Experiment 1 Identification of a Compound: Physical Properties
I NTRODUCTION
Gold is yellow, roses are often red, salt is white and crystalline, lead is "heavy," and water is
a colorless, clear liquid that freezes at 0°C and boils at 100°C. These are physical
properties of substances that are often used for their identification. The more common
physical properties include color, odor, density, solubility, melting point, boiling point, and
state of the substance (solid, liquid, or gas). Physical properties can be observed or
measured without any knowledge of the chemical reactivity of the substance. Additional
tests, tests that reveal more information about its chemical or physical properties, may be
necessary to confirm the purity of the substance. The purpose of this experiment is to
identify a substance based on its solubility, density, melting point, and/or boiling point, all of
which are physical properties and intensive properties (a property independent of sample
size) of a substance. In Experiment 2, the chemical properties of a substance are used for
its identification.
Solubility
Solubility: the maximum mass (usually in grams) that dissolves in a fixed mass (usually 100
g) of solvent at a given temperature. A substance has different solubilities in different
solvents, reflecting the differences in the molecular compositions and molecular structures
of the substance and the solvent. For instance, some substances, such as table salt, are
soluble in water but insoluble in gasoline. In this experiment, we examine the solubility of a
substance in three different solvents: water, ethanol, C2H5OH, and acetone, (CH3)2CO. If
further solubility tests are required then cyclohexane, C6HI2, is used as a solvent.
Qualitatively each substance is recorded as being soluble (complete dissolution) or
insoluble.
Density
Density: the ratio of the measured mass of the substance to its measured volume. A
substance with a large density has a large mass occupying a small volume. We commonly
say that lead is "heavy." What we really mean is that lead has a large density; the volume of
a lead object need not be large for it to have a large mass.
Melting Point
Melting point: the specific temperature that a solid changes completely into a liquid . At the
melting point the solid and liquid phases coexist; any lowering of the temperature results in
the formation of only solid, and any raising of the temperature results in the existence of
only the liquid phase.
Boiling Point
Boiling point: the temperature at which the vapor pressure (the pressure exerted by a vapor
when it is in dynamic equilibrium with its liquid) of a liquid equals atmospheric pressure
Atmospheric pressure: the force (or weigh) that a column of air exerts over an area of the
earth's surface
Intermolecular forces: interactive attractions and repulsions between molecules
When a liquid is heated, there is a temperature at which bubbles form spontaneously and
continue to form until all of the liquid has been converted to vapor. This constant
temperature is called the boiling point of the liquid, a temperature at which the liquid and
vapor phases coexist. The boiling point of a liquid depends on the prevailing atmospheric
pressure. The boiling point values listed in Table 1.1 are measured at normal atmospheric
pressure (defined as 1 atmosphere of pressure).
The melting and boiling points of a substance are characteristic of the magnitude of the
forces acting between molecules, called intermolecular forces. The greater the
magnitude of the intermolecular forces (i.e., the stronger the attraction between
molecules), the higher the melting and boiling points of the substance. As the
magnitude of the intermolecular forces is different for each substance, each has a
characteristic melting point and boiling point.
Table 1.1
Physical Properties of Some Common Laboratory Chemicals
Compound
Acetone
Acetamide
Acetanilide
Anthracene
Benzamide
Benzoic acid
Benzoin
2-Butanone
Cyclohexane
Cyclohexene
Ethanol
Ethyl acetate
Heptane
n-Hexane
1-Hexane
Methanol
Naphthalene
1-Propanol
2-Propanol
Water
Density Melting
3
o
(g/cm ) Point ( C)
0.79
-95
1.00
82.3
1.22
114
1.28
216
1.08
132
1.07
122
1.31
137
0.81
-86
0.79
6.5
0.81
-104
0.79
-117
0.90
-84
0.68
-91
0.66
-95
0.67
-140
0.79
-94
0.96
80.5
0.80
-127
0.79
-90
1.00
0
Boiling Water
o
Point ( C)
56
s (b)
221
s
304
290
249
344
80
81
83
79
77
98
69
63
65
218
97
82
100.0
s
s
s
s
s
s
s
-
Ethanol
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
Acetone
Cyclohexane
-
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
PROCEDURE
Obtain an unknown listed in Table 1.1 from your instructor.
A. Solubility
1. Solubility in Water. To a small test tube add a pinch (a sample about the size of a
grain of rice) of your solid or 3 drops of your liquid unknown to 1 mL of water. Agitate
the contents. Does your unknown dissolve? Record your observations. Describe the
solubility of your unknown as soluble(s) or insoluble(i) in each solvent.
2. Solubility in Ethanol and Acetone. Repeat the test with ethanol and acetone as
solvents. (Caution: Avoid breathing vapors; acetone is flammable--keep away from
flames
3. Solubility in Cyclohexane. If the solubility of the unknown is still uncertain, repeat
the solubility tests using cyclohexane as a solvent.
B. Density of Liquid Unknown
Complete two trials to determine the density of your liquid unknown.
