Experiment 2: SUBSTANCES
AND MIXTURES OF SUBSTANCES
Many materials are actually
mixtures of pure substances. The separation of mixtures into their component
substances is often a most difficult operation. In fact, the essential
distinction between mixtures and "pure" substances is whether or not they
can be separated by physical means. Physical means of separation are those
techniques which utilize the physical properties of a substance (e.g.,
melting point, solubility). Some physical means of separation are
filtration, distillation, decantation, evaporation, and fractional crystallization.
In this experiment you will
separate a mixture of three substances - sodium chloride (NaCl, table salt),
benzoic acid (C6H5COOH, a common food preservative),
and silicon dioxide (SiO2, sand) - into pure substances.
The separation will be accomplished by utilizing the differences in the
solubilities of these materials in water. Additionally, you are asked to
determine quantitatively the relative amounts of each of these substances
in your sample (percent composition).
Solubility is defined as
the amount of the solute that will dissolve in a given amount of solvent.
The extent to which a substance dissolves depends mainly upon the physical
properties of the solvent and the solute and also to some extent upon the
temperature. In this case the solvent is water, a strongly polar substance,
and the three possible solutes each have distinct structure and thus distinct
Solubility is a physical
property and (like all other chemical and physical properties) ultimately
depends upon the structure and bonding of the substances involved. Exactly
how chemical and physical properties depend upon the structure of molecules
is a major focus of this course and is treated in detail in the textbook.
Briefly, silicon dioxide is a nonpolar substance with a very high molar
mass and is thus insoluble in water at all temperatures. Benzoic acid is
a polar covalent compound that is slightly soluble in water. The ionic
substance sodium chloride dissolves readily even in cold water. The temperature
dependence of the solubility of benzoic acid and sodium chloride in water
is shown in Figure 4.
Figure 4. Solubility
as a Function of Temperature
This exercise includes five
Pre-Lab questions which are to be answered before the laboratory period
and turned in at the beginning of the laboratory period.
These Pre-Lab questions are
to be completed independently before you come to lab. They may be turned
in on notebook paper.
Distinguish unambiguously between
a mixture and a substance and give an example (not from the experiment)
Is it possible to have a heterogeneous
pure substance? If so, please give an example.
Classify each of the following
as an element, compound, or mixture: (a) Kool-Aid, (b) helium, (c) mud,
(d) cement, (e) sugar.
In the eighteenth century there
was considerable conflict over the "discovery" of new elements. If you
were president of one of the scientific societies of the day, what evidence
would you find convincing that a new element had in fact been discovered?
Salad dressing is a mixture
with which you are familiar (basically, vinegar, oil, and spices). Please
outline, in good detail, a procedure by which this mixture might be separated
into its component substances.
How to use a Meker burner
Note the three controls
available: a) the bench stopcock is not open until the paddle handle
is turned to almost 90º (pointing at the hose); b) the needle
valve on the bottom of the burner is closed by turning to the right (clockwise
as viewed from below); c) the air inlet control is a sleeve at the bottom
of the burner tube that can be rotated to cover or uncover the air inlet
vents. Before lighting, close the air inlet most of the way and open
the needle valve a couple of turns. Open the bench gas stopcock and
hold the cup of your flint striker above the burner for 3-4 seconds to
catch some of the gas. Squeeze the handles of the striker to produce
a spark to light the gas. The size of the flame is adjusted with
the needle valve. The air inlet is opened to produce a steady flame.
A hotter flame is produced with a high air/gas ratio (open the air vent
and/or reduce the amount of gas), while a cooler flame is produced with
a lower air/gas ratio (close down the air vent and/or increase the amount
In your discussion section you
should write a careful analysis of systematic errors. For each substance,
you should propose a specific reason for that result being too high or
too low, as the case may be.
Carefully weigh a clean, dry
250-mL beaker to the nearest milligram (0.001 g). The various beakers that
you use should be marked in some way on the white labeling spot in order
to identify one from another. Transfer about 5 to 6 g (about a tablespoonful)
of the mixture into the beaker and reweigh the beaker and contents. From
these two masses, the exact mass of the sample of the mixture may be calculated.
