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Group VA and VIA Elements eLearn.Punjab

4. On large scale, it can be prepared by heating a mixture of
phosphorite (bone ash) and sand in an electric furnace. The phosphorus
pentoxide formed is treated with hot water to obtain phosphoric acid.

Ca3 (PO4 )2 (s) + 3SiO2 (s) → 3CaSiO3 (s) + P2O5 (s)

P2O5 (s) + 3H2O(l) → 2H3PO4 (aq)

Properties of Orthophosphoric Acid
It is a colourless, deliquescent crystalline solid which melts at 41°C. It is soluble
in water.

Reactions
1. It is a weak tribasic acid. It reacts with NaOH to give three series

of salts.

i. H3PO4 (aq) + NaOH (aq) → NaH2PO4 (aq) + H2O(l)

ii. NaH2PO4 (aq) + NaOH (aq) → Na2HPO4 (aq) + H2O(l)
iii. Na2HPO4 (aq) + NaOH (aq) → Na3PO4 (aq) + H2O(l)

2. On heating, it loses water and converted into pyro and metaphosphoric
acid.

240 C 316 C
− H 2O − H 2O
2H PO → H P O → 2HPO3 4

Orthophosphoric
42 7 3
Pyrophosphoric Metaphosphoric
acid acid acid

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