Laboratory
2
Separation of the Components of
a Mixture
Objectives
•
Use extraction methods to separate the components of a mixture and determine the mass percent of each component of the mixture.
•
Learn the basic concepts of solubility (solute, solvent, insoluble, precipitate, soluble).
•
Illustrate simple organic and inorganic compounds and be able to identify
as such when given the chemical formula.
Introduction
Outside of the chemistry laboratory, we rarely encounter pure substances. The air
we breathe is a mixture of different gases, predominantly nitrogen and oxygen.
,QIDFWLWLVFKDOOHQJLQJWR¿QGH[DPSOHVRISXUHVXEVWDQFHVLQRXUVRFLHW\³$OXminum” foil, for example, is not pure aluminum but a mixture of Al and Al2O3, a
“nickel” coin is 25% Ni and 75% Cu, and a “silver dollar” coin has a Mn core with
an outer coating of brass that is comprised of Cu, Zn, Mn, and Ni. Mixtures with
a uniform substance throughout are called homogenous, and ones with varying
compositions of two or more distinct components and are called heterogeneous.
Inside the chemistry laboratory you are provided with many very pure substances,
(although an extremely careful analysis would reveal that these too contain some
impurities and are often mixtures). Where do pure substances come from? Usually
from the separation of a mixture.
Separation of a mixture is a process that depends on a difference in a physical
property of the components. In Lab 1 and lecture you learned about the property of
13
Laboratory 2
density. This property can be used to separate components in a mixture. During the Gold
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gravel in a shallow pan. The less dense materials would wash away, leaving the denser
substances (like gold!) in the bottom of the pan.
In this experiment you will separate the components of a mixture by using differences in
solubility. Solubility is the amount of a substance, the solute, that will dissolve in a given
amount of another substance, the solvent. Solubility is a property you are quite familiar
with. Every time you dissolve sugar in a cup of tea you are “experimenting” with solubility.
Other kitchen experiments involving solubility you may have tried include adding salt to
oil (salt dissolves in water; does it dissolve in oil?), or adding salt to a pot of water and then
boiling away the liquid (is salt left in the pot, or does it “vanish”?).
Discussion
2.1 Organic and Inorganic Compounds
The heterogeneous solids that you will be separating in this lab are comprised of inorganic
and organic substances (X, Y, and Z). Organic chemistry is described as the chemistry of
compounds of carbon. Organic materials contain carbon and hydrogen atoms that are usually combined with other non-metals like nitrogen and oxygen. Inorganic materials refer
to non-organic substances, though it is possible to have carbon atoms in inorganic materials
like carbonates (CO3í) and cyanides (CNí). You will see organic compounds from time to
time throughout general chemistry and this lab, but there are also entire courses dedicated
to the study of organic chemistry. You should be able to look at a chemical formula and
determine if it is organic or inorganic. In this lab course the organic waste and inorganic
waste produced are always disposed of separately.
Question 2.1: Identify the following compounds as organic or inorganic:
14
SrBr2
CH4
C6H6
NaCl
RuO2
Br2O
CH3CH2OH
C(graphite)
NiCO3
KCN
The separation for this lab will be achieved by using the differences in solubility among the
components. Solubility will be discussed more in-depth later in the course, but we encounter this property regularly. We know that salt will dissolve in water and sugar will dissolve
in our coffee and tea to make homogeneous solutions. Contrarily, we also know that oil
will not mix with water and the two will remain separated in a heterogeneous solution.
Solubility is greater when the solute and solvent are more alike. The organic component X
will tend to dissolve in organic solvents, and the inorganic component Y will tend to dissolve in water or other similar solvents. Most metal carbonates are insoluble, that is to say
they do not dissolve readily in most solvents, including water (and these are designated as
component “Z”). Researchers have extensively studied the solubility of many compounds,
and this data is compiled in the CRC Handbook of Chemistry and Physics for us to reference. Organic components (designated “X” in this experiment) tend to dissolve in organic
solvents, and inorganic components (designated “Y”) tend to dissolve in water or other
similar solvents.
Separation of the Components of a Mixture
Use the CRC Handbook of Chemistry and Physics, available online. Information will be
found in either the table of “Physical Constants on Inorganic Compounds” or “Physical
Constants of Organic Compounds.” This may also be available in print form at the Science
and Engineering Library.
