Chemistry

How to tell if a Substance is an Acid or a Base



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There are several simple methods to determine if a substance is an acid or a base. They are litmus paper, pH paper, phenolphthalein solution, portable hand held pH meter, or pH wax pencil. Litmus is a mixture of blue dyes that has been used to test for acids and bases for over 300 years. If the litmus paper remains pink or red, the substance is an acid. If the litmus paper turns blue, the substance is a base. The color of the pH paper must be compared to a color chart. The various colors indicate pHs from 0-14. A pH of 0-6 indicates the substance is an acid. Strong acids have lower pH numbers. A pH of 8-14 indicates the substance is a base. Strong bases have higher pH numbers. A pH of 7 is neutral; water is neutral. The color from a pH wax pencil must also be compared to a chart. The pH pencil is used on solid substances. A few drops of phenolphthalein are added to a solution. If the solution remains colorless or unchanged, the solution is an acid. If the solution turns pink or magenta in color, the solution is a base. Substances that behave like vinegar or lemon juice are acids. Substances that behave like lye or baking soda are bases. Acids taste sour; bases taste bitter and are slimy. Never taste a substance in the laboratory because it may be poisonous or otherwise injurious to your health.

The pH of some common substances:

Oranges 3.5, tomatoes 4.2, soap 8, detergents 10.8, household ammonia 11, lye 13.5.

Acids can dissolve most metals with the production of heat and hydrogen gas. Beet juice and cabbage juice change color in acid and base solutions. Acids form hydrogen (H+) ions when they are dissolved in water. Bases form hydroxide (OH-) ions when they are dissolved in water. A conductivity meter will light up a light bulb in either case so it is not a method to determine if a substance is an acid or a base. Pepsi and Coca Cola are acids. You wouldn't use phenolphthalein to test Pepsi or Coke because the drink is so dark that you could not see the color change. Milk of magnesia is a base that does not form hydroxide ions.

What is acid rain? Water should have a pH of 7. Sulfur and nitrogen oxides emitted from cars, power plants, and industries can lower the pH of rain water to 4-4.5. Acid rain lowers the pH of lakes so that fish can no longer survive. Acid rain dissolves monuments and building facades.

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