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Difficulty: EasyHydrogen: Preparation, Properties, Isotopes, and Water Hardness

Water hardness is classified as either temporary or permanent depending on the dissolved salts present. Which of the following chemical compounds causes temporary hardness in water that can be removed simply by boiling?

  1. Calcium hydrogentrioxocarbonate(IV)Answer
  2. B
    Calcium tetraoxosulfate(VI)
  3. C
    Magnesium chloride
  4. D
    Calcium chloride

Answer

Calcium hydrogentrioxocarbonate(IV)
The correct compound is Calcium hydrogentrioxocarbonate(IV). Temporary water hardness is uniquely caused by dissolved hydrogentrioxocarbonate(IV) salts of calcium or magnesium. Heating or boiling the water decomposes this soluble compound into insoluble calcium trioxocarbonate(IV) precipitate, water, and carbon(IV) oxide gas, thereby removing the calcium ions responsible for hardness.

Step-by-Step Solution

1
Identify the cause of temporary water hardness.
Temporary hardness is caused by dissolved hydrogentrioxocarbonate(IV) salts of calcium and magnesium, such as Ca(HCO3)2\text{Ca(HCO}_3)_2.
Hydrogentrioxocarbonate(IV) ions decompose thermally when heated.
2
Analyze the chemical effect of boiling on calcium hydrogentrioxocarbonate(IV).
Ca(HCO3)2(aq)→ΔCaCO3(s)+H2O(l)+CO2(g)\text{Ca(HCO}_3)_2\text{(aq)} \xrightarrow{\Delta} \text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}
Boiling converts the soluble hydrogentrioxocarbonate(IV) into insoluble calcium trioxocarbonate(IV), which precipitates out as fur or scale.

Key Concept

Temporary Water Hardness and Removal by Boiling
Estimated Time:45s
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Topics

Hydrogen: Preparation, Properties, Isotopes, and Water Hardness

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