xxn ACIDS, BASES, AND SALTS 305 



acids to form salts and water. These compounds are both 

 included under the name of bases. We may hence write 



Base + Acid = Salt + Water, 



and may define a base as the oxide or hydroxide of a metal, 

 capable of neutralising an acid with the formation of a 

 salt and water. 



Alkalies. The soluble bases, the student has probably ob- 

 served, are characterised by alkaline properties, i.e., are known 

 as alkalies. 



Insoluble hydroxides can be prepared by the addition of an 

 alkali to the solution of a metallic salt. Thus, if caustic soda 

 solution be added to, say magnesium sulphate, a double de- 

 composition occurs with the formation of the insoluble magnesium 

 hydroxide, Mg(OH) 2 , as a white precipitate and sodium sulphate 

 which remains in solution. 



2NaOH + MgSO 4 = Na 2 S0 4 + Mg(OH) 2 . 



On strongly heating this hydroxide, the oxide itself is obtained, 

 Mg(OH) 2 = MgO + H 2 O. 



The slaking of lime is due to the conversion of the calcium 

 oxide into calcium hydroxide, 



CaO + H 2 O = Ca(OH) 2 



Acid Salts and Basic Salts. If the quantity of base is 

 insufficient to completely neutralise the whole of the acid, we 

 frequently obtain acid salts (p. 280), as acid sodium sulphate, 

 NaHSO 4 , thus : 



NaOH + H 2 SO 4 = NaHSO 4 + II 2 O. 



Acid salts are also called hydrogen salts. 



In other salts, on the contrary, a greater quantity of the base 

 is present than is required for the neutralisation of the acid. 

 Such salts are termed basic salts. 



Ammonia. Certain other compounds, however, besides the 

 metallic oxides, may act as bases, thus we may obtain salts from 

 ammonia, which, by testing with red litmus, is seen to be an 

 alkali. 



EXPT. 308. Prepare salts from the three acids, using the 

 ammonia liquid of the laboratory in place of caustic soda, &c. 



x 



