EXERCISE 3 



When a substance is acted upon in such a way that the particles of which it is made are com- 

 pletely rearranged, a chemical change is produced. We cannot always recognize a chemical change at 

 once, but certain appearances usually indicate such 

 a change. Any one substance may undergo many 

 different chemical changes, just as any one sub- 

 stance or object may undergo many physical changes. 



Problem. To show some of the differept chemical 

 changes that a single substance may undergo. 



What to use. Sodium carbonate (washing soda) 

 solution; water; hydrochloric acid (dilute 10%); 

 barium chlorid ; phenolphthalein ; 5 test tubes and 

 rack ; I beaker. 



What to do. Have about I inch of the solution 



indicated in the different tubes. Pour solution from the beaker into the test tubes, drop by drop. 

 Note what happens immediately after the addition of sodium carbonate to the solution in each tube. 

 After a few minutes shake the contents of each tube gently and note results. 



Record. Describe the results of adding sodium carbonate to each tube. 



Questions. Which of the changes you have described would you consider physical and which 

 chemical ? Why ? 



NOTE I. The alteration that is brought about by the action of one thing or substance upon another is called the 

 reaction. 



NOTE 2. The discharge of bubbles of gas from a solution is called effervescence. 



NOTE 3. The production of solid particles in a solution is called precipitation, which means " throwing down.." 



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