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Bases

A base is a proton acceptor or electron pair donor.
 
Properties of Bases
  1. They taste bitter.
  2. They feel slippery, i.e. soapy to touch.
  3. They turn red litmus blue.
  4. They neutralise acids.
  5. They absorb carbon dioxide to form carbonates.
  6. They react with ammonium salts to liberate ammonia.

Different Concepts of Bases

  1. Arrhenius concept: According to Arrhenius, a base is a substance that increases the concentration of hydroxide ions (OH) when dissolved in water.
     
    Description: 29779.png
  2. Bronsted–Lowry concept: According to this concept, a base is a proton acceptor.
     
    When HCl loses a proton, it forms a base, Cl. When the base Cl accepts a proton, it reforms the acid, HCl. Hence, these two are a conjugate pair.
  3. Lewis concept: According to this definition, a base is an electron-pair donor.

Classification of Bases

Bases are classified into different types as shown in Table.

 

Classification of Bases

Parameters

Types

Strength of bases

(a) Strong base: Complete dissociation in aqueous solution, thereby producing a high concentration of hydroxyl (OH) ions, e.g. sodium hydroxide, potassium hydroxide.
NaOH (aq)  Na+ + OH (contains only ions)
 
(b) Weak base: Partial dissociation in aqueous solution, thereby producing a low concentration of hydroxyl ions, e.g. ammonium hydroxide, calcium hydroxide
NH4OH (aq)  NH +4 + OH (contains molecules and ions)

Concentration

(a) Concentrated base: Has a relatively high percentage of base in its aqueous solution
 
(b) Dilute base: Has a relatively low percentage of base in its aqueous solution

Acidity (number of hydroxyl ions which can be
produced per
molecule of the base in aqueous solution)

(a) Monoacidic base: Ionises in aqueous solution to produce one hydroxyl ion per molecule of the base, e.g. NaOH, KOH
NaOH (aq)  Na+ + OH
 
(b) Diacidic base: Ionises in aqueous solution to produce two hydroxyl ions per molecule of the base, e.g. Ca(OH)2, Zn(OH)2
Ca(OH)2 (aq)  Ca2+ + 2OH
 
(c) Triacidic base: Ionises in aqueous solution to produce three hydroxyl ions per molecule of the base, e.g. Al(OH)3
Al(OH)3 (aq)   Al3+ + 3OH

Preparation of Bases

  1. From metals: Metals react with oxygen to form a basic oxide. This basic oxide dissolves in water to give an alkali.
     
    4Na + O2 Description: 32750.jpg 2Na2O
     
    Na2O + H2O Description: 32750.jpg 2NaOH
  2. From salts: Aqueous solutions of salts with a strong base precipitates the respective metallic hydroxide.
     
    AlCl3 + 3NaOH Description: 32750.jpg 3NaCl + Al(OH)3
  3. Decomposition of salts: On heating carbonates or nitrates, a basic oxide is formed.
     
    2Pb(NO3)2 Description: 32750.jpg 2PbO + 4NO2 + O2
This basic oxide on dissolution in water forms an alkali.

Chemical Properties of Bases

  1. Reaction with ammonium salts: Alkalis react with ammonium salt to liberate ammonia.
     
  2. Reaction with metallic salt solutions: Alkalis react with certain metallic salt solutions to precipitate insoluble hydroxides.
     
    CuCl2 + 2NaOH Description: 32760.jpg 2NaCl + Cu(OH)2
  • Sodium hydroxide (NaOH) is called caustic soda, and potassium hydroxide (KOH) is called caustic potash.
  • Certain substances act as both an acid and a base. Such substances are called amphoteric substances.


Uses of Bases
  1. Sodium hydroxide is used in the manufacture of soaps.
  2. Calcium hydroxide is used in the manufacture of bleaching powder. It is also used in the preparation of plaster and mortar.
  3. Magnesium hydroxide, sodium bicarbonate and calcium carbonate are used as antacids.
  4. Aluminium hydroxide is used as a foaming agent in fire extinguishers.
  5. Calcium hydroxide is used in softening water and to reduce the acidity of the soil.
  6. Ammonium hydroxide is used to remove grease stains from clothes. It is also used in the preparation of nitric acid and ammonium chloride.




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