366 Common Science 



gas) and compress it in iron tanks. These iron tanks 

 of carbon dioxid (CC^) are shipped to soda-water foun- 

 tains and soda-bottling works. Here the compressed 

 carbon dioxid is dissolved in water under pressure, - 

 this is called " charging " the water. When the charged 

 water comes out of the faucet in the soda fountains, or 

 out of the spout of a seltzer siphon, or out of a bottle 

 of soda pop, the carbon dioxid that was dissolved in 

 the water under pressure bubbles up and escapes, - 

 the soda water effervesces. 



Sometimes there is compressed carbon dioxid down 

 in the ground. This dissolves in the underground water, 

 and when the water bubbles up from the ground and 

 the pressure is released, the carbon dioxid foams out 

 of the water ; it effervesces like the charged water at a 

 soda fountain. 



But the most useful and best-known effervescence is 

 the kind you got when you stirred the baking soda in 

 the lemonade. Baking soda is made of the same ele- 

 ments as caustic soda (NaOH), with carbon dioxid 

 (CO 2 ) combined with them. The formula for baking 

 soda could be written NaOHCO 2 , but usually chemists 

 put all of the O's together at the end and write it 

 NaHC0 3 . Whenever baking soda is mixed with any 

 kind of acid, the caustic soda part (NaOH) is used up 

 in neutralizing the acid. This leaves the carbon dioxid 

 (CO 2 ) part free, so that it bubbles off and we have 

 effervescence. Baking soda mixed with an acid always 

 effervesces. That is why sour milk muffins and pan- 

 cakes are light as well as not sour. The effervescing 

 carbon dioxid makes bubbles all through the batter, 



