All acids contain hydrogen. Acids dissociate in water to produce hydrogen ions. Basically, acids are just ionic compounds that contain hydrogen. A strong acid completely ionizes, breaking down into positive hydrogen ions and negatively charged ions.
Acids are measured using the pH scale, which measures the amount of hydrogen ions present in a solution. A pH of less than seven is acidic. Pure water has a pH of seven, which is neutral. Water contains both hydrogen ions and hydroxide ions, but water is neutral because the amount of hydrogen and hydroxide ions balance each other. A higher percentage of hydrogen ions in solution results in a lower pH. Acids donate hydrogen ions, which are also called protons. Some common acids are nitric acid (HNO3), sulfuric acid (H2SO4) and carbonic acid (H2CO3).