Identification of a Pure Substance
Purpose:
Determine the identity of an unknown pure substance by determining several of its physical
properties and comparing the results with the values for known substances.
Background:
Gold is yellow, roses are red, salt is white and crystalline, lead is “heavy”, and water is a
colorless, clear liquid. Physical properties are often used for the identification of a chemical
substance. Characteristic physical properties such as color, physical state, solubility, melting
temperature, and density are commonly used to help identify pure substances. In this experiment
you will be given an unknown liquid. You will determine several physical properties of this
liquid and compare your experimental values with the values in the following table to determine
the identification of your unknown liquid.
Substance
Density (g/mL @
20oC)
Melting
Temperature (oC)
Boiling
Temperature (oC)
Solubility
in Water
Solubility
in Alcohol
Acetone
0.79
- 95
56
vs
vs
Cyclohexane
0.78
6.6
81
i
vs
Ethanol
0.79
- 117
78
vs
vs
Ethyl acetate
0.90
- 84
77
ss
vs
Methanol
0.80
- 98
65
vs
vs
2-methyl-2-propanol
0.79
26
83
vs
vs
2-methyl-1-propanol
0.81
-108
107
ss
vs
Nitrobenzene
1.20
5.7
211
ss
vs
1-propanol
0.80
- 126
97
vs
vs
2-propanol
0.79
- 90
82
vs
vs
Tricholorotrifluoroethane
(TTE)
1.50
-35
48
i
vs
Water
1.00
0
100
vs
vs
Equipment:
Balance
Bunsen burner
Stirring rod
Test tube holder
10-mL graduated cylinder
Thermometer
safety goggles (worn at all times during the lab)
Iron ring and ring stand
Medium test tubes, 18 x 150
Styrofoam cup
Test tube rack
250-mL beaker
Materials:
Unknown liquid
Distilled water
Boiling chip
Ice
Procedure:
Part One Density
Tare a clean, dry 10 mL graduated cylinder on the electronic balance. Add 10 mL of your
unknown liquid to the graduated cylinder. Use a micropipet to adjust the volume. Record the
volume of the liquid in the graduated cylinder to the nearest 0.1 mL. Weigh the graduated
cylinder and your unknown liquid to the nearest 0.01 gram. Calculate the density of the
unknown liquid. Record the density of the unknown liquid. Return the liquid to the sample
container for the other determinations.
Part Two Solubility in Water
The solubility of a substance depends upon temperature. Solubility can be expressed as grams of
solute per 100 grams of solvent for a specified temperature. Your solubility tests will be less
quantitative and will be reported simply as very soluble (vs), slightly soluble (ss), or insoluble
(i).
Using a clean medicine dropper, add 20 drops of water to a medium test tube. Use the
micropipet to add 20 drops of the unknown, 2 or 3 drops at a time, shaking the tube after each
addition. If all of the drops mix completely forming a single phase, the substance is very soluble.
If, after the first few drops the unknown does not seem to dissolve but forms two layers, the
substance is said to be insoluble. If at first a few drops seem to dissolve but later two layers form
as more is added, the substance is said to be slightly soluble. Record your results.
Part Three Solubility in Alcohol
Repeat the procedure used in part two of this experiment substituting isopropyl alcohol as the
solvent. Record your results.
Part Four Melting or Freezing Temperature
The temperature at which the liquid and solid phases of a substance are both present is called its
melting or freezing temperature.
Measure about 10 mL of the unknown substance into a clean, dry medium test tube. Clamp the
test tube on a ring stand and place a thermometer into the liquid. Obtain an ice-slush mixture in
a styrofoam cup. Lower the test tube into the ice bath and continuously monitor the temperature.
Stir the liquid gently with the thermometer. Note the time and temperature when crystals first
appear. Continue to monitor the temperature and record the freezing temperature based upon
your observations of crystal formation and constant temperature.
If the liquid does not begin to form crystals by the time the temperature has become nearly zero,
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record the freezing temperature to be below 0oC. If the liquid did freeze, record the freezing
temperature when the temperature remained relatively constant as the crystals formed. Leave the
thermometer in the test tube. Save the entire system for the boiling temperature measurement.
Part Five Boiling Temperature
Add a boiling chip to the unknown liquid in the test tube. Half-fill a 250 mL beaker with tap
water and place it on a wire gauze on a ring stand. Lower the test tube into the beaker of water
and slowly begin heating the water with a Bunsen flame. CAUTION: Keep the burner flame
away from the mouth of the test tube. The unknown may be flammable. Continuously monitor
the temperature and note the temperature when the liquid first starts to boil. Continue to take
monitor the temperature until a constant boiling temperature is observed. Stop heating the water
bath and remove the test tube from the water bath. Return any remaining unknown liquid to the
original vial and hand in to Mr. Gensits.
If your unknown does not boil by the time the tap water starts boiling, stop heating, remove the
test tube from the hot water, and record the boiling temperature to be greater than 100oC. If the
liquid did boil, record the boiling temperature when the temperature remained relatively constant
as the liquid boiled.
Part Six Identifying the Unknown
Using the physical properties determined from your unknown substance, choose the substance
listed in the table on page one of the experiment which best fits the data. Account for any
differences in the listed data and your experimentally determined values. (Note: In the table
densities are given @ 20oC and the boiling temperatures are measured for 1 atmosphere
pressure.)
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