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Determination of Melting Point and Boiling Point Experiment

 

a. Determination of Melting Point Experiment

 

The melting point of a substance is defined as the temperature at which a substance changes from the solid state to the liquid state. A pure substance has a sharp melting point. Therefore, melting point is a characteristic physical constant of a substance.

 

Procedure

The most convenient apparatus for the determination of melting point is Thiele's tube. It is a hard glass test tube which is fused with the two ends of a V—shaped glass tube at the middle and at the bottom as shown in the figure. The tube can be closed with a one holed cork which carries a sensitive mercury thermometer. The cork is provided with a side slit. About three fourth of Thiele's tube is filled with a high boiling liquid like conc. sulphuric acid or liquid paraffin.

 

The substance, the melting point of which is to be determined, is powdered well. A capillary tube of about 5cm length, one end of which is fused, is taken and the powdered substance is introduced into it so as to get a column of about 0.5 cm.

 

The bulb of the thermometer is wetted with tube liquid paraffin or conc. sulphuric acid. Now the capillary tube is allowed to cling to the thermometer in such a way that the bulb of the thermometer and the specimen are as close as possible. The apparatus is now closed with the cork in such a way that the thermometer is introduced into the liquid. The bulb of the thermometer should be at a level of the upper side tube. The open end of the capillary tube should be above the liquid level. The apparatus is clamped to an iron stand and it is heated gently by waving the flame at the side tube. Due to the movement of the heated liquid molecules, the liquid will be uniformly heated up. Development of high pressure during heating is prevented by the escape of vapours through the slit on the cork. During heating the mercury thread slowly rises and at a particular time the solid substance in the capillary tube suddenly shrinks and melts. The temperature at this time is noted as the melting point of the substance.

 

Result : Melting point of the substance = ____°C

 

b. Determination of Boiling Point Experiment

Boiling point of a liquid is defined as the temperature at which the vapour pressure of the liquid becomes equal to the atmospheric pressure exerted on the surface of the liquid. Since the atmospheric pressure varies at different places, the boiling point of a liquid also varies. Boiling point is a characteristic physical property of a liquid. The boiling point of a liquid varies with the presence of impurities in it. The boiling point of a liquid can be determined in a boiling point apparatus. The apparatus consists of a pyrex glass tube which is provided with a side tube. The side tube is connected to an adapter which in turn is introduced into a receiver.

 

The liquid, the boiling point of which is to be determined, is taken in the apparatus. A few porcelain pieces are introduced into the liquid to promote uniform heating and to prevent bumping of the liquid. The mouth of the apparatus is closed with a one holed cork carrying a thermometer.

 

The boiling point apparatus is slowly heated. The liquid boils and the vapours escape through the exit. The steady reading on the thermometer at which the liquid distills constantly gives the boiling point of the liquid. The first few drops of the condensed liquid are discarded and the rest of the liquid is collected. The apparatus is then cooled and the experiment is repeated with the distilled liquid to get concordant results.

 

Results: Boiling point of the liquid = ____°C


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