PREPARATION OF PURE COMPOUNDS 5 



is bored to carry a thermometer. The position of the thermometer 

 is so adjusted that the bulb is just below or opposite the side tube and 

 not touching the walls. The side tube is connected by a cork to a 

 clean, dry condenser, which is supported by a clamp, and a slow stream 

 of cold water is allowed to flow through the condenser. A receiver 

 (flask) is placed at the other end of the condenser (Fig. i). 



A water condenser is not used for liquids boiling above I2O ; the 

 vapours are condensed by being passed through a simple tube (the 

 inner tube of the above condenser). The vapours of liquids boiling at 

 very high temperatures are condensed in the side tube of the distilling 

 flask. If the vapours condense to a solid on cooling, the solid is melted 

 by a flame so that the liquid runs into the receiver and does not block 

 up the side tube. 



Fio. i. 



Liquids boiling below 1 00 are heated on a water-bath, liquids 

 boiling above 1 00 are heated directly with a flame, which is moved 

 round and round under the bottom of the flask until boiling begins. 

 When boiling commences it must be kept on continuously and 

 vigorously and not interrupted by the removal of the flame or by 

 draughts. 



When the vapour from the boiling liquid reaches the thermometer, 

 the temperature is seen to rise rapidly, and then becomes stationary 

 at a definite temperature. This is the boiling-point of the liquid. 

 Drops of condensed liquid are usually seen to fall from the end of the 

 thermometer into the flask. The heating is continued until all the 

 liquid boiling at this temperature has distilled over into the receiver. 

 The portion remaining in the distilling flask contains the impurities. 



