An inorganic compound ' X ' of an alkali metal on heating gives a...

An inorganic compound ' $\mathbf{X}$ ' of an alkali metal on heating gives a reddish-brown gas ' $\mathbf{Y}$ ' and a binary solid ' $\mathbf{Z}$ '. This solid is less soluble in water and its solution is basic. ' $\mathbf{X}$ ' does not give a positive silver nitrate test. ' $\mathbf{X}$ ' can be identified as