1030 Experiment No. 2, Physical Separation Techniques

General Chemistry I
CHEM-1030
Laboratory Experiment No. 2
Physical Separation Techniques
(revised 05/21/2015)
Introduction
When two or more substances that do not react chemically are blended together, the components of the
mixture retain their individual identities and properties, whether the mixture is homogeneous or
heterogeneous. The separation of the components of a mixture is a problem frequently encountered in
chemistry. The basis of separation theory is the fact that components of a mixture have different
physical and chemical properties. The components are pure substances, either elements or compounds.
Under the same conditions of pressure and temperature, the properties of every sample of a pure
substance are identical. Each sample of a pure substance melts at the same temperature, boils at the
same temperature, has the same solubility in a given solvent, etc.
Although these and other characteristics can be used to identify a particular substance, we will be
concerned in this experiment with the separation of a mixture into its three known components, not
with the identification of the three substances. Methods used to separate mixtures rely on differences
in the components’ physical properties. Several separation techniques are described below. Those
methods prefaced by an asterisk will be employed in this experiment.
*Extraction: The removal of a substance from a mixture by using a solvent that will dissolve one
component but not the other.
*Filtration: The process of removing or “straining” a solid (sometimes called a precipitate) from a
liquid by the use of filter paper or other porous material.
*Decanting: Pouring a liquid from a solid-liquid mixture, leaving the solid behind.
*Sublimation: The physical process by which some substances can pass directly from the solid state
to the gaseous state without the appearance of the liquid state. Not all substances possess this
characteristic. If one component of a mixture sublimes, this property may be used to separate it from
the other components of the mixture. Iodine (I2), naphthalene (C10H8, mothballs), ammonium chloride
(NH4Cl) and dry ice (solid CO2) are substances that sublime readily.
Centrifuging: The process of separating a suspended solid from a liquid by spinning the mixture at
high speed to accelerate the settling-out process.
Chromatography: The separation of a mixture by the distribution of its components between a
stationary phase such as paper or aluminum oxide powder and a moving phase such as water. Some
examples are gas chromatography, paper chromatography and thin-layer chromatography.
Distillation: This process is used to purify an impure liquid by heating it to the boiling point and
condensing its vapors to collect the purified liquid. It is easy to remove liquids from nonvolatile solids
because nonvolatile solids remain behind during distillation. Separating a mixture of two liquids is
1
only possible if they have significantly different boiling temperatures. Heat-sensitive liquids can be
separated by decreasing the pressure over a liquid mixture to reduce all boiling temperatures.
The scheme used to separate the components of a mixture is based on differences in the physical
properties (ability to sublime and solubility in water) of the three components. Chemists frequently
illustrate a separation procedure with a flow chart like the one depicted in the prestudy exercise. By
knowing the physical properties of each component, they decide what techniques will allow them to
separate the mixture. In this experiment’s prestudy, you will look up the pertinent physical properties
of the three components and create a flow chart describing the separation procedure.
Experimental
In this experiment, you will separate a three-component mixture of table salt (sodium chloride, NaCl),
ammonium chloride (NH4Cl) and sand (silicon dioxide, SiO2) into its individual components.
Measuring the masses of the original mixture and the masses of the pure components will allow you to
calculate the percent by mass of each substance in the original mixture.
A. Sublimation of NH4Cl: Weigh a clean, dry, evaporating dish to 0.001 g on a hanging pan
balance. It is not necessary to zero in the balance before you use it. Since all materials in this
experiment will be weighed in containers, any weighing errors due to the balance not being zeroedin will cancel out. After you weigh the evaporating dish, check your balance for consistency.
Move the balance masses back to zero and reweigh the dish two more times to make sure you get
the same reading. Show your results to the instructor before you proceed. Enter the three masses
and their average on your data sheet. If your balance does not read consistently, you may need to
use another balance. Do not move or adjust your balance once you have started using it. Be sure
to use the same balance for every subsequent weighing of the evaporating dish.
