Which of the following common substances is correctly classified as a deliquescent compound that absorbs moisture from the air until it forms an aqueous solution?

Introduction / Context:
Deliquescent substances are important in both laboratory practice and industry because they can absorb moisture from the atmosphere and eventually dissolve in the absorbed water to form a solution. This question asks you to identify which substance among the options behaves in this characteristic way. Understanding deliquescence helps in storage of chemicals and in designing drying agents or humidity control systems.

Given Data / Assumptions:

• We are asked for a deliquescent compound, not merely a hygroscopic or drying agent.
• Deliquescent substances absorb so much moisture that they eventually form a liquid solution.
• The options include several well known laboratory chemicals.
• Normal atmospheric humidity is assumed.

Concept / Approach:
Deliquescent compounds are usually ionic solids with strong affinity for water. When exposed to moist air, they absorb water vapour continuously until they dissolve in the absorbed water and form a concentrated solution. Magnesium chloride and certain other salts such as calcium chloride show this behaviour. Some substances, like phosphorous pentoxide, are strongly hygroscopic and can remove water but do not necessarily form a stable solution in the same way. Quick lime reacts chemically with water, while common salt is not significantly deliquescent under ordinary conditions.

Step-by-Step Solution:

Verification / Alternative check:
An alternative check is to consult typical lists of deliquescent substances from school level chemistry, which commonly mention magnesium chloride, calcium chloride, and zinc chloride as examples. These salts are troublesome to store because they liquefy in moist air. Quick lime and phosphorous pentoxide are usually listed as hygroscopic or drying agents rather than deliquescent salts. This supports the choice of magnesium chloride as the correct answer.

Why Other Options Are Wrong:

• Quick lime: It reacts with water to form calcium hydroxide; this is slaking, not simple deliquescence.
• Sulphuric acid: It is a liquid, strongly hygroscopic, but the question focuses on solid compounds that become solutions.
• Phosphorous pentoxide: Powerful drying agent, reacts with water to form acids, generally classified as hygroscopic rather than deliquescent in school chemistry.
• Sodium chloride: Ordinary table salt does not significantly absorb moisture to dissolve under normal conditions.

Common Pitfalls:
Learners often confuse hygroscopic substances, drying agents, and deliquescent compounds. Not every substance that absorbs water is deliquescent. The key feature of deliquescence is the formation of a liquid solution from a solid because of absorbed moisture. Another pitfall is to assume that all chlorides or all drying agents behave the same way. It is important to remember specific standard examples like magnesium chloride and calcium chloride.

Final Answer:
The correctly identified deliquescent compound is Magnesium chloride.