According Arrhenius theory, any compound that can donate hydroxide ions (OH–) when dissolved in water, is called as a base.
Substance + Water → Metal ion + OH–
NaOH(aq) → Na+(aq) + OH–(aq)
In general, hydroxides of alkali metals (LiOH, NaOH, KOH, RbOH, CsOH) and alkaline earth metals (Mg(OH)2 , Ca(OH)2 , Sr(OH)2 , Ba(OH)2) are considered as alkalies. Alkalies are water soluble bases.
KOH(aq) → K+(aq) + OH–(aq)
Ca(OH)2(aq) → Ca2+(aq) + OH–(aq)
If base is dissolved in water , pH of water becomes more than 7, at 25ºC.
|Note: What is pH ?
We can measure a solution is acidic or basic by using a scale called pH.
Technically pH = – log (H+)
At 25ºC ; If any solution has pH<7, solution is acidic
If solution pH > 7, solution is basic
If pH = 7, solution is neutral
Later Chemists found some compounds whose solution in water is basic but their molecules do not contain OH– ions. for example : Ammonia (NH3), Pyridine (C5H6N ), Methyl amine (CH3NH2). As you can see their molecular formula does not contain hydroxyl ions (OH–), but if these compounds are dissolved in water; pH >7
Therefore , there are some bases , they do not contain hydroxide ion in their molecular formula.
Ammonia when dissolved in water forms ammonium hydroxide which is a weak base.
NH3 + H2O → NH4OH (aq)
Oxides, carbonates (along with hydroxides) of alkali metals and alkaline earth metals are also considered as basic in nature.
Example: CaO, MgO, Na2O, K2O etc
Classification of bases:
Classification I : On the basis of strength:
The bases which undergoes complete ionisation in aquesous solution are called as strong bases.
Example: NaOH, KOH, CsOH etc
2. Weak bases:
The bases which undergoes partial ionisation in aqueous solution are called weak bases.
Example: NH4OH, CH3NH2 , pyridine etc.
Classification II : on the basis of acidity:
Based on acidity bases are:
1. Mono acidic base
2. Di acidic base
3. Tri acidic base
1.Mono acidic bases:
Bases which produces only one hydroxide (OH-) ion in aqueous solutions are called mono acidic bases.
Example: NaOH, KOH etc
2. Di acidic bases:
Bases which produces two hydroxide ions in aqueous solutions are called di acidic bases.
Example: Ca(OH)2, Mg(OH)2 etc
3. Tri acidic bases:
Bases which produces three hydroxide ions in aqueous solutions are called tri acidic bases.
Example: Al(OH)3, Fe(OH)3 etc
Classification III : on the basis of their nature :
- Orgnic Bases : They contain carbon in their molecular formula. Amines of organic chemistry are organic bases. Examples : Methyl amine CH3NH2 , Ethyl amine C2H5NH2 etc. They are organic molecules which act as base.
- Inorganic Bases: They are inorganic molecules which act as base . examples : NaOH, Ca(OH)2 etc
Physical Properties of Bases:
- Bases are bitter to taste, soapy to touch.
- Bases are good conductors of electricity in aqueous solution. In aqueous solution, they release ions, which conduct electricity.
- Bases liberates heat on dilution.
Chemical Properties of Bases:
- Reaction with active metals:
Bases react with metals to liberate hydrogen gas
Example: Sodium hydroxide react with zinc and liberate hydrogen and sodium zincate.
NaOH + Zn → Na2ZnO2 + H2
- Reaction with non-metal oxides:
Bases react with non-metallic oxides to form salt and water. This is similar to a neutralization reaction between an acid and a base.
Example: Calcium hydroxide reacts with carbon dioxide to form calcium carbonate and water
Ca(OH)2 + CO2 → CaCO3 + H2O
From this reaction, it can be concluded that non-metallic oxides are acidic in nature.
- Reaction with acids:
Bases reacts with acids to form salts and water.
Potassium hydroxide reacts with hydrochloric acid to form potassium chloride and water.
KOH(aq) + HCl(aq) → KCl(aq) + H2O(l)
Indicators in presence of Bases:
Bases turns red litmus to blue.
Phenolphthalein turns pink in presence of bases.
Methyl orange turns to yellow in presence of bases.
Uses of Bases:
|Sodium hydroxide (NaOH)||
|Calcium hydroxide (Ca(OH)2)||
|Magnesium hydroxide (Mg(OH)2)||
|Ammonium hydroxide (NH4OH)||