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JUNIOR GRADE SCIENCE 



again for a further twenty minutes, cool and weigh. The weight ought to 

 be the same as in last weighing. 



v. Add a little water to the residue in the crucible, and note the effect. 



vi. Gas given off when chalk is heated. Place some very finely 

 powdered chalk in a hard glass test-tube fitted as shown in Fig. 117, and 



heat as strongly as possible with a 

 blow-pipe flame, even if the test-tube 

 is destroyed. A gas is given off 

 which collects in the receiving tube. 

 Test with a little lime-water and with 

 litmus. 



The chalk will not be entirely 

 decomposed before the hard glass 

 tube is destroyed, but sufficient gas 

 is obtained for the purpose of identi- 

 fication. 



Fro. 117. Carbon dioxide produced by 

 heating chalk. 



Chalk undergoes a change when 

 heated. It is easy to prove by 

 putting some powdered chalk upon 

 a piece of moist red litmus paper 

 that this substance is unable to change the colour of the paper. If, 

 however, some powdered chalk be strongly heated on a piece of platinum 

 foil in a laboratory burner and then placed on a piece of moist red litmus 

 paper, the red colour is changed to blue. The chalk undergoes some 

 change when heated, or it would not acquire this new property. The 

 same chemical change as occurs when a little powdered limestone, or 

 some chalk, is heated on platinum foil takes place on a large scale in 

 the limekiln. In other words, when chalk and limestone are strongly 

 heated they are changed into quick-lime. The change is brought about 

 by driving carbon dioxide out of them. 



when heated 



CHALK splits up LIME and 

 into 



Changes in mass when chalk is heated. That chalk does undergo 

 a change when heated has already been learnt. But by studjing the 

 changes in mass which take place when chalk is strongly heated several 

 important conclusions are arrived at. To obtain satisfactory numerical 

 results it is desirable to heat a fair amount of powdered chalk in a muffle 

 furnace, though a very small quantity of chalk can be completely 

 converted into quicklime when heated in a good laboratory burner. 

 After heating the chalk in a crucible in a muffle furnace for about an 

 hour the crucible is taken out, allowed to cool again, and then re weighed. 

 This plan is continued until there is no further loss of mass on reheating. 

 A simple proportion sum will now show how much 100 grams of chalk 



CARBON 

 DIOXIDE. 



