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

Match each chemical species or substance involved in water chemistry with its primary role or behavior regarding water hardness.

  • Magnesium hydrogencarbonate, Mg(HCO3)2\text{Mg(HCO}_3)_2Causes temporary hardness and decomposes thermally on boiling to form an insoluble precipitate
  • Magnesium tetraoxosulfate(VI), MgSO4\text{MgSO}_4Causes permanent hardness and remains soluble upon boiling
  • Sodium trioxocarbonate(IV), Na2CO3\text{Na}_2\text{CO}_3Removes both temporary and permanent hardness by precipitating calcium and magnesium ions as carbonates
  • Sodium permutit / Zeolite, Na2Z\text{Na}_2\text{Z}Removes both temporary and permanent hardness by replacing calcium and magnesium ions with sodium ions

Answer

Magnesium hydrogencarbonate causes temporary hardness that decomposes on boiling; magnesium tetraoxosulfate(VI) causes permanent hardness unaffected by boiling; sodium trioxocarbonate(IV) removes all hardness via chemical precipitation; sodium permutit removes all hardness via ion exchange.
Magnesium hydrogencarbonate is a soluble hydrogencarbonate salt causing temporary hardness removed by boiling. Magnesium tetraoxosulfate(VI) is a soluble sulfate salt causing permanent hardness that cannot be removed by boiling. Sodium trioxocarbonate(IV) removes all hardness types by precipitating calcium/magnesium ions as insoluble carbonates. Permutit softened water by exchanging hardness cations for soluble sodium ions.

Step-by-Step Solution

1
Identify the cause of temporary hardness.
Magnesium hydrogencarbonate, Mg(HCO3)2\text{Mg(HCO}_3)_2, dissolves in water to cause temporary hardness. Upon boiling, hydrogencarbonate ions decompose to insoluble carbonate precipitates: Mg(HCO3)2(aq)→MgCO3(s)+H2O(l)+CO2(g)\text{Mg(HCO}_3)_2(aq) \rightarrow \text{MgCO}_3(s) + \text{H}_2\text{O}(l) + \text{CO}_2(g).
Temporary hardness is specifically caused by hydrogen carbonate salts of calcium and magnesium, which are thermally unstable.
2
Identify the cause of permanent hardness.
Magnesium tetraoxosulfate(VI), MgSO4\text{MgSO}_4, causes permanent hardness because sulfate salts do not undergo thermal decomposition upon boiling.
Permanent hardness is caused by soluble tetraoxosulfate(VI) or chloride salts of calcium and magnesium.
3
Analyze the action of washing soda (Na2CO3\text{Na}_2\text{CO}_3).
Adding washing soda introduces CO32−\text{CO}_3^{2-} ions, which combine with dissolved Mg2+\text{Mg}^{2+} or Ca2+\text{Ca}^{2+} ions to precipitate them as MgCO3\text{MgCO}_3 or CaCO3\text{CaCO}_3.
Precipitation of divalent metallic cations as insoluble carbonates removes both temporary and permanent hardness.
4
Analyze the action of sodium permutit (zeolite).
Permutit acts as an ion exchanger: Na2Z(s)+Mg2+(aq)→MgZ(s)+2Na+(aq)\text{Na}_2\text{Z}(s) + \text{Mg}^{2+}(aq) \rightarrow \text{MgZ}(s) + 2\text{Na}^+(aq).
Hardness-causing divalent cations are bound to the zeolite matrix while harmless sodium ions are released into solution.

Key Concept

Classification of water hardness causes (hydrogencarbonate vs sulfate salts) and chemical removal mechanisms (boiling, precipitation, ion exchange).
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Topics

Hardness of Water: Causes, Types, and Removal

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