Acids and Bases

Acids and bases are two kinds of chemicals that behave in opposite ways. Acids are usually sour. Bases are usually bitter and feel slippery. When an acid and a base meet, they often cancel each other out. Almost every liquid you can think of, from orange juice to soap to pool water, is either an acid, a base, or neutral.

The difference comes down to tiny particles called ions. When an acid dissolves in water, it releases extra hydrogen ions. When a base dissolves in water, it releases extra hydroxide ions. The more of these ions a liquid has, the stronger the acid or base.

Scientists measure acids and bases using something called the pH scale. The scale runs from 0 to 14. A liquid with a pH of 7, like pure water, is neutral. Anything below 7 is an acid, and the lower the number, the stronger the acid. Anything above 7 is a base, and the higher the number, the stronger the base. Each step on the scale is ten times stronger than the last. A pH of 4 is ten times more acidic than a pH of 5, and one hundred times more acidic than a pH of 6.

You meet acids every day. Lemons and limes contain citric acid. Vinegar is acetic acid. Soda is full of carbonic acid, which is what makes the bubbles. Your own stomach is a pool of hydrochloric acid strong enough to help break down the food you eat. Rain is slightly acidic too, because carbon dioxide in the air mixes with water in the clouds.

Bases are just as common. Baking soda is a mild base. So is soap, which is why it feels slippery between your fingers. Toothpaste is a base because it helps cancel out the acids that bacteria make on your teeth. Bleach and drain cleaners are strong bases, which is why they are dangerous to touch.

When an acid and a base mix, they trade ions and form water and a salt. This is called neutralization. If your stomach feels sour after too much pizza, an antacid tablet works by being a mild base that neutralizes the extra acid. The same idea explains why gardeners add lime, a base, to soil that has turned too acidic for their plants. Acids and bases do not just fight each other. Together, they keep the chemistry of life in balance.