SECTION II. : CHEMISTRY. 



CHAPTER XVL 



32. PHYSICAL AND CHEMICAL CHANGES. 



i. Physical changes. (a) Heat a piece of platinum wire in the flame of 

 a Bunsen burner. The wire soon becomes red-hot. Remove the wire from 

 the flame and it quickly resumes its former appearance. 



(f>) Magnetise a sewing- needle. Though the needle now possesses a new 

 property, it is still easily recognised as a needle and is clearly not a new 

 substance. 



(c) Place a piece of ice in a test-tube, heat it gently and observe the 

 changes. Notice that the solid ice changes, first to liquid water, and on 

 further heating to a gas, steam. By reversing the processes the steam can 

 be reconverted into water, and the water to ice. 



ii. Chemical changes. (a) Roll a piece of paper to form a pipe-light. 

 Light the paper and allow it to burn. Observe the changes. The paper 

 disappears and in its place is a black charred residue. During the burning, 

 smoke is formed. 



(6) Consider the burning of a lump of coal in an ordinary fire. Note the 

 properties of the coal before it is placed on the fire. As burning proceeds 

 smoke and flame are produced. Compare the ash left after the burning 

 is complete with the original coal. 



(c) Heat one end of a piece of magnesium ribbon in the flame of a Bunsen 

 burner. The metal soon bursts into flame and burns brilliantly. Compare 

 the white ash formed with the original ribbon. There is no doubt that there 

 has been a complete change of substance. 



Physical and chemical changes. All substances are subject to 

 two kinds of change. Hitherto only those changes which influence 

 the properties of substances, leaving their composition unaltered, have 

 been studied. It has been seen that a body, such as a piece of iron, 

 may gradually increase in temperature, changing from cold iron to 

 hot, and, becoming hotter and hotter, may change in colour, passing 

 from a dull grey to red, and from red to almost white, becoming in- 

 candescent and emitting light rays. But, if left to itself, the iron will 



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