34 2 Common Science 



paint is phosphorescent for the same reason, there is 

 a chemical change going on that releases energy in the 

 form of light. 



When you poured the hydrochloric acid on the zinc 

 to make hydrogen, the flask became warm ; the chemical 

 change going on in the flask released heat energy. 



Application 76. Explain why pouring cold water on cold 

 quicklime makes the slaked lime that results boiling hot; 

 why a glowworm shines in the dark ; why a piece of carbon 

 and a piece of zinc placed in a solution of sal ammoniac will 

 make electricity run through the wire that connects them; 

 why fire is hot. 



Inference Exercise 

 Explain the following : 



481. A baking potato sometimes bursts in the oven. 



482. Turpentine is used in mixing paint. 



483. Sodium is a metal; chlorine is a poisonous gas; yet salt, 



which is made up of these two, is a harmless food. 



484. When bricklayers mix water with cement and lime, the re- 



sulting mortar boils and steams. 



485. Green plants will not grow in the dark. 



486. Parts of the body are constantly uniting with oxygen. This 



keeps the body warm. 



487. Water will not always put out a kerosene fire. 



488. Delicately colored fabrics should be hung in the shade to dry. 



489. A match glows when you rub it in the dark. 



490. Candy hardens when it cools. 



SECTION 52. Explosions. 



What makes a gun shoot? 

 What makes an automobile go ? 



Usually we think of explosions as harmful, and they 

 often are, of course. Yet without them we could no 

 longer run automobiles ; gasoline launches would stop 

 at once; motorcycles would no longer run; gasoline 



