Acids, Bases, and the pH Scale

By the end of this lesson, you will be able to compare and contrast acids and bases and describe where each one falls on the pH scale compared to water.

Think about a summer afternoon in Texas when you squeeze fresh lemons into a pitcher of lemonade. That sour taste comes from an acid, a substance that releases hydrogen ions when it dissolves in water and tastes sour. Now think about the baking soda sitting in your kitchen cabinet, or the slippery feel of soap on your hands. Those are bases, substances that release hydroxide ions in water and often feel slippery and taste bitter. You should never taste or touch a chemical in the lab to test it, but you already know these properties from foods and cleaners at home.

Scientists compare acids and bases using the pH scale, a set of numbers from 0 to 14 that tells how acidic or basic a solution is. Pure water sits right in the middle at a pH of 7, which we call neutral. Anything with a pH lower than 7 is an acid, and the lower the number, the stronger the acid. Lemon juice is around 2, and stomach acid is close to 1. Anything with a pH higher than 7 is a base, and the higher the number, the stronger the base. Baking soda water is about 9, and drain cleaner can reach 13.

Acids and bases do share some traits. Both dissolve in water, both can conduct electricity in solution, and both can be dangerous in strong concentrations. They also change the color of indicators, which are chemicals like litmus paper that change color in acids or bases. When an acid and a base are mixed in the right amounts, they neutralize each other and move toward the pH of water.

Try it: If a sample of rainwater tests at pH 5, is it an acid, a base, or neutral, and how does it compare to pure water?