98 ANSWERS TO PRACTICAL QUESTIONS 



35. Steel articles (ire blued to protect from rusting, by 

 heating in a sand-bath. Explain. 



A thin coating of oxide is formed on the surface of the 

 metal. 



36. Give the formulas for copperas and white lead. 



1. FeSO 4 = FeO,SO 3 . 



2. PbCO 3 = PbO,C0 3 . 



37. Why is Hg used for filling thermometers ? 



Because it is fluid at all ordinary temperatures. 



38. What oxides are formed by the combustion of Na 9 

 K, Zn, 8, Fe, Pb, Cti, P, etc. ? Which are bases ? Anhy- 

 drides ? Give the common name of each. 



(1.) Na 2 O is formed when ISTa oxidizes in dry air, or oxy- 

 gen at a low temperature. This takes up water with great 

 avidity, forming HNaO (NaHO), sodium hydroxide. Na 2 O 2 is 

 made when Na is heated to 200 C. HNaO is. the caustic soda 

 of commerce, and is an alkaline base. 



(2.) K in a similar manner, depending upon the tempera- 

 ture, forms K 2 O, K 2 O 2 , and K 2 O 4 . The first, with water, 

 forms the ordinary caustic potash, HKO, of commerce. It is 

 an alkaline base. 



(3.) ZnO is the only known oxide of zinc. It forms salts. 



(4.) Seven compounds of S and O are known, but only two 

 are of interest the familiar anhydrides, SO 2 and SO 3 . 



(5.) The oxides of iron are four in number : (1) the mon- 

 oxide, or ferrous oxide, FeO, from which the green ferrous salts 

 are derived ; (2) the sesquioxide, or ferric oxide, Fe 2 O 3 , yield- 

 ing the yellow ferric salts; (3) the magnetic or black oxide, 

 Fe 3 O 4 , which does not form any definite salts ; (4) ferric acid, 

 H 2 FeO 4 , a weak acid, forming colored salts with potassium. 



(6.) Pb forms two oxides, the monoxide and the dioxide. 

 The former is the well-known litharge, which is the base of 

 the lead salts. 



(7.) Cu has two oxides the cuprous (Cu a O) and cupric 

 (CuO), both of which form salts, thus giving rise to two series, 



