Science: Air and Water: Properties of Water and Temporary Hardness (For CBSE, ICSE, IAS, NET, NRA 2022)

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Properties of Water

  • It has a unique property of dissolving a large number of substances starting from solids such as common salt, sugar, to gases like oxygen, carbon dioxide etc. it is called a universal solvent.
  • Water forms lather with soap which is used for cleaning purposes. It is called soft water. Sometimes water from some sources like rivers or hand pumps does not produce any lather with soap. It is called hard water.
  • The hardness of water is due to the presence of salts of magnesium and calcium in water.

Temporary Hardness

Temporary hardness of water is due to the presence of soluble bicarbonates of calcium and magnesium. It is also called carbonate hardness. It can be removed by boiling and by soda lime process.

By Boiling

Upon boiling hard water, calcium or magnesium bicarbonate present in it are decomposed to give magnesium or calcium carbonate. These carbonate salts are insoluble in water. They settle down easily and water can be decanted.

Ca (HCO3)2 CaCO3 + H2O + CO2

Mg (HCO3)2 MgCO3 + H2O + CO2

By Soda Lime Process (Clark՚s Method)

When a calculated amount of lime is added to hard water, then the soluble bicarbonates are converted to insoluble carbonates as follows:

Ca (HCO3)2 + Ca (OH)2 CaCO3 + 2H2O

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

Permanent Hardness

Permanent hardness of water is due to the presence of soluble chlorides and sulphates of calcium and magnesium. It is also known as non-carbonate hardness.

Various Sources of Water Pollution

Various sources of water pollution are:

  • Factories and industries which release various toxins, untreated effluents and heavy metals and industrial solvents into natural water bodies.
  • Agricultural farms releases fertilizers and pesticides which leads to eutrophication and biomagnification. (See lesson 30- Section 30.6 . 3 b and 30.6. 3c for details)
  • Mining exposes heavy metals and Sulphur which were buried deep in the earth into water bodies.
  • Sewage pipe and storm water drains release various pathogens, disinfectants and detergents
  • Air pollution releases pollutants like Sulphur dioxide, oxides of nitrogen etc. which are washed down by rains.
  • Food processing units and their waste includes fats and greases.

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