EXAMINATION OF COMMON SUBSTANCES 



101 



Saturated solutions. When any given amount of water has dis- 

 solved as much of a solid as it can be made to, without warming, it is 

 said to be saturated. But though cold water, for instance, may be 

 saturated with, say, nitre, it can if warmed be made to dissolve more 

 nitre, and the more the solution is heated the greater the quantity of 

 nitre which can be made to dissolve. As cold water will not dissolve 

 so much nitre as hot, it can be seen why a hot saturated solution 

 deposits nitre on cooling. 



Most other solids behave in a similar manner to nitre, but there are 

 some exceptions. Lime is less soluole in hot water than in cold.' 



44. THE SOLUBILITY OF NITRE IN WATER AT 

 DIFFERENT TEMPERATURES. 



Solubility may be defined as the mass of a given substance which 

 dissolves in 100 grams of a given liquid at a given temperature. 



i. The solubility of nitre in water at a temperature of 60 C. Heat 

 a beaker of water to about 65 C. and regulate the name so as to maintain 

 it at this temperature. Take less than 10 c.c. 

 of water in a test-tube, and add finely powdered 

 nitre till saturated. Place the test-tube in the 

 beaker of hot water, and continue to add nitre 

 till some of it remains undissolved. To make 

 sure of this it will be necessary momentarily 

 to remove the test-tube from the hot water 

 and shake it, replacing it immediately. 



When the saturated solution has been pre- 

 pared, the flame should be removed and the 

 thermometer placed in the test-tube. When 

 the temperature falls to 60 C. the clear liquid 

 should be decanted into a weighed basin, and 

 covered with a clock glass. Care must be taken 

 not to transfer any of the undissolved solid. 

 The basin, contents and glass should be weighed 

 and then placed on a water bath (Fig. 78), 

 evaporated to complete dryness, cooled and 

 weighed. Re- heat and weigh till a constant weight is obtained, 

 your results in the following form : 



Wt. of basin, glass and solution - - 59-68 gm. 

 Wt. of basin and glass - - - - 45 79 gm. 



Wt. of solution 13-89 gm. 



Wt. of basin and nitre .... 52-99 gm. 



Wt. of nitre 7-20 gm. 



Wt. of water 6-69 gm. 



- 7-2 gm. nitre. 

 7-2 x 100 



FIG. 78 The shallow basin 

 has solution in it and is kept 

 hot by the steam rising froic 

 the water in the glass beaker. 



Record 



Since 6.69 gm. of water dissolve 

 100 gm. of water will dissolve 



The solubility of nitre in water at 60 C. 

 is 108 gm. 



6-69 



:108 gm. 



gm. 



