ON SCIENCE IN SCHOOL CERTIFICATE EXAMINATIONS. 491 



E. Chemistry. 



Note. — §§ 1-4 should be taken during Autumn Term. 



1. Washing soda a crystalline substance which degenerates (especially in 

 warm weather) into a shapeless powder. Distillation shows changes to be due 

 to loss of 'water ot crystallisation.' Water derivable from other crystals (but 

 not all) and from vegetable and animal substances (e.g. a potato) where its 

 presence is not apparent. First notions of chemical combination between 

 substances. 



Crystallisation from solution in water. Manufacture of common salt, cane 

 and beet sugar; plaster of Pans; ' sympathetic inks.' Variations in solubility. 

 Crystalloids and colloids. Other solvents (e.*;. petrol, solvent naphtha in water- 

 proofing, turpentine, &c.) and their uses. 



Soluble and insoluble substances in soil. Residue from evaporation of tap- 

 water ; formation of sea-water. 



2. Use of soda in cookery leads to discovery that it turns the juice of 

 pickling cabbage green. (The juice is extracted by pounding in a mortar.) 

 Vinegar (preferably ' white ' vinegar) turns the juice red. Soda and vinegar 

 can ' overcome ' one another's effects. Caustic soda, mild and caustic potash, 

 ammonia and lime, being found to turn the juice green, are classed with washing 

 soda as alkalis ; acids are found to turn it red. Other vegetable extracts found 

 to show colour changes with acids and alkalis, e.g. litmus. Other ' indicators ' : 

 phenolphthalein, methyl orange. 



Neutralisation ; careful study by means of burette, different boys working 

 with different acids and alkalis. Evaporation of neutral solutions reveals 

 presence of common salt when mild or caustic soda is neutralised by hydrochloric 

 acid, and other ' salts ' in the other cases. Sa/lts named from acid and alkali 

 (e.g. sulphate of ammonia). Manufacture of sulphate of ammonia for manure, 

 and of sal-ammoniac. 



3. How does caustic differ from washing soda? On addition of acid the 

 latter yields a heavy gas which extinguishes flames, turns lime-water cloudy 

 and ultimately clears it again. The cloudy matter, when collected, returns the 

 gas if acid is added. Chalk is known to yield the same gas when ' burnt ' 

 to make lime. Finally, caustic soda is made by boiling washing soda with 

 lime, the latter becoming converted into chalk. (Similar statements apply to 

 mild and caustic potash.) Thus, washing soda, mild potash, and chalk are to 

 be classed together, and also caustic soda, caustic potash and lime. But there 

 are two ' limes ' — quicklime and slaked lime. Dry ' heavy gas ' liberates water 

 from caustic eoda, caustic potash and slaked lime, but not from quicklime ; 

 hence the analogy is with slaked lime. 



4. The ' heavy gas ' is produced in breathing, and also in the burning of 

 coal-gas, candles, &c. Burning of these substances in a jar demonstrates its 

 production together with water, and shows, further, that one-fifth of the air 

 is consumed. The burning of metals (e.g. magnesium), and of phosphorus, 

 sulphur, &c., the rusting of iron, the 'drying ' of boiled oil, &c., also remove 

 the 'active ' one-fifth of the air and leave four-fifths 'inactive.' Consideration 

 of the mode of manufacture of red lead suggests that if heated it may restore 

 the absorbed active constituent. Oxygen and nitrogen ; argon. Manufacture of 

 oxygen from liquid air. Properties and uses of oxygen. Oxides. 



The ' heavy gas ' is produced without water when pure carbon is burnt in 

 oxygen. It is, therefore, an oxide of carbon. Confirmation by burning mag- 

 nesium in gas. Oxygen passed over red-hot carbon (as in a domestic fire and 

 in the smelting furnace) produces a gas which burns to form the heavy gas. 

 The latter must, therefore, contain more oxygen (compare litharge and red 

 lead) ; hence the names carbon monoxide and carbon dioxide. 



5. Oxides and oxidation in nature and industry. Oxides of iron, copper, 

 magnesium, aluminium, &c. : ochres and other painter's colours ; ' drying ' of 

 oils ; linoleum. 



6 Is water also an oxide? Affirmative answer obtained by passing steam 

 over hot magnesium. Discovery of hydrogen. Production in bulk by passing 

 steam over hot iron ; properties. Known to be produced also when the plumber 

 'kills spirits of salt ' with zinc. Composition of water confirmed by burning a 



