CHAPTER XI 

 HOW MATTER CHANGES 



147. Physical Changes. A small bar of iron rubbed 

 upon a magnet becomes a magnet itself; that is, it acquires 

 the property of attracting pieces of iron or steel. Left un- 

 touched for a time, it loses its new property; but the iron 

 was iron and nothing else all the time. Mercuric iodide, a 

 red powder, on being heated becomes yellow. When cooled 

 again, it returns to its original red color; it is the same sub- 

 stance as before. If a piece of wood is changed to sawdust, 

 or a piece of stone crushed to powder, the wood and stone 

 retain their former characteristics though they may never 

 again become a single body. If sugar or salt is dissolved in 

 water and the solution is exposed to the air, the water evapo- 

 rates and the sugar or salt reappears unchanged. Such 

 changes as these are physical changes. Condensation and 

 evaporation, freezing and melting, expanding and contract- 

 ing, are all physical changes. 



A physical change is one that does not permanently 

 change the properties of a substance nor produce any change 

 in its composition. 



148. Chemical Changes. A lump of sugar placed on a 

 hot stove melts. A cloud of white smoke rises from it and 

 after a time a crisp, black solid is all of the sugar that 

 remains on the stove. This residue looks like charcoal; it 

 does not dissolve in water; it has no sweet taste. It has 

 permanently lost its characteristic properties of whiteness, 

 solubility, and sweetness. It is not sugar. 



A chemical change is one that causes a loss of characteris- 

 tic properties because of a change in the composition of the 

 substance. 



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