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Difficulty: EasyWater of Crystallization, Deliquescence, Efflorescence, and Hygroscopy

When transparent crystals of washing soda, Na2CO3⋅10H2O\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}, are left exposed to dry laboratory air, they gradually lose water of crystallization to become a white powdery monohydrate, Na2CO3⋅H2O\text{Na}_2\text{CO}_3 \cdot \text{H}_2\text{O}. Which phenomenon is demonstrated by this observation?

  1. A
    Deliquescence
  2. EfflorescenceAnswer
  3. C
    Hygroscopy
  4. D
    Hydrolysis

Answer

Efflorescence is demonstrated when washing soda crystals lose water of crystallization upon exposure to dry air.
Efflorescence is the phenomenon where a hydrated compound loses its water of crystallization spontaneously when exposed to air because its hydration vapor pressure exceeds atmospheric moisture vapor pressure.

Step-by-Step Solution

1
Analyze the observed physical change.
Hydrated washing soda crystals (decahydrate) lose 9 molecules of water of crystallization to form a monohydrate powder in dry air.
The vapor pressure of water in the hydrated crystal is greater than the partial pressure of water vapor in the surrounding air.
2
Match the observed behavior with the appropriate chemical term.
The spontaneous loss of water of crystallization to the atmosphere is defined as efflorescence.
Efflorescent salts lose moisture to dry air until an equilibrium is reached.

Key Concept

Efflorescence in hydrated salts
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Topics

Water of Crystallization, Deliquescence, Efflorescence, and Hygroscopy

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