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

A sample of well water contains dissolved calcium hydrogentricarbonate(IV), Ca(HCO3)2\text{Ca(HCO}_3)_2, and magnesium tetraoxosulfate(VI), MgSO4\text{MgSO}_4. After boiling the water sample and filtering off the precipitate formed, which type of hardness is removed, and which salt remains in the filtrate causing persistent hardness?

  1. Temporary hardness is removed; magnesium tetraoxosulfate(VI) remains in the filtrate.Answer
  2. B
    Permanent hardness is removed; calcium hydrogentricarbonate(IV) remains in the filtrate.
  3. C
    Temporary hardness is removed; calcium hydrogentricarbonate(IV) remains in the filtrate.
  4. D
    Permanent hardness is removed; magnesium tetraoxosulfate(VI) remains in the filtrate.

Answer

Temporary hardness is removed, and magnesium tetraoxosulfate(VI) remains in the filtrate.
Boiling thermal decomposition converts soluble calcium hydrogentricarbonate(IV) into insoluble calcium trioxocarbonate(IV) precipitate, thereby removing temporary hardness. Soluble magnesium tetraoxosulfate(VI) remains dissolved in the filtrate, responsible for the remaining permanent hardness.

Step-by-Step Solution

1
Identify the nature of the dissolved salts present in the water sample.
Ca(HCO3)2\text{Ca(HCO}_3)_2 causes temporary hardness, while MgSO4\text{MgSO}_4 causes permanent hardness.
Hydrogentricarbonate salts of calcium and magnesium cause temporary hardness, whereas soluble sulfates and chlorides cause permanent hardness.
2
Analyze the chemical effect of boiling on the water sample.
Ca(HCO3)2→ΔCaCO3(s)+H2O(l)+CO2(g)\text{Ca(HCO}_3)_2 \xrightarrow{\Delta} \text{CaCO}_3(s) + \text{H}_2\text{O}(l) + \text{CO}_2(g)
Heat decomposes soluble calcium hydrogentricarbonate(IV) into insoluble calcium trioxocarbonate(IV), which precipitates out, removing temporary hardness.
3
Determine the species remaining in solution after filtration.
Magnesium tetraoxosulfate(VI), MgSO4\text{MgSO}_4, remains unchanged in solution as the filtrate.
Magnesium tetraoxosulfate(VI) is thermally stable under boiling conditions and cannot be precipitated by heating alone, retaining permanent hardness.

Key Concept

Temporary vs. Permanent Water Hardness Removal
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Topics

Hydrogen: Preparation, Properties, Isotopes, and Water Hardness

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