WATER 95 



Physical and Chemical Changes. If we throw some 

 water into the air, it will fall to the ground, but the com- 

 position of the water is not changed. It is still water. 

 The water of the ocean is changed to vapor and carried 

 long distances by winds, to 'fall again as water in the form 

 of rain or snow, but it is water during the entire change 

 of location. These changes are called physical changes. 

 In the experiment we have just performed, however, we 

 have a different kind of change. The result was no longer 

 water but two substances of entirely different character : 

 one of them a strong supporter of burning or combustion, 

 and the other a substance which will burn. How dif- 

 ferent from water which is so much used to extinguish 

 flames ! The electrolysis of water is a chemical change. 

 If we place a piece of paraffin in an evaporating dish and 

 apply heat to it, a change will take place in the paraffin. 

 It melts and becomes a liquid, but the substance is still 

 paraffin and possesses the same properties as it did before, 

 with the addition of the ordinary properties of liquids. 

 A change which does not alter the nature or properties 

 of a substance, but only the form or appearance, is called 

 a physical change. A chemical change is one in which 

 the properties are changed and an entirely new substance 

 is formed. When ice is changed to water or water to ice, 

 is there any change in the real nature of the substance ? 

 What kind of change is vaporization? Burn a piece of 

 wood. What kind of change is demonstrated here? 



Experiment 26. Into a test tube pour a little solution of ordi- 

 nary table salt and in another test tube a similar amount of silver 

 nitrate solution. Note the appearance of each. Now pour the 

 contents of one tube into the other. The solid that slowly falls 

 to the bottom of the tube is called a precipitate. What kind of 

 change is this? 



