Base

Base

A base is a substance that furnishes hydroxide ions (OH ) when dissolved in water.
For example : sodium hydroxide NaOH (aq), In its aqueous solutions dissociates as:
      NaOH (aq) → Na⁺(aq) + OH^-(aq)

Base is a substance which :
i. bitter in taste
ii. turns red litmus paper into blue
iii. gives hydroxyl ions (OH-) in aqueous solution
iv. can accept proton (Bronsted & lowry concept)
v. can donate electrons (Lewis theory)

» Oxides & hydroxides of metals are bases.
» Water soluble bases are called alkali e.g. NaOH, KOH, etc.
» All alkalies are bases but all bases are not alkalies because all bases are not soluble in water.

Strength of bases :
Strong bases : NaOH, KOH
Weak bases : NH₄OH, Fe(OH)₃

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 like:
1. LiOH Lithium hydroxide
2. NaOH Sodium hydroxide
3. KOH Potassium 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) + NH^-₄(aq) + OH^-(aq)

Examples of weak bases :
(i) NH4OH
(ii) CU(OH)₂
(iii) Cr(OH)₃
(iv) Zn(OH)₂

Acidity of a Base

The number of removable hydroxyl (OH^-) ions from a base is called acidity of a base.
Acidity of NaOH = 1
Acidity of KOH = 1
Acidity of Ca(OH)₂ = 2