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Question

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

When white anhydrous copper(II) tetraoxosulfate(VI) powder is exposed to moist air, it turns blue by absorbing water vapor without forming a solution. Under the same conditions, solid sodium hydroxide pellets absorb moisture from the atmosphere until they completely dissolve to form a liquid solution. Which terms correctly classify the behavior of anhydrous copper(II) tetraoxosulfate(VI) and solid sodium hydroxide, respectively?

  1. Hygroscopic and deliquescentAnswer
  2. B
    Deliquescent and hygroscopic
  3. C
    Efflorescent and deliquescent
  4. D
    Hygroscopic and efflorescent

Answer

Anhydrous copper(II) tetraoxosulfate(VI) is hygroscopic, while solid sodium hydroxide is deliquescent.
Hygroscopic substances absorb moisture from the atmosphere without dissolving or forming a liquid solution, as seen with anhydrous copper(II) tetraoxosulfate(VI) turning blue (CuSO4+5H2O→CuSO4⋅5H2O\text{CuSO}_4 + 5\text{H}_2\text{O} \rightarrow \text{CuSO}_4\cdot 5\text{H}_2\text{O}). Deliquescent substances, such as solid sodium hydroxide (NaOH\text{NaOH}), absorb moisture from the atmosphere and dissolve in that absorbed water to form a saturated solution.

Step-by-Step Solution

1
Analyze the behavior of anhydrous copper(II) tetraoxosulfate(VI) in moist air.
It absorbs atmospheric water vapor to form hydrated copper(II) tetraoxosulfate(VI) (blue) without forming a liquid solution. Substances that absorb atmospheric moisture without dissolving are classified as hygroscopic.
Hygroscopy involves water absorption without phase change into a liquid solution.
2
Analyze the behavior of solid sodium hydroxide in moist air.
It absorbs sufficient atmospheric water to dissolve entirely and form a saturated solution. Substances that absorb water from the air and dissolve in it are classified as deliquescent.
Deliquescence occurs when a substance absorbs water until its vapor pressure matches atmospheric humidity, dissolving the solid.
3
Match the respective terms in order.
Hygroscopic and deliquescent.
The question asks for the behavior of anhydrous copper(II) tetraoxosulfate(VI) followed by sodium hydroxide.

Key Concept

Distinction between Hygroscopy, Deliquescence, and Efflorescence
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Topics

Water of Crystallization, Efflorescence, Deliquescence, and Hygroscopy