Litmus paper measures the acidity or base (alkalinity) of a substance it comes in contact with. It turns red when dipped in acid and blue in a base. Acids are sour tasting. They can corrode the metals that they touch. A base tastes bitter and feels slippery. Both acids and bases are dangerous in strong concentrations and can burn the skin. In chemistry, an acid is defined as a "proton donor," and a base is a substance that "accepts protons." Lime juice and vinegar are acids; ammonia and baking soda are basic.
The acidity or alkalinity of a substance is measured on the pH scale. The scale goes from 0 to 14, with a reading of 7 indicating neutral pH. Numbers below 7 indicate acidity, and above 7 indicate base. Remember that this is a logarithmic scale, so each number is 10 times stronger than the preceding. For example, pH 3 is 10 times more acidic than pH 4. The tool most commonly used in schools to measure pH is litmus paper.
Set up a simple experiment with litmus paper. Litmus paper packages come with a color chart that gives the pH scale. Gather a variety of materials and solutions. Try to get some materials that you know to be acidic, such as lime juice, orange juice and vinegar. Use baking soda and ammonia as your basic (alkaline) materials. Distilled water is pH neutral. Give your students plastic cups and have them pour each solution in its own cup. Have them prepare a three-column chart with headings "Name of Material," "Color of Litmus Paper" and "pH Value." Dip one end of the litmus paper in a cup. Compare its color to the color on the chart. Note the observation on the chart, such as: Vinegar, Orange, 2.0.
Litmus paper is a paper soaked in a dye obtained from lichens. This dye is a pH indicator and gives the paper its color-changing ability. You can make your own pH indicator with red cabbage juice. Put finely sliced red cabbage in a pot and boil it over a low flame for 30 minutes, or pour hot water over the cabbage slices and let them stand for a 30 minutes. You will get a purple-colored liquid. Cool and strain it. Pour a few drops of the juice on the substance to be tested and watch it change color. You also can make paper test strips. Take a coffee filter and dip it in the red cabbage juice. Hang out to dry. When the paper is totally dry cut it into strips and use it as you would use litmus paper strips. Store the strips in an air-tight box to extend their life. Make a color chart by comparing the colors with the color chart of a commercial litmus paper package. You also can compare the colors with the readings obtained by using a pH meter on the same materials to make a color chart.