336 SMITH'S INTERMEDIATE CHEMISTRY 



nesium or aluminium powder, however, will burn when placed on a 

 cake of solid carbon dioxide and set on fire with burning magne- 

 sium ribbon : 



2Mg + CO 2 -* 2MgO + C. 



The gas extinguishes burning wood, oil, or candles, and 15 to 

 16 per cent of it in air is sufficient to extinguish ordinary com- 

 bustibles. For this reason some fire extinguishers contain a dilute 

 solution of bicarbonate of sodium (NaHC0 3 , p. 366) and sulphuric 

 acid: 



2NaHC0 3 + H 2 S0 4 ? Na 2 S0 4 + 2H 2 C0 3 -> 2H 2 + 2CO 2 1- 



When the instrument is inverted, these materials are mixed, 

 and water and carbon dioxide are forced out by the pressure of the 

 gas. 



Chemical Properties Carbonic Acid. 2. Carbon dioxide, 

 when dissolved in water, combines in part to form carbonic acid: 



The gas is therefore often called the anhydride (Greek, without 

 water) of carbonic acid. The solution has all the properties of an 

 acid, although, as the acid is very little ionized (p. 189), it exhibits 

 them rather feebly. It tastes slightly sour, turns blue litmus 

 faintly red, and neutralizes bases. The last action is easily shown 

 by shaking the gas with limewater (solution of calcium hydroxide, 

 a base) : 



Ca(OH) 2 + H 2 C0 3 - CaCO 3 | + 2H 2 0. 



The carbonate of calcium is precipitated and the liquid becomes 

 milky in appearance. This action is used by sugar refiners for 

 removing the lime employed in purifying the sugar. Manu- 

 facturers of white lead (carbonate of lead) also employ carbon 

 dioxide, because of its entering into double decomposition to give 

 carbonates. The same property is utilized in making bicarbonate 

 of sodium and washing soda (carbonate of sodium, p. 367). 



