On heating, some solid substances change from solid to vapour state without passing through liquid state. The technique used for the purification of such solid substances based on the above principle is known as

During the preparation of Mohr's salt solution (Ferrous ammonium sulphate), which of the following acid is added to prevent hydrolysis of $$\mathrm{Fe}^{2+}$$ ion?

Which amongst the following compounds/species is least basic?

Which amongst the following compounds will show geometrical isomerism?