Problem 2 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is not efflorescent? (a) \(\mathrm{CuSO}_{4}\) (b) \(\mathrm{NaOH}\) (c) \(\mathrm{Na}_{2} \mathrm{SO}_{4}\) (d) \(\mathrm{MgSO}_{4}\)

Step-by-Step Solution

Answer

(b) \(\mathrm{NaOH}\) is not efflorescent.

Step 1: Understand Efflorescence

Efflorescence is the process where hydrate salts lose their water of crystallization when exposed to the atmosphere at ordinary temperature. Only salts that have water of crystallization are considered for efflorescence.

Step 2: Consider Each Compound

Evaluate each compound to identify whether it can lose water of crystallization: (a) \(\mathrm{CuSO}_{4}\) typically exists as \(\mathrm{CuSO}_{4} \, \cdot \, 5\mathrm{H}_{2}\mathrm{O}\), a common hydrate that can undergo efflorescence.(b) \(\mathrm{NaOH}\) usually doesn't exist as a hydrate in this context, hence is not relevant to efflorescence.(c) \(\mathrm{Na}_{2} \mathrm{SO}_{4}\) exists as \(\mathrm{Na}_{2} \mathrm{SO}_{4} \, \cdot \, 10\mathrm{H}_{2}\mathrm{O}\), known to effloresce.(d) \(\mathrm{MgSO}_{4}\) can occur as \(\mathrm{MgSO}_{4} \, \cdot \, 7\mathrm{H}_{2}\mathrm{O}\), which is a hydrate capable of efflorescence.

Step 3: Identify the Non-Efflorescent Compound

From the evaluation, only \(\mathrm{NaOH}\) is not a salt with water of crystallization involved in efflorescence. It does not meet the criteria for losing water as it normally does not contain any.

Key Concepts

Hydrate Salts

Hydrate salts are fascinating chemical compounds that include water molecules within their crystalline structure. These special crystals have water molecules embedded in them, and these molecules play a crucial role in maintaining the structure and properties of the salt.

• These salts are represented by a compound formula followed by a dot and then a number, with the formula for water, such as \( \mathrm{CuSO}_{4} \cdot 5\mathrm{H}_{2}\mathrm{O} \).
• The number represents how many water molecules are attached to each molecule of the compound, detailing a specific ratio.
• These attached water molecules are called "water of crystallization." Without them, the structure might collapse, or the properties of the salt might change significantly.

The role of these water molecules is crucial, as they also impact the physical appearance and stability of the hydrate.
Efflorescence is direct evidence of hydrate salts, as they lose this water over time when exposed to the atmosphere under typical conditions, leaving behind a powdered form.

Water of Crystallization

Water of crystallization is a term used to describe water molecules that are an integral part of the crystal structure of many salts. These water molecules are not just mixed in randomly but are actually part of the solid structure itself.

• This water can literally hold the crystal lattice together, serving as a 'glue' that helps to maintain the crystal shape and structure.
• The amount of water of crystallization is specific to each compound, and the removal of this water can sometimes lead to a change in properties.
• For example, when the water is removed due to exposure or heat, the resulting solid can turn powdery — a process we call efflorescence.

Compounds like \(\mathrm{Na}_{2} \mathrm{SO}_{4} \cdot 10\mathrm{H}_{2}\mathrm{O}\) clearly illustrate how these waters of crystallization are calculated and factored into the total composition of the salt.
Even the color can change upon losing the water; some blue copper sulphate crystals, when losing their water, turn white. This emphasizes the significant role that water of crystallization plays in determining the properties of hydrates.

Chemical Properties of Compounds

The chemical properties of compounds are determined by their composition and structure, and hydrate salts are no exception. The presence of water of crystallization significantly affects a compound's behavior and chemical reactivity.

• These properties can include aspects like solubility, color, and the way the compound interacts with heat and other chemicals.
• Hydrates, due to their water molecules, often have different physical forms and reactivity compared to their anhydrous states.
• Compounds with water of crystallization can react differently to environmental conditions such as humidity and temperature, often forming different products if they lose their water molecules.

Understanding these properties helps in practical applications, as the behavior under standard air exposure often results in efflorescence.
Reactions such as these provide insight into real-world applications where the integrity of the compound is vital, such as in construction materials or pharmaceuticals.