Question

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

• A. Baking soda and dead burnt plaster.
• B. Washing soda and soda ash
• C. Washing soda and dead burnt plaster.
• D. Baking soda and soda ash

Answer 1

The correct option is B

Washing soda and soda ash

Explanation for Correct Option:

Option (B): Washing soda and soda ash:

Step 1: Finding the compound X

We know that all the states of washing soda, that is Sodium carbonate$\left({\mathrm{Na}}_2{\mathrm{CO}}_3\right)$ are white, and they are also water-soluble.

Therefore, compound X is Sodium carbonate or washing soda.

Step 2: Finding the compound Y

When this hydrated Sodium carbonate which has 10 water molecules as water of crystallization is heated it loses 9 water of crystallization and forms ${\mathrm{Na}}_2{\mathrm{CO}}_3.{\mathrm{H}}_2\mathrm{O}$

$\underset{\mathrm{Sodium}\mathrm{carbonate}}{{\mathrm{Na}}_2{\mathrm{CO}}_3.10{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)}\stackrel{\mathrm{Heat}}{\to }\underset{\mathrm{Sodium}\mathrm{carbonate}}{{\mathrm{Na}}_2{\mathrm{CO}}_3.{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)}$

Therefore, compound Y is ${\mathrm{Na}}_2{\mathrm{CO}}_3.{\mathrm{H}}_2\mathrm{O}$

Step 3: Finding the compound Z

When ${\mathrm{Na}}_2{\mathrm{CO}}_3.{\mathrm{H}}_2\mathrm{O}$ is heated above $100°\mathrm{C}\left(373\mathrm{K}\right)$ it loses the last water of crystallization it has and forms an anhydrous white powder of ${\mathrm{Na}}_2{\mathrm{CO}}_3$

$\underset{\mathrm{Sodium}\mathrm{carbonate}}{{\mathrm{Na}}_2{\mathrm{CO}}_3.{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)}\underset{\mathrm{T}>373\mathrm{K}}{\overset{\mathrm{Heat}}{\to }}\underset{\underset{(\mathrm{Soda}\mathrm{ash})}{\mathrm{Sodium}\mathrm{carbonate}}}{{\mathrm{Na}}_2{\mathrm{CO}}_3\left(\mathrm{s}\right)}$

Therefore, the compound Z is ${\mathrm{Na}}_2{\mathrm{CO}}_3$(soda ash)

Explanation for Incorrect Options:

Option (A): Baking soda and dead burnt plaster.

• Baking soda$\left({\mathrm{NaHCO}}_3\right)$ does not exist in hydrated form, hence upon heating, it won't form a monohydrated compound as well.

Option (C): Washing soda and dead burnt plaster.

• Washing soda after losing the 9 hydrated water of crystallization, when heated, it produces soda ash.
• It does not produce burnt plaster as dead burnt plaster is anhydrous Calcium sulphate $\left({\mathrm{CaSO}}_4\right)$

Option (D): Baking soda and soda ash

• Baking soda$\left({\mathrm{NaHCO}}_3\right)$ does not exist in hydrated form, hence upon heating, it will not form a monohydrated compound as well.
• Hence upon heating, it cannot form soda ash.

Therefore, option (B) is correct.