A colourless solution Y effervesces with dilute acid. With cold $\mathrm{MgSO}{}_4$ solution Y gives no precipitate, but on boiling a white precipitate forms and gas is evolved. Y contains

Carbonate gives white $\mathrm{MgCO}{}_3$ immediately in the cold. Hydrogencarbonate gives no precipitate in the cold but on boiling it decomposes: $2\mathrm{HCO}{}_3{}^{-}\to \mathrm{CO}{}_3{}^{2-}+\mathrm{CO}{}_2+\mathrm{H}{}_2\mathrm{O}$, and the carbonate formed precipitates $\mathrm{MgCO}{}_3$. Sulphate and acetate do not effervesce with dilute acid.