160 PRINCIPLES OF CHEMISTRY 



and oxygen, as was mentioned before. Priestley, in 1772, and Scheele, 

 somewhat later, obtained oxygen by heating nitre to a red heat. The 

 best examples of the formation of oxygen by the heating of salts is given 

 in jwtassium chlorate, or Berthollet's salt, so called after the French 

 chemist who discovered it. Potassium chlorate is a salt composed of 

 the elements potassium, chlorine, and oxygen, KC10 3 . It occurs as- 

 transparent colourless plates, is soluble in water, especially in hot 

 water, and resembles common table salt in some of its physical properties; 

 it melts on heating, and in melting begins to decompose, evolving oxygen 

 gas. This decomposition ends in all the oxygen being evolved from 

 the potassium chlorate, potassium chloride being left as a residue, accord- 

 ing to the equation KC1O 3 =KC1 + O 3 . 12 This decomposition proceeds 

 at a temperature which allows of its being conducted in a vessel 

 made of glass. However, in decomposing, the molten potassium 

 chlorate swells up and boils, and gradually solidifies, so the evolution of 

 the oxygen is not regular, and the glass vessel may crack. In order 

 to overcome this inconvenience, the potassium chlorate is crushed 

 and mixed with a powder of a substance which is incapable of com- 

 bining with the oxygen evolved, and which is a good conductor of heat. 

 Usually it is mixed with manganese peroxide. 13 The decomposition of 

 the potassium chlorate is then considerably facilitated, and proceeds at 

 a lower temperature (because the entire mass is then better heated, 

 both externally and internally), without swelling up, and is therefore 

 more convenient than the decomposition of the salt alone. This 

 method for the preparation of oxygen is very convenient ; it is generally 

 employed when a small quantity of oxygen is required. Further, potas- 

 sium chlorate is easily obtained pure, and it evolves much oxygen. 100 

 grams of the salt give as much as 39 grams, or 30 litres, of oxygen. 

 This method is so simple and easy, 14 that a course of practical chemistry 



of chromic acid (for instance, potassium dichromate, K.)Cr.)O 7 ) give oxygen with 

 sulphuric acid ; first potassium sulphate, K.^SO.^ is formed, and then the chromic acid set 

 free gives a sulphui'ic acid salt of the lower oxide, Cr.,05. 



12 This reaction is not reversible, and is exothermal that is, it does not absorb heat, 

 but, on the contrary, evolves 9713 calories per molecular weight KC1O 5 , equal to 122 

 parts of salt (according to the determination of Thomsen, who burnt hydrogen in a 

 calorimeter either alone or with a definite quantity of potassium chlorate mixed with 

 oxide of iron). It does not proceed at once, but first forms perchlorate, KCIO^ (see 

 Chlorine and Potassium). It is to be remarked that potassium chloride melts at 788, 

 potassium chlorate at 372, and potassium perchlorate at 010. 



13 The peroxide does not evolve oxygen in this case. It may be replaced by many oxides 

 for instance, by oxide of iron. It is necessary to take the precaution that no combustible 

 substances (such as bits of paper, splinters, sulphur, &c.) fall into the mixture, as they 

 might cause an explosion. 



14 The decomposition of a mixture of melted and well-crushed potassium chlorate 



