CHAPTER VI 



ELEMENTS AND COMPOUNDS 



98. Physical and Chemical Changes. If we heat a 

 flat-iron on a stove, it becomes hot. It may even become 

 red hot and give off light as well as heat. But if it is 

 taken from the stove, it becomes cold again, and looks 

 just as it did before the heating. A lump of coal thrown 

 into the air comes down again, still a lump of coal. The 

 water of the ocean is changed into steam, then carried 

 away by air currents, and finally falls again as rain or 

 snow. The iron is changed in temperature, the coal and 

 water are changed in position, and the water is changed in 

 physical state (cf. 69); but the changes do not really 

 alter the iron, coal, and water. We call such changes 

 physical changes. 



But if a piece of iron is left in moist air, it rusts (cf. 

 48) . If water is added to quicklime, it combines with the 

 lime, forming "slaked" lime; it is no longer water. If 

 coal is burned, it disappears, as carbon dioxide and steam 

 (cf. 52). Such changes do alter the nature of substances. 

 They are called chemical changes, because they are stud- 

 ied in the science of Chemistry. Chemical changes are 

 also called reactions. 



Digestion consists of the physical and chemical changes 

 that take place in food, in order that it may "be taken up 

 by the blood for the use of the body. 



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