acid

An acid is a substance that, dissolved in water, raises the concentration of hydrogen ions (H⁺) above what pure water has, which puts its pH below 7. A lower pH means a more acidic solution (more H⁺). Whether an acid is strong or weak is a different idea: a strong acid ionizes completely in water, a weak acid only partly. On the pH scale each step of one is a factor of ten, so a solution at pH 2 has ten times the hydrogen ions of one at pH 3. Acids turn litmus red and react with many metals, but for a test the useful fact is the direction: lower pH means more acidic. The words are easy to mix up with a base, which is the opposite — above 7 and with fewer hydrogen ions. The pH scale usually runs from 0 (most acidic) to 14 (most basic), with neutral pure water at 7.