TESTS FOR CERTAIN SALTS 195 



If a salt turns out to be a calcium salt and also a car- 

 bonate, it must be calcium carbonate. If we get a test for 

 a sodium salt, and also for a chloride, the substance tested 

 must be sodium chloride. 



1. Chlorides. We test for a chloride (including hydrochloric 

 acid) by dissolving the substance in distilled water, adding enough 

 dilute nitric acid to give the solution an acid reaction (cf. 215), and 

 by then adding a drop or two of silver nitrate solution. If a chloride 

 is present, there will be a white precipitate of silver chloride, an insoluble 

 solid. If we shake up the precipitate, it will clot together. If we pour 

 some of it upon filter paper, and expose it to sunlight, it becomes dark. 

 This last fact is the basis of modern photography (cf. 188). 



2. Sulphates. We test for sulphates (including sulphuric acid) 

 by adding to the solution of the substance some dilute nitric acid, and 

 then a few drops of a solution of barium chloride or barium nitrate. If 

 a sulphate is present, there will be a white precipitate of barium sul- 

 phate. 



3. Nitrates. To test for a nitrate we first make a saturated solution 

 of ferrous sulphate. To about 5 c.c. of this solution, in a test tube, we 

 add a few drops of the solution to be tested, and mix it, by shaking, 

 with the ferrous sulphate solution. We then tilt the test tube, and pour 

 down its side about 5 c.c. of concentrated sulphuric acid. The heavy 

 acid slides underneath the solution, and when we hold the tube up- 

 right, we find a brown layer or ring between the solution and the acid, 

 provided a nitrate is present. 



4. Carbonates (either the solids or their solutions) are tested for by 

 means of dilute hydrochloric or nitric acid. Effervescence takes place, 

 and we prove that the escaping gas is carbon dioxide by passing it 

 into lime water. A white precipitate of calcium carbonate is formed 

 (cf. 126). 



5. Phosphates. We test for phosphates in solution by adding to 

 the solution, first, a few drops of concentrated nitric acid, and then 

 a solution of ammonium molybdate. A yellow solid is precipitated, if 

 phosphates are present. 



6 Iron Salts. Solutions suspected of containing iron are treated 



