THE ELEMENTS OF PLANT FOOD 31 



and liberate CO 2 . Carbonate of lime (CaCO 3 ) may be 

 treated with HC1 to obtain CO 2 for laboratory nse. 

 The reaction is : 



CaCO 3 + 2 HC1 = CaCl 2 + H 2 CO 3 

 H 2 CO 3 = H 2 O + CO 2 



In this case carbonic acid is formed, but is so unsta- 

 ble that it immediately breaks up into H 2 O and CO 2 . 

 Since CO 2 is heavier than air, it may be collected by 

 pouring as one would a liquid. If care be used, it may 

 be transferred from one vessel to another in this way. 



Note. The gas produced in fermented liquors and that which 

 causes bread to rise is CO 2 . Yeast is an active ferment which when 

 mixed with flour and water under proper conditions produces CO 2 . 

 This gas makes the bread light by forming little bubbles throughout the 

 mass of the dough till the baking hardens the dough so that it retains 

 the little spaces. 



The gas in soda water is CO 2 . It gives a sharp, prickly 

 taste to carbonated water and a pungent sensation in 

 the nose as it escapes. 



COo is not a supporter of combustion. Test this 

 property by putting a lighted splinter or match into 

 a jar of the gas, and note how quickly it is extin- 

 guished. 



CO 2 sometimes accumulates at the bottom of a well, 

 a cistern, or a silo. Before going down into such a 

 place, one should lower a lighted lantern or candle. If 

 the light is extinguished, there is sufficient CO 2 present 

 to suffocate any one going into it. 



The limewater test is the usual one for CO 2 . Pour 

 into a glass or test tube some clear limewater, Ca(OH) 2 . 

 Through a straw or glass tube blow into the limewater 



