(a)(i) Outline the steps involved in the purification of water for town supply.
(ii) Give two cations that can cause hardness in water.
(iii) State two disadvantages of hardness of water.
(iv) List two sources of water pollution.
(b)(i) Explain briefly why water is a good solvent for sodium chloride but not for oil.
(ii) State the function of H\(_2\)SO\(_4\) in each of the following reaction equations:
(i) C\(_2\)H\(_5\)OH\(_{(l)}\) \(\to\) C\(_2\)H\(_{4(g)}\)
(ii) MgO\(_{(s)}\) + H\(_2\)SO\(_4\)\(_{(aq)}\) —> MgSO\(_{4(aq)}\) + H\(_2\)O\(_{(l)}\)
(iii) C\(_{(s)}\) + conc. H\(_2\)SO\(_4\) --> CO\(_{(g)}\) + SO\(_{2(g)}\) + H\(_2\)O\(_{(l)}\)
(c) (i) Give the two products formed when chlorine water is exposed to sunlight.
(ii) I. Which of the compounds is suitable for the preparation of standard alkaline solution?
II. Give a reason for your answer in (c)(ii)I
(a)(i) Purification of water for town supply
- Screening to remove large floating debris.
- Sedimentation/coagulation: a coagulant such as alum is added to make fine suspended particles clump and settle.
- Filtration through beds of sand and gravel to remove remaining suspended matter.
- Chlorination (sterilization) to kill harmful micro-organisms.
- Storage in covered reservoirs and distribution.
(ii) Cations causing hardness: Ca2+ and Mg2+.
(iii) Disadvantages of hard water: it wastes soap (forms scum); it deposits scale/fur in kettles, boilers and pipes.
(iv) Sources of water pollution: industrial effluents; sewage/domestic waste (also agricultural run-off, oil spillage).
(b)(i) Water is a polar solvent. Sodium chloride is ionic, so its Na+ and Cl- ions are strongly attracted to and become surrounded (solvated) by the polar water molecules, and it dissolves. Oil is non-polar (covalent) and has no charged centres for water to attract, so water does not dissolve it (like dissolves like).
(ii) Function of H2SO4
- C2H5OH → C2H4: dehydrating agent (removes water).
- MgO + H2SO4 → MgSO4 + H2O: acid (undergoes neutralization).
- C + conc. H2SO4 → CO + SO2 + H2O: oxidizing agent.
(c)(i) When chlorine water stands in sunlight the hypochlorous acid decomposes and the two products are hydrochloric acid (HCl) and oxygen (O2):
2Cl2 + 2H2O → 4HCl + O2
(ii) Neither of these two products (an acid and oxygen) gives an alkaline solution, so neither is suitable for preparing a standard alkaline solution; both leave an acidic/neutral solution. A primary standard alkali (e.g. anhydrous Na2CO3) would instead be needed, which is not among these products.
(a)(i) Purification of water for town supply
- Screening to remove large floating debris.
- Sedimentation/coagulation: a coagulant such as alum is added to make fine suspended particles clump and settle.
- Filtration through beds of sand and gravel to remove remaining suspended matter.
- Chlorination (sterilization) to kill harmful micro-organisms.
- Storage in covered reservoirs and distribution.
(ii) Cations causing hardness: Ca2+ and Mg2+.
(iii) Disadvantages of hard water: it wastes soap (forms scum); it deposits scale/fur in kettles, boilers and pipes.
(iv) Sources of water pollution: industrial effluents; sewage/domestic waste (also agricultural run-off, oil spillage).
(b)(i) Water is a polar solvent. Sodium chloride is ionic, so its Na+ and Cl- ions are strongly attracted to and become surrounded (solvated) by the polar water molecules, and it dissolves. Oil is non-polar (covalent) and has no charged centres for water to attract, so water does not dissolve it (like dissolves like).
(ii) Function of H2SO4
- C2H5OH → C2H4: dehydrating agent (removes water).
- MgO + H2SO4 → MgSO4 + H2O: acid (undergoes neutralization).
- C + conc. H2SO4 → CO + SO2 + H2O: oxidizing agent.
(c)(i) When chlorine water stands in sunlight the hypochlorous acid decomposes and the two products are hydrochloric acid (HCl) and oxygen (O2):
2Cl2 + 2H2O → 4HCl + O2
(ii) Neither of these two products (an acid and oxygen) gives an alkaline solution, so neither is suitable for preparing a standard alkaline solution; both leave an acidic/neutral solution. A primary standard alkali (e.g. anhydrous Na2CO3) would instead be needed, which is not among these products.
