Acid and Base

Acid and base are terms used by chemists to describe the very different actions and opposing properties of certain chemicals when they are dissolved in water. A solution of acid produces a burning sensation on the skin and has a sour taste. A base solution feels slippery and tastes bitter. Both occur naturally and some acids are essential to life.

Acids and bases are both biologically important compounds. As compounds, however, they are complete opposites because of what happens when they are put into a solution, or liquid mixture. At the subatomic level of protons and electrons, any substance that releases a hydrogen ion (a positively charged hydrogen atom) when in solution is called an acid. Conversely, any substance that combines with or gains a hydrogen ion in solution is called a base. While this may sound complicated, many acids and bases are actually quite familiar to us. The vinegar used in a salad dressing or in ketchup and pickles gets its flavor from acetic acid. We have only to place an aspirin on our tongues for a moment to recognize its distinctly acidic, sour taste. We also know immediately that lemons and other citrus fruit contain some form of natural acid. On the other hand, when we wash with soap, we can feel the slipperiness of a base substance, and when we take an antacid tablet for an upset stomach we can experience its neutralizing effect on acids.

People have been using both acids and bases ever since they first started making food and drink. When wine turns sour it changes to vinegar, a diluted or weak form of acetic acid. Spilling some vinegar, or even lemon juice, on a cut lets us feel immediately that it is a mildly burning acid. A common base material that is found in nature is limestone, which people eventually learned to roast and obtain lime. Today we sprinkle this white powder on some soils that are too "acidic" for certain plants to thrive.

Acids come in a variety of strengths, from the fiercely strong hydrochloric acid found in the human stomach, to the mild strength of tomatoes, and the very mild strength found in our own saliva. Strong acids however, are poisonous and can cause severe burns. Acid and base strength are measured on a pH scale ranging from 1 (strongest acid) to 14 (strongest base). Since the strength of a particular substance depends on its concentration of hydrogen ions and whether it releases or attracts them in solution, the pH scale was devised to measure this concentration of hydrogen ions. On a scale of 1 to 14, a solution with a pH of 7 is considered neutral. Very potent acids like sulfuric acid and hydrochloric acid have a pH of 1; lemon juice has a pH of 2; vinegar has a pH of 3; tomatoes are 4; black coffee is 5; and urine is 6. Pure water (not rainwater which is slightly acidic) has a neutral pH of 7. Pure water is between the highest acid (1) and the highest base (14), and is actually neither.

Continuing on the pH scale to the base side, seawater has a pH of 8; baking soda has a pH of 9; milk of magnesia is 10; household ammonia is 11; lime is 12; hair remover is 13; and lye is 14. On this scale, a change of only 1 point means a change of ten times the concentration. Thus, lemon juice is ten times more acidic than vinegar, and stomach acid is ten times more acidic than lemon juice. An easy way of distinguishing acids from bases is to use litmus paper. It was discovered that certain organic extracts, such as litmus—which is obtained from lichen, a plant-fungus organism—turns red when dipped in acid and blue when dipped in a base.

Acids play a major biological role at the most fundamental level, since the organic acids called amino acids are necessary for life. Amino acids are known to be the building blocks of proteins. The pH level is also important to life, since every cell is sensitive to it and will not tolerate too great a change from its proper pH level. Because of this, most living systems have several mechanisms to make sure that their internal pH remains fairly constant. An organism's habitat or external environment must also have a range of pH that is suited to it or it will suffer or even die. Acid rain, which results from air pollution, has damaged certain vulnerable forests. It has also made some lakes too acidic (as much as a pH of 5) for trout and other fish. Finally, both acids and bases have been put to a number of practical uses in industry and in the production of consumer goods. We have acid in our batteries and bases in our soaps.