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bases

  • Bases are compounds which react with acid to form a salt and water as only products.

  • Bases that soluble in water are called alkalis.

  • In aqueous solution, alkali it produces hydroxide ions (OH-).

  • In short, alkalis are substances that form hydroxide ions (OH-(aq)) in water

Example

  • 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.]

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

Chemical Properties Of Bases/Alkalis

  • Alkalis react with acids to form a salt and water — this is a neutralisation reaction:

  • Alkalis, when warmed with ammonium salts, give off ammonia gas:

  1. Reaction between Alkalis and Acids

                  Acid + Alkali → Salt + Water

  • Example:

     Potassium hydroxide + Nitric Acid → Potassium Nitrate + Water

                                KOH + HNO3 → KNO3 + H2O

2. Alkali heat with Aluminium salt

                  Ammonium Salt + Alkali → Salt + Ammonia + Water

  • Example:
    Ammonium Chloride + Sodium Hydroxide → Sodium chloride + Water + Ammonia

  NH4Cl + NaOH → NaCl + H2O + NH3

Ammonia as an Alkali

  • By nature, ammonia is a covalent compound.

  • Ammonia exists as gas at room temperature.

  • Ammonia gas is soluble in water.

  • Physical properties of ammonia:

    • Pungent smell

    • Colourless

    • Turn litmus from red to blue

The Role Of Water In Showing Alkaline Properties

  • Alkali shows alkaline properties only in the presence of water.

  • When water is present, ionisation of alkali produces OH-, ions that are responsible for the alkali properties

  • Without the presence of water, there are no free-moving hydroxide ions. Therefore, in the absence of, alkalis do not show alkalinity 

  • Diagram below shows the illustration to investigate the role of water in showing alkaline properties of ammonia.

  • The ammonia dissolve in water can turn red litmus paper to blue whereas the dry ammonia gas of the ammonia gas dissolve in propanone give no effect to the litmus paper.

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Strength Of Alkalis

  • Similar to strength of acids, the strength of an alkali is defined by its ability to ionise and release hydroxide ions (OH-) in the solution.

  • In a solution of strong alkali, all the alkali molecules are ionised in the water to produce hydroxide  ions.

  • In a solution of weak alkali, only small portion of the molecules are ionised to release hydroxide ions.

  • Table below shows some example of strong/weak alkalis:

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