Animated Solution for Chemistry - s and p-Block Elements: The substance not likely to contain ${\text{CaCO}}_3$, is

Have you ever wondered what common materials like marble statues, sea shells, and chalk have in common? They are all fundamentally connected by a single, incredibly versatile chemical compound: Calcium Carbonate (${\text{CaCO}}_3$). In this problem, we are tasked with identifying which of the given substances does not belong to this carbonate family. Let's embark on a chemical journey through each of these materials to uncover their true identities.

Let's start with a marble statue. Marble is a beautiful, metamorphic rock that has been used by sculptors and architects for millennia. Geologically, marble forms when limestone is subjected to immense heat and pressure deep within the Earth. Since limestone is primarily composed of the mineral calcite, marble is also overwhelmingly made of calcium carbonate (${\text{CaCO}}_3$). Therefore, a marble statue definitely contains our target compound.

Next, consider sea shells. When you walk along a beach and pick up a shell, you are holding a marvel of biological engineering. Marine organisms, such as mollusks and oysters, extract calcium ions (${\text{Ca}}^{2+}$) and carbonate ions (${\text{CO}}_3^{2-}$) from seawater to construct their hard, protective exoskeletons. Through a process called biomineralization, they secrete these ions to form crystalline structures of aragonite or calcite, both of which are naturally occurring forms of calcium carbonate (${\text{CaCO}}_3$).

Now, let's look at dolomite. Dolomite is a very common rock-forming mineral that often forms massive geological deposits. Unlike pure limestone, dolomite is a double carbonate. Its chemical formula is $\text{CaMg}({\text{CO}}_3{)}_2$, which can also be written as ${\text{CaCO}}_3\cdot {\text{MgCO}}_3$. As the formula explicitly shows, it contains a significant proportion of calcium carbonate alongside magnesium carbonate. Thus, dolomite is also a member of the carbonate family.

Calcined Gypsum

Finally, we arrive at calcined gypsum. To understand what this is, we first need to look at its parent mineral, gypsum. Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula ${\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}$. Notice the crucial difference here: the anion is a sulfate (${\text{SO}}_4^{2-}$), not a carbonate (${\text{CO}}_3^{2-}$).

When gypsum is subjected to a process called calcination—which involves heating the mineral to high temperatures (around $150^{\circ }\text{C}$) in the absence of air—it loses a portion of its water of crystallization. This thermal decomposition reaction produces Plaster of Paris (calcium sulfate hemihydrate):

If heated even further, it loses all its water to become anhydrous calcium sulfate, known as dead burnt plaster (${\text{CaSO}}_4$). In either form, calcined gypsum remains a sulfate compound. It contains absolutely no carbonate ions.

By systematically analyzing the chemical composition of each option, we have established that marble, sea shells, and dolomite are all rich in calcium carbonate. Calcined gypsum, on the other hand, is a derivative of calcium sulfate. Therefore, calcined gypsum is the substance that is not likely to contain ${\text{CaCO}}_3$.