05 Chemical Bonds
06 Acids, Bases and Salts
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6.1.2 Bases and Alkali


  1. Bases are compounds which react with acid to form a salt and water as only products.
  2. Bases that soluble in water are called alkalis.
  3. In aqueous solution, alkali it produces hydroxide ions (OH). In short, alkalis are substances that form hydroxide ions (OH(aq)) in water
    Sodium hydroxide NaOH gives Na+(aq) and OH(aq) ions,
    NaOH → Na+ + OH

    calcium hydroxide Ca(OH)2 gives Ca2+(aq) and 2OH(aq)  ions.

    Ca(OH)2 → Ca2+ + 2OH

    Ammonia give NH4+ and OH

    NH3 + H2O → NH4+ + OH

    [Note: an alkali is a base soluble in water.]

  4. In alkaline solution there are more OH ions than H+ ions.

Physical Properties of Alkali

The following are the physical properties of alkali
  1. Alkalis are bitter in taste. 
  2. Alkalis turn litmus from red to blue.
    Like acid, alkali can change the colour of litmus. In alkali solution, the colour of litmus turn blue.
  3. Alkalis are soapy to touch. 
  4. Alkalis has pH value more than 7
    pH value is a measurement of the concentration of hydrogen ions in a solution. Alkali has very low concentration of solution hydrogen ion, even lower than water. Hence the pH value of alkali is higher than 7. (Note: The pH value of water is 7. The lower the concentration of hydrogen ions, the higher the pH value.
  5. Alkalis can conduct electricity
    When a base dissolve in water, it will dissociate and form hydroxide ions. The present of the freely move ions make alkali an electrolyte.