204 COMMON SUBSTANCES 



At once hold a cool glass beaker or tumbler over the flame 

 (Fig. 135) and note the condensing of water vapor upon it. 

 This water vapor is given off when the H unites with the 

 O, H 2 + = H 2 0. 



236. Sulphuric Acid. Sulphuric acid, H 2 SO 4 , is made 

 in great quantities by the union of the elements of SO 2 , 

 H 2 O, and O. Being a very strong acid, it is used to 

 break up the salts of many other acids, setting those 

 acids free. In this way HC1 is made from NaCl, HNO 3 

 from NaNO 3 , etc. H 2 SO 4 forms with different metals an 

 important group of salts called sulphates. 



237. Carbon Dioxide. Whenever carbon is burned in 

 a good supply of air, a gas called carbon dioxide (CO 2 ) 

 is formed. CO 2 is a colorless, odorless gas ; it is heavier 

 than air, and is sometimes called carbonic acid gas. 

 In the earth CO 2 occurs widely in carbonates, chiefly as 

 CaCOg (limestone, marble, etc.). It is given off, in its 

 free gaseous state, from burning wood, coal, kerosene, 

 illuminating gas, etc. ; also from the lungs of animals, 

 mixed with the air breathed out. Carbon dioxide occurs 

 (in a very small quantity) in the atmosphere, where it 

 forms an important part of the food of plants. The gas 

 which causes the " lightness " of bread and cakes is gen- 

 erally CO 2 , and the same gas causes the effervescence 

 of soda water and bottled tonics. 



Experiment 129. Into a large test tube put a few bits of mar- 

 ble, and add HC1. Stop the test tube, running a delivery tube to 

 the bottom of a loosely covered jar, as in Experiment 125. When 

 the gas has flowed freely for two or three minutes remove the tube 

 from the jar, carefully covering the latter. In this way fill two 

 jars. The gas is CO 2 . CaCO 3 + 2 HC1 = CaCl 2 + H 2 O + CO 2 . 



