Match each chemical compound or process related to water hardness with its correct descrip… — JAMB UTME

Match each chemical compound or process related to water hardness with its correct description or function.

Calcium hydrogencarbonate ( $\text{Ca(HCO}_3\text{)}_2$ )Causes temporary hardness in water
Calcium tetraoxosulfate(VI) ( $\text{CaSO}_4$ )Causes permanent hardness in water
BoilingRemoves temporary hardness only
Addition of washing soda ( $\text{Na}_2\text{CO}_3$ )Removes both temporary and permanent hardness

Answer

Calcium hydrogencarbonate matches with causes temporary hardness; Calcium tetraoxosulfate(VI) matches with causes permanent hardness; Boiling matches with removes temporary hardness only; Addition of washing soda matches with removes both temporary and permanent hardness.

Calcium hydrogencarbonate is soluble and decomposes upon heating to form insoluble calcium carbonate, causing temporary hardness. Calcium tetraoxosulfate(VI) stays dissolved when boiled, causing permanent hardness. Boiling specifically precipitates temporary hardness salts. Washing soda contains carbonate ions which precipitate calcium ions from both hydrogencarbonates and sulfates, removing both temporary and permanent hardness.

Step-by-Step Solution

Identify the cause of temporary hardness

Calcium hydrogencarbonate (

\text{Ca(HCO}_3\text{)}_2

) causes temporary hardness because it thermally decomposes when heated.

Temporary hardness is due to soluble hydrogen trioxocarbonates of calcium and magnesium.

Identify the cause of permanent hardness

Calcium tetraoxosulfate(VI) (

\text{CaSO}_4

) causes permanent hardness because boiling does not precipitate it.

Permanent hardness is due to soluble sulfates and chlorides of calcium and magnesium.

Determine the effect of boiling

Boiling removes temporary hardness by converting dissolved hydrogencarbonates into insoluble calcium carbonate precipitate.

Thermal decomposition:

\text{Ca(HCO}_3\text{)}_2\text{(aq)} \rightarrow \text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}

Determine the effect of adding washing soda

Washing soda (

\text{Na}_2\text{CO}_3

) removes both temporary and permanent hardness.

Carbonate ions precipitate

\text{Ca}^{2+}

ions as

\text{CaCO}_3\text{(s)}

regardless of whether they originated from hydrogencarbonate or sulfate salts.

Key Concept

Causes, types, and chemical methods for removal of water hardness

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