University of Virginia Physics Department Acid-Base Tea Party (Demonstration only) A Physical Science Activity

Adapted from an activity in Macmillan's Physical Science by Denise Eby and Robert B. Horton (1988).

2003 Virginia SOLs

• PS.1
• PS.2

Motivation for Learning

Demonstration - Discrepant Event

Materials

• 500 mL 0.1 M solution of sodium hydroxide NaOH (4g NaOH in 1 liter of water)
• 6 mL of concentrated hydrochloric acid (HCl)
• Less than 1 mL of phenolphthalein
• 4 clear glasses
• 1 clear pitcher or 1000 mL beaker

Procedure

1. Prior to students reporting to class, set out the 4 glasses all in a row and prepare your 0.1 M NaOH solution. Put 500 mL of NaOH in the pitcher or the large beaker.
2. Leave the first two glasses empty.
3. Add just a few drops of phenolphthalein to the third glass.
4. Add 6 mL of concentrated HCl to the fourth glass. (CAUTION: Do not let students handle the fourth glass in any way!)
5. When your students arrive, begin class by telling them "today, we are going have a tea party."
6. Begin pouring the NaOH solution into the glasses beginning with first glass. Of course, nothing will happen in the first or second glass. If students complain that the tea is clear, then tell them "we are pretending."
7. As you pour the "tea" into the third glass the phenolphthalein will react and turn the "tea" pink. At this point you need to act as if you are totally surprised by this turn of events.
8. Begin with the first glass and pour the "tea" back into the pitcher. Again as you pour the first two glasses back into the pitcher, nothing will happen. When you pour the third glass of pink "tea" into the pitcher, all of the tea will turn pink. Again act surprised and say "Oh well, pink tea will just have to do."
9. Start pouring the now pink "tea" into the glasses starting with the first glass. When you pour the pink "tea" into the fourth glass, the concentrated HCl will reverse the reaction with the phenolphthalein and the "tea" will be clear again. At this point, you need to act totally perplexed.
10. Finally, pour the pink "tea" now in the first three glasses back into the pitcher also containing pink "tea". Then pour the clear "tea" in the fourth glass back into the pitcher of pink "tea". All the "tea' turns colorless. Now your students should be ready to begin their study of Acid-Base Chemistry!

Background Information

This demonstration is designed to hook your students on acid-base chemistry. It requires a little acting on your part as you proceed with your pretend tea party. Be sure to keep your students back from the demonstration. Wear your safety goggles. The demonstration can be done with more or less NaOH by adjusting the amount of HCl used also. Because phenolphthalein detects bases, you can use any amount over 6 mL of HCl and it will turn the whole pitcher clear again by making the solution acidic, but you cannot use less than 6 mL or the solution will remain basic and the color will still be bright pink.