Acids and Bases

The term acid is derived from the Latin word acidus meaning sour. Bases are bitter in taste. An acid is a substance that has the tendency to lose a proton and a base is a substance that has the tendency to accept a proton. An acid in water gives hydrogen ions. A base in water gives Hydroxyl ion.

Key Features of Acids

Acids can be defined in many ways but generally their aqueous solutions have the following properties.

• They are sour in taste.
• They turn blue litmus red.
• They react with certain metals and liberate hydrogen gas.
• They react with oxides and hydroxides of metals forming salt and water.
• Their aqueous solutions conduct electricity

Key features of Bases

Bases are defined in various ways but generally substances having the following characteristics are called bases.

• They have a bitter taste.
• Their aqueous solutions have a soapy touch.
• They turn red litmus blue.
• They react with acids to form salt and water.
• Their aqueous solutions conduct electricity.

Weak and Strong acids and bases

• Acids such as Hydrochloric Acid (HCl), Sulphuric Acid (H₂SO₄) and Nitric Acid (HNO₃) which are almost completely ionised in aqueous solution are termed as strong acids. Acids such as acetic acid (CH₃COOH) is partially ionised and is called a weak acid.
• Similarly, bases like NaOH and KOH are almost completely ionised in aqueous solution and are therefore called strong bases. Ammonium Hydroxide is partially ionised and is called a weak base.

Important Notes on Acids and Bases

• Acid-base indicators are dyes or mixtures of dyes which are used to indicate the presence of acids and bases.
• Acidic nature of a substance is due to the formation of H+(aq) ions in solution. Formation of OH–(aq) ions in solution is responsible for the basic nature of a substance.
• When an acid reacts with a metal, hydrogen gas is evolved and a corresponding salt is formed. When a base reacts with a metal, along with the evolution of hydrogen gas a salt is formed which has a negative ion composed of the metal and oxygen.
• When an acid reacts with a metal carbonate or metal hydrogen carbonate, it gives the corresponding salt, carbon dioxide gas and water.
• Both acidic and basic solutions in water conduct electricity because they produce hydrogen and hydroxide ions respectively.

pH Scale

The acidity or basicity of a solution is usually expressed in terms of function of the hydrogen ion concentration. This function is called pH of a solution. pH of a solution may be defined as the negative logarithm (to the base 10) of hydrogen ion concentration expressed in moles per litre.

pH = – log₁₀ [H+]

For pure water and neutral solutions, at 298 K, concentration of hydrogen ions is 1 x 10-7 mol L-1. These hydrogen ions are formed by ionisation of some of the water molecules.

H₂O (l) ↔ H(aq) ⁺ + OH(aq) ^–

Thus, for pure water at 298 K,

Sp pH of distilled water is :

-log (1 x 10^-7) = 7

The approximate values of pH for some familiar solutions are as follows:

Red Cabbage Juice as Acid Base Indicator

Red cabbage juice is known as an acid/base indicator because it has pigments in it that react differently to acids and bases. These pigment change color when exposed to an acid or a base. Cabbage juice is naturally neutral and it has a purplish color. When acid is added to it, it turns pink. If a base is added, it turns green.

For example, the juice will turn pink when lemon juice is added to it. It changes to green when toothpaste is mixed with it, because toothpaste is basic in nature. The tooth paste should be basic due to the reason, that some acids are formed by the bacteria in our mouth and these are neutralized by the bases present in the tooth paste.