spatula.
Drain any liquid water in the evaporating dish and add a couple more ice
cubes. Stir the contents of the beaker with a glass rod. Place the
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evaporating dish back on the beaker and apply heat again. Continue
heating and scraping off solid until no more solid collects. Place all the
naphthalene that you scrape off in a collecting jar on the lab cart.
Let the beaker cool to room temperature and then weigh the beaker
with the contained solid. Add 25 mL of distilled water to the solid in the
beaker. Heat and stir for 5 minutes. Weigh a second dry 150 mL beaker
with 2 or 3 glass-bead boiling chips in it and assemble the apparatus
shown below where a funnel is supported by a iron ring and the funnel tip
is arranged so that the filtrate will run down the wall of the beaker:
Fold a piece of filter paper in halves then quarters and place it inside
the funnel. Wet the paper with water and adjust the paper so that is lies
flat on the funnel. Position the second 150 mL beaker under the funnel as
shown in the apparatus above. Pour the mixture through the filter, first
decanting most of the liquid into the beaker and then transferring the wet
solid into the funnel with a rubber policeman (stirring rod with a rubber tip
on it). Collect all the liquid (filtrate).
Rinse the beaker with 5 to 10 mL of distilled water, pour over the
residue in the funnel, and add the liquid to the filtrate; do this again with
an additional 5 to 10 mL of distilled water.
Place the beaker containing the filtrate and its contents on a wire
gauze with an iron ring and ring stand. Heat the beaker with a Bunsen
burner gently. Try to control the flame to prevent boiling over. As the
volume of liquid is reduced, solid NaCl will appear. Reduce the flame to
avoid bumping of the solution and spattering of the solid. When all the
liquid is gone, cool the beaker to room temperature. Weigh the beaker,
glass-beads, and the solid residue. Record.
Weigh an evaporating dish. Transfer the sand from the filter paper
into the evaporating dish. Heat the sand to dryness in the evaporating dish
with a Bunsen burner, using an iron ring and ring stand. Heat gently to
avoid spattering; when dry the sand should be freely flowing. Let the sand
cool to room temperature. Weigh the evaporating dish and the sand.
Record.
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5
Name
Date
Report for Mixture Separation Experiment:
Data:
1.
weight of 150 mL beaker
2.
weight of beaker and mixture
3.
weight of mixture (#2 – #1)
4.
weight of beaker and solid after
sublimation
5.
weight of naphthalene (#2 – #4)
6.
weight of second 150 mL beaker
7.
weight of beaker and NaCl
8.
weight of NaCl (#7 – #6)
9.
weight of evaporating dish
10.
weight of dish and sand
11.
weight of sand (#10 – #9)
6
Calculations:
12.
weight recovered solids
13.
percentage of solids recovered
14.
percentage of naphthalene
15.
percentage of NaCl
16.
percentage of sand
(#5 + #8 + #11)
([#12 ÷ #3] × 100)
% = ([#5 ÷ #3] × 100)
% = ([#8 ÷ #3] × 100)
% = ([#11 ÷ #3] × 100)
Questions:
1.
Dry cleaners remove oil and grease spots from clothing by using an
organic solvent called perchloroethylene. What separation
technique do the cleaners use?
2.
Ice cubes stored in the freezer compartment of a refrigerator for a
long period of time lose their shape and shrink in size. Why?