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One of the simplest activities to show how acids and bases react with each other (and to demonstrate their different properties) is to make a baking soda and vinegar 'volcano'. To make a big eruption, use a small plastic bottle (the size 20-oz soft drinks come in works well). Fill the bottle halfway (1 to 1.5 cups) with vinegar. To start the eruption, drop a baking soda 'bomb' into the bottle--wrap one tablespoon of baking soda into a small piece of tissue paper, tying the ends with thread. You should see an instant eruption! The baking soda, a base, neutralizes the acid in vinegar. This releases carbon dioxide gas, which causes the fizzing action in your volcano. (An acidic solution is neutralized when a base is added to it, and a basic solution is neutralized by the addition of an acid.)
For another reaction experiment, put an Alka-Seltzer tablet in the bottom of a clear plastic film canister (the kind where the cap fits inside instead of closing over the outside). Fill the canister with warm water and then quickly put the cap on and watch the acid-base reaction!
The pH scale is used to measure the amount of H+ ions in a solution. Acids have a pH below 7; bases have a pH above. Strong acids have the lowest pH levels (0-4) and strong bases have the highest pH levels (10-14). Neutral solutions have a pH of 7 and they are neither acidic nor basic. Distilled water is neutral, because the H+ and OH- ions are balanced.
Litmus is a natural acid-base indicator extracted from a type of lichen. If you have red and blue litmus paper, you can test different solutions for whether they are acids or bases. Blue litmus paper turns red when a solution is acidic; red litmus paper turns blue in basic solutions. Try testing window cleaner, toilet bowl cleaner, orange juice, and apple juice--pour a little of each into separate test tubes or small glasses or jars. Use the litmus paper to determine which are acids and which are bases. Here are the pH levels of some other substances that you might test: lemon juice (2), vinegar (3), milk (6), egg whites (8), baking soda (9), and ammonia (10). Human blood has an ideal pH of 7.4; even slight fluctuations can seriously affect our bodies.
You can also make your own pH indicator--use a blender to mix one part chopped red cabbage with two parts boiling water and use the juice to test different solutions. Acids will turn the pigments in the indicator to a reddish color; bases will turn the pigments bluish or yellow-green.