§ 56. COPPER. § 57. ARSENIC. 73 



ed, a yellow precipitate of plumbic chromate, PbCrO^, is 

 formed. 



Lead is precipitated from its solutions by metallic zinc 

 in the presence of free acid. 



COPPER. Cu. 63.5. 



56. Compounds of copper with phosphoric, oxalic, car- 

 bonic, tartaric, and silicic acids, and sulphur and cyanogen, 

 are insoluble, or sparingly soluble in water. The sulphide 

 and silicate are insoluble in dilute acids. 



Cupric salts give a blue or a greenish-blue color to so- 

 lutions containing them. 



Reactions* — Solutions containing coj^per give a green- 

 ish precipitate, CuO, H^O or CuH^O^, with sodic or am- 

 monic hydrate. This precipitate is dissolved by an excess 

 of ammonic hydrate, giving a deep blue solution ; the 

 reaction is very delicate. 



Solutions of copper give a red precipitate with potassic 

 ferrocyanide, Cu^Fe^Cyg. 



Copper is precipitated from its solutions by zinc in the 

 presence of free sulphuric or hydrochloric acid; free 

 nitric acid hinders the reduction, but does not prevent it. 



ARSENIC. As. 75. 



57. When a solution containing arsenic is treated with 

 dilute sulphuric acid and metallic zinc, in a small flask 

 closed with a cork through which passes a glass tube 

 drawn out to a small jet at the end, and the escaping gas 

 is lighted, after it has been evolved long enough to expel 

 the oxygen from the flask ^ a bluish flame is produced, 

 which deposits black, shiny spots on a cold porcelain sur- 

 face. The arsenic was evolved as arseniuretted hydrogen, 

 AsIIg. This reaction is very delicate, and is known as 

 Marsh's test. 



4 



