268 ELEMENTARY GENERAL SCIENCE CH^P. 



bustion. It is very soluble in water forming an acid solution. If 

 passed into a solution of caustic soda and the resulting solution 

 evaporated to dryness a white solid is obtained which is found to be 

 salt. Since the same results follow if this experiment is repeated, 

 substituting hydrochloric acid for "salt gas," we are justified in 

 calling "salt gas" by the name of hydrochloric acid gas. We may 

 therefore state that Common salt and sulphuric acid yield hydro- 

 chloric acid gas and sodium sulphate. 



Composition of " Salt-" or Hydrochloric Acid-Gas. When the acid 

 gas is passed over heated copper oxide, water is formed and can be 

 collected. Water contains hydrogen, therefore probably this hydro- 

 gen was obtained from the " salt gas." The copper oxide becomes 

 changed into a green substance ; probably the copper of the oxide 

 has combined with the other part of the " salt gas " to form the green 

 compound. 



The " salt gas " is proved to contain hydrogen, to the extent of one- 

 half its volume, by the action of a piece of sodium upon a quantity 

 of the gas in a tube over mercury. Half the amount of hydrogen is 

 found left in the tube. The other constituent of the ' ' salt gas " com- 

 bines with the sodium to form salt again. The name given to this 

 other constituent is chlorine. 



By means of the eudiometer and the voltameter (see Chap. XVI.), 

 the hydrochloric acid gas can be proved to be made up of one volume 

 of hydrogen and one volume of chlorine combined to form two volumes 

 of hydrochloric acid gas. 



Preparation and Properties of Chlorine. Chlorine is obtained from 

 hydrochloric acid by heating it with manganese dioxide. The gas 

 being heavier than air is collected by downward displacement as 

 in Expt. 283. 



Chlorine is a very heavy, yellowish-green gas with an irritating, 

 choking odour. It is soluble in water and the solution smells like 

 the gas. It possesses powerful bleaching properties. Though it 

 does not itself burn, it is capable of supporting the combustion of 

 many bodies, with the formation of chlorides. The weight of a litre 

 of chlorine is about 3*18 grains, it is hence about 35 '5 times as heavy 

 as hydrogen. 



Various Compounds of Chlorine. Potassium chlorate can be 

 obtained by passing chlorine for some time through a boiling solution 

 of caustic potash and allowing the solution to crystallise. The 

 chlorates are salts of an acid known as chloric acid. Potassium 

 chlorate is the most important chlorate ; it is used in the manu- 

 facture of matches and fireworks. 



Potassium hypochlorite is obtained by passing chlorine through a 

 cold solution of caustic potash. If lime is used instead of caustic 

 potash, a mixture called bleaching -powder is obtained. 



QUESTIONS ON CHAPTER XVIII. 



1. How is hydrochloric acid obtained? Give a short account of its 

 chief properties. 



2. What are chlorides? How may they be obtained? Give 

 examples. 



