1 Fill cup A with 200 mL of water.
2 Put 10-12 drops of phenolphthalein solution in cup A with water
3 Add Sodium Carbonate to cup B
4 Pour cup A into cup B
5 Insert the straw into the cup and blow OUTWARD.
6 The color will start to change back to colorless.
Phenolphthalein is an acid/base indicator. It is colorless when it is an acid and pink when it is a base.
Phenolphthalein is added to water, which is then poured into a second cup B containing a base (sodium carbonate). If the pH of the resulting solution is right, you can make the water turn from clear to red to clear again, as long as you like.
When I exhaled into the solution, the carbon dioxide in my breath reacted with the water to form Carbonic Acid. When enough carbonic acid is formed it neutralizes the water, which is a base and makes the solution an acid. Therefore with the phenolphthalein, the color disappears in the now acidic solution.
As you keep blowing your breath into the solution, there is now an excess of hydrogen ions or acid.
The principle is the same as for the disappearing ink formula.
These are easy instructions for making sodium carbonate, also known as washing soda or soda ash, from baking soda.
MAKE SODIUM CARBONATE
Simply heat baking soda in a 200°F oven for about an hour. Carbon dioxide and water will be given off, leaving dry sodium carbonate. This is the soda ash.