CH. ii] Chalk and its Composition 25 



found in the water through which carbon dioxide was 

 blown. 



What has happened chemically is that calcium car- 

 bonate in presence of carbon dioxide and water becomes 

 converted into calcium bicarbonate which is soluble in 

 water but readily decomposes on boiling into calcium 

 carbonate once more. 



A much quicker way of dissolving calcium carbonate, 

 and one largely used in laboratories, is to treat it with a 

 strong acid, when decomposition takes place and carbon 

 dioxide is evolved. 100 parts of calcium carbonate give 

 rise to 44 parts of carbon dioxide just as it did on heat- 

 ing, but the remainder, instead of appearing as calcium 

 oxide, appears as a salt. Thus if sulphuric acid is used 

 the reaction is : 



Calcium carbonate -r sulphuric acid = calcium sulphate -r carbon dioxide 

 100 44 parts bj weight. 



This reaction is so important that it must be studied in 

 an actual experiment. Weigh out 0-5 gram of calcium 

 carbonate^ in a 50 c.c. conical flask, put in a small tube 

 of strono: hvdrochloric acid, cover the calcium carbonate 

 with water, and then stop the flask with a cork bored 

 with two holes, one to admit a tube passing to the 

 bottom of the flask, the other to hold a tube that just 

 dips into the flask and then connects with a wide tube 

 holding calcimn chloride, a powert'ul agent for absorbing 

 water vapour (Fig. 9). CarefuUy wipe the whole appara- 

 tus with a soft duster, leave it standing for a time in the 

 balance case and then weigh, Xext tilt the acid gently 



^ Whiting is a stifficiently pure form. If it is not convenient to weigh 

 the gas as described above, the volume can be measured and the weight 

 calculated in the usual way. 1 c.c. of CO* at 0' C. and 760 nun. pressure 

 weighs 0-002 gram. 



