Understanding Water Molecules in Ferrous Sulphate Crystals

Many ionic compounds form crystals that contain water molecules trapped within their lattice structure. This water is known as water of crystallization or hydration. The number of water molecules associated with one formula unit of the salt is fixed for a specific compound.

The question asks for the number of water molecules present in a Ferrous Sulphate crystal. Ferrous sulphate is an ionic compound with the chemical formula $\text{FeSO}_4$. When ferrous sulphate crystallizes from an aqueous solution, it typically forms a hydrated crystal. This hydrated form is commonly known as green vitriol.

The chemical formula for hydrated ferrous sulphate is $\text{FeSO}_4 \cdot 7\text{H}_2\text{O}$.

In this formula:

• $\text{FeSO}_4$ represents one formula unit of ferrous sulphate.
• $\cdot 7\text{H}_2\text{O}$ indicates that seven water molecules are associated with each formula unit of ferrous sulphate in the crystalline structure.

Therefore, the number of water molecules present in a Ferrous Sulphate crystal (specifically, the common hydrated form) is 7.

This water of crystallization gives hydrated ferrous sulphate its crystalline shape and color (green). When hydrated ferrous sulphate is heated, it loses its water of crystallization and turns into anhydrous ferrous sulphate, which is a pale grey or white powder.

Detailed Analysis

Let's look at the common hydrated salts and their water of crystallization:

Based on the chemical formula $\text{FeSO}_4 \cdot 7\text{H}_2\text{O}$, the number of water molecules per formula unit of ferrous sulphate crystal is 7.

Revision Table: Key Concepts for Ferrous Sulphate Crystals

Additional Information: Water of Crystallization

Water of crystallization is the water present in many crystalline substances. It is part of the crystal structure, chemically bound to the metal atom or coordinated to the ions, and not just water molecules adsorbed on the surface. The number of water molecules is stoichiometric, meaning it is in a fixed ratio to the salt molecules.

Properties influenced by water of crystallization:

• Crystal Shape: The presence of water molecules influences how the ions pack together, determining the specific geometric shape of the crystal.
• Color: The hydration state can significantly affect the color of a compound. For example, anhydrous copper sulphate ($\text{CuSO}_4$) is white, while hydrated copper sulphate ($\text{CuSO}_4 \cdot 5\text{H}_2\text{O}$) is blue. Similarly, anhydrous ferrous sulphate is pale grey, while hydrated ferrous sulphate is green.
• Physical Properties: Melting point and other physical properties are different for hydrated and anhydrous forms of the same salt.

When hydrated salts are heated, they lose their water of crystallization, undergoing dehydration. This process is often reversible; if the anhydrous salt is exposed to moisture, it can reabsorb water and return to its hydrated form and color, a process called hydration.

Some substances that do not crystallize with water can absorb moisture from the atmosphere, a phenomenon called hygroscopy. If a substance absorbs enough moisture to dissolve in it, it is said to be deliquescent.

Understanding water of crystallization is important in chemistry, particularly when dealing with the preparation and properties of salts.