Step 1:  Boil [500 mL tap water + 10 mL phenolphthalein indicator solution]  in a Florence flask with a long glass tube (as shown).   Steam should be coming out the top.

Step 2:  Invert flask, inserting tube into graduated cylinder or  beaker of [700 mL tap water  +  10 mL concentrated NH₄OH]   (^csolution 2).

Step 3:  The pressure in the flask will be lower than the air pressure:  water will be pushed into the flask.

The color change is noted as the phenolphthalein changes from acidic (colorless) to basic (purple). 

TEACHER NOTES

^aSTOCK    phenolphthalein STOCK Solution:  750 mL distilled water  +  0.5 g phenolphthalein
                                                                               (add 6M concentrated HCl dropwise until clear)

^bSOLUTION 1    500 mL tap water + 10 mL phenolphthalein STOCK solution

^cSOLUTION 2    700 mL tap water  +  10 mL concentrated NH₄OH

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