EXPERIMENT #2: SEPARATION OF A
MIXTURE OF SOLIDS
Purpose:
The purpose of this experiment is to learn methods of separating a mixture of
substances and to perform calculations involving mass percentages.
Special Apparatus and Chemicals:
Bunsen burner
2 Evaporating dishes
Iron ring and stand
Electric hot plate
Tongs
Watch glass
Clay triangle
2-3g of unknown mixture
Discussion:
Since chemistry is the scientific study of the composition, structure, and properties of
matter and the transformations that it undergoes, chemists are often called upon to determine
the composition of a material. If the material is a mixture, the components of the mixture
may have to first be separated in order to quantitatively determine the composition.
Materials may be classified as either pure substances or a mixtures. Pure substances are
those materials defined by a fixed composition and a set of characteristic properties. A pure
substance may be either an element or a compound. Mixtures are combinations of two or
more pure substances in a variable ratio. Mixtures may be classified as heterogenous or
homogenous. Homogenous mixtures (often called solutions) have a uniform composition
throughout; heterogenous mixtures have a variable composition in different regions of the
material
Mixtures may be separated into pure substances based on differences in the physical
properties of its component pure substances. Physical properties are those properties that can
be measured without changing the basic nature of the substance. Some examples of physical
properties are melting and boiling points, color, odor, density, solubility, and hardness. In
this experiment we will use differences in the properties of sublimation and solubility to
separate a heterogenous mixture of solid NH,CI, NaCl, and
Because ammonium chloride (NH CI) undergoes sublimation at 340°C and NaCl and
SiO2 do not, the NH C1 may therefore be separated from a mixture containing NaCl and
SiO2 by heating the mixture containing all three above 340°C. A substance is said to
undergo sublimation when it goes from the solid phase directly to the gas phase without
passing through the liquid state. Some common substances that undergo sublimation under
normal conditions are CO2 (dry ice), iodine, naphthalene (moth balls), and ice (below the
SiO2
Experiment 2
53
freezing point, 0°C). Upon heating the mixture containing NH,CI, the ammonium chloride
will be seen leaving the sample as a white smoke. As soon as the ammonium chloride leaves
the dish, the sublimed NH Cl condenses back to a solid which is seen as white smoke.
Both NH,Cl and NaCl are soluble in water but Sio, is not. NH,Cl and/or NaCl
could be extracted from a mixture with SiO2 by adding water to preferentially dissolve the
NH,Cl and/or NaCl. The aqueous solution of NH,Cl and/or NaCl that forms could be
removed from the still solid SiO2 by decantation or filtration. Decantation is the process of
carefully pouring a liquid from a solid. Filtration is the process of using a porous membrane
(i.e. filter paper), allowing a liquid to pass through leaving the solid behind on the filter.
Once you have determined the mass of the components in a mixture, and the total
mass of the sample is known, the percent by mass of the components in the mixture may be
determined. The composition of mixtures is often reported as a mass percent. "Percent"
means per centum the amount out of 100. Percents are calculated by taking a fraction or
ratio and multiplying by 100. To calculate a mass percent take the mass fraction and
multiply by 100. For example if a mixture contains two components A and B with a total
sample mass of 2.454g and
there is 1.002g of A in the mixture then the
mass percent of A in the mixture may be
1.002 g A
calculated as follows:
2.454 g total * 100 = 40.83% A
Concept:
The unknown mixture that you will separate contains 3 components, NaCl (sodium
chloride, "table salt"), SiO2 (silicon dioxide, sand" or "silica") and NH,Cl (ammonium
chloride). The separation of these three compounds will be accomplished by first heating the
mixture to remove NHCl by sublimation. Next, extracting the NaCl from the mixture with
water, and finally removing the remaining water from the NaCl and SiO2. See scheme
below:
NH,CI, NaCl, and Sio
(Dish #1)
Sublimation
NH CI
NaCl and Sioz
(Dish #1)
Gas
Extraction
Decanting
NaCI
(Dish #2)
sioz
(Dish #1)
54
Separation of a Solid Mixture
Procedure:
Weigh a clean, dry evaporating dish. Add between 2 to 3 grams of your unknown
mixture and weigh the dish and the sample and determine the sample mass to the full limit of
precision of your balance. Put the evaporating dish with the mixture on a ring stand IN THE
FUME HOOD and heat the mixture over a bunsen burner until white smoke stops forming.
