CRYSTALS 



89 



We have learned (c/. 69) that when ice is placed in water, the heat 

 needed to melt the ice (80 calories for each gram) comes from the 

 water ; hence the water is cooled to its freezing point, C. Now, when 

 ice is mixed with salt, the salt dissolves in the water formed by the 

 melting of the ice. We thus get a salt brine. The heat needed to 

 melt the ice comes from the brine, and cools it to its freezing point, 

 22 C. If cream or ' ' ices " are placed in this brine, heat is taken out 

 of them until they are frozen. 



94. Solubility. By the solubility of a solid we mean 

 the weight of it that will dissolve in a definite amount, 

 say, 100 grams, of the solvent. We know that if we add 

 too much sugar to our tea or coffee, some of it will not 

 dissolve, even though we stir the liquid vigorously. The 

 tea or coffee is then said to be " saturated " with sugar. 

 If a liquid is hot, it can usually dissolve more solid than 

 when cold. 



A solid that is soluble in water may be insoluble in 

 another solvent. Thus, salt does not dissolve in alcohol. 

 Camphor and shellac dissolve in alcohol, but not in water. 



The accompanying table shows that substances differ greatly in 

 solubility, and also that the solubility of solids generally increases as 

 the temperature rises. 



95. Crystals. The table of solubility shows us that 

 water at 100 C. can dissolve more of the substances 



