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Chemistry - Water (Hard and Soft Water, Water Pollution)

Grade 8ICSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Hard Water is water that does not readily form a lather with soap because it contains dissolved salts of calcium and magnesium, such as chlorides, sulfates, and bicarbonates.

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Soft Water is water that easily forms a rich lather with soap, such as distilled water or rainwater, as it is free from dissolved calcium and magnesium salts.

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Temporary Hardness is caused by the presence of bicarbonates of calcium and magnesium: Ca(HCO3)2Ca(HCO_3)_2 and Mg(HCO3)2Mg(HCO_3)_2. It can be removed by simple boiling or by adding lime (Clark's process).

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Permanent Hardness is caused by the presence of chlorides and sulfates of calcium and magnesium, such as CaCl2CaCl_2, MgCl2MgCl_2, CaSO4CaSO_4, and MgSO4MgSO_4. It cannot be removed by boiling.

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Removal of Permanent Hardness requires the addition of washing soda (Na2CO3Na_2CO_3), which converts soluble calcium/magnesium salts into insoluble carbonates.

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Soap reacts with hard water to form an insoluble precipitate called 'scum' (calcium or magnesium stearate), which wastes soap and sticks to fabrics: 2C17H35COONa+MgSO4→(C17H35COO)2Mg↓+Na2SO42C_{17}H_{35}COONa + MgSO_4 \rightarrow (C_{17}H_{35}COO)_2Mg \downarrow + Na_2SO_4.

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Water Pollution is the contamination of water bodies by harmful substances like domestic sewage, industrial waste (PbPb, HgHg), and agricultural runoff (pesticides and fertilizers).

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Eutrophication is the process where excessive nutrients (nitrates and phosphates) in water lead to the overgrowth of algae (algal bloom), depleting dissolved oxygen and harming aquatic life.

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Potable water is water that is safe for human consumption, free from pathogens and harmful chemicals.

📐Formulae

Ca(HCO3)2→BoilingCaCO3↓+H2O+CO2↑Ca(HCO_3)_2 \xrightarrow{\text{Boiling}} CaCO_3 \downarrow + H_2O + CO_2 \uparrow

Mg(HCO3)2+Ca(OH)2→CaCO3↓+MgCO3↓+2H2OMg(HCO_3)_2 + Ca(OH)_2 \rightarrow CaCO_3 \downarrow + MgCO_3 \downarrow + 2H_2O

CaCl2+Na2CO3→CaCO3↓+2NaClCaCl_2 + Na_2CO_3 \rightarrow CaCO_3 \downarrow + 2NaCl

MgSO4+Na2CO3→MgCO3↓+Na2SO4MgSO_4 + Na_2CO_3 \rightarrow MgCO_3 \downarrow + Na_2SO_4

2C17H35COONa+Ca2+→(C17H35COO)2Ca↓+2Na+2C_{17}H_{35}COONa + Ca^{2+} \rightarrow (C_{17}H_{35}COO)_2Ca \downarrow + 2Na^+

💡Examples

Problem 1:

A sample of water contains Mg(HCO3)2Mg(HCO_3)_2. Explain with a chemical equation how the hardness can be removed by boiling.

Solution:

Mg(HCO3)2→ΔMgCO3↓+H2O+CO2↑Mg(HCO_3)_2 \xrightarrow{\Delta} MgCO_3 \downarrow + H_2O + CO_2 \uparrow

Explanation:

When water containing Magnesium Bicarbonate (temporary hardness) is boiled, it decomposes into insoluble Magnesium Carbonate (MgCO3MgCO_3), which precipitates out. The water left behind becomes soft.

Problem 2:

Why is hard water not suitable for use in industrial boilers?

Solution:

It leads to the formation of 'scales' (CaCO3CaCO_3 and CaSO4CaSO_4 deposits) inside the boiler tubes.

Explanation:

The salts in hard water precipitate upon heating, forming a thick, heat-insulating layer called scale. This wastes fuel, reduces efficiency, and can cause the boiler to overheat and explode.

Problem 3:

What happens when washing soda is added to water containing CaCl2CaCl_2?

Solution:

CaCl2+Na2CO3→CaCO3↓+2NaClCaCl_2 + Na_2CO_3 \rightarrow CaCO_3 \downarrow + 2NaCl

Explanation:

Washing soda (Na2CO3Na_2CO_3) reacts with the soluble Calcium Chloride to form insoluble Calcium Carbonate (CaCO3CaCO_3) as a precipitate. Since the Ca2+Ca^{2+} ions are removed from the solution, the water becomes soft.