Animated Solution for Chemistry - s-Block Elements: A hydrated solid $X$ on heating initially gives a monohydrated compound $Y$. $Y$ upon heating above $373\text{ K}$ leads to an anhydrous white powder $Z$. $X$ and $Z$, respectively, are

Have you ever wondered what happens when you heat a crystal that looks perfectly dry, yet contains water hidden within its structure? This hidden water is known as the water of crystallization, and the way a salt loses this water upon heating can tell us exactly what that salt is. Let's embark on a chemical detective journey to identify the mysterious compounds $X$, $Y$, and $Z$.

Our first clue is that compound $X$ is a hydrated solid. Let's look at our options. Baking soda is sodium bicarbonate (${\text{NaHCO}}_3$). Does it have water of crystallization? No, it doesn't. It's an anhydrous powder in its natural state. So, we can immediately cross out any option suggesting $X$ is baking soda.

What about dead burnt plaster? This is anhydrous calcium sulphate (${\text{CaSO}}_4$), which is formed by strongly heating gypsum (${\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}$). However, when gypsum is heated, it first forms Plaster of Paris (${\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}$), which is a hemihydrate, not a monohydrate. Since our mystery compound $X$ forms a monohydrate $Y$, gypsum doesn't fit the profile.

This leaves us with washing soda. The chemical formula for washing soda is sodium carbonate decahydrate, written as ${\text{Na}}_2{\text{CO}}_3\cdot 10{\text{H}}_2\text{O}$. It's a highly hydrated solid, making it a perfect candidate for compound $X$. Let's see how it behaves when the heat is turned up.

When washing soda is gently heated to a temperature below $373\text{ K}$ (which is $100^{\circ }\text{C}$), it doesn't lose all its water at once. Instead, it undergoes a partial dehydration, losing exactly nine water molecules:

The resulting compound, ${\text{Na}}_2{\text{CO}}_3\cdot {\text{H}}_2\text{O}$, is sodium carbonate monohydrate. This perfectly matches the description of our intermediate compound $Y$!

Now, what happens if we crank up the heat? If we heat this monohydrate above $373\text{ K}$, the thermal energy becomes sufficient to break the bond holding the final water molecule.

The water evaporates away, leaving behind a completely dry, white powder of anhydrous sodium carbonate (${\text{Na}}_2{\text{CO}}_3$). In the chemical industry, this anhydrous form is famously known as soda ash. This is our final product, $Z$.

By carefully tracing the thermal decomposition steps, we've successfully identified that the initial hydrated solid $X$ is washing soda, and the final anhydrous powder $Z$ is soda ash.