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

Match each chemical substance with its characteristic atmospheric behavior or hydration property when exposed to ambient laboratory air or heat:

  • Na2CO3⋅10H2O\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O} (Washing soda crystals)Undergoes efflorescence by losing water of crystallization to dry atmosphere, forming a powdery monohydrate.
  • Anhydrous CaCl2\text{CaCl}_2 (Calcium chloride pellets)Undergoes deliquescence by absorbing atmospheric moisture until it dissolves to form an aqueous solution.
  • Concentrated H2SO4\text{H}_2\text{SO}_4 (Sulfuric acid)Exhibits hygroscopic behavior by absorbing moisture from air without forming a liquid solution.
  • CuSO4⋅5H2O\text{CuSO}_4 \cdot 5\text{H}_2\text{O} (Blue vitriol)Contains water of crystallization that is lost upon heating, causing a blue-to-white color change.

Answer

Washing soda crystals undergo efflorescence; anhydrous calcium chloride undergoes deliquescence; concentrated sulfuric acid is hygroscopic; copper(II) sulfate pentahydrate loses its water of crystallization upon heating.
Washing soda crystals (Na2CO3⋅10H2O\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}) undergo efflorescence when exposed to dry air. Anhydrous calcium chloride (CaCl2\text{CaCl}_2) absorbs moisture until it forms a solution, which is deliquescence. Concentrated sulfuric acid (H2SO4\text{H}_2\text{SO}_4) absorbs moisture without dissolving a solid matrix, illustrating hygroscopic behavior. Copper(II) sulfate pentahydrate (CuSO4⋅5H2O\text{CuSO}_4 \cdot 5\text{H}_2\text{O}) loses its water of crystallization thermally to yield a white anhydrous salt.

Step-by-Step Solution

1
Analyze the behavior of Na2CO3⋅10H2O\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}.
It loses 9 molecules of water of crystallization to dry air, forming a powdery monohydrate Na2CO3⋅H2O\text{Na}_2\text{CO}_3 \cdot \text{H}_2\text{O} (efflorescence).
Efflorescence occurs when the vapor pressure of the hydrated salt is greater than the partial pressure of water vapor in the atmosphere.
2
Analyze anhydrous CaCl2\text{CaCl}_2.
It absorbs moisture from air and eventually dissolves in the absorbed water to form a saturated solution (deliquescence).
Deliquescence occurs when a solid compound's saturated solution has a lower vapor pressure than the atmospheric water vapor pressure.
3
Analyze concentrated H2SO4\text{H}_2\text{SO}_4.
It absorbs moisture from the atmosphere without changing state to form a new dissolved phase, acting as a liquid hygroscopic substance.
Hygroscopic substances absorb water vapor from the surrounding environment without dissolving into a liquid solution from a solid state.
4
Analyze CuSO4⋅5H2O\text{CuSO}_4 \cdot 5\text{H}_2\text{O}.
Heating drives off the 5 water molecules of crystallization, converting blue crystalline pentahydrate to white anhydrous copper(II) sulfate.
Water of crystallization is chemically bound in the crystal lattice and can be expelled thermally, altering optical/crystalline properties.

Key Concept

Water of Crystallization, Deliquescence, Efflorescence, and Hygroscopy
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Topics

Water of Crystallization, Deliquescence, Efflorescence, and Hygroscopy

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