234 NON-METALS AND THEIR COMBINATIONS. 



vapor, until 20.2 per cent, is reached, when the residue in the flask passes over 

 unchanged. Other acids for example, sulphuric, nitric, hydriodic, hydro- 

 brornic show a similar property. 



Neither the dry gas (HC1) nor the liquefied gas has any marked acid char- 

 acter. They do not conduct electricity and have no action on dry litmus- 

 paper or on zinc, but the presence of water causes strong acid properties to be 

 developed. This is explained on the Dissociation Theory, which holds that 

 only hydrogen ions have acid properties. Water is required for the ionization 

 of HC1, and without it the gas lacks acid character. A solution of the gas in 

 liquids like benzene and toluene, which have scarcely any ionizing power, has 

 practically no effect on zinc, which is freely attacked by an aqueous solution 

 of the gas. The same is true of hydrobromic and hydriodic acids (Chapter 

 15). 



Nearly all chlorides are soluble in water. Of those ordinarily met with 

 only two are insoluble in water, namely, silver and mercurous chlorides, and 

 one is difficultly soluble in cold water, but more readily in hot water, namely, 

 lead chloride. 



Tests for hydrochloric acid and chlorides. 

 (Sodium chloride, NaCl, may be used.) 



1. To hydrochloric acid, or to solution of chlorides, add silver 

 nitrate : a white, curdy precipitate is produced, which is soluble in 

 ammonia water, even when very dilute, but insoluble in nitric acid : 



AgN0 3 + Nad = AgCl. -f NaNO 3 . 

 Ag' + NO/ + Na' + Cl' AgCl '+ Na' + NO/. 



2. Add solution of mercurous salt (mercurous nitrate) : a white 

 precipitate of mercurous chloride (calomel) is produced, which black- 

 ens on the addition of ammonia : 



HgN0 3 + NaCl = HgCl + NaNO 3 . 

 Hg' + N0 3 ' + Na- + Cl' HgCl + JNV + NO/. 



3. Add solution of lead acetate : a white precipitate of lead chloride 

 is formed, which is soluble in hot, or in much cold water, and is, there- 

 fore, not formed in dilute solutions. Its composition is PbCl 2 . 



Pb(C 2 H 3 2 ) 2 + 2NaCl PbCl 2 + 2Na(C 2 H 3 O 2 ). 



Pb" + 2(C,H 3 2 )' + 2Na- + 201' - PbCl 2 + 2Na' + 2(C 2 H 3 O 2 )'. 



4. To a dry chloride add strong sulphuric acid and heat : hydro- 

 chloric acid gas is evolved, which may be recognized by the odor, or 

 by its action on silver nitrate, when a drop of the solution on the end 

 of a glass rod is held in the gas. (The insoluble chlorides of silver, 

 lead, and mercury do not give this reaction.) 



5. Chlorides treated with sulphuric acid and manganese dioxide 

 evolve chlorine. 



Test 1, combined with test 5, is the most decisive proof of hydro- 



