Scrape enough to turn the water a pinkish color. Test for acid or base by adding a few drops of the liquid you want to test. Once you know whether something is an acid or a base, try adding the opposite to see if the color changes again. You can also make a pH tester with red cabbage juice. Lemon juice and vinegar both contain acids.
Bases have no strong taste in foods, and often feel slippery or soapy to the touch.
Liquid bleach is a base. Water and soil are tested for their pH. If a swimming pool is too acid, it will burn your eyes. If a pond is too acid, the fish will die. Something acid can be made less so by adding a base. Fold the postcard with shiny side in, lengthwise into three equal sections.