Examkin

Question

Difficulty: MediumHardness of Water: Causes, Types, and Removal

A sample of well water forms a white precipitate when boiled and requires a large amount of soap to produce lather. Which soluble compound present in the water is responsible for this temporary hardness?

  1. Ca(HCO3)2\text{Ca(HCO}_3)_2Answer
  2. B
    CaSO4\text{CaSO}_4
  3. C
    MgCl2\text{MgCl}_2
  4. D
    Na2SO4\text{Na}_2\text{SO}_4

Answer

Calcium hydrogentrioxocarbonate(IV), Ca(HCO3)2\text{Ca(HCO}_3)_2, is responsible for temporary hardness.
Temporary water hardness is caused by dissolved hydrogentrioxocarbonate(IV) salts such as calcium hydrogentrioxocarbonate(IV), Ca(HCO3)2\text{Ca(HCO}_3)_2. Heating the water decomposes this soluble compound into insoluble calcium trioxocarbonate(IV), carbon dioxide gas, and water, thereby removing the calcium ions responsible for hardness.

Step-by-Step Solution

1
Identify the type of water hardness based on the behavior upon heating.
Because boiling removes the hardness by forming a precipitate, the water sample exhibits temporary hardness.
Temporary hardness is caused by hydrogentrioxocarbonate(IV) salts of calcium or magnesium, which undergo thermal decomposition.
2
Determine the chemical formula and reaction involved.
Ca(HCO3)2(aq)→ΔCaCO3(s)+H2O(l)+CO2(g)\text{Ca(HCO}_3)_2(aq) \xrightarrow{\Delta} \text{CaCO}_3(s) + \text{H}_2\text{O}(l) + \text{CO}_2(g)
Soluble calcium hydrogentrioxocarbonate(IV) breaks down on heating to yield insoluble calcium trioxocarbonate(IV), effectively precipitating out calcium ions.

Key Concept

Temporary hardness in water is caused by dissolved calcium and magnesium hydrogentrioxocarbonates(IV) and can be removed by boiling.
Rate this question

Topics

Hardness of Water: Causes, Types, and Removal

More questions from this topic