I learned this experiment from my younger sister, Julie Millard, who is a professor of chemistry at Colby College in Waterville, Maine. She has two children in elementary school, and likes to visit their classes to do chemistry demonstrations. She showed me this experiment and gave me some goldenrod paper to do it myself.
Goldenrod paper is a special golden yellow paper coated with a dye that acts as an acid-base indicator. It turns bright red when exposed to a base such as ammonia or baking soda, and back to yellow with an acid such as vinegar or lemon juice.
The experiment teaches the differences between acids and bases. Historically, acids were recognized by their sour taste in water and because they could attack and dissolve some metals. The word acid is derived from acetum (Latin, "Vinegar"). Bases were usually soapy to the touch and could react with acids in water to form salts.
To do this experiment, you need:
Procedure: Before class, write a secret message on the goldenrod paper with the crayon. It will be invisible because it's the same color as the paper. You might want to write "Hello!" Or "Wow!"
Put some baking soda in water and stir it with a Q-tip to form a solution. Then wipe the Q-tip over the hidden words on the paper. The paper will turn red, except for the words.
Then rub the red area with a Q-tip soaked in vinegar. It will bubble, then slowly turn back to yellow.
Finally, put some more of the baking soda solution on the words and the red color will return.
If you want to expand the experiment, you can ask the students to test the paper with ammonia and lemon juice to find out which is an acid and which is a base. If using ammonia, make sure the room is well ventilated.
Scientific question: Which substances are acids and which are bases? The students will report their answers. Even if they don't understand all the chemical processes taking place, they will remember their results, and which substances fall into each category.
Scientific method: The lesson is nondogmatic, testable, consistent and objective. These words can be used throughout the lesson at each step in the process. After determining which substances are acid and which are basic, you can do the experiment in several different orders, write down which substances you are testing, and the results obtained.
Learning cycle: For invitation, you can invite the students to write their own secret messages on their own paper. For exploration, you can let them take turns testing their papers with the two to four substances you are using. For concept introduction, you can explain what acids and bases are, and why they act the way they do when combined. For application, you can demonstrate the experiment in the most dramatic order (baking soda, then vinegar). This will enable you to keep the dynamic alive.