98 LABORATORY LESSONS IN GENERAL SCIENCE 



2. Follow very closely the results as the teacher carries 

 out the following directions : To a solution of baking 

 soda in an evaporating dish add drop by drop some dilute 

 hydrochloric acid (HC1) till, as the solution is stirred thor- 

 oughly, it does not affect the color of litmus paper, i.e., 

 till it is neutralized. If too much acid is added at any time, 

 more of the soda solution may be added drop by drop. 

 When the solution becomes neutral, evaporate it to dryness, 

 heating till no more vapors pass off. Identify the solid 

 that remains by tasting it. 



SOME CARBONATES 



1. In the lesson on Combustion what was discovered re- 

 garding the effect of the expired breath upon limewater 

 (CaO 2 H 2 ) ? What was the explanation of the change in its 

 appearance? The precipitate (the solid formed as result of 

 chemical change) is calcium carbonate (CaCOs). 



2. Force air out from the lungs through a glass tube that 

 reaches down into lOc.c. of limewater in a test tube, and 

 continue until the limewater becomes clear as at first. The 

 solid particles of the carbonate of "lime" apparently dis- 

 solve. Test the liquid with litmus paper to determine its 

 nature. 



To explain what has occurred, it may be supposed that car- 

 bon dioxide combines with the water as shown in the equation 

 CO 2 + H 2 O > H 2 COs (carbonic acid), and this in turn with 

 the limewater thus : CaO 2 H 2 +-H 2 CO 8 -> H 2 O + CaCO 3 . 

 But when an excess of H 2 CO3 has been formed by reason of 

 continued breathing of CO 2 into the limewater, the further 

 change whereby the limewater becomes clear again may be 



