How decaying plants consume oxygen?


  1. No. iv. How decaying plants consume oxygen?

Answer. Decaying plants being bio-degradable consume O2 present in water for decomposition process. As a result, O2 depletes in water that causes death of aquatic life.

  1. No. v. What is function of fertilizers?

Answer. Fertilizers are used to make up the deficiency of nitrogen, phosphorus, etc., of the soil because of intensive cultivation of crops in the recent years. They provide nutrients for the growth of plants.

  1. No. vi. How pesticides cause water pollution?

Answer. Pesticides are chemicals that are used either directly to kill or control the growth of pests. They are used in agricultural fields. When they run-off from these fields enter into pounds, streams or rivers, the salts containing phosphate ions causes the rapid growth of algae. When algae dies, bacteria consumes oxygen of the water for decomposition of algae. As & result, oxygen, depletes in the water that affects other organisms living in the water. Thus, pesticides cause water pollution.

Test Yourself 15.3 (WATER POLLUTION)

  1. No. i. Which salts are responsible for hardness of water?

Answer. Salts of divalent cations like Mg2+, Ca2 and anions like Cl-, S042-, HCO3– and COs2; for example, gypsum (CaSO4.2H2O) and lime stone (CaCO3) are responsible for hardness of water. Gypsum is sparingly soluble in water, while limestone is insoluble in water. However, in the presence of carbon dioxide small quantity of limestone is soluble in water according to the chemical reaction.

CaCO3(s) + CO2(g) + H2O(1)            Ca(HCO3)2(aq)

MgCO3(s) + CO2(g) + H2O(1)           Mg(HCO3) 2(aq)

  1. No. ii. Explain the chemistry of removing the temporary hardness by boiling water.

Answer. Temporary hardness of water is easily removed by boiling water. On boiling calcium bicarbonate Ca (HCO3)2 decomposes to produce insoluble calcium carbonate, which precipitates out of the solution and water becomes soft.

Ca (HC03) 2 ———> CaCO3 + H2O + CO2

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