Write examples to show that ${\mathrm{H}}_2{\mathrm{S}\mathrm{O}}_4$ is both, a drying agent as well as a dehydrating agent.

During the laboratory preparation of $\mathrm{H}\mathrm{C}\mathrm{l}$ gas, conc.${\mathrm{H}}_2{\mathrm{S}\mathrm{O}}_4$ is used as _____ agent.

Sometimes, a sticky mass of _____ is formed during the laboratory preparation of $\mathrm{HCl}$ gas.