Which of the following substances is NOT hygroscopic?

Analysis

The question asks us to identify which of the listed substances is not hygroscopic. To answer this, we must first understand what "hygroscopic" means in chemistry.

Concept: Hygroscopic Substances

A hygroscopic substance is one that has the ability to attract and hold water molecules from the surrounding environment through either absorption or adsorption. When these substances are exposed to air, they absorb moisture, often becoming damp or even dissolving in the water they have absorbed (a process known as deliquescence).

• Common examples: Silica gel (used in shoe boxes to keep them dry), concentrated sulfuric acid, and common salt (sodium chloride) which often becomes "clumpy" in humid weather due to moisture absorption.
• Non-hygroscopic substances: These are materials that do not naturally attract or hold water from the atmosphere. They remain stable and dry under normal atmospheric conditions.

Step-by-Step Evaluation

1. Salt (Sodium Chloride): Common table salt is hygroscopic. If you leave it in an open container during the rainy season in Nigeria, it absorbs moisture from the air and becomes damp.
2. Sugar (Sucrose): Sugar is also hygroscopic. It attracts moisture from the air, which is why sugar cubes or granulated sugar can become sticky or clump together in humid conditions.
3. Silica gel: This is a classic example of a desiccant. It is highly hygroscopic and is specifically manufactured to absorb moisture to protect goods from humidity.
4. Aluminum: Aluminum is a metal. Metals like aluminum form a thin, protective oxide layer on their surface when exposed to air, but they do not absorb water molecules into their structure. Aluminum is not hygroscopic; it remains dry and does not change its physical state due to atmospheric humidity.

Conclusion

Based on the chemical properties of these substances, Aluminum is the only one that does not absorb moisture from the atmosphere.

Exam Tip: When you see questions about "hygroscopic" or "deliquescent" substances, think of things that get "wet" or "sticky" when left out in the open air (like salt or sugar). Metals like Aluminum or iron (unless rusting) generally do not behave this way.