Obtain a vial containing an unknown mixture and record its number on the data page. Pour all the
mixture into the evaporating dish and weigh them together on the same balance. Repeat the
weighing two more times as you did for the empty dish. The difference between the averages of
the two mass determinations is the mass of your starting mixture. Place the evaporating dish and
mixture on a hot plate in the fume hood. With the hot plate on its highest setting, heat the mixture
until you no longer see white ammonium chloride fumes being evolved. (The white “smoke” you
see is ammonium chloride that reforms as solid particles in the cooler air above the dish.) Stir the
mixture twice during this time to facilitate sublimation of the ammonium chloride. (If any solid
adheres to your glass rod, return it the dish.) If all the ammonium chloride is not removed at this
point, your subsequent results will be in error. Test for complete sublimation of ammonium
chloride by holding the tip of a glass rod above the mixture in the evaporating dish for 5 s. If any
ammonium chloride still remains in the mixture, a faint white deposit will form on the glass rod.
If you see this deposit, heat the mixture for five more minutes and test it again.
When you are sure no ammonium chloride remains in the mixture, grasp the evaporating dish with
tongs and hold a wire gauze square beneath the dish to lift it from the hotplate. Allow the
evaporating dish and its contents to cool to room temperature at your workstation and record their
combined mass three times on the same balance you used before. From the averages of the masses
you measured, calculate the mass lost by the system, which is the mass of NH4Cl that was in the
original mixture.
2
B. Extraction of NaCl: Pencil your initials on the frosted white patch on the side of a clean, dry 400
mL beaker and weigh the beaker together with a watch glass three times to 0.001 g. Write your
initials in pencil on a piece of filter paper and fold the paper twice into a one-quarter “pie slice”.
Weigh the paper three times to 0.001 g and set it aside. Add about 5 mL of distilled or deionized
water to the cooled NaCl-SiO2 mixture in the evaporating dish and stir gently for 30 s. Assemble
the setup shown in Figure 1 to filter the sand-salt solution through the weighed filter paper in a
funnel into the weighed beaker. The solution that passes through the filter paper in the funnel
should appear clear. (Do not stir or poke the sand in the funnel; you may rip the fragile, wet filter
paper.) Add 5 mL more distilled water to the evaporating dish and stir to dislodge any remaining
sand. Stir and pour the water and remaining sand through the funnel. Add another 5 mL of
distilled water to the evaporating dish and pour this through the funnel. If all the sand is now gone
from the evaporating dish, pour a fourth 5 mL portion of water directly onto the sand in the funnel.
If any sand remains in the evaporating dish, pour the last portion of water into the dish to remove
any sand residues, then pour all the water and remaining sand into the funnel.
Figure 1
ring stand
small iron ring
The tip of the funnel should touch the side
of the beaker for maximum flow rate.
C. Drying of SiO2: Carefully place your wet filter paper cone with sand on an unweighed watch
glass. Do not unwrap the paper cone. (Take care not confuse your filter paper and watch glass with
those of another student.) Place the watch glass in the laboratory drying oven set to 100 °C. After
about 30 minutes, the sand should be dry. Using rubber hand protectors or tongs, remove the watch
glass, dried filter paper and sand from the oven. Allow them to cool to room temperature and weigh
the filter paper and sand three times on the same balance you used earlier to weigh the filter paper
alone. Calculate the mass of recovered SiO2. Ask your instructor if you can throw the sand and filter
paper away as ordinary trash.
D. Drying of NaCl: While the SiO2 is drying in the oven, place the weighed beaker containing the
NaCl solution on a hot plate. Place the watch glass on top of the beaker. Heat the solution to
evaporate the water using the highest hotplate setting. At some point, the salt-water mixture will sizzle
and pop, but the watch glass will keep the spattering sodium chloride solution inside the beaker. Make
sure all condensation evaporates from the sides of the beaker. If the salt appears to be dry but there is
still water on the beaker walls or underneath the watch glass, continue heating for ten minutes after all
visible water drops are gone. Using beaker tongs or other hand protection, remove the beaker and
watch glass from the hotplate and let them cool to room temperature. Weigh the beaker, watch glass
and salt together three times on the same balance you used to weigh the beaker and watch glass alone.
Subtract the average masses to find the mass of NaCl in the original mixture. Afterwards, dissolve the
solid NaCl residue with tap water and rinse it down the drain.
3
Percent Calculation for Report: For your report, determine the percent by mass of each component
on your mixture using the following formula:
% of component A 
mass of component A
x 100%
mass of original sample
(Equation1)
Safety
Chemical splash goggles and a waterproof apron must be worn at all times during this and all
chemistry experiments, from the very beginning to the very end of the time you spend in the
laboratory.