CAUTION: Sublimation must be done in the hood because a large amount of NH CI
smoke is produced. Be careful to make sure that all of the NH,Cl has been removed from
the dish, check to make sure none has re-condensed on the side of the dish. Crucible tongs
may be used to move the dish and a wire gauze pad should be used as a hot pad. Cool the
evaporating dish to room temperature and weigh the dish and sample after heating. (NEVER
WEIGH HOT OBJECTS). The mass difference before and after heating is the amount of
NH,Cl sublimed. Calculate the mass percent of NH CI in the original sample.
Alternative Heating Method. If bunsen burners are unavailable, the sublimation step
may be carried out by placing the sample in a evaporating dish or Petri dish bottom and
heating on an electric hot plate turned on high. Remember to do this step in the HOOD.
Add 20 mL of distilled water to the solid remaining in the evaporating dish and stir to
dissolve the NaCl. Weigh the second evaporating dish and watch glass empty. After letting
any SiO2 present settle to the bottom, decant (pour oft) the liquid from the first dish to the
second. Be careful not to transfer any of the Sio, sediment to the second dish. Add 10
more mL of water to the first dish, stir, and then again decant liquid to the second dish.
Repeat with a second 10 mL portion of water. Now the first dish contains wet SiO, and the
second an aqueous solution of Naci.
Heat both dishes carefully on a hot plate until all of the water evaporates. When the
dish containing NaCl is near to dryness, cover with the watch glass to prevent splattering as
the last of the water evaporates. After all the water has evaporated from both dishes (make
sure the watch glass is completely dry also), cool to room temperature, and weigh both
dishes. Calculate the amount of NaCl and Sio, that you have recovered from the sample.
Calculate the mass percentage of NaCl and SiOthat were in the original sample.
DISPOSAL: The NaCl left in the evaporating dish may be washed down
the sink since it is non-toxic and water soluble. The leftover SiO2 should
be dumped in the trash can.
Add the mass of NH,CI, NaCl, and SiO2 that you determined to be in your sample.
Calculate the percent of matter that you recovered. If your combined total is not between
99.0% and 100.0%, explain what mistakes may have occurred in your experiment. Some
common mistakes include not completely drying the NaCl or SiO2, letting some of the NaCl
solution splash out of the dish during evaporation, and material popping out of the dish
during sublimation or evaporation. Consult your instructor to find out if you should repeat
the procedure.
Experiment 2
55
Name:
Section:
Date:
Experiment #2 Pre-Lab Questions
1.
Classify each of the following as a pure substance or a mixture:
(a)
ammonium chloride
(b)
silica
(©)
gasoline
(d)
dry ice
(e)
iodine crystals
2.
Define the terms "pure substance" and "mixture".
3.
In this exp what particular method did you separate ammonium chlon
from the mixture? What safety precaution do you need to know when you use this
method of separation?
Experiment 2 57
4.
What other safety precautions do you need to be aware of before you start the
Experiment 2 57
4.
What other safety precautions do you need to be aware of before you start the
experiment?
5.
Consider that you started with 2.101 g sample of a mixture containing NH,CI, NaCl, and
SiO, and sublimed away 0.810 g of NH,CI. When you removed all of NaCl by dissolving
in water, 1.008 g of silica was present in the evaporating dish. What was the mass
percentage of NaCl in the mixture? Show all work.
58
Separation of a Solid Mixture
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