OXIDATION 



59 1 2 



Oxidation. Term in chemistry. 

 It is applied in the strict sense to 

 changes which result in the forma- 

 tion of new compounds with oxy- 

 gen. The term oxidation has, how- 

 ever, been extended to kindred 

 changes, such as new unions with 

 chlorine or with some other ele- 

 ment. The element or compound 

 which brings about the changes is 

 called an oxkliser or oxidising 

 agent. See Oxygen. 



Oxide Ores. Ores from which 

 the bulk of the world's metals is 

 obtained, and in which the metals 

 occur combined with some other 

 element or elements, forming a 

 mineral not necessarily suggesting 

 any metallic character. The more 

 important of these oxide ores are 

 those of iron, the haematites ; 

 magnetic or black iron ore ; mag- 

 netite ; siderite ; chalybite or 

 spathic iron ore, from which a 

 large proportion of all the iron of 

 the world is obtained ; cuprite or 

 copper ruby ore, carrying 88'8 p c. 

 of the metal ; zincite or red zinc 

 ore ; and cassiterite or tinstone, 

 which is practically the only ore of 

 tin from which the metal is ex- 

 tracted. See Metallurgy ; Ores. 



Oxides. In chemistry, com- 

 pounds which oxygen forms with 

 other elements. Fluorine is the 

 only well-known element with 

 which oxygen does not combine, 

 but in some cases oxygen com- 

 bines with the same element in 

 different proportions. 



Oxides are classified thus: (1) 

 Acid-forming oxides, those which 

 when combined with water form 

 acids ; this class is also known as 

 acid anhydrides, i.e. acids without 

 water. Nitrogen pentoxide (N 2 O 5 ) 

 with water yields nitric acid 

 (HN0 3 ). (2)* Basic oxides, pro- 

 duced when metals burn in air 

 or oxygen. When combined with 

 water basic oxides form hydroxides 

 or hydrated oxides. An example is 

 potassium oxide (K 2 0), which 

 with water produces potassium 

 hydrate (KOH). (3) Peroxides 

 which contain more oxygen than 

 basic oxides ; part of the oxygen is 

 loosely combined, and is given off 

 on heating. Barium peroxide 

 (Ba0 2 ) and manganese dioxide 

 (Mn0 2 ) are examples. (4) Neutral 

 or indifferent oxides, such a swater 

 (H 2 0), carbon monoxide (CO), 

 nitrous oxide (N 2 O), and lead sub- 

 oxide (Pb 2 0). See Oxygen. 



Oxlip (Primula elatior). Peren- 

 nial herb of the natural order 

 Primulaceae. It is a native of 

 Europe and Siberia. In Britain it is 

 restricted to the counties of 

 Bedford, Cambridge, Suffolk, and 

 Essex. Its flowers are similar in 

 size and colour to those of the 

 primrose, with short individual 



w. 



Oxlip. Leaves and flower-spray ; 

 inset, roots 



stalks springing from the top of a 

 stout, tall stem like that of the 

 cowslip. What is commonly known 

 as the oxlip in gardens is a hybrid 

 between the primrose and cowslip. 



Ox-Pecker OR RHINOCEROS BIRD 

 (Buphaga). Bird found in Africa. 

 They are dull brown above and 

 light brown beneath, and about the 

 size of a starling. Insectivorous, 

 they get their name from the habit 

 of settling on the backs of the 

 ox and rhinoceros to search for 

 parasitic insects. 



Oxus. Ancient name of the river 

 now called the Amu Daria (q.v.). 



Oxy chlorides. Metallic chlor- 

 ides which also contain oxygen. 

 They are formed when certain 

 metallic chlorides are added to 

 water, e.g. when bismuth chloride 

 (BiCl 3 ) or antimony chloride (SbCl 3 ) 

 are added to water the oxy- 

 chlorides, BiOCl and SbOCl, are 



Oxygen. Diagram showing the 



Linde-Hampson process of oxygen 



production. See text 



produced. Zinc chloride also forms 

 an oxychloride on keeping, or on 

 evaporating a solution of the salt. 

 Some chlorides such as ferric 

 chloride (FeCl 3 ), cupric chloride 

 (CuCl 2 ), and bismuth chloride 

 (BiCl 3 ) yield oxychlorides when 

 heated in dry air. 



Oxygen. Most widely dis- 

 tributed of the chemical elements, 

 chemical symbol 0, atomic weight 

 16. It is a colourless, odourless, 

 and tasteless gas, and exists in the 

 free state in the atmosphere, of 

 which it forms about 21 p.c. by 

 volume. Oxygen also occurs in 

 enormous quantities in the com- 

 bined state, as eight-ninths, by 

 weight, of water consists of oxygen, 

 and oxides form a large part of 

 the earth's crust. It is essential 

 to life. 



Oxygen was discovered in 1774 

 by Priestley, who called it " de- 

 phlogisticated air,"' and simulta- 

 neously by Scheele (1742-86), a 

 Swedish apothecary, who named it 

 " empyreal " or " fire-air." Lavoi- 

 sier, regarding it as the essential 

 constituent of acids, gave it the 

 name oxygene (acid-former). 



The gas was first liquefied in 

 1877 by Cailletet and Pictet, and 

 in liquid form is pale blue. The 

 characteristic of oxygen is its 

 power of supporting combustion. 



The methods of preparing oxy- 

 gen are as follows: 



(1) By heating mercuric oxide 

 (HgO) or red oxide of mercury in a 

 glass retort, Priestley's method. 



(2) By strongly heating man- 

 ganese dioxide (Mn0 2 ) in an iron 

 retort. One-third of its oxygen is 

 given off, a lower manganese oxide 

 (Mn 3 4 ) being formed. Other 

 oxides as lead dioxide (PbO-j), 

 barium dioxide (Ba0 2 ), and chro- 

 mium trioxide (Cr0 3 ), lose part 

 of their oxygen when heated in the 

 same way. 



(3) When potassium chlorate 

 (KC10 3 ) is heated it gives off its 

 oxygen, but in order to obtain the 

 evolution of oxygen at -a lower 

 temperature, it is mixed with one- 

 eighth its weight of manganese 

 dioxide. This is the method 

 usually followed in the laboratory, 

 and was employed for making large 

 quantities of oxygen before the 

 discovery of cheaper processes. 



(4) When barium dioxide 

 (Ba0 2 ) is heated it gives off an 

 atom of oxygen, and yields the 

 lower oxide (BaO). This process 

 has been employed on a very large 

 scale for the preparation of oxygen, 

 the advantage being its economical 

 working. It is known as the Brin 

 process, and was originally patent- 

 ed in 1880. The barium monoxide 

 which is formed also has the 

 property of absorbing oxygen when 



