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| Acids and Bases - pH Scale |
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| Arrehenius' Concept of Acids and Bases |
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| An acid is a substance which can furnish hydrogen ions in its solution. |
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| Examples: |
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| Hydrogen atom contains one proton and one electron. H+ ion is formed by loss of this electron. Therefore H+ ion is simply a proton. Charge density of this unshielded proton is very high. Therefore it is not likely to exist independently as H+ ion. In an aqueous solution H+ ion is considered to be present in hydrated form in combination with water molecules as H3O+. |
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| H3O+ ion is called hydronium ion. |
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| Acids like HCl and HNO3 which are almost completely ionised in aqueous solutions are termed as strong acids whereas acids such as CH3COOH which are weakly ionised are called weak acids. |
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| A base is a substance which can furnish hydroxyl ions in its aqueous solutions. |
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| Example: |
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| Bases which are almost ionised in aqueous solution are called strong bases. |
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| Example: sodium hydroxide (NaOH) and potassium hydroxide (KOH) |
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| The bases such as ammonium hydroxide (NH4OH) which are only slightly ionised are called weak bases. |
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| In 1923, a Danish Chemist J.H Bronsted and an English chemist T.M Lowry independently proposed new definitions for acids and bases. They proposed that: |
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 An acid is a substance that can donate a proton. |
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A base is a substance that can accept a proton. |
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| These definitions are more general because according to these definitions even ions can behave as acids or bases. Moreover these definitions are not restricted to reactions taking place in aqueous solution only. In order to understand this concept of acids and bases, let us take some specific examples: |
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| From the above equations, it is obvious that an acid base reaction according to Bronsted-Lowry concept involve transfer of proton from an acid to a base. A substance can act as an acid only if another substance capable of accepting a proton is present. |
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