108 JUNIOR GRADE SCIENCE 



48. EXAMINATION OF COMMON SALT. 



i. Examine as described in Chapter XVII. 



ii. Make a solution of salt in water, and evaporate it by gently heating in 

 a basin. When all theTwater has disappeared examine the residue. Note 

 the shape of the small crystals, using a lens. 



Properties of common salt. Common salt crystallises in six-sided 

 solids, or cubes (Fig. 84). When the crystallisation is brought about 

 by evaporating a solution of salt, the crystals are very small. Some 



natural crystals, known as rock salt, are, 

 however, of a considerable size. There 

 is no water of crystallisation in the 

 crystals, and consequently when they are 

 heated no steam is given off. The crack- 

 ling noticed when salt crystals are heated 

 in a tube is spoken of as decrepitation, 

 and is due to the breaking up of the 

 crystals into smaller pieces. 



When exposed in damp air, salt becomes 



FIG. 84.-A crystal of salt. moist ' n which M* {i said to * 

 deliquescent. 



Common salt dissolves in water, and the amount dissolved by cold 

 water is almost as great as by warm water. The solution has no 

 action on litmus papers, and is said to be neutral. 



Rock salt is found in the earth in some countries as layers of varying 

 thickness. The largest amount is found in the salt mines of Austria, 

 but considerable quantities are found in Carrickfergus, Co. Antrim, 

 and in Cheshire. Common salt is also present in large quantities in 

 sea water, but is mixed with other substances. 



Salt is used largely to prevent the decomposition of meat, and 

 enormous quantities are employed in the manufacture of sodium 

 carbonate. 



49. EXAMINATION OF LEAD. 



i. Describe its physical properties. 



ii. Effect of heating lead strongly in air. Heat a fire-clay crucible 

 to redness, and then drop small pieces of lead foil in it one at a time. 

 After adding the lead, stir with a piece of iron wire and continue heating 

 for at least ten minutes, stirring all the time. Allow to cool, and observe 

 any changes. 



Lead. When freshly scraped with a knife lead has a rather bluish 

 colour. The bright metal, however, soon tarnishes in air. It tarnishes 