Organic or
Inorganic
Compound
Solubility
in Other
Solvents
Solubility in
Water
Classify as X,
Y, or Z
Iron(III) oxide, Fe2O3
Lead nitrate, Pb(NO3)2
Naphthalene, C10H8
Nickel(II) carbonate,
NiCO3
Safrole, C10H10O2
Zinc acetate,
Zn(C2H3O2)2
2.2 Separating a Heterogeneous Mixture
Add
dichloromethane
X
YZ
XY
Solid mixture with
organic, X (green) and
inorganic, Y and Z (orange)
X
Y
Z
X
©Hayden-McNeil, LLC
Addition of the organic solvent dichloromethane to the mixture will dissolve the organic
compound, X. The process of dissolving a compound out of a mixture is called extraction.
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must then be removed from the remaining solid.
Z
Y
Organic solvent has
dissolved X, leaving Y
and Z (orange) as solids
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is a mechanical device that rotates test tubes causing the solid, more dense, component
of the mixture to quickly gravitate to the bottom of the test tube. The liquid may then be
poured off, or decanted, to separate the two. This process is shown on the following page.
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Laboratory 2
Y Z
Z Y
X
Y
Centrifuge
Z
X
Z
Y
X
Decant
Dry
+
X
ZY
Y Z
X
X
X
ZY
Y Z
©Hayden-McNeil, LLC
Once the solid and liquid are separated, the next component may be removed by the same
process but by now using water as the solvent. The water-soluble inorganic component Y
is removed in this process, leaving the water-insoluble component Z.
Question 2.2: Choose a solvent that will separate each of the following pairs and
state which compound will dissolve and which will remain solid.
a. Fe2O3 and Pb(NO3)2
b. Fe2O3 and safrole (C10H10O2)
c. Pb(NO3)2 and naphthalene (C10H8)
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16
Separation of the Components of a Mixture
Solid sample in test tube
components X, Y, and Z
Add dichloromethane
and centrifuge
X
YZ
XY
X
X
Part B
ZY
Y Z
Decantate solvent
with component X
Solid components
Y and Z
X
X
Evaporate and weigh
component X
Steps 7–8
ZY
Y Z
Add water
and centrifuge
Dry and weigh
component Z
Decantate water with
component Y
Evaporate and
weigh component Y
Steps 11–12
Y
Y
Solid component Y
Z Z
©Hayden-McNeil, LLC
Part C
Part D
Solid component Z
Materials Required
Equipment
4-mL test tube
glass wool
watch glasses
crucible and cover
10-mL graduate cylinder
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medicine dropper
test tube holder
Bunsen burner
thin-walled rubber tubing
ring stand, ring, clay triangle
Common Equipment
analytical balances
centrifuge
Chemicals
dichloromethane, CH2Cl2
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distilled water, H2O
unknown sample
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Laboratory 2
Cautions
Dichloromethane is a suspect carcinogen. Be careful to avoid burns from the ring, triangle
DQGWKHRSHQÀDPHFume hoods must be on.
Procedure
Answer questions in your lab notebook as you go along. Discussions with your peers and
TAs are encouraged.
A. Obtain weights of sample and containers.
1. Weigh a 4-mL test tube.
Question 2.3: Which balance should you use: top-loading or analytical?
2. Place a plug of glass wool in the open end of the test tube and reweigh. Remove the
plug and set it aside on a small watch glass.
3. Add approximately 0.6 g of sample to the test tube and reweigh.
4. Weigh a large watch glass on the same balance used in step 1.
5. Weigh a crucible and cover on the same balance used in step 1.
B. Extract the sample with the organic solvent, dichloromethane.
$GGDSSUR[LPDWHO\P/RIGLFKORURPHWKDQHWRWKHWHVWWXEH¿OODERXWKDOIZD\FRQtaining the sample and stir for 2 minutes to dissolve the organic component. Centrifuge
the test tube for 30 seconds and use a micropipet to remove the decantate (the liquid
above the solid). Dispense the decantate into the weighed large watch glass. Be careful
not to remove any solid with the decantate or to spill the liquid.
