Chemistry notes hardness of water


Methods of softening water



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Methods of softening water
Removal of temporary hardness from water
i)
Boiling (physical method)
Boiling decomposes calcium and magnesium hydrogen carbonates to their respective carbonates, water and carbon dioxide gas. The calcium and magnesium ions are removed as from water as insoluble carbonates and then deposited as solids at the bottom of boilers.
Ca(HCO
3
)
2(aq)
CaCO
3(s)
CO
2(g)
H
2
O
(l)
ii)
Addition of calcium hydroxide (lime water) Calcium hydroxide reacts with the soluble magnesium or calcium hydrogen carbonates to form insoluble magnesium or calcium carbonates.
Ca(HCO
3
)
2
(aq) + Ca(OH)
2
(aq) 2CaCO
3
(s) + 2H
2
O(l)
iii)
Adding sodium carbonate (washing soda the calcium or magnesium ions are removed
πΆπ‘Ž(𝐻𝐢𝑂
3
)
2
(π‘Žπ‘ž) + π‘π‘Ž
2
𝐢𝑂
3
(π‘Žπ‘ž) β†’ 2π‘π‘Žπ»πΆπ‘‚
3
(π‘Žπ‘ž) + πΆπ‘ŽπΆπ‘‚
3
(𝑠)
iv)
Addition of aqueous ammonia The soluble hydrogen carbonates are converted into insoluble carbonates. This enables magnesium and calcium ions to be removed from wate. i.e;
2NH
4
OH
(aq)
Mg(HCO
3
)
2(aq)
MgCO
3(s)
(NH
4
)
2
CO
3(aq)
H
2
O
(l)

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