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| What is an Acid? |
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| The word 'acid' is derived from a Latin word, which means "sour". The sour taste of most of the fruits and vegetables is due to various types of acids present in them. The digestive fluids of most of the animals and humans also contain acids. |
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| An acid is a compound, which on dissolving in water yields hydronium ions (H3O+) as the only positive ions. The characteristic property of an acid is due to the presence of these hydronium ions. |
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| Formation of Hydronium ion |
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| According to the classical concept all acids dissolve in water to give hydrogen ions (H+). A hydrogen ion is a proton, since hydrogen atom has only one proton and one electron. These hydrogen ions cannot exist independently in water and combine with neutral water molecules to form hydronium ions (H3O+). This can be explained on the basis of the lone-pair of electrons present in the oxygen atom of the water molecule. |
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| The hydrogen ion or proton liberated from the acid combines with a water molecule by forming a covalent bond. Since hydrogen ion does not have any electrons at all, the pair of electrons required for bond formation can only come from the oxygen atom of the water molecule. Thus the lone pair of electrons from the oxygen atom is shared with the hydrogen ion to establish a covalent bond. |
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| This covalent bond is of a special type and is called the co-ordinate bond. Here both the atoms share the pair of electrons, which belongs to only one of the combining atoms (Fig.8.1). |
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| Lone-pair of electrons |
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| Pair of electrons left on the atom after the formation of covalent bonds e.g. in water molecule oxygen atom has 2 lone-pair of electrons. In ammonia molecule nitrogen atom has one lone pair of electrons (Fig.8.2). |
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