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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

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