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.

Chemical Properties of Bases/Alkalis

  1. Alkalis react with acids to form a salt and water — this is a neutralisation reaction:
  2. Alkalis, when warmed with ammonium salts, give off ammonia gas:

Reaction between Alkalis and Acids

Acid + Alkali  Salt + Water

Potassium hydroxide + Nitric Acid  Potassium Nitrate + Water

KOH + HNO3  → KNO3  +  H2O

Alkali heat with Ammonium Salts

Ammonium Salt + Alkali  Salt + Ammonia + Water

Ammonium Chloride + Sodium Hydroxide  Sodium chloride + Water + Ammonia

NH4Cl + NaOH → NaCl +  H2O + NH3

More examples:

2NH4Cl +    Ca(OH)2  → CaCl2 + 2H2O + 2NH3
(NH4)2SO4 + 2NaOH→ Na2SO4 + 2H2O+ 2NH3

Ammonia as an Alkali

  1. By nature, ammonia is a covalent compound.
  2. Ammonia exists as gas at room temperature.
  3. Ammonia gas is soluble in water.
  4. Physical properties of ammonia:
    1. Pungent smell
    2. Colourless
    3. Turn litmus from red to blue

Oxide: Basic, Acidic, Neutral, or Amphoteric

Basic Oxide

  1. Most oxides of metal, especially alkali and alkaline earth metals, are basic oxides.
  2. Basic oxides that dissolve in water are called alkalis. 
  3. A basic oxide is an oxide that shows basic properties in opposition to acidic oxides and that either
    1. reacts with water to form an alkali; or
    2. reacts with an acid to form a salt.


  1. Sodium oxide, which reacts with water to produce sodium hydroxide
  2. Magnesium oxide, which reacts with hydrochloric acid to form magnesium chloride
  3. Copper(II) oxide, which reacts with nitric acid to form copper nitrate
Acidic Oxide
  1. Most non-metal oxides dissolve in water to form acids, and are called acidic oxides.
  2. An acidic oxide is an oxide that either
    1. reacts with water to form an acid; or
    2. reacts with a base to form a salt.

Examples :

  1. Carbon dioxide which reacts with water to produce carbonic acid.
  2. Sulfur dioxide, which does not form the non-existent sulfurous acid but does react with bases to form sulfites.
  3. Silicon dioxide, which does not react with water but will react with bases to form silicates


    Not all oxide of non-metal are acidic oxide. For example, carbon monoxide (CO) is neutral.

    Amphoteric Oxide

    1. The metal oxides that can behave as both acids and bases and are said to be amphoteric oxides.
    2. Examples of amphoteric oxides are lead oxide, aluminium oxide and zinc oxide.