288 



ELEMENTARY GENERAL SCIENCE 



CHAP. 



Cottonwool 



EXPT. 296. Into a flask A (Fig. 135) fitted with india-rubber 

 stopper and tubes, as shown, pour some dilute hydrochloric acid, 

 and weigh the flask with the contained 

 acid. Weigh a small piece of tube 

 B (i) empty (ii) with some chalk. 

 Place this in the flask, taking care 

 not to allow the acid to reach the 

 inside of the tube. Replace the 

 cork and shake the flask so as to 

 allow the acid and chalk to react. 

 The carbon dioxide formed escapes 

 through the tube C in which cotton- 

 wool is packed to retain moisture. 

 When the action is at an end draw a 

 current of air through the flask in 

 order to displace the carbon dioxide 

 still left inside. Weigh the flask 

 again, and subtract the weight so 

 obtained from that of the flask + 

 weight of tube and chalk. The re- 

 sulting number is evidently the 

 weight of the carbon dioxide evolved. 

 Calculate the loss for 100 grams of 



ChalA 



dilute hydrochloric 

 tfcut 



FIG. 135. To illustrate 

 Experiment 296. 



Qf 



equal to the loss of weight on heating, 



so that we are justified in concluding that the same constituent 

 was evolved, and we may write : 



Chalk = lime and carbon dioxide. 



By the action of the gas upon lime dissolved in water we also 

 obtain a white product, and if sufficient of this be collected and 

 examined it is found that by the action of acids it also gives 

 off" the carbon dioxide, and that it forms lime on heating. It 

 is therefore chalk, and the formation is merely the reverse of 

 what was written above, i.e., lime and carbon dioxide form 

 chalk, so that the turbidity of the lime-water is due to the 

 formation of chalk. 



Compounds which, like chalk, evolve carbon dioxide when 

 acted upon by an acid, are known as carbonates, and many 

 carbonates exist, all possessing many characteristics in common. 

 Many of these on heating also give off carbon dioxide, the 



