With the aid of an indicator, an acid/base reaction will occur and will show color changes on a piece of paper.
Materials & Pre-Demo Preparation
- large piece of paper
- phenolphthalein indicator
- 0.1 M sodium hydroxide
- 5 ml vinegar
- paintbrush and cotton balls
- Prior to the demo, use a paintbrush dipped in the phenolphthalein indicator to paint a picture on the large piece of paper. Make sure the picture is dry before the experiment--the picture will disappear as it dries off.
Draw a picture on the paper using the phenolphthalein indicator. Let it dry--it will appear colorless on the paper. Next, dip a cotton ball in the sodium hydroxide solution and wipe it over the paper where the drawing is. The drawing should appear. To make the drawing disappear again, dip a cotton ball in vinegar, and wipe it over the picture once again. The drawing should disappear.
Introduction & Commentary
How many of you like to draw or paint pictures? Which do you like better--crayons or paint? Well, do you think it's possible to draw a picture without crayons, paint, a pencil, pen, etc.? I'm going to show you a way to make colors and drawings without any of those things I mentioned.
The piece of paper is slightly acidic, and the phenolphthalein indicator is also acidic, as can be seen from the way it is colorless on the paper. The sodium hydroxide, however, is a base containing hydroxide ions (OH-), so when it is added to the picture, it turns the indicator a bright pinkish color. Vinegar, when added to the picture, will make the drawing disappear because it is a weak acid, leading to a colorless picture once again.
Vinegar or acetic acid + Sodium Hydroxide ---> Sodium Acetate + Water
HC2H3O2 + NaOH ----> NaC2H3O2 + H2O
- There are no particular safety precautions for this experiment.
Dispose of the remaining vinegar, indicator, and sodium hydroxide down the sink. Throw the picture away in the garbage.
- Charles Ophardt, Professor Emeritus, Elmhurst College; Virtual Chembook