EXERCISE 12 



In Exercise 2 we learned that there are certain substances (phenolphthalein and litmus were exam- 

 ples given) called indicators for acids and alkalies ; we use such an indicator to test, or try out, the 

 acidity or alkalinity of a substance. In the study of living things we' often need an indicator to test 

 the presence of some particular substance, and different indicators 

 are accordingly used. 



In Exercise 3 we also learned that a given substance may react 

 in a distinct way with other substances ; for example, sodium carbonate 

 (washing soda) and barium chlorid together produce a white precipitate. 

 But we do not know whether sodium carbonate can produce a pre- 

 cipitate with substances other than barium chlorid ; nor do we know 

 whether barium chlorid can produce a precipitate with something else 

 besides sodium carbonate. 



Problem. To find the effect of carbon dioxid on limewater. 



What to use. Gas generator (see figure) ; limewater with container. 



What to do. Prepare some carbon dioxid gas by means of apparatus such as shown in diagram ; 

 pour dilute hydrochloric acid on marble chips through the thistle tube. Allow the gas to bubble 

 through the limewater. 



Record. Note the changes that take place in the limewater. 



NOTE i . Chemists have made thousands of experiments to learn the effects of various gases on limewater, and similar 

 ones to learn the effects of carbon dioxid upon various substances. Whereas carbon dioxid will produce a white precipitate 

 with various solutions, it is the only gas that will produce a precipitate in limewater. We may therefore be sure of the 

 presence of this gas when we see limewater turn turbid, or milky. 



Question. Write out a method for determining the presence or absence of carbon dioxid gas in 

 the breath ; test for the presence or absence of carbon dioxid gas in illuminating gas. 



H 



NOTE 2. Just as we used the symbol H 2 O, or > O, to represent water, we denote carbon dioxid by the shorthand 

 method of CO 2 , or C < ? 



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