Waste Disposal
None of the materials used in this experiment are considered to be hazardous waste for disposal
purposes. (Ammonium chloride is an eye irritant and slightly toxic if ingested.) You may rinse
sodium chloride residues down the drain with water. Your instructor will tell you how to dispose of
the sand you isolate from the mixture.
Cleanup
Before you leave the laboratory, wipe down all your work surfaces with a damp sponge. Do not leave
any solid or liquid residues on any balance or work surface.
4
General Chemistry I
CHEM-1030
Laboratory Experiment No. 2
Physical Separations Techniques
Data page
Unknown Mixture Number: __________
1
Mass of empty evaporating dish (three times)
Average Mass
2
Mass of evaporating dish plus original mixture (three times)
Average Mass
3
Mass of original mixture (by subtraction)
4
Mass of evaporating dish plus NaCl and SiO2 (three times)
5
Mass of NH4Cl lost (by subtraction)
6
Mass of dry filter paper (three times)
Average Mass
7
Mass of filter paper plus dry SiO2 (three times)
Average Mass
8
Mass Of SiO2 in original sample (by subtraction)
9
Mass of beaker and watch glass (three times)
Average Mass
10
Mass of beaker, watch glass and dry NaCl (three times)
Average Mass
11
Mass of NaCl in original sample (by subtraction)
5
Average Mass
6
General Chemistry I
CHEM-1030
Laboratory Experiment No. 2
Physical Separation Techniques
Report and Calculations
Use Equation 1 to calculate the percent by mass of each mixture component. For full credit, show all
calculation setups in this report. Answers must have the correct units and the correct number of
significant figures.
1.
Calculate the percentage of ammonium chloride in the mixture.
(4 points)
2.
Calculate the percentage of silicon dioxide in the mixture.
(4 points)
3.
Calculate the percentage of sodium chloride in the mixture.
(4 points)
7
4.
It is almost certain that the mass of the individual components added up will be less than the
mass of the original mixture. Calculate the percentage loss of your mixture in two ways:
a) Calculate the percentage lost by subtracting the three component loss percentages you
calculated in questions 1, 2 and 3 from 100%.
(2 points)
b) Calculate the percentage lost directly from the total mass of material lost. The total mass
lost is the difference between the mass of the original mixture and the sum of the masses of
the recovered components of the mixture.
(2 points)
5.
Would incomplete drying of the sand increase or decrease the apparent percent value of the
sand? Explain the reason for your answer.
(2 points)
6.
Would incomplete sublimation of the ammonium chloride increase or decrease the apparent
percent value of ammonium chloride? Explain the reason for your answer.
(2 points)
8
General Chemistry I
CHEM-1030
Laboratory Experiment No. 2
Physical Separations Techniques
Prestudy (first of two pages)
1. Use the CRC Handbook of Chemistry and Physics or other suitable on-line or print reference to
look up the physical properties of ammonium chloride (NH4Cl), silicon dioxide (SiO2, sand, quartz),
and sodium chloride (NaCl, table salt) and enter them in the table. Cite each reference.
(4 points)
Substance
Formula
sodium
chloride
NaCl
ammonium
chloride
NH4Cl
silicon
dioxide
(crystabolite)
SiO2
Melting Point
(C)*
Solubility†
Appearance
Reference
white powder
*Do not use the triple point (tp) for ammonium chloride; use the sublimation point (sp).
†
State whether each substance is soluble or insoluble in water
2. A student is assigned a 3.589 g mixture of iron filings, calcium chloride and sand. She separates the
mixture and recovers 0.897 g of iron, 0.923 g of sand and 1.686 g of calcium chloride. Calculate the
percentage of each of the three components recovered from the original mixture and the percent of
material lost during the separation process. Show your setups clearly.
(4 points)
9
PRESTUDY (second of two pages)
3. Using the physical properties you entered in the table on the previous page and the experimental
procedures you read on pages 2 and 3, complete the flow chart below. In the oval spaces, name the
reagents and/or conditions necessary to affect each indicated separation step. In the rectangular boxes,
write the names and formulas of the separated components.
(2 points)
Mixture of NaCl, SiO2, NH4Cl
heat to 340C
SiO2 and NaCl
NH4Cl vapor
10