Examkin

Question

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

A water sample contains both temporary hardness due to dissolved Ca(HCO3)2\text{Ca(HCO}_3)_2 and permanent hardness due to dissolved MgSO4\text{MgSO}_4. Which sequence correctly arranges the operational steps required to completely soften this water sample using slaked lime followed by washing soda?

  1. 1Add a stoichiometric amount of calcium hydroxide to react with calcium hydrogentrioxocarbonate(IV), forming insoluble calcium carbonate.
  2. 2Filter the mixture to remove the precipitated calcium carbonate solid.
  3. 3Add sodium trioxocarbonate(IV) solution to the filtrate to react with dissolved magnesium tetraoxosulfate(VI), forming insoluble magnesium carbonate.
  4. 4Perform a final filtration to remove the magnesium carbonate precipitate, yielding soft water.

Answer

The correct operational sequence is: first, adding slaked lime to precipitate calcium carbonate from temporary hardness; second, filtering off the solid precipitate; third, adding washing soda to precipitate magnesium carbonate from permanent hardness; and fourth, performing a final filtration to obtain clear, soft water.
The removal process follows a logically structured chemical workflow. Temporary hardness caused by Ca(HCO3)2\text{Ca(HCO}_3)_2 is treated first with a calculated amount of slaked lime (Clark's process), producing insoluble CaCO3\text{CaCO}_3, which is filtered out. The remaining filtrate contains permanent hardness from MgSO4\text{MgSO}_4, which is treated with washing soda (sodium trioxocarbonate(IV)) to precipitate MgCO3\text{MgCO}_3. A second filtration removes this precipitate, leaving pure soft water.

Step-by-Step Solution

1
Identify the chemical reaction for temporary hardness removal.
Adding Ca(OH)2\text{Ca(OH)}_2 precipitates temporary hardness: Ca(HCO3)2+Ca(OH)2→2CaCO3↓+2H2O\text{Ca(HCO}_3)_2 + \text{Ca(OH)}_2 \rightarrow 2\text{CaCO}_3\downarrow + 2\text{H}_2\text{O}.
Slaked lime specifically removes hydrogentrioxocarbonate(IV) salts causing temporary hardness.
2
Separate the solid calcium carbonate formed.
The solid CaCO3\text{CaCO}_3 is filtered out of the solution.
Precipitates must be removed so they do not redissolve or contaminate subsequent steps.
3
Identify the chemical reaction for permanent hardness removal.
Adding Na2CO3\text{Na}_2\text{CO}_3 precipitates permanent hardness: MgSO4+Na2CO3→MgCO3↓+Na2SO4\text{MgSO}_4 + \text{Na}_2\text{CO}_3 \rightarrow \text{MgCO}_3\downarrow + \text{Na}_2\text{SO}_4.
Soluble carbonate ions from washing soda precipitate divalent magnesium cations as insoluble trioxocarbonate(IV) salts.
4
Conduct final liquid-solid separation.
Filtering removes insoluble MgCO3\text{MgCO}_3, producing soft water.
Soluble sodium tetraoxosulfate(VI) remaining in solution does not react with soap to form scum.

Key Concept

Sequential chemical removal of temporary and permanent hardness using Clark's process followed by precipitation with washing soda.
Rate this question

Topics

Hardness of Water: Causes, Types, and Removal

More questions from this topic