Acids, Bases and Salts Part - 3 | Class - 10th

BASES:

Substances which change red litmus solution blue are called bases. They are bitter in taste.

They releases hydroxide ions (OH^-) when dissolved in water. They have a soapy touch (e.g. Washing soda, caustic soda and caustic potash).

CLASSIFICATION OF BASES:

Ø    Based on Ionisation:

v    Strong Bases: These are bases which ionise completely in aqueous solution eg. NaOH, KOH.

v    Weak bases: These are bases which ionise partially in aqueous solution eg. NH4OH, Ca(OH)₂.

Ø  Based on their Acidity:

v    Monoacidic Base: It is a base which ionises in water to give one hydroxide ion per molecule eg. NaOH, KOH.

v    Diacidic Base: It is a base which ionises in water to give two hydroxide ions per molecule eg.  Ca(OH)₂, Mg(OH)₂.

v Triacidic Base: It is a base which ionises in water to give three hydroxide ions per molecule eg. Al(OH)₃, Fe(OH)₃.

Ø  Based on the Concentration:

Depending upon the amount of water added in a base, bases are classified as concentrated and dilute Bases.

v    Concentrated alkali: It is an alkali having a relatively high percentage of alkali in its aqueous solution.

v    Dilute alkali: It is an alkali having a relatively low percentage of alkali in its aqueous solution.

Chemical properties of Base:

Ø    Reaction of Base with Metals:

When alkali (base) reacts with metal, it produces salt and hydrogen gas.

Alkali + Metal → Salt + Hydrogen

v    Sodium aluminate and hydrogen gas are formed when sodium hydroxide reacts with aluminium metal.

             2NaOH + 2Al + 2H₂O → 2NaAlO₂ + 2H₂

v    Sodium hydroxide gives hydrogen gas and sodium zincate when reacts with zinc metal. 
             2NaOH + Zn → Na₂ZnO₂ + H₂

Ø    Reaction of Non-Metallic Oxides with Base:

Non- metallic oxides react with base to produce Salt and Water

 Non-metallic oxide + Base → Salt + Water

v    Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

F    Non Metallic oxides are said to be acidic in nature because on reacting with a base they produce Salt and Water.

Ø    Action of Alkalis/Base with Ammonium Salts:

 Alkalis combine with ammonium salts to liberate ammonia.

 Alkali + Ammonium salt   →  Salt  + Water + Ammonia

v    Ca(OH)₂ + NH₄Cl  →  CaCl₂ + H₂O + NH₃

USES OF BASES:

Ø    Magnesium hydroxide is used as a medicine for stomach troubles.

Ø    Ammonium hydroxide is used to remove grease stains from clothes.

Ø    Sodium hydroxide is used in the manufacture of soap.

Ø    Calcium hydroxide is used in white washing the buildings.