1. Determine Mass and Volume. Determine the mass (∓0.001 g) of your smallest
laboratory beaker. Pipet 2 mL of the liquid sample into the dry, clean beaker. Measure
the combined mass of the liquid sample and beaker. (Note: Use a rubber pipet bulb in
drawing the liquid into the pipet.) Calculate the density of the liquid.
C. Melting Point of a Solid Unknown
1. Prepare the Sample. Fill a capillary melting point tube to a depth of 0.5 cm with the
solid unknown. Compact the solid to the bottom of the tube by dropping it several times
onto the lab bench through a glass tube at least 25 cm in length or by vibrating the
sample with a triangular file. Attach the tube to a 360°C thermometer with a rubber band
(or band of rubber tubing). Place the sample alongside the thermometer bulb.
2. Set up the Melting Point Apparatus. Obtain a 360°C thermometer. Assemble the
melting point apparatus below. Note that cooking oil is the liquid that is heated in the
50mL beaker.
:
3. Determine the Melting Point. Slowly heat the oil bath (at a rate of about 5°C per
minute) until the solid melts. (Caution: The oil bath may be at a temperature greater
than 100°C---do not touch!) Cool the oil bath until the sample solidifies; very slowly (at a
rate of about 1°C per minute) heat the bath until the solid again melts. Cycle the cooling
and heating of the oil bath until the melting point of the solid has been accurately
determined. Complete two measurements for the melting point of your unknown.
*Disposal: Dispose of the capillary tube into the broken glass container and the pour the
cooled oil back into the oil containers.
D. Boiling Point of Liquid Unknown
1. Assemble the Boiling Point Apparatus.
Place 10 drops (0.5 mL) of the liquid unknown
into a "'small" (75-mm) test tube and a boiling
stone. (Caution: The liquid may be flammable-extinguish all flames!) A piece of fire resistant
wire gauze with a small hole is placed on a ring.
Lower a thermometer (capable of measuring to at
least 150 C) until the bulb is 0.5-1.0 cm above
the surface of the liquid. Center the thermometer
by adjusting the thermometer clamp so that the
bulb does not contact the inner walls of the test
tube.
2. Measure the Boiling Point Adjust your
burner to a very low flame and manipulate it
beneath the hole in the gauze until a gentle
boiling of the liquid takes place. Gentle boiling is
essential for the success of this determination.
Violent boiling will result in a rapid loss of the
sample and possibly increase the danger of its becoming ignited by the burner flame.
(Caution: If fire occurs, remove the burner and extinguish the flame by blowing across
the opening of the test tube. Let it burn out if it doesn't blow out.) Record the boiling
point of the liquid when the vapor condenses on the thermometer bulb and drips off in a
series of steady drops and the temperature no longer rises. Complete two
measurements for the boiling point of your unknown.
3. Extinguish the Bunsen flame before disassembling the apparatus. Allow plenty
of time for the test tube to cool before disassembling and cleaning.• Disposal: Dispose
of the leftover unknown substance in the "Waste Organics “container in the fume hood.
Name _____________________________
Identification of a Compound: Physical Properties
Experiment1 : Prelab Assignment
1. What is the purpose of this lab?
2. From your general experiences, would you select water or gasoline to try
dissolving the following?
a. Table sugar (sucrose), used to satisfy your sweet tooth
b. Table salt (sodium chloride), thrown over your shoulder for good luck!
c. Motor oil that has dripped from your automobile onto the garage floor
d. Road tar on the side of your automobile
e. Fertilizer that has spilled from an overturned truck
f. Rubber from the tire marks left on your driveway
g. Blood from a "squished" mosquito
h. Adhesive from a pricing sticker that adheres to a glass vase
2. A student's liquid unknown boils at approximately 65°C and is soluble in water and
ethanol. Its measured density is 0.80 g/ml. Which substance in Table 1.1 is the
student's unknown?
3. What physical property, measurable in this experiment, distinguishes 1-propanol from
2-propanol? (Keep in mind the state of these compounds)
4. During the boiling point determination, when should the boiling point of a liquid be
recorded?
5. Identify the essential steps for placing a solid sample in a capillary tube.
Name _______________________________
Identification of a Compound: Physical Properties
Experiment1 : Prelab Assignment
Data & Calculations Table: (Show work if applicable.)
A. Solubility
Unknown Number
I. Solubility in water
2. Solubility in ethanol
3. Solubility in acetone
4. Solubility in cyclohexane, if necessary
B. Density of Liquid Unknown
1. Mass of beaker and liquid (g)
2. Mass of beaker (g)
3. Mass of liquid (g)
4. Volume of liquid (mL)
5. Density of liquid (g/mL)
6. Average density of liquid (g/mL)
C. Melting Point of Solid Unknown
1. Observed melting point (°C)
2. Average melting point (°C)
D. Boiling Point of Liquid Unknown
1. Observed boiling point (°C)
2. Average boiling point (°C)
E. Name of Unknown . Please justify your answer.
Conclusion Questions:
I. How does atmospheric pressure affect the boiling point of a liquid?
2. Suppose that in Part B, the liquid unknown remaining in the pipet is "blown out" after
delivery. Will the reported density of the liquid be too high or too low? Explain.
3. Besides the physical properties used to identify a substance in this experiment, what
other physical properties might be used to characteristically identify a substance?