While the chemical concept of acid-base chemistry is definitely a high school level subject, kids of any age can learn that many substances are classified as either acidic or basic. They can also understand that these designations are important to chemists as they attempt to understand the properties of materials. Knowing about the fundamental properties of a substance or mixture helps chemists formulate life-saving pharmaceuticals, create nutritious, enjoyable food products, and make safe cosmetics and soaps. Part of understanding a substance or mixture is classifying it as an acid or a base.
One of the best ways to explore acid-base properties is by using a substance known as an acid-base indicator. An acid-base indicator changes colors, and the color of the indicator depends on the acid or base nature of the substance you mix with the indicator. Thus, a simple color change gives chemical information.
Indicators are an interesting phenomenon in analytical chemistry in and of themselves. Color changes can give information about the amount of oxygen in a water sample, the presences of ammonia in a fish tank, or whether or not a woman is pregnant. Acid-base indicators are just one of the many ways chemists use color to make tests simple and easy to use.
You can make your own acid-base indicator with something remarkably common and inexpensive- red cabbage! Simply chop up the cabbage into small pieces, add boiling water, and let it sit for at least ten minutes. Next, filter out the solid plant material by pouring the liquid through a coffee filter in a funnel.
The result is a liquid formulation of an acid-base indicator called flavin. Exposing the flavin solution to acids like lemon juice or vinegar will cause it to turn red. (When neutral, it is purple.) Adding a base such as baking soda, ammonia, or lye will cause it to turn blue to yellow-green. You can see a difference in the extent and quality of the color change depending on how much acid or base you add.
Depending on the ages of the children involved, you may want to limit your experiments to food products like lemon juice, vinegar, baking soda, milk, etc. Ammonia has dangerous fumes, and lye is caustic enough to cause a chemical burn. Hydrochloric acid (also called muriatic acid) is used for stripping paints and finishes and can be purchased at the hardware store. Hydrochloric acid is stomach acid, which makes it an interesting biological compound. However, it should not be used unless kids are mature enough to handle highly corrosive substances and have the manual dexterity to do so!
Whenever you do chemistry experiments with kids, teach them proper lab safety. Have them wear goggles and protective clothing, even if working with household chemicals. Professional chemists take safety seriously, and do not rely on their own judgment in "deciding" whether to use goggles and a lab coat; they simple wear safety gear for all laboratory experiments. By teaching kids proper lab safety early, you instill a valuable lesson in professionalism.