Title; To determine the purity of benzoic acid

crystals by melting point determination.

Thermometer Capillary tube

Benzoic acid crystalsAluminium block

Bunsen burner

122

Procedure • Seal one end of a capillary tube.• Dip the open end in the benzoic acid and

catch a small quantity of the crystals.• Tap the closed end off the desk top until

the crystals have lodged there.• Place the capillary tube in the melting

point apparatus as shown above.• Heat the aluminium block until the crystals

melt.• Note the temperature (123 degrees for

benzoic acid).

Discussion • If the crystals melt exactly at their

correct melting point they are very pure. If they melt at a lower or higher temperature and take 4 or 5 degrees to melt, they are impure.