SULPHUR. 211 



about 60 c.c. of acid and 900 c.c. of water. This corresponds to 10 

 per cent, of H 2 SO 4 , and a specific gravity of 1.067 at 25 C. 



Great care should be taken in diluting concentrated sulphuric acid. 

 The acid should always be poured slowly into water with constant stir- 

 ring. It is dangerous to pour the hot acid into water and foolhardy 

 to pour water into the hot acid. Ignorance or disregard of these rules 

 may lead to sad consequences. 



When diluted sulphuric acid acts on metals, hydrogen is liberated and 

 escapes as a gas ; but if these same metals are acted on by concentrated sul- 

 phuric acid, which usually requires heating, other products are formed, such 

 as sulphur dioxide, hydrogen sulphide, or sulphur. This is due to the fact 

 that the concentrated acid acts as an oxidizing agent toward the hydrogen that 

 would otherwise be evolved, and itself is reduced. Certain metals do not act 

 on dilute sulphuric acid at all, but only on the hot concentrated acid, and 

 under these conditions hydrogen is never evolved. This is illustrated in the 

 preparation of sulphur dioxide by heating the concentrated acid with copper. 

 Other substances, as charcoal, sulphur, etc., that can be oxidized, act in the 

 same way on the hot, strong acid. 



Most sulphates are soluble in water. There are practically only three which 

 are insoluble in water or dilute acids, namely, barium, strontium, and lead. 

 Calcium sulphate is slightly soluble in water, and more so in hydrochloric 

 acid. A solution of it is used as a reagent. 



Most sulphates are more or less easily decomposed when heated, but those of 

 potassium, sodium, lithium, calcium, strontium, and barium can stand red 

 heat. 



Sulphuric acid, like all dibasic acids, has two modes of ionizing according 

 to concentration (see page 200). In more concentrated solutions, the ions H* 

 and HSO/ predominate; in dilute solutions, 2H* and SO/ X predominate. 

 Upon diluting a more concentrated solution, HSO/ ions dissociate further, 

 thus: 



H- + SO/'. 



The same thing takes place when SO/ 7 ions are removed by precipitation, 

 which has an effect equivalent to diluting the acid. 



Tests 1 and 2 below are examples of reversible reactions that run practically 

 to completion, because of the removal of one of the factors by precipitation, 

 due to its insolubility (page 114). These tests, like all similar ones that fol- 

 low, are explained in terms of the ionic theory on pages 193 and 200. 



Tests for sulphuric acid and sulphates. 



1. When a solution of barium chloride is added to dilute sulphuric 

 acid, or a solution of any sulphate, a white precipitate of barium 

 sulphate is obtained, which is insoluble in all dilute acids : 



Na 2 S0 4 + BaCl 2 == BaSO 4 + 2NaCl 

 2Na- + SO/' + Ba" + 2C1' = BaSO 4 + 2Na + 2C1'. 



