Acids are solutions that have an excess of H+ ions. Bases, or alkali, are solutions that have an excess of OH- ions. Strong acids are defined by the characteristic of having low pH levels, usually between zero and four on the pH scale. Strong bases are defined by the characteristic of having high pH levels, usually between 10 and 14 on the pH scale. Weak acids and weak bases have pH levels close to a seven on the pH scale; seven is considered neutral.
Acidic compounds are materials that break into hydrogen (H+) ions and other compounds when they are placed in aqueous solutions. Basic compounds are materials that break into hydroxide (OH-) ions and other compounds when they are placed in aqueous solutions. In other words, if an ionic compound is put into water, it will break into two ions. If an ion is H+, the solution will be acidic; if an ion is OH-, the solution will be basic.
There are many examples of acids and bases that are used every day. Pure water is always considered neutral, with a pH of 7. Milk, rainwater and soda are examples of weak acids with pH levels between six and four. Vinegar, lemon juice, stomach acids and hydrochloric acid are examples of strong acids with pH levels between three and zero. Egg whites, baking soda and antacids are examples of weak bases with pH levels between eight and 10. Ammonia, mineral lime, Drano and sodium hydroxide are examples of strong bases with pH levels between 11 and 14.
The terms "acids" and "bases" were first designated by Robert Boyle in the 17th century. He characterized acids based on their sour taste, ability to corrode metals and their ability to change litmus to red. Bases were characterized by their slippery feel and ability to change litmus to blue. Svante Arrhenius suggested that water dissolves compounds, separating them into individual ions. Acids were compounds that can dissolve in water and release hydrogen ions; bases were compounds that can dissolve in water and release hydroxide ions. Johannes Bronsted and Thomas Lowry refined Arrhenius' theory in 1923. They posited that acids are substances that donate hydrogen ions, and bases are substances that accept hydrogen ions.
The pH scale was invented by a Danish biochemist by the name of Soren Sorensen in 1909. The pH scale is used to measure the acidity of substances. The scale has values ranging from zero to 14; the closer a substance is to zero on the pH scale, the more acidic the substance is. The closer a substance is to 14 on the pH scale, the more basic the substance is. Substances that have a pH level of seven are considered to be neutral, neither acidic nor basic.