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More Deep Purple Magic

In this activity, students are fooled by an acid-base colour-changing reaction, and learn how chemistry can explain this magical result.

Phenolphthalein is an indicator — a chemical which changes colour depending on whether it meets an acid or a base. It turns purple if it meets something basic, such as ammonia; it stays colourless if it meets an acid like vinegar or a neutral substance like water.

In this magic trick, the phenolphthalein in beakers 1 and 3 will turn purple when you mix it with the ammonia in beakers 2 and 4. When you pour the two purple solutions into the large beaker, the acidic vinegar neutralizes the basic ammonia. The phenolphthalein becomes colourless.

Vocabulary:

  • Acid: Acids are compounds with excess H+ ions. The term comes from the Latin word acidus that means “sharp” or “sour”. Some basic properties of acids are that they taste sour, react with metal, change the color of litmus paper from blue to red, and have a low pH (0 – 7). Examples of acids are vinegar and lemon juice.
  • Base: A compound that has an excess of OH- ions. Another word for base is alkali. Some basic properties of bases are that they can be used as household cleaners, turn red litmus paper blue, and have a high pH (7 – 14). Some examples of bases are ammonia and baking soda.
  • Indicator: A substance that indicates the degree of acidity or alkalinity of a solution through characteristic color changes.

Objectives

  • Explain the importance of observation when doing science.

  • Explain the chemistry behind a simple acid-base reaction.

Materials

  • Per Demo or Group:
    2 large beakers
    4 small beakers
    Phenolphthalein
    Ammonia
    Vinegar
    Water
    *all chemicals and solutions are readily available from science supply stores.

Key Questions

  • Were all the beakers of “water” the same to begin with?
  • What is an acid?
  • What is a base?
  • What happens when an acid is mixed with a base?
  • Why might an indicator like phenolphthalein be useful in chemistry?

What To Do

Preparation:
Do the following preparation out of students’ view.

Hint: ammonia evaporates quite quickly, so cover the ammonia beakers if you’re preparing more than a few minutes before you do the trick.

  1. Label four small beakers 1 to 4.
  2. In 1 and 3, put in 5 drops of phenolphthalein.
  3. In 2 and 4, put in 5 drops of ammonia.
  4. In one large beaker, put in 20 drops of vinegar.
  5. Fill the other large beaker with water (for pouring into small beakers).

Instructions:

  1. Pour water into each beaker (1-4) so each is half full.
  2. Announce that you will now “magically” change the colour of the “water” by mixing two beakers of water together. Ask students to observe carefully and describe the colour as you pour 1 into 2, and 4 into 3. Watch as they both turn purple-pink!
  3. “Shtick” at the ready, ask students in a mysterious voice to report what they see when you mix together the two coloured solutions.
  4. Dump both solutions together into the large beaker (laced with vinegar) and watch as they become colourless again!

Extensions

  • Can you predict the colour phenolphthalein will turn when added to milk? How about orange juice? Try it out with some other common solutions.
  • Try making your own cabbage juice acid-base indicator.

Other Resources

Science World | YouTube | Using Purple Cabbage to do some Chemistry!