Which of the following calcium compounds is commonly used as a drying agent to remove moisture?

Introduction / Context:
Drying agents are substances used in chemistry laboratories and industrial processes to remove moisture from gases and liquids. Among simple inorganic compounds, certain calcium salts are known for their hygroscopic or drying properties, while others are not. This question tests your ability to recognise which calcium compound is widely used as a drying agent for gases and in some cases for organic solvents.

Given Data / Assumptions:

• The focus is on calcium based compounds.
• Options include calcium oxide, calcium carbide, gypsum, calcium carbonate, and calcium hydroxide.
• We assume typical laboratory usage and textbook descriptions of drying agents.
• The agent should effectively absorb water and help remove moisture.

Concept / Approach:
Calcium oxide, often called quicklime, reacts readily with water to form calcium hydroxide. Because of this strong affinity for water, it is used as a drying agent for certain gases like ammonia. Gypsum is calcium sulfate dihydrate and already contains water of crystallisation, so it is not effective as a drying agent. Calcium carbonate and calcium hydroxide are not as efficient in absorbing moisture. Calcium carbide is mainly used to generate acetylene gas and is not a standard drying agent. Therefore, calcium oxide is the correct choice.

Step-by-Step Solution:
Step 1: Recall that calcium oxide (quicklime) is produced by heating limestone, driving off carbon dioxide from calcium carbonate. Step 2: Calcium oxide has a strong tendency to react with water, forming calcium hydroxide and releasing heat. Step 3: Because of this reactivity towards water, calcium oxide can absorb moisture from gases and is used as a drying agent in suitable situations. Step 4: Gypsum, being calcium sulfate dihydrate, already contains water and is used in plaster, not as a moisture remover. Step 5: Calcium carbonate is stable and does not strongly absorb water, while calcium hydroxide is used for making limewater and in neutralisation, not primarily as a drying agent. Step 6: Calcium carbide reacts with water to produce acetylene gas, which is a source of fuel, not a safe or standard drying agent. Hence, calcium oxide is the correct answer.

Verification / Alternative check:
Laboratory manuals often list quicklime (calcium oxide), phosphorus pentoxide, and concentrated sulfuric acid as common drying agents for gases, each with suitable ranges of application. Demonstrations may show how quicklime warms up when water is added, illustrating its strong reaction with moisture. In contrast, gypsum is used for plaster of Paris and construction materials, not in drying tubes. These practical references confirm that calcium oxide is the best known calcium compound used specifically as a drying agent.

Why Other Options Are Wrong:
Calcium carbide: Used to generate acetylene gas by reaction with water; it is not designed to dry gases and can be dangerous when in contact with moisture.
Gypsum: Contains water in its crystal structure and is used in building materials; it does not act as an efficient drying agent.
Calcium carbonate: A stable salt found in limestone and chalk; it does not strongly absorb water and is not commonly used to dry gases.
Calcium hydroxide: Used in limewater, whitewash, and neutralisation reactions; it is not the preferred drying agent compared to quicklime.

Common Pitfalls:
Students sometimes choose gypsum because they associate it with plaster and construction, where drying or setting occurs, but this setting is due to hydration and crystallisation, not drying in the chemical sense. Others may pick calcium carbonate simply because it is a familiar calcium compound. To avoid confusion, remember that quicklime rapidly reacts with water and is specifically used to remove moisture from certain gases, making calcium oxide the correct drying agent in this context.

Final Answer:
Calcium oxide is the calcium compound commonly used as a drying agent.