5 WATER. (iii) Temporary hard water (iv) Permanent hard water

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1 5 WATER Question-Answers. Q. 1. Give two examples of : (i) Underground water (ii) Surface water. (i) Underground water Well water, spring water. (ii) Surface water River and lake water Q. 2. Briefly describe the water cycle in nature. An enormous amount of water evaporates in the air and rises up. In the upper regions of the atmosphere, water vapour condenses to form clouds. The clouds cause rain. Some rainwater flows over the earth s surface in the form of streams, rivers and sea. Some amount of rainwater collects within the earth. Thus, the balance of water on the earth is maintained due to the water cycle. Q. 3. State five physical properties of water. (i) Water is colourless and odourless. (ii) Freezing point of water is 0 C and its boiling point is 100 C. (iii) Specific heat of water is very high. (iv) Density of water is 1 g/cc at 4 C. (v) Water can dissolve a large number of substances. It is a good solvent. Q. 4. (i) What is potable water? (ii) Why is distilled water not potable? (iii) State three chief qualities of potable water. (i) The water which is fit for human consumption is called potable water. (ii) Distilled water is not potable because it will dissolve essential salts present in our body. It may lead to the deficiency of vital salts which in turn can make us sick. (iii) The three chief qualities of potable water are (a) It must be clear, colourless and odourless. (b) It must be free from harmful bacteria and suspended impurities. (c) It must contain small amount of mineral salts. Q. 5. What do you understand by the following terms? (i) Soft water (ii) Hard water (iii) Temporary hard water (iv) Permanent hard water 1

2 (i) Soft water A sample of ground water which freely lathers with soap solution is called soft water. (ii) Hard water A sample of ground water, which instead of freely forming lather with soap solution, forms sticky scum (or precipitate), is called hard water. (iii) Temporary hard water A sample of ground water which ordinarily forms scum with soap solution, but on boiling and then treating with soap solution produces lather, is called temporary hard water. (iv) Permanent hard water A sample of ground water which on boiling and then treating with soap solution does not lather but forms scum, is called permanent hard water. Q. 6. Name two chemicals which are responsible for temporary hardness of water. Calcium hydrogen carbonate (Ca(H ) and magnesium hydrogen carbonate [Mg(H ]. Q. 7. Name four chemicals which are responsible for permanent