Give the preparation of phosphine $(PH_{3})$. State the physical and chemical properties of phosphine $(PH_{3})$. State the uses of phosphine.

(N/A) Preparation: Phosphine is prepared by the reaction of calcium phosphide with water or dilute $HCl$.
$Ca_{3}P_{2} + 6H_{2}O \rightarrow 3Ca(OH)_{2} + 2PH_{3}$
$Ca_{3}P_{2} + 6HCl \rightarrow 3CaCl_{2} + 2PH_{3}$
In the laboratory,it is prepared by heating white phosphorus with concentrated $NaOH$ solution in an inert atmosphere of $CO_{2}$.
$P_{4} + 3NaOH + 3H_{2}O \rightarrow PH_{3} + 3NaH_{2}PO_{2}$ (Sodium hypophosphite)
To obtain pure $PH_{3}$,it is absorbed in $HI$ to form phosphonium iodide $(PH_{4}I)$,which on treatment with $KOH$ releases pure phosphine: $PH_{4}I + KOH \rightarrow KI + H_{2}O + PH_{3}$.
Physical Properties: $PH_{3}$ is a colourless gas with a rotten fish smell and is highly poisonous. It is sparingly soluble in water.
Chemical Properties: Phosphine explodes in contact with traces of oxidizing agents like $HNO_{3}$,$Cl_{2}$,and $Br_{2}$ vapours. It is weakly basic and forms phosphonium compounds with acids,e.g.,$PH_{3} + HBr \rightarrow PH_{4}Br$ (Phosphonium bromide). It reacts with metal salts to form phosphides: $3CuSO_{4} + 2PH_{3} \rightarrow Cu_{3}P_{2} + 3H_{2}SO_{4}$ and $3HgCl_{2} + 2PH_{3} \rightarrow Hg_{3}P_{2} + 6HCl$.
Uses: Phosphine is used in Holme's signals. Containers containing calcium carbide $(CaC_{2})$ and calcium phosphide $(Ca_{3}P_{2})$ are pierced and thrown into the sea; the evolved gases ($C_{2}H_{2}$ and $PH_{3}$) burn and serve as a signal. It is also used in the preparation of smoke screens.