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MELTING POINT AND BOILING POINT1
Melting point is the temperature at which a solid and liquid exist in equilibrium with each
other at a given pressure, or it is the temperature at which a solid begins to change to a liquid
under a pressure of one atmosphere. The physical properties of a compound depend largely upon
the kind of bonds which hold its atoms together in the molecule. Melting point is no exception.
Melting occurs when a temperature is reached at which the thermal energy of the particles is
great enough to overcome the intercrystalline forces that hold them in position. In general,
melting points of ionic compounds are greater than melting points of polar compounds, which
are greater than melting points of purely covalent, non-polar compounds of similar molecular
weight. The electrostatic forces holding the ions together in an ionic substance are powerful and
are overcome only at a very high temperature. Sodium chloride has a melting point of 801°C.
Dipole-dipole interaction is the attraction of the positive end of a polar molecule for the negative
end of another polar molecule. The forces binding polar molecules are weaker than interionic
forces and therefore less energy is required to overcome these forces and melt the solid.
However, as a result of dipole-dipole interaction, polar molecules are generally held to each
other much more strongly than are non-polar molecules of comparable molecular weight. There
must, however, be forces between the molecules of a non-polar compound since such
compounds can solidify. Such attractions are called Van der Waals forces.
Organic compounds are less polar than inorganic compounds and therefore generally
have lower melting points than inorganic compounds with corresponding molecular weight. In
addition to polarity, the molecular weight of the compound is also important. Thus an increase
in molecular weight tends to increase the melting point.
The purpose of the first part of the laboratory course is to introduce you to many of the
techniques used to purify and separate compounds. Melting point (and, to a much lesser extent,
boiling point) is one important way of establishing whether the desired purification
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For a general review see: Practical Organic Chemistry, 5th Ed., by A.I. Vogel, Longman, 1989.
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has been obtained. It is also a possible means of identification of a specific compound. This will
be discussed further below. A melting point range of 1°C is usually indicative of a pure sample.
Melting points of mixtures of compounds also give a great deal of information.
A phase diagram (see Fig. 1 on page 4) can be drawn to show the melting point behaviour
of mixtures of two pure compounds, A (melting point 54°C) and B (melting point 62°C). Such
diagrams are drawn from experimental data obtained when the melting points of mixtures of
A and B of varying compositions are taken. The various areas of this diagram have been
labelled. Below the line CE (i.e. a temperature of 20°C for this system) no liquid can exist. This
line CE is known as the eutectic line. The point D, at which the mixture of A and B behaves
like a pure compound and has a sharp melting point, is known as the eutectic temperature.
As the temperature of a pure compound is raised, there is a sharp transition from solid to
liquid; however, as the temperature of an impure compound is raised, there is a gradual transition
from solid to liquid. Consider a compound B, contaminated by another compound A, in such a
way that the resulting mixture is 75% pure B and 25% A. The phase diagram in Figure 1 can be
used to give information about the melting point range. By following the vertical dotted line
from 75% B, you can see that the mixture remains solid until the eutectic temperature, T (= 20°C
for this system) is reached and then liquid appears gradually until, in this example, the entire
mixture is liquid at about 40°C.
We can draw two conclusions then, about the effect of
impurities on the melting point of pure compounds.
1. Impurities cause a melting point range to be observed, rather than a sharp transition
from solid to liquid, with the size of the range often indicating the amount of impurity. (Note,
however, that a sharp melting point would also be observed, fortuitously, at the eutectic point.)
Often when impurities are present in small amounts a temperature close to the melting point of
the compound must be reached before there is enough liquid to be seen in the common methods
of melting point determination. The temperature at which liquid is first observed is often
referred to as the sintering temperature; therefore, in recording melting point, two temperatures
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should always be noted, sintering temperature and the temperature at which all the solid finally
becomes liquid.
2. Impurities cause a depression or lowering of the melting point of a pure compound.
These two facts can be used in identifying unknown compounds in the following way.
Suppose we have an unknown compound X which we suspect to be the known compound A. If
these two chemicals are mixed together and melting points are taken, two possibilities exist. If X
really is compound A, a sharp melting point at the melting point temperature of compound A
will be observed. If X is not compound A, it will act as an impurity and cause a melting point
range, terminating at a temperature below the melting point temperature of compound A.
Procedure for determining melting point.
Take a very small amount (a few grains) of a sample, and take its melting point on the
melting point apparatus. Record each melting point you determine twice - the first time heating
quickly to get an approximate melting point, and the second time heating much more slowly. In
this way you can observe and note both the "sintering" temperature (i.e. the temperature at which
liquid begins to appear) and the temperature at which the sample completely liquefies.
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Boiling point
The boiling point of a liquid is that temperature at which the vapour pressure of the liquid equals the
atmospheric pressure. Boiling point can also be used to indicate purity and to identify a compound but it is
not as satisfactory a criterion as melting point, for several reasons. First, the boiling point can be greatly
affected by atmospheric pressure changes whereas melting point shows a negligible change for the
fluctuations in atmospheric pressure. Secondly, impurities tend to have an erratic and often unpredictable
effect on the boiling point whereas impurities have a much more predictable effect on the melting point (see
earlier discussion). In the third place, super-heating and/or "bumping" of the liquid can occur and this will
often lead to erroneous boiling point results. Despite these problems, the temperature at which a compound
boils can give useful information about its structure. The type of bond existing in the compound and the
molecular weight of the compound both affect the boiling point of the compound in much the same manner as
they affect the melting point (see page 1).
Procedure for determining boiling point
Set up an apparatus as shown in Figure 2 on page 4 with the thermometer bulb just above the liquid
level and a groove cut in the stopper supporting the thermometer. Before you begin to heat your liquid be
sure to add 2 or 3 boiling chips to ensure smooth boiling and to avoid super-heating. Gas bubbles in the
boiling chips escape and break the surface of the liquid, and thus prevent superheating. DO NOT CONFUSE
THIS BUBBLING WITH THE ACTUAL BOILING OF THE LIQUID. Whenever heating is resumed
after boiling has been interrupted for any reason be sure to add fresh boiling chips, for liquid will have entered
the pores of the previous ones on cooling; however, to avoid dangerous frothing or spillage of the hot liquid,
never add boiling chips to a hot solution.
The liquid is considered to be boiling when quiet bubbling is observed, together with vapour
condensation on the sides of the test tube and the thermometer bulb.
Record the temperature change once a minute until the temperature remains constant for several
minutes. This constant temperature is the boiling temperature of the liquid at atmospheric pressure.
Note that, as for the melting point determinations, this temperature is an "uncorrected" value (i.e. no
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account is taken of thermometer stem correction) but this is true for most such values recorded in the scientific
literature.