A salt which loses its mass when exposed to air is

Analyze the question: The question asks for the term describing a salt that loses mass when left in open air. Losing mass means the salt gives off something, usually water of crystallization.

Explain the concept: Here are the key terms.

• Deliquescent: absorbs so much moisture from the air that it dissolves in it and becomes a solution (mass increases). Examples are NaOH and CaCl₂.
• Hygroscopic: absorbs moisture from the air without necessarily dissolving (mass increases). Examples are concentrated H₂SO₄ and anhydrous CaCl₂.
• Efflorescent: loses water of crystallization to the air and crumbles into powder (mass decreases). Examples are Na₂CO₃·10H₂O (washing soda) and Na₂SO₄·10H₂O (Glauber's salt).
• Effervescent: gives off bubbles of gas, as in fizzing. It has nothing to do with water in the air.
• Fluorescent: emits light when exposed to radiation. It is not related to this question.

Step-by-step solution:

1. The salt loses mass, so it must be giving something off to the air.
2. The only common process in which a salt does this is loss of water of crystallization.
3. For example: Na₂CO₃·10H₂O → Na₂CO₃·H₂O + 9H₂O (the water escapes, so the mass decreases).
4. This process is called efflorescence.

Deliquescent and hygroscopic substances gain mass, so A and E are out. B and D do not describe mass change due to exposure to air.

Answer: C. Efflorescent

Exam tip: Remember it this way: Efflorescent = Exits water (loses it), while Deliquescent = Dissolves in absorbed water. Hygroscopic and deliquescent substances gain mass, and efflorescent ones lose mass.