Add about 50 mL of distilled
water to the beaker and place the beaker and contents on wire gauze on
the ring stand as shown in Figure 5.
Figure 5. Set-up for
heating beaker on ring stand.
Heat the mixture to near boiling
and stir the mixture with a stirring rod to make sure that all soluble
material is dissolved. At the boiling temperature, all benzoic acid and
sodium chloride should be in solution (there should be no white solids
visible). If necessary, add small amounts of water until
the dissolution is complete. At this point the substances soluble
in hot water have been extracted from the insoluble sand. Use of
excessive amounts of water in this and later steps will greatly increase
the length of time necessary to complete the lab.
Place about 50 mL of distilled
water in a beaker and bring it to a boil. This boiling water will be used
in step 6.
Quickly decant the supernatant
liquid in the first beaker while it is hot into another pre-weighed 250-mL
beaker, guiding the solution by pouring down a glass stirring rod. Use
the beaker tongs to hold the beaker of hot solution. Do not let the sand
get into the second beaker.
In order to assure that all
of the soluble materials are separated from the sand, wash the sand in
the first beaker with about 15 mL of boiling water and decant the washings
into the second beaker. If necessary, use a second rinse with boiling
water to get all of the soluble material. Note the precaution in
Place the second beaker that
now contains a water solution of benzoic acid and salt into an ice bath
and let it cool. Observe the benzoic acid crystallizing out of solution.
Heat the sand in the first beaker
with a gentle flame from a gas burner until the sand is completely
dry. The sand has a tendency to splatter if heated too rapidly. The possibility
of loss of sample can be reduced if the beaker is covered with a watch
glass (weigh it first) and the heating is carried out very slowly.
When the sand is completely
dry, allow the beaker to cool to room temperature. Reweigh this beaker
which now contains only the sand. Calculate the mass of sand in the sample.
(NOTE: You may proceed to the next step while the beaker and sand are cooling.)
Assemble the Buchner funnel
with a filter flask and connect the filter flask to a trap and vacuum source
as shown in Figure 6. Unlike the photo, the filter flask rather than
the vacuum trap should be clamped to the ring stand.
Figure 6. Filter flask
Place a circle of filter paper
in the Buchner funnel (it must lie flat), wet it with distilled water using
a wash bottle, and turn on the vacuum source.
Swirl the contents of the second
beaker and quickly pour the contents into the Buchner funnel.
Wash the solid (benzoic acid)
with about 5 mL of chilled distilled water to ensure that the benzoic acid
is free of sodium chloride.
Continue suction for about five
minutes beyond the point at which water stops dripping from the funnel
to help dry the precipitate.
Disconnect the rubber tubing
from the filter flask before turning off the vacuum. This prevents water
from backing up into the filter flask.
Using a spatula, transfer the
suction-dried benzoic acid into a clean, pre-weighed 150-mL beaker. Let
it dry further in the oven at 80o C. Cool the beaker to room
temperature and weigh it. From the total mass, calculate the mass of benzoic
acid in the sample.
Transfer the filtrate solution
from the filter flask to a clean, dry pre-weighed beaker. Using a wash
bottle, rinse the filter flask into the beaker with a small amount of water
to assure that all of the sodium chloride solution is transferred.
Place the beaker containing
the salt solution on a wire gauze resting on an iron ring on the ring stand,
Figure 5. Heat the solution gently with a burner so that the liquid does
not boil over. Later reduce the flame to avoid overheating. It may prove
helpful to cover the beaker with a pre-weighed watch glass at this point.
When the sodium chloride is
completely dried, remove the flame and cool the beaker and contents to
room temperature. Weigh the beaker and its contents to the nearest 0.001
g. From this datum and the masses of the empty beaker and the watch glass,
calculate the mass of sodium chloride in the sample.
Calculate the percentage of
each substance in the mixture. Also calculate the total percentage of the
sample recovered in the operation.
Obtain the known percentages
from your instructor and write a careful, detailed discussion of systematic
errors (explain why each component is either too high or too low).
Place the recovered benzoic
acid in the labeled waste jar, and clean up all of your equipment and put
it away (except for the vacuum trap and ring stand).