The Role of Water in Showing Acidic and Alkaline Properties


6.1  The Role of Water in Showing Acidic and Alkaline Properties


  • Acid is a chemical compound that produces hydrogen ions, \(H^+\) or hydroxonium ions \(H_3O^+\) when dissolved in water. 
  • The dissociation of acid in water produces hydrogen ions and an anion.
  • An example of dissociation of hydrochloric acid in water is as follows; 

\(HCl(aq) \xrightarrow[]{H_2O} H^+ (aq) + Cl^- (aq)\)

  • Without the presence of hydrogen ions, that means there are no acidic properties on the chemicals.


Basicity of acids

  • Refers to the number of hydrogen ions, \(H^+\) that can be produced by an acid molecule that ionises in water. 








One \(H^+\) per acid molecule Two \(H^+\) per acid molecule Three \(H^+\) per acid molecule

Hydrochloric acid, 


Sulphuric acid, 


Phosphoric acid, 




  • The base is a chemical compound that can neutralise an acid to produce salt and water. 
  • Example of formation of salt and water is:
    • \(HCl (aq) + NaOH(aq) \rightarrow NaCl (aq)+ H_2O(l)\)
  • Most bases are not soluble in water.
  • Bases that are soluble in water are known as alkalis.
  • Bases that insoluble in water are zinc oxide, zinc hydroxide, copper(II) oxide and copper(II) hydroxide.
  • An alkali is a chemical compound that dissolves in water and produces freely moving hydroxide ions, \(OH^- \).
  • Without the presence of hydroxide ions, the chemical substances do not show alkaline or basic properties. 
  • Example of alkali is potassium hydroxide:
    • \(KOH(aq) \xrightarrow[] {H_2O}K^+(aq) + OH^-(aq)\)
  • Examples of akali;
    • Sodium hydroxide, \(NaOH\)
    • Potassium hydroxide, \(KOH\)


Definition of Alkali
Chemical compound that dissolves in water and produces freely moving hydroxide ions, \(OH^-\)


Uses of acids, bases and alkalis in our daily life.




Sulphuric acid

Make paints, polymer, fertilisers and detergent

Hydrochloric acid

To clean metal before electroplating

Benzoic acids

To preserve food




Make fertilisers, nitric acid, and to maintain latex in liquid form

Magnesium hydroxide

To make toothpaste, gastric medicine

Calcium hydroxide

To make cement, limewater and to neutralise acidic soil


Colour of indicators in acidic and alkaline solutions 


Indicator Colour of indicator in acidic solution Colour of indicator in alkaline solution
Methyl orange Red Yellow
Universal indicator Red for strong acid and yellow for weak acid Purple for strong alkali and blue for weak alkali
Blue litmus paper Red Blue
Red litmus paper Red Blue