Acids and bases are assessed on the basis of where they fall on the pH (potency of the hydrogen ion) scale. This scale assesses whether a substance adds hydrogen ions, which makes it an acid, or takes in (absorbs) hydrogen ions, which makes it a base. This scale goes from 1 to 14, with 7 being neutral, meaning the substance neither adds nor takes in hydrogen ions.
Acids are substances whose pH score falls below 7. A pH of 1 is the strongest acid strength, meaning a substance adds the most hydrogen ions, and a pH of 6 is the weakest acid strength, meaning a substance adds few hydrogen ions. One example of a weak acid is lemon juice, while hydrochloric acid is a much stronger acid.
Bases are substances whose pH score falls above 7. A base with a pH of 14 takes in the most hydrogen ions, and a base with a pH of 8 takes in the fewest hydrogen ions. Because acids add hydrogen ions and bases take in hydrogen ions, when a base is combined with an acid, the mixture's pH moves closer to 7. Acids have the same effect of moving the pH score closer to 7 when added to a base.
Because of this property, a compound with a pH greater than 7 will taken in hydrogen ions and move the acid closer to being a neutral substance. However, this balance depends on the relative strength of the acid and the base in question. If a weak base is added to a strong acid, then the pH of the acid will only move slightly toward 7. Alternatively, if a very strong base is combined with a weak acid, then the acid will become basic. To perfectly neutralize the substance, the strength of the acid and strength of the base need to be equal.