Melting Point of an Organic Compound Video Lecture | Chemistry for JAMB

Melting Point of an Organic Compound
The melting point of a solid is defined as
the temperature at which the solid exists
in equilibrium with its liquid under an external
pressure of one atmosphere. On heating a solid,
its molecule absorbs energy in the form of
heat. As a result, the force of attraction
between the molecules decreases and the molecules
become increasingly separated. At a particular
temperature, called the melting point of the
solid, the separation of the molecules increases
by a large amount and the solid melts and
is converted into liquid.
Our aim here is to determine the melting point
of the given compound, naphthalene.
Materials Required:
Liquid paraffin in a 100 ml beaker, powdered
naphthalene, thin walled capillary tube of
approximately 8-10 cm length and 2 mm diameter,
thread, glass plate, thermometer, stand with
clamp, stirrer, spatula, hot plate.
Procedure:
Take a capillary tube and seal one end by
heating it in the flame of the Bunsen burner.
Now, using the spatula, make a heap of the
powdered naphthalene on the glass plate.
Push the open end of the capillary tube into
the heap. Some substance will enter into the
capillary tube.
Now tap the sealed end of the capillary tube
on the glass plate gently and fill the capillary
tube up to 2-3 mm.
Attach the capillary tube to the thermometer
using the thread.
Take the 100 ml beaker containing liquid paraffin
and place it over the hot plate.
Clamp the thermometer carrying the test tube
to the iron stand and immerse them in the
bath of liquid paraffin.
Start heating the liquid paraffin bath slowly
and stir the bath gently using the stirrer
to ensure uniform heating.
Note the temperature t1 when the naphthalene
starts melting.
Continue heating and note the temperature
t2 when the naphthalene in the capillary tube
is completely melted.
The average of the two temperatures t1 and
t2 gives the melting point of naphthalene.
MeltingpointofNaphthalene= (t_1+t_2)/2=(75+85)/2=80 degree celsius
Precautions:
Use dry and powdered sample for the determination
of melting point.
Packing of the powder should be uniform without
any big air gaps between the solid particles.
The liquid paraffin bath must be heated very
slowly and the bath is stirred gently to ensure
uniform heating.
The bulb of the thermometer and the capillary
attaching to it should not touch the sides
or the bottom of the beaker.