So the circle with A in the center should be blackened in the answer row for sample question 1. Acids contain many hydrogen atoms. Some acids, like sulfuric and hydrochloric acid, can be very dangerous. However, hydrochloric acid, as a component of stomach acid, is used in digestion. Bases contain a combination of hydrogen and oxygen atoms called hydroxyl groups.
Household cleaning agents such as ammonia, borax, lye, and detergents are common examples of bases.
Many different kinds of acids and bases are used by scientists. The pH of a liquid tells whether it is an acid or a base and how weak or strong it is. A liquid can have a pH number from 1, the stongest acid, to 14, the strongest base. Any solution with a pH less than 7 is an acid, while a solution with a pH greater than 7 is a base. Pure water is one of the few substances whose pH is seven.