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ICSE Class 9 Chemistry Question 12 of 31

Water — Question 5

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Question

Question 5

Give equations to show what happens when temporary hard water is

(a) boiled

(b) treated with slaked lime

Answer

(a) By boiling carbon dioxide is driven off and the soluble hydrogen carbonates are converted into insoluble carbonates and could be removed by filtration or decantation.

Calcium carbonate and magnesium carbonate are precipitated leaving the water soft.

Ca(HCO3)2 →Boil \xrightarrow{\text{Boil}\space} CaCO3 ↓ + H2O + CO2 ↑

Mg(HCO3)2 →Boil \xrightarrow{\text{Boil}\space} MgCO3 ↓ + H2O + CO2 ↑

(b) Lime stone is first thoroughly mixed with water in a tank and then fed into another tank containing the hard water. Revolving paddles thoroughly mix the two solutions.

Most of calcium carbonate settles down. If there is any solid left over, it is removed by a filter. This process goes by the name 'Clark's process'.

Ca(HCO3)2 + Ca(OH)2 ⟶ 2CaCO3 ↓ + 2H2O

Mg(HCO3)2 + Ca(OH)2 ⟶ MgCO3 ↓ + CaCO3 ↓ + 2H2O