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

• Correct answer
  A

  washing soda and soda ash.
• B

  baking soda and soda ash.
• C

  washing soda and dead burnt plaster.
• D

  baking soda and dead burnt plaster.

Solution

'X' is washing soda $\left(\mathrm{N}{\mathrm{a}}_2\mathrm{C}{\mathrm{O}}_3.10{\mathrm{H}}_2\mathrm{O}\right)$ which on heating gives a monohydrate $\left(\mathrm{N}{\mathrm{a}}_2\mathrm{C}{\mathrm{O}}_3.{\mathrm{H}}_2\mathrm{O}\right).$ The monohydrate on further heating gives an anhydrous residue of $\mathrm{N}{\mathrm{a}}_2\mathrm{C}{\mathrm{O}}_3$



$\therefore \mathrm{X}\mathrm{ }$ and $\mathrm{ }\mathrm{Z}$ are washing soda and soda ash.

Hence, the correct answer is option: 'A'.