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Bases

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Bases - Lesson Summary

According Arrhenius theory any substance that can produce hydroxide ions when dissolved in water is called as a base.
        Substance + Water → Metal ion + OH
Example:
        NaOH (aq)  → Na+ (aq) + OH(aq)

A base is said to be an alkali if it is soluble in water. In general hydroxides of alkali metals and alkaline earthmetals are considered as alkalies.
Example:
        KOH  (aq) → K+(aq) + OH (aq) 
        Ca(OH)2(aq) → Ca+2 (aq) + OH (aq)

It is not a necessary that a base should contain hydroxide ion.
There are some bases even they does not contain hydroxide ion, can be considered as bases.
Example: Ammonia (NH3)
Ammonia when dissoled in water forms ammonium hydroxide which is a weak base.
NH3 + H2O → NH4OH (aq)

Oxides of alkali metals and alkaline earthmetals are also considered as basic in nature.
Example: CaO, MgO, Na2O, K2O...etc

Classification of bases:
Classification based on the strength:
Based on the extent of ionisation bases are classified into strong bases and weak bases.

Strong bases:
The bases which undergoes complete ionisation in aquesous solution are called as strong bases.
Example: NaOH, KOH...etc

Weak bases:
The bases which undergoes partial ionisation in aqueous solution are called weak bases.
Example: NH4OH, NH3...etc

Classification of based on acidity:
Based on acidity bases can be classified into different types. They are:
Mono acidic base
Di acidic base
Tri acidic base

Mono acidic bases:
Bases which produces only one hydroxide (OH-) ion in aqueous solutions are called mono acidic bases.
Example: NaOH, KOH...etc

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

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

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.

Indicators in pressence of bases:
Bases turns red litmus to blue.
Phenolphthalein turns pink in pressence of bases.
Methyl orange turns to yellow in pressence of bases.

Chemical properties:
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.
Example:
Potassium hydroxide reacts with hydrochloric acid to form potassium chloride and water.
KOH + HCl → KCl + H2O

Uses of Bases:
        •  Mild bases neutralise the acidity in the stomach.
        • Sodium hydroxide is used in the manufacture of soaps, paper and synthetic fibres like rayon.
        • Calcium hydroxide is used in the manufacture of bleaching powder. Bleaching powder is used as a disinfectant.
        • Magnesium hydroxide is used as an antacid to neutralize the acid in the stomach.
        • Ammonium hydroxide is used in the preparation of fertilizers like ammonium phosphate and ammonium sulphate.

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