Why does $PC{l_3}$ fume in air?

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Hint: Phosphorus trichloride is a colorless liquid with yellow fumes. It is highly corrosive in nature and on reacting with water it forms phosphorus acid and an acidic gas.

Complete step by step answer:
The phosphorus trichloride is a phosphorus halide having the chemical formula $PC{l_3}$. The phosphorus trichloride is a colorless liquid which gives yellow fumes. The phosphorus trichloride is pungent and irritating in odour which resembles the odour of hydrochloric acid. The phosphorus trichloride is a very hazardous acid which can cause severe burns to skin, mucous membrane and eyes. If phosphorus trichloride is inhaled or absorbed by the skin it causes severe toxic affect. phosphorus trichloride reacting with water gives hydrochloric acid which is a very corrosive gas. The phosphorus trichloride is formed by reacting yellow phosphorus and chloride
On further chlorination of phosphorus trichloride phosphorus pentachloride is formed.
The reaction is shown below.
$PC{l_3} + C{l_2} \to PC{l_5}$
In this reaction, the phosphorus trichloride is reacted with chlorine to give phosphorus pentachloride.
When phosphorus trichloride comes in contact with the air, it reacts with the moisture present in the air to form phosphorus acid and a white fume of hydrochloric acid. This is why phosphorus trichloride fumes in air.
The reaction is shown below.
$PC{l_3} + 3{H_2}O \to {H_3}P{O_3} + 3HCl$
In this reaction, one mole of phosphorus trichloride reacts with three mole of water to form one mole of phosphorus acid and three mole of hydrochloric acid.

Phosphorus trichloride is used for the preparation of phosphorous oxychloride which is formed by oxidation of phosphorus trichloride. The phosphorus trichloride reaction with nitrous acid results in an explosion.