Explanation Acids and bases can be distinguished by yet another simple property, their ability to cause color changes in certain dyes. An acid- base indicator is a dye, which undergoes color changes when solutions containing acids or bases are added. For example, amber color of tea is lightened by the addition of lemon juice (citric acid). Ed cabbage juice changes to green and then to yellow when a base is added. The green and yellow colors change back to red when acid is added. Litmus is a common laboratory dye used as an acid-base indicator and is produced from certain species of lichens. Litmus turns red in acidic solution and blue in basic solution. Phenolthalein is another acid-base indicator that remains colorless in acidic solutions and turns pink in basic solutions. When Arrhenius developed his ionic theory of solutions, he also gave the classic definitions for acids and bases. An acid is a substance that produces hydrogen ions, H+, when it dissolves in water. For example, nitric acid when dissolved in water produces H+ and NO3- ions as shown below. A base is a substance that produces hydroxide ions, when dissolved in water. For example, sodium hydroxide dissolves in water to give OH- ions. The above definitions, though useful, are somewhat limited. In 1923, Johannes N. Bronsted and Thomas M. Lowry independently found that many reactions involve nothing more than the transfer of a proton (H+) between reactants, and realized that they could use this idea to expand the definitions of acids and bases. From this viewpoint, acid - base reactions are proton - transfer reactions. For example, consider ammonia, which is a base, when dissolved in water gives OH- ions. The reaction actually involves a transfer of a proton from water molecule to the ammonia molecule as shown below. Once the proton left the water molecule, it leaves behind an OH- ion. When this proton is added to ammonia, NH4+ ion is formed. So H2O is said to have donated a proton to NH3 or NH3 is said to have accept a proton from H2O. So, Bronsted and Lowry defined an acid as the species (molecule or ion) that donates a proton to another species, and a base as the species that accepts a proton in a proton-transfer reaction. Thus, in the above reaction, H2O molecule, that donated proton, is the acid while NH3 molecule is a base since it accepts a proton. The dissolution of nitric acid in water is also a proton-transfer reaction. To understand it is a proton-transfer reaction, we need to understand the structure of hydrogen ion (H+). This ion consists of a proton in association with water molecule, which is what (aq) means. This is not a weak association, however, because the hydrogen nucleus ( proton) would be expected to attract electrons to itself. The H+ ion is better thought of as a proton chemically bonded to water molecule to give H3O+ ion, with other water molecules less strongly associated with this ion, which we represent by phase- labeled formula H3O+(aq). Written in this form, it is usually called as the hydronium ion. It is important to understand that hydrogen ion, H+, and hydronium ion, HO3+, represent precisely the same physical ion. For simplicity, it is often written as H+(aq) ion, but when it is needed to express the proton-transfer aspect of a reaction, explicitly, it is written by the formula H3O+(aq). The above nitric acid reaction with water can be written as It may be noted that the above reaction involves a transfer of a proton (H+) from HNO3 to H2O. STRONG AND WEAK ACIDS AND BASES: Acids and bases are classified as strong and weak, depending on whether they are strong or weak electrolytes. A strong acid is an acid that ionizes completely in water; it is a strong electrolyte. Hydrochloric acid, nitric acid and sulfuric acid are examples of strong acids. A weak acid is an acid that only partially ionizes in water; it is a weak electrolyte. An example of weak acid is hydro cynic acid, HCN(aq). This hydrogen cyanide molecule reacts with water to produce a small percentage of ions in solution. A strong base is a base that is present in aqueous solutions entirely as ions, one of which is OH-; it is a strong electrolyte. The ionic compound, sodium hydroxide, NaOH, is an example of strong base. It dissolves in water as Na+ and OH-. The hydroxides of Group IA and IIA elements, except beryllium hydroxide, are strong bases. A weak base is a base that is only partially ionized in water; it is a weak electrolyte. Ammonia is an example of weak base.