Moisture Masters
Distinguish efflorescence, deliquescence, and hygroscopy
What this lesson covers
Why it matters
Leave washing-soda crystals out and they crumble to powder. Leave caustic soda out and it turns into a puddle. Different substances treat the moisture in air in completely different ways.
Predict first
Washing-soda crystals left in dry air slowly turn to powder. Why?
Efflorescent solids LOSE water of crystallisation to the air (washing soda). Deliquescent solids ABSORB so much water they dissolve into a solution (NaOH, CaCl₂). Hygroscopic substances absorb water from the air but do NOT dissolve into a solution — they may be solid (silica gel, quicklime) or liquid (concentrated sulphuric acid).
- They lose their water of crystallisation to the air — efflorescence — correct
- They absorb water from the air and swell
- They react with oxygen in the air
What you do
Sort each substance by how it behaves in air: efflorescent, deliquescent, or hygroscopic.
Check yourself
A deliquescent substance differs from a hygroscopic one because it…
Both absorb moisture, but a deliquescent solid takes up enough to turn into a solution; a hygroscopic one stays solid.
Which substance is efflorescent?
Washing soda loses its water of crystallisation to the air, crumbling to powder.
- absorbs so much water that it dissolves into a solution — correct
- loses its own water to the air
- does not interact with water at all
- Washing soda (Na₂CO₃·10H₂O) — correct
- Sodium hydroxide
- Silica gel