Acids have long been associated with the sour taste of some fruits such as lime and lemon. However, the definition of an acid has now been modified to express and reflect the behaviour of acid in solution.
An Acid is defined as a substance which when dissolved in water produces hydrogen or hydroxonium ion as the only positive ion.
(i) HCl(aq) → H+(aq)) + OH–(aq)
(ii) HCl(aq) → H3O+ (aq) + OH–(aq)
Arrhenius definition of an Acid:
An Acid is a substance that dissociates in water to produce hydrogen ions (H+) or protons.
HCl(aq) → H+(aq) + OH–(aq)
Brønsted–Lowry Concept of Acids:
An acid is a proton donor e.g
HCl + NH3 → NH4+ + Cl–
HCl donates its proton to form ammonium ion
Lewis Concept of Acid:
An acid is an electron-pair acceptor.
Lewis acids do not have hydrogen atoms in their molecules but have empty valence orbital on their central atoms e.g. BF3, AlCl3, FeCl3 etc.
AlCl3 + Cl– → AlCl4–
In the above reaction, the empty valence orbital on the aluminium atom in AlCl3 accepts a pair of electrons from Cl–.
Types of Acid:
Two types of Acids are:
1. Organic Acid.
2. Inorganic Acid or Mineral Acids.
1. Organic Acid:
Organic Acids occur naturally in plants and animals. Most of the fruits we eat are acidic, for example, ethanoic acid is found in lemon and orange juice. The most common organic acids are the carboxylic acids, whose acidity is associated with their carboxyl group −COOH.
A list of common organic acids and their sources are in the table below.
2. Inorganic Acids:
Inorganic Acidsare called mineral acids since they can be prepared from mineral ores. e.g Hydrochloric acid (HCl)
Table: List of Common Organic Acids
|Citric acid||Lime, Lemon|
|Ascorbic acid (vitamin C)||Orange|
|Palmitic acid||Palm oil|
|Fatty acid||Fats and Oils|
|Methanoic acid||Bees and ants’ stings|
|Malic acid||Grapes, Watermelons, |
Table: List of Organic Acids
The most common inorganic acids are;
|Tetraoxosulphate (VI) Acid||H2SO4|
|Trioxonitrate (V) Acid||HNO3|