THE SULPHATES 



77 



nesium, calcium, etc., instead of the sulphates of soda, potash, magnesia, 

 lime, etc. The latter terms are, however, always employed in agriculture, 

 and it is very important that the student should be familiar with them and 

 understand their meaning. 



When the solution of a free base or acid is added to the 

 solution of a salt of a different base or acid, reaction occurs. 

 Thus, if solution of caustic potash be added to a solution of 

 sulphate of sodium, sulphate of potassium is formed and caustic 

 soda set free, to an extent depending upon the amount of 

 caustic potash added, till an equilibrium is established between 

 the substances. But when the combined base or acid is 

 volatile and therefore escapes from the liquid, or is insoluble 

 and therefore is precipitated, more of the free base or acid will 

 take its place, until, if sufficient of the free base or acid has 

 been used, complete change has occurred. To a solution of 

 sulph/V^ of sodium, i.e. the salt produced by the action of sul- 

 ^\mxous acid and caustic soda on each other, add strong 

 sulphuric acid. Note that sulphurous acid is set free as indi- 

 cated by its smell, and being unstable, decomposes, gaseous 

 sulphurous anhydride being given off. To solution of sulphate 

 of copper add solution of caustic soda till the mixture is alkaline 

 in reaction. Note that the blue hydroxide of copper is precipi- 

 tated, and that this, when the mixture is heated, loses water and 

 is converted into the familiar black oxide. Filter the mixture 

 and evaporate the now colourless filtrate ; crystals of sulphate 

 of sodium should be obtained. Write equations to represent 

 the changes. These facts are of great importance because 

 they provide methods for preparing acids and bases which 

 cannot be directly or conveniently prepared from the elements. 

 Insolubility or volatility conditions, not only the formation 

 of a base or an acid, but also the formation of a salt. The 

 sulphate of barium is insoluble in water. If, then, to a solution 

 of sulphuric acid or a sulphate, a solution of baryta or a soluble 

 barium salt be added, sulphate of barium will be precipitated. 

 A.S this is the only insoluble barium salt unattacked by, and 



