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

When exposed to moist atmospheric air, certain solid compounds absorb sufficient water vapor to completely dissolve in it and form a liquid solution. Which of the following substances exhibits this phenomenon known as deliquescence?

  1. Calcium chloride (CaCl2\text{CaCl}_2)Answer
  2. B
    Concentrated tetraoxosulfate(VI) acid (H2SO4\text{H}_2\text{SO}_4)
  3. C
    Sodium trioxocarbonate(IV) decahydrate (Na2CO3⋅10H2O\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O})
  4. D
    Copper(II) oxide (CuO\text{CuO})

Answer

Calcium chloride (CaCl2\text{CaCl}_2) is deliquescent because it absorbs atmospheric water vapor to form a liquid solution.
Calcium chloride (CaCl2\text{CaCl}_2) is a deliquescent solid. When exposed to air with sufficient humidity, its surface vapor pressure is much lower than the partial pressure of water vapor in the atmosphere, causing it to absorb water continuously until it fully dissolves into an aqueous solution.

Step-by-Step Solution

1
Define deliquescence
Deliquescence is the process in which a solid substance absorbs moisture from the atmosphere until it completely dissolves in the absorbed water, forming a concentrated solution.
Understanding the precise chemical definition distinguishes deliquescent solids from hygroscopic liquids or efflorescent hydrated salts.
2
Evaluate the options based on their behavior in moist air
Solid calcium chloride (CaCl2\text{CaCl}_2) absorbs water vapor and turns into a liquid solution, making it deliquescent.
Calcium chloride has a very low saturated solution vapor pressure compared to atmospheric water vapor pressure.

Key Concept

Atmospheric Phenomena of Salts: Deliquescence vs Hygroscopy vs Efflorescence
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Topics

Water of Crystallization, Deliquescence, Efflorescence, and Hygroscopy

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