Properties of Bases

The properties of bases include (i) Taste and Touch, (ii) Action on Indicators, (iii) Conduction of electricity and dissociation of bases, (iv) Reaction of Bases with Metals, (v) Reaction of Bases with Mon-metal Oxides, and (vi) Reaction of Bases with Acids.

1. Taste and Touch

Bases have a bitter taste and their solutions are soapy to touch.

2. Action on Indicators

Each indicator shows characteristic colour in presence of bases. The colours shown by three commonly used indicators in presence of bases are:

  1. Litmus - Blue
  2. Phenolphthalein - Pink
  3. Methyl orange - Yellow

3. Conduction of electricity and dissociation of bases

Aqueous solutions (solution in water) of bases conduct electricity which is due to the formation of ions. Like acids, bases also dissociate on dissolving in water. Bases produce hydroxyl ions (OH–) which are responsible for their characteristic properties.

The bases which are soluble in water and give OH– ions in their aqueous solution are called alkalies. All alkalies are bases but all bases are not alkalies. On the basis of the extent of dissociation occurring in their solution, bases are classified as strong and weak bases.

Strong Bases

These bases are completely dissociated in water to form the cation and hydroxide ion (OH–). For example, potassium hydroxide dissociates as:

KOH (aq) → K+ (aq) + OH– (aq)

There are only eight strong bases. These are the hydroxides of the elements of the Groups 1 and 2 of the periodic table.

  1. LiOH - Lithium hydroxide
  2. NaOH - Sodium hydroxide
  3. KOH - Potassium hydroxide
  4. RbOH - Rubidium hydroxide
  5. CsOH - Caesium hydroxide
  6. Ca(OH)2 - Calcium hydroxide
  7. Sr(OH)2 - Strontium hydroxide
  8. Ba(OH)2 - Barium hydroxide

Weak Bases

Weak bases do not furnish OH– ions by dissociation. They react with water to furnish OH– ions.

NH3 (g) + H2O (l) → NH4OH

NH4OH (aq) ⇌ NH4+ (aq) + OH– (aq)

or

NH3 (g) + H2O (l) ⇌ NH4+(aq) + OH– (aq)

The reaction resulting in the formation of OH– ions does not go to completion and the solution contains relatively low concentration of OH– ions. The two half arrows are used in the equation to indicate that equilibrium is reached before the reaction is completed.

Examples of weak bases:

  1. NH4OH
  2. Cu(OH)2
  3. Cr(OH)3
  4. Zn(OH)2

4. Reaction of Bases with Metals

Like acids, bases also react with active metals liberating hydrogen gas. For example, sodium hydroxide reacts with zinc as:

Zn (s) + 2NaOH (aq) → Na2ZnO2 (aq) + H2 (g) ↑

5. Reaction of Bases with Mon-metal Oxides

Bases react with oxides of non-metals like CO2, SO2, SO3, P2O5 to form salt and water.

Base + Non-metal oxide → Salt + Water

For example,

Ca(OH)2 (aq) + CO2 (g) → CaCO3 (s) + H2O (l)

6. Reaction of Bases with Acids

Reaction between acids and bases are called neutralization reactions and result in the formation of salt and water.

HCl (aq) + KOH (aq) → KCl (aq) + H2O (l)

H2SO4 (aq) + 2NaOH (aq) → Na2SO4 (aq) + 2H2O (l)

Caustic nature

Strong bases like sodium hydroxide and potassium hydroxide are corrosive towards organic matter and break down the proteins of the skin and flesh to a pasty mass. This action is called caustic action and it is due to this property that sodium hydroxide is called caustic soda and potassium hydroxide is called caustic potash.