لخّصلي

The melting point of a solid, the temperature at which it transitions to liquid at atmospheric pressure, is a useful physical constant for identification. Strong intermolecular forces in a solid lead to high melting points, while weak forces result in low melting points. Pure solids melt sharply within a 1-2°C range, while impurities lower and broaden this range. Insoluble impurities don't affect the melting point. Mixture melting points help identify unknown compounds; if a mixture of two compounds melts sharply at the same temperature as each individual compound, they are identical. Otherwise, the mixture's melting point will be lower and its range broader. Melting point determination involves heating a sample in a capillary tube immersed in an oil bath, noting the temperature range of melting. Factors affecting accuracy include particle size, amount of sample, and heating rate, with the latter being most critical. The experiment aims to determine melting points of pure and impure solids and identify an unknown substance using its melting point.

The melting point of a solid is the temperature at which transition from
solid to liquid occurs at atmospheric pressure; or the temperature at which
solid and liquid phases are in equilibrium at a pressure of one atmosphere.
The melting point is practically unaffected by changes in external pressure,
making it a convenient physical constant for the identification of solids.
Many organic compounds are solids at room temperature as a result of
strong intermolecular forces which hold the individual molecules together
in a crystal lattice. The nature and strength of these intermolecular forces
are responsible for the observed differences in melting point. In general, if
the forces are strong, the melting point will be high, and if they are
relatively weak, the melting point will be low.
A pure solid has a sharp melting point and will melt within a narrow range
of 1-2 oC. Soluble impurities affect the melting point of a solid in the
following manner:
a. Lower the melting point of the substance, with the upper limit
considerably below the true melting point. The presence of an
impurity in the molten compound, reduces its vapor pressure thus
lowering the melting point of the compound (Figure 5a). The greater
the amount of impurity, the greater is the melting point depression
(Figure 5b).
b. Broaden the melting point range. Depending on the amount of
impurity, the melting process may extend over a range of 2-20 oC or
more. Insoluble impurities (e.g., glass, sand ...etc.) do not affect the
melting point or the melting point range.
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liquid pure
impure
mp
presure
solid
temperature
Figure 5a.
Vapor pressure-temperature diagram.
100 80 60 40 20 0 mol % X
0 20 40 60 80 100 mol % Y
Figure 5b.
Temperature-composition diagram.
Mixture melting points can be used in the following manner to determine
whether two compounds are the same or different even though they have
similar melting points. Assuming that a given organic compound (A) melts
sharply at 120 oC, and benzoic acid (compound B) also has a melting point
of 120 oC. Is compound (A) benzoic acid or a different compound?
If compound (A) is benzoic acid, then a mixture melting point of (A) and
(B) will melt sharply at 120 oC, i.e. the same as each individual compound
alone. If, on the other hand, compound (A) is not benzoic acid, then the
mixture melting point of (A) and (B) will be lowered and the melting
range will be broadened. Since they are different compounds, each behaves
as an impurity in the other.
GENERAL PROCEDURE
Apparatus. A simple device for determining melting points is shown in
Figure 6. It consists of a thermometer fitted through a cork and suspended
into a long-necked flask which is three quarters filled with a high boiling
and stable liquid like paraffin oil, di-butylphthalate or silicon oil.
The thermometer bulb should be
about 1 cm above the bottom of
the flask. The sample in the
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capillary tube is fastened to the
thermometer with a rubber band
placed above the level of the oil.
The capillary tube should be
close to and on a level with the
thermometer bulb.
Figure 6.
Melting point apparatus.
Capillary melting point tubes are about 6-7 cm in length and 1 mm in
diameter. They are sealed by rotating one end of the capillary tube in the
edge of a small hot flame. The dry solid is ground to a fine powder on a
piece of paper with a spatula. The open end of the capillary is then pushed
into the powder which is forced down the capillary tube by gently tapping
the closed end on the bench top. This is repeated several times until the
solid is densely packed at the bottom of the tube to a height of 2-3 mm.
Procedure. To determine the melting point of a solid, a small amount of
the powdered substance is introduced into a capillary tube which is then
attached to a thermometer and placed in the oil bath. The bath is heated
rapidly to within 20 oC of the expected melting point then slowly, and at a
constant rate of 2-3 degrees per minute, close to the melting point. The
temperature at which the solid begins to melt, and that at which it is
completely liquid, is recorded as the melting point range of that substance.
The melting point range is affected by a number of factors in addition to
that of purity. Particle size, amount of material used, density of packing in
the capillary tube, thickness of the capillary tube and the rate of heating of
the liquid bath, are all factors that should be carefully considered to ensure
an accurate melting point. The rate of heating is the most critical factor
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affecting experimental results, and should be carefully monitored,
particularly close to the expected melting point.
OBJECTIVES

1. Determining the melting point of a pure organic solid.


2. Determining the melting point of an impure organic solid (mixture).


3. Identifying an unknown from its melting point.
   Having done this experiment you will have seen the effect of an impurity on
   the melting point of a solid substance and the use of the melting point in
   characterizing organic solids.
   EXPERIMENTAL
   MATERIA