Question 2.4: At this point in the lab you have a test tube with solid and a watch
glass with liquid. Which contain the components Y and Z? Where is component X?
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7. To ensure that all of the organic component has been separated, wash the sample with
dichloromethane, as you did in step 6, two more times. Set the watch glass at the front
of your hood so that the solvent will evaporate while you continue with the rest of the
experiment.
8. When the solvent has completely evaporated, weigh the watch glass containing the
solid organic component, X.
18
Separation of the Components of a Mixture
C. Extract the sample with the inorganic solvent, water.
9. To the sample in the test tube (components Y and Z), add approximately 2 mL of waWHUWRWKHWHVWWXEH¿OODERXWKDOIZD\DQGVWLUIRUWZRPLQXWHVWRGLVVROYHWKHVROXEOH
inorganic component, Y. Centrifuge the test tube for 30 seconds and use a micropipet
to remove the decantate. Dispense the decantate into the weighed crucible. Again, be
careful not to remove any solid.
Question 2.5: Which component is now in the decantate?
10. To ensure that all of component Y has been separated, wash the sample with distilled
water, as you did in step 9, two more times.
11. Place the crucible on a clay triangle, ring, and ring stand. Put the cover on the crucible,
leaving a small gap to let steam escape. Heat gently to evaporate the water. Heating too
rapidly will cause spattering and loss of sample. (See Appendix D for use of the gas
burner.)
12. When the water has all evaporated, allow crucible, cover, and soluble inorganic component, Y, to cool before weighing.
D. Weigh the remaining solid.
13. The solid remaining in the test tube is the insoluble metal carbonate, component Z.
Replace the glass wool plug and heat the test tube gentlyRYHUDORZR[\JHQÀDPH
to dry the solid. If heated too strongly, the carbonate will decompose. Cool to room
temperature.
14. Weigh the test tube containing solid carbonate, Z, with the glass wool in place.
Waste Disposal
Dissolve the solid organic component in 5 mL of CH2Cl2 and place in the organic solvents
beaker. The water-soluble inorganic component can be rinsed down the sink. Dispose of
the insoluble inorganic compound in the metal salts beaker. The waste disposal sheets must
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containers.
19
Laboratory 2
20
Laboratory 2 Report Sheet
Separation of the Components of a Mixture
Name: _________________________________ Date: ______________ TA Name: ____________________
Mass of test tube and sample
Mass of test tube
Mass of sample
Mass of watch glass and organic solid
Mass of watch glass
Mass of organic solid, X
Percent of organic component, X
Mass of crucible, cover, and soluble inorganic
Mass of crucible and cover
Mass of soluble inorganic, Y
Percent of soluble inorganic component, Y
Mass of test tube and glass wool, and insoluble
inorganic, Z
Mass of test tube and glass wool
Mass insoluble inorganic component, Z
Percent of insoluble inorganic component Z
Show a sample calculation for each calculation performed.
21
Laboratory 2
22
Report Sheet
Laboratory 2 Answer Clinic
Separation of the Components of a Mixture
Question 2.1: Identify the following compounds as organic or inorganic:
SrBr2
CH4
C6H6
NaCl
RuO2
Br2O
CH3CH2OH
C(graphite)
NiCO3
KCN
Answer
The organic substances include CH4, C6H6, and CH3CH2OH. The remaining substances in this
question are inorganic.
Question 2.2: Choose a solvent that will separate each of the following pairs and state which
compound will dissolve and which will remain solid.
a. Fe2O3 and Pb(NO3)2
b. Fe2O3 and safrole (C10H10O2)
c. Pb(NO3)2 and naphthalene (C10H8)
Answer
Fe2O3 and Pb(NO3)2. The Pb(NO3)2 is soluble in water; Fe2O3 will remain a solid.
Fe2O3 and safrole (C10H10O2). The safrole is soluble in organic solvents, like dichloromethane;
Fe2O3 will remain a solid.
Pb(NO3)2 and naphthalene (C10H8). The Pb(NO3)2 is soluble in water; naphthalene will remain a
solid.
Question 2.3: Which balance should you use: top-loading or analytical?
Answer
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the analytical balance should be used.
23
Laboratory 2
24
Answer Clinic