The number of hydroxyl groups in pyrophosphoric acid is ......... .

Explain the molecular structure and properties of phosphorus pentachloride $(PCl_5)$.

Write the main differences between the properties of white phosphorus and red phosphorus.

Among the members of $V$ $A$ group ($N$,$P$,$As$,$Sb$,and $Bi$),which of the following properties shows an increase as we go down from nitrogen to bismuth?

Explain the nature of hydride compounds of group-$15$ elements. Explain the nature of oxide compounds of group-$15$ elements. Write a note on halide compounds of group-$15$ elements.

What happens when excess water is added to a solution of $BiCl_3$?