PREPARATION OF PURE COMPOUNDS 



DETERMINATION OF THE MELTING-POINT. 



A small quantity of the finely powdered substance is introduced 

 into a melting-point tube; this is attached! to a thermometer and the 

 two are heated together in a bath until the substance is seen to melt. 

 The first determination of an unknown substance is usually only 

 approximate: it is repeated, heating rapidly to within 10 and then 

 more slowly. 



A small beaker containing water is used as a bath if the melting- 

 point is below 1 00, and the thermometer in a cork is held in the 

 centre of it by a clamp. The beaker is heated over a gauze by a 

 small flame and the liquid is stirred with a circular glass stirrer. 



A flask of about 50 ex. capacity with a long neck (10-20 cm.) filled 

 about two-thirds with strong sulphuric acid is more generally used. 

 The thermometer is secured in a cork into which a notch is cut to 



FIG. 14. 



allow hot air to escape when the flask is heated and to see the gradua- 

 tions if the mercury reaches this level. The flask is held by a clamp 

 and heated with a flame directly, the burner being inclined at an angle 

 and held by the hand so that it is heated round and round and not 

 directly in the centre (Fig. 1 4). After frequent use the acid becomes dark 

 in colour, but it will become clear again if a tiny crystal of potassium 

 nitrate be added. 



Paraffin wax is most generally used when the melting-point of a 

 substance is above the boiling-point of sulphuric acid (290). The 

 solid substance is introduced and melted until it fills two-thirds of the 

 space. It becomes brown after being used several times and must be 

 renewed. 



