Experiment #3: Sublimation and Melting Point Determination
Background
The vapor pressure of a solid varies with its temperature. There are some solids that
can pass directly into vapor without passing through the liquid phase. This is called
sublimation. This process occurs if the vapor pressure of a substance is greater than the
atmospheric pressure at the melting point. Since the vapor can solidify, the vaporization-
solidification cycle can be used as a purification process. Sublimation, therefore, is a
technique used to purify solid mixtures. The purification, however, can be done only if the
impurities are non-volatile or have significantly lower vapor pressure than the pure
compound.
Non-polar substances are symmetrical compounds having high melting points and
high vapor pressures. The case by which a non-polar substance can break away from the
solid state is determined by strength of intermolecular forces. Symmetrical compounds
have relatively even distribution of electron density and a small dipole moment. A smaller
dipole moment means a higher vapor pressure due to lower electrostatic attractive forces in
the crystal.
Melting point determination can be employed to ascertain the identity and purity of
an unknown compound.
Materials:
Bunsen Burner Iron Ring
Beaker Wire Gauze
Watch Glass Capillary Tubes
Thermometer Rubber Band/Thread
Iron Stand and Clamp Cooking Oil (unused)
Cork Benzoic Acid
Pre-Lab Assignment:
1. What is Sublimation? Melting point?
- Sublimation is at transitioning state or a change in a state of matter. Through
this, a substance changes to a gas state without passing through a liquid phase.
Melting point, temperature at which a solid forms of a pure substance (becomes
a liquid).
2. In this experiment, why is it preferable to use an oil bath instead of water bath?
- Oil bath is preferred than water bath because the initial boiling point of oil used
is more than 100 degrees Celsius which is greater than the boiling point of water.
Vapor generated by oil are less as compared to water at normal working
temperature, less chance of exposure to the latent heat of water and its
relatively uniform rise in temperature.
3. Why is it necessary to grind sample?
- In an experiment like sublimation, it is necessary to grid the sample because this
allows efficient and uniform packing of sublimate in the capillary tube.
4. Why is it important to pack the sublimate uniformly?
- Packing the sublimate uniformly is important because the tube containing
uniformly packed sublimate will melt consistently and evenly to give sharp and
reproducible melting point determination.
Post-Lab Assignments:
1. What are the limitations of sublimation as purification method?
- Sublimation basically requires that the compound we are trying to separate is
volatile while everything in the mixture is not volatile.
- Sublimation is not relating to crystallization
2. Give other methods of determining the melting point of a compound.
- Another method is to use a very small amount of solid placed in a capillary tube
sealed at one end. The tube can be strapped to a thermometer via elastics and
immersed in liquid than can be slowly heated.
- Another method is to slowly heat the solid and record the temperature at
regular intervals.
Data Sheets
Experiment 3
Sublimation and Melting Point Determination
1.
a. Weigh of Salicylic used 5.02g
b. Weigh of Evaporating dish + Salicylic 97.95g
c. Weigh of empty Evaporating dish 92.93g
d. Weigh of sublimate of Salicylic 0.23g
e. Weigh of Naphthalene used 5.03g
f. Weigh of Evaporating dish + Naphthalene 97.96g
g. Weigh of empty Evaporating dish 92.93g
h. Weigh of sublimate of Naphthalene 1.3g
2.
a. Recovery Percentage of Salicylic
0.23 g
x 100 %=4.58 %
5.02 g
b. Recovery Percentage of Naphthalene
1.3
x 100 %=25.84 %
5.03
