Chemical Change and Energy 309 



sputters and drives hydrogen out of the water, and the 

 poison gas chlorine (Cl), combined with each other. 

 And salt is neither dangerous to put in water like sodium, 

 nor is it a greenish poison gas like chlorine. 



Mixtures. But sometimes elements can be mixed 

 without their combining to form compounds, in such 

 a way that they keep most of their original properties. 

 Air is a mixture. It is made of oxygen (0) and nitrogen 

 (N). If they were combined, instead of mixed, they 

 might form laughing gas, the gas dentists use in put- 

 ting people to sleep when they pull teeth. So it is well 

 for us that air is only a mixture of oxygen and nitrogen, 

 and not a compound. 



You found that things burned brilliantly in oxygen. 

 Well, things burn in air too, because a fifth of the air 

 is oxygen and the oxygen of the air has all its original 

 properties left. Things do not burn as brightly in air 

 as they do in pure oxygen for the same reason that a 

 teaspoonful of sugar mixed with 4 teaspoonfuls of boiled 

 rice does not taste as sweet as pure sugar. The sugar 

 itself is as sweet, but it is not as concentrated. Like- 

 wise the oxygen in the air is as able to help things burn 

 as pure oxygen is ; but it is diluted with four times its 

 own volume of nitrogen. 



A solution is a mixture, too ; for although substances 

 disappear when they dissolve, they keep their own 

 properties. Sugar is sweet whether it is dissolved or 

 not. Salt dissolved in water makes brine; but the 

 water will act in the way that it did before. It will 

 still help to make iron rust; and salt will be salty, 

 whether or not it is dissolved in water. That is why 



