Problem 8 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is not efflorescent? (a) \(\mathrm{NaOH}\) (b) hydrated \(\mathrm{CuSO}_{4}\) (c) hydrated \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) (d) all of these

Step-by-Step Solution

Answer

(a) NaOH is not efflorescent.

Step 1: Understanding Efflorescent Substances

Efflorescent substances are those that lose water of crystallization when exposed to air, often appearing as a powdery surface.

Step 2: Identify Efflorescent Substances

Evaluate the options: (a) NaOH can absorb moisture, but it does not lose water of crystallization as it doesn't form hydrates; (b) Hydrated CuSO4, commonly known as blue vitriol, loses its water of hydration when exposed to air; (c) Hydrated Na2CO3, also known as washing soda, also loses water of crystallization in air.

Step 3: Determine the Correct Answer

Since both (b) hydrated CuSO4 and (c) hydrated Na2CO3 are known efflorescent compounds, and (a) NaOH does not form a hydrate to lose water of crystallization, option (a) NaOH is not efflorescent.

Key Concepts

Water of crystallization

Water of crystallization refers to the specific amount of water molecules that are chemically bound within a crystalline structure of a compound. This water is not merely absorbed but is integral to maintaining the structure and properties of the compound. When a compound contains water of crystallization, it is often referred to as a "hydrate."The water associated is found in a fixed ratio relative to the other elements in the crystalline structure. For instance:

• In copper sulfate pentahydrate, \(\text{CuSO}_4 \cdot 5\text{H}_2 \text{O} \), each formula unit of copper sulfate includes five molecules of water.

Water of crystallization contributes to the physical characteristics of hydrates. It can affect color, stability, and solubility. When such hydrates are exposed to air, they may lose their water of crystallization and transform into an anhydrous form, often resulting in a change of appearance and chemical properties. This process is key in understanding why some substances like hydrated copper sulfate appear blue when hydrated, but change to a light white color upon losing their water of crystallization.

Hydrated compounds

Hydrated compounds are chemical compounds that include water molecules integrated into their solid structure. This water is crucial because it stabilizes the lattice structure of the compound. These compounds showcase interesting behaviors among which are:

• Color change: For instance, when cobalt chloride is hydrated, it appears pink, and when it loses water and becomes anhydrous, it turns blue.
• Humidity and moisture indicators: Some hydrates are used as indicators for moisture in the air.

When hydrated compounds are exposed to air and lose their water content, they are said to undergo efflorescence. This behavior can be observed every day, such as the faint white powder seen on some surfaces, indicating the mineral's exposure and dehydration. However, it is vital to note that not all substances react the same way; for example, sodium hydroxide, \(\text{NaOH}\), does not form hydrates and thus behaves differently.

Chemical properties of NaOH

Sodium hydroxide (NaOH), also known as caustic soda or lye, is a highly caustic base and alkali that decomposes proteins at ordinary ambient temperatures and may cause severe chemical burns. NaOH has distinct chemical properties:

• High solubility: It is very soluble in water, releasing heat during dissolution because of its exothermic nature.
• Delinquency: Although NaOH doesn't form hydrates, it is hygroscopic and deliquescent, meaning it can absorb moisture from the air to the point of dissolving.
• Strong base: It ionizes completely in water, releasing hydroxide ions \(\text{OH}^-\) and thus significantly raising pH levels in solutions.

These features make NaOH distinct from substances that are efflorescent, such as certain hydrated salts, which lose water rather than absorb it. Unlike hydrated compounds, NaOH does not retain water of crystallization and thus does not undergo efflorescence, highlighting its unique chemical behavior and importance in various industrial and laboratory applications.