CHAPTER XXIV. 

 ALKALIES. 



71. CAUSTIC SODA. 



DURING the previous work a number of substances have been found 

 which turn reddened litmus blue, e.g. solutions of washing soda. Hme, 

 magnesium oxide, sodium oxide. Some of these must be examined 

 more fully. 



i. Sodium oxide. Burn a piece of sodium metal in a jar of oxygen (p. 120). 

 Shake up with a little water to dissolve the sodium oxide formed. Note the. 

 soapy feeling of the solution. Try its action on litmus. Evaporate the 

 solution to drjness. 



ii. Sodium and water. Into a basin containing distilled water put, one 

 at a time, a few pieces of sodium about the size of a small pea. Note the 

 action, soapy feeling and effect on litmus. Evaporate, just to dryness, 

 but no further. The white solid obtained is called caustic soda. 



iii. Effect of air on caustic soda. Leave a stick of caustic soda exposed 

 in the laboratory till the end of the lesson and make observations. 



iv. Effect of caustic soda on other substances. Examine the effect 

 of a solution of caustic soda on each of the following : (a) a solution of green 

 vitriol, (6) a solution of blue vitriol, (c) a piece of aluminium metal. 



Caustic soda. When a small piece of the metal sodium is added 

 to water in a basin it remains on the surface and races about over it 

 till it is entirely dissolved. The solution formed feels soapy when 

 rubbed between the fingers, and turns red litmus blue. On evaporation 

 a white solid is obtained, which is caustic soda or sodium hydroxide. 

 Caustic soda is commonly supplied in the form of white sticks. It 

 is very deliquescent and easily soluble in water, giving out heat. It 

 has a corrosive action on most organic substances. It can be made to 

 combine with certain oils and fats, forming soap. When a solution 

 of caustic soda is allowed to act on aluminium an inflammable gas is 

 given off. 



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