Explanation

(b)
Microcosmic salt is $Na\left( NH_{4} \right)HPO_{4}.$ It is white crystalline solid.
It is obtained when $NH_{4}Cl\ and\ Na_{2}HPO_{4}$ are dissolved in hot water and cooled.
$\ \ \ \ \ \ \ \ \ \ NH_{4}Cl + Na_{2}HPO_{4}\ \rightarrow Na\left( NH_{4} \right)HPO_{4} + NaCl$
It is separated by fractional crystallisation.
It is used for the detection of certain basic radicals which forms coloured mixed phosphate with $NaPO_{3}.$
$Na\left( NH_{4} \right)HPO_{4}\overset{\ \ \ \ \ \mathrm{\Delta}\ \ \ \ \ }{\rightarrow}\ NaPO_{3} + NH_{3} + H_{2}O$
$\ \ \ \ \ NaPO_{3} + CoO\ \rightarrow NaCo\ .\ PO_{4}$
blue bead