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

The non-efflorescent compound is (b) NaOH.

Step 1: Understanding Efflorescence

Efflorescence is a process where hydrated salts lose water of crystallization when exposed to air, turning into a powdery substance. This typically occurs with salts that have a high affinity for water vapor from the atmosphere.

Step 2: Evaluate Each Compound

Check each compound for their efflorescent nature: - (a) \(CuSO_4\) is typically found as a pentahydrate form (blue), which loses water and turns white upon exposure to air, making it efflorescent. - (b) \(NaOH\) is hygroscopic and can absorb moisture from the air, but it does not usually lose water. - (c) \(Na_2SO_4\) often exists in its decahydrate form (Glauber's salt) which can lose water in the air and is hence efflorescent. - (d) \(MgSO_4\) often found as heptahydrate (Epsom salt) and can lose water, showing efflorescence.

Step 3: Identify the Non-Efflorescent Compound

From the evaluation, \(NaOH\) is identified as the substance that does not undergo efflorescence, as it is more prone to absorbing moisture rather than losing it.

Key Concepts

Hydrated Salts

Hydrated salts are fascinating compounds that include water molecules within their crystal structure. This particular water is not just trapped; it forms a part of the crystal lattice. When you observe hydrated salts, you're essentially seeing crystals that appear dry but actually contain certain amounts of water known as water of crystallization.
These salts can change their characteristics based on environmental conditions. For example, copper sulfate ( CuSO_4 ) often appears as blue crystals when hydrated because of the water within it. Same goes for magnesium sulfate, commonly known as Epsom salt, which is a heptahydrate. These crystalline structures change when water is lost or absorbed, which is a process we will explore as water of crystallization and hygroscopic behavior.

Water of Crystallization

Water of crystallization is a crucial concept when it comes to understanding hydrated salts. It refers to the fixed number of water molecules that are integrated into the crystal lattice of a salt.
Each different hydrated salt has a specific number of water molecules aligned in its structure. For instance:

• CuSO_4 ·5H_2O - has five water molecules.
• Na_2SO_4 ·10H_2O - also known as Glauber's salt, carries ten water molecules.

These molecules are loosely attached and can evaporate off if the salt is exposed to the air, which leads to a process called efflorescence where the salt becomes anhydrous and crumbles into powder.
Understanding the concept of water of crystallization helps in comprehending how some substances can naturally release moisture and become non-crystalline over time.

Hygroscopic Substances

Hygroscopic substances are a bit different from efflorescent ones; they have an inherent desire to draw in water from the environment. These materials can absorb moisture from the air without even dissolving in it.
Common examples of hygroscopic substances include sodium hydroxide ( NaOH ) and calcium chloride ( CaCl_2 ). Unlike efflorescent substances that lose water to form a dry powder, hygroscopic materials do the opposite; they gain water, sometimes becoming a liquid.
That's why you might find substances like NaOH stored in airtight containers. Left exposed, they can absorb so much moisture that they dissolve in the water they absorb. This behavior makes them essential in controlling humidity in packaging and storage.