The basic property of salts used as drying agents is by?

Analyze the question: The question asks which property lets certain salts work as drying agents (desiccants), meaning they remove moisture from substances or from the air.

Explain the concept:

• Efflorescence: a hydrated salt loses its water of crystallization to the air and turns into a powder (e.g. Na₂CO₃·10H₂O). The salt gives out water, so it cannot dry anything.
• Hygroscopy: a substance absorbs moisture from the atmosphere but does not necessarily dissolve (e.g. anhydrous CaCl₂, CaO, conc. H₂SO₄, silica gel). This is the principle behind drying agents.
• Deliquescence: a related property where a substance absorbs so much moisture that it dissolves in it (e.g. NaOH, anhydrous CaCl₂, MgCl₂).
• Melting point and solubility: neither of these has any direct link to absorbing water.

Step-by-step solution:

1. A drying agent must take water from its surroundings.
2. Efflorescence releases water, so A is ruled out.
3. High melting point (B) describes thermal stability, not water absorption.
4. Low solubility (D) has nothing to do with attracting water. Many drying agents are actually very soluble.
5. Hygroscopy (C) is the ability to absorb water vapour from the air, which is exactly what a drying agent does.

Answer: C. hygroscopy

Exam tip: Remember that efflorescence = emits water and hygroscopy = holds/absorbs water. If a question mentions drying agents or desiccators, think "hygroscopic."