238 NON-METALS AND THEIR COMBINATIONS. 



The explanation of this action is that chlorine first forms a hypo- 

 chlorite, which, as stated above, decomposes by heating into chlorate 

 and chloride. The change will be clearer if written in two steps : 

 6KOH + 6C1 = 3KC1 -f 3KC1O + 3H 2 O. 



3KC10 = 2KC1 + KC10 3 . 



In recent years large quantities of chlorates, especially potassium 

 chlorate, are made by passing an electric current, under proper con- 

 ditions, through an alkaline solution of potassium chloride. 



Perchloric acid, HC1O 4 . This is a colorless liquid, which, in the pure 

 state, decomposes, and often explodes spontaneously when kept. A 70 

 per cent, aqueous solution is stable. Although it contains more oxygen than 

 the other acids of chlorine, it is the most stable one of all. It can be prepared 

 by distilling a mixture of potassium perchlorate and concentrated sulphuric 

 acid in a vacuum. It was seen in the chapter on oxygen that potassium chlor- 

 ate, when heated, gives the perchlorate, chloride, and oxygen. The perchlor- 

 ate, being difficultly soluble in water, can be separated easily from the far more 

 soluble chloride. 



Tests for chlorates and hypochlorites. 

 (Potass, chlorate, KC1O 3 , and bleaching powder, Ca(ClO) 2 .CaCl 2 , may be used.) 



1. Chlorates liberate oxygen when heated by themselves. 



2. Chlorates liberate chlorine dioxide, C1O 2 , a deep-yellow explo- 

 sive gas, on the addition of strong sulphuric acid. 



2KC10 3 + H 2 S0 4 = K 2 S0 4 + 2HC1O 3 . 

 3HC10 3 = HC10 4 + H 2 + 2C10 2 . 



This test should be made only on a quantity about the size of a pea. 



3. Chlorates deflagrate when sprinkled on red-hot charcoal. 



4. Hypochlorites are strong bleaching agents, and evolve a pecu- 

 liarly smelling gas (chlorine) on the addition of acid (see page 236). 



QUESTIONS. State the names and general physical and chemical properties 

 of the four halogens. How is chlorine found in nature, and why does it not 

 occur in a free state? State the general principle for liberating chlorine from 

 hydrochloric acid, and explain the action of the latter on manganese dioxide. 

 Mention of chlorine : its atomic weight, molecular weight, valence, color, odor, 

 action when inhaled, and solubility in water. How does chlorine act chemi- 

 cally upon metals, hydrogen, phosphorus, water, ammonia, hydrocarbons, and 

 coloring matters? Mention two processes for making hydrochloric acid; state 

 its composition, properties, and tests by which it may be recognized. What is 

 aqua regia? State the composition of hypochlorous and chloric acids. What 

 is the difference in the action of chlorine upon a solution of potassium hydrox- 

 ide at ordinary temperature and at the boiling-point ? How many pounds of 

 manganese dioxide, and how many of hydrochloric acid gas are required to 

 liberate 142 pounds of chlorine ? 



