A substance is said to be hygroscopic when it

Analyze the question: It asks for the definition of a hygroscopic substance, which is about how a substance behaves toward moisture in the air.

Concept: Three terms are easy to mix up:

• Hygroscopic: absorbs moisture from the atmosphere, but does not necessarily dissolve in it. Examples: concentrated H₂SO₄, quicklime (CaO), anhydrous CaCl₂ (in limited humidity), silica gel.
• Deliquescent: absorbs so much atmospheric moisture that it dissolves and forms a solution. Examples: NaOH, KOH, anhydrous CaCl₂, FeCl₃, MgCl₂.
• Efflorescent: loses its water of crystallization to the air. Examples: Na₂CO₃·10H₂O, Na₂SO₄·10H₂O.

Working through the options:

• A. Absorbs moisture and goes into solution. This is the definition of deliquescent, so it is not the best fit.
• B. Absorbs moisture but does not go into solution. This is the standard textbook definition of hygroscopic.
• C. Changes colour. This is not the definition of any of these terms.
• D. Not affected by moisture. This is the opposite of hygroscopic.
• E. Loses water of crystallization. This is efflorescence.

Note: In a broad sense, every deliquescent substance is also hygroscopic, since it absorbs moisture. The distinction NECO tests, however, is that "hygroscopic" means absorbing moisture without dissolving, while "deliquescent" means absorbing it and dissolving.

Tip: Link the words to what happens:

• Deliquescent = Dissolves
• Eflorescent = Exits (water leaves)
• Hygroscopic = Holds moisture (absorbs only)