Specification Point 2.31:
Know that acids in aqueous solution are a source of Hydrogen Ions and Alkalis in a Aqueous Solution are a source of Hydroxide Ions

Acids and Alkalis

  • When atoms, or groups of atoms, lose or gain electrons, they form charged particles called ions.
  • These ions can be either positively or negatively charged.


  • When acids dissolve in water to form an aqueous solution, they produce hydrogen ions (H+).
  • These H+ ions make the aqueous solution acidic.

 Example: Hydrochloric Acid

HCl (aq)   →    H+ (aq)    +    Cl (aq)

Acids are often produced from non-metal oxides, e.g. Sulfur oxides make sulfuric acid.

Acids can be of two types:

i. Strong acid. Those which ionizes fully in water.

ii. Weak acid. Those which ionizes partially/incompletely in water. In this case they form an equilibrium between the acid and the ions in solution.
E.g. ethanoic acid CH3COOH ⇌ CH3COO + H+

If an acid ionizes to give just one hydrogen ion, it is monobasic aid. Similarly, dibasic acids are those which provides hydrogen ions. Tribasic acids provide three hydrogen ions.


  • When alkalis dissolve in water to form an aqueous solution, they produce hydroxide ions, (OH).
  • These OH ions makes the aqueous solution an alkali.

    Example: Sodium Hydroxide

NaOH (aq)   →    Na+ (aq)    +    OH (aq)

Alkalis can also be strong or weak. Those which ionize completely are strong alkali and those which doesn’t ionize completely are weak alkalis.

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Author: Jamie

Jamie got a First class degree in Chemistry from Oxford University before going on to teach chemistry full time as a professional tutor. He’s put together these handy revision notes to match the Edexcel IGCSE Chemistry specification so you can learn exactly what you need to know for your exams.