SS1 Chemistry Lesson Note on Bases: Definition; Strength and Characteristics of Bases

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BASES

  1. Definition, Characteristics and Preparation of bases
  2. Reactions and Uses of bases
  3. Relative acidity and alkalinity (the pH scale)

PERIOD 1:    BASES

DEFINITION OF BASES

A base is a substance which will neutralize an acid to yield salt and water only. It is either an oxide or hydroxide of a metal, e.g sodium oxide, magnesium hydroxide, etc, while; an alkaline is a basic hydroxide which is soluble in water. Bases that are soluble in water are referred to as alkalis. Examples are sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide (Ca(OH)2)

Oxides of heavy metals like PbO, ZnO and CuO are insoluble in water and are therefore bases not alkalis. CaO and MgO are slightly soluble and are alkalis. Like acids, alkalis may be strong or weak.

STRENGTH OF AN ALKALIS:

The strength of an alkali is the degree of its ionization in water. Strong alkalis are completely ionized in water. Examples of strong alkalis are potassium hydroxide, sodium hydroxide, calcium hydroxide, strontium hydroxide etc. Weak alkalis are partially ionized in water. example of a weak alkali is aqueous ammonia.

CHARACTERISTICS OF BASES

  1. Bases are soapy to touch, e.g. NaOH
  2. They have bitter taste, e.g. lime water
  3. They turn red litmus blue
  4. Concentrated form of the caustic alkalis, NaOH and KOH are corrosive
  5. They are electrolytes
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PREPARATION OF BASES

  1. Combustion of a reactive metal in air. When electropositive metals are heated in oxygen, they form metallic oxides.

2Ca  + O2                 →         CaO(s)

  1. By reaction of metals with water (steam)

Ca +  H2O       →        Ca (OH)2    +   H2

  1. Decomposition of metal hydroxides by heating

Cu(OH)2      →      CuO  +  H2O

  1. Precipitation or double decomposition reaction

CuSO4   +   2NaOH      →    Cu(OH)2   + Na2SO4

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