A hydrated solid X on heating initially gives a monohydrated compound Y. Y upon heating above 373 K leads to an anhydrous white powder Z. X and Z, respectively, are;

Step-by-step solution

'X' is washing soda N a 2 C O 3 . 10 H 2 O which on heating gives a monohydrate N a 2 C O 3 . H 2 O . The monohydrate on further heating gives an anhydrous residue of N a 2 C O 3 ∴ X   and   Z are washing soda and soda ash.