EXERCISE 4 



Just as we can reverse the change from ice to- liquid water or from fluid to solid water, we can 

 reverse a chemical change in some cases, but not in all. A boiled egg cannot be made raw again ; a 

 piece of charcoal cannot be worked up into wood ; dried oil paint cannot be turned back into 

 fresh paint. 



Problem. How may I show a reversible chemical reaction ? 



What to use. The colored phenolphthalein solution of the last experiment ; 10 per cent hydrochloric 

 acid ; the beaker of washing soda ; some litmus solution or litmus paper ; test tubes ; rack. 



What to do. Add hydrochloric acid, drop by drop, to colored phenolphthalein solution, counting 

 the drops carefully. Shake tube gently after the addition of each drop. Note when color disappears. 

 Then add, drop by drop, counting each drop, sodium carbonate ; again, hydrochloric acid ; again, sodium 

 carbonate. 



Record. Tell what happens when the measured amount (that is, number of drops counted) of the 

 substance used in the experiment has been added to the solution in the test tube. 



NOTE i. The hydrochloric-tf^zV/, or sour, solution and the carbonate, or alkaline, solution counteract or neutralize one 

 another. A given amount of acid substance will neutralize a certain quantity of alkaline substance, and vice .versa. The 

 quantity of a solution required to neutralize another depends upon the relative concentration of acid and alkali. 



NOTE 2. Phenolphthalein is colorless in a neutral or acid solution, but it turns red in an alkaline solution. Substances 

 that react differently in two kinds of solution may thus be used as " indicators." 



NOTE 3. Litmus solution, made from a vegetable dye, or litmus paper, which is made by saturating paper with the 

 dye, is a common indicator used in chemical and biological laboratories. 



Repeat the above experiment, using litmus solution or litmus paper in place of phenolphthalein. 

 Record results. 



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