`PH_3` has lower boiling point than `NH_3`. Why?

Explanation

Due to smaller size and higher electronegativity of nitrogen, `NH_3` shows strong intermolecular H -bonding in liquid state as shown below.

On the other hand no such H-bonding is possible in phosphorus because both H and P have same electronegativity and as such the P-H bond is non-polar. Thus ammonia exists as associated molecules and boils at a higher temperature than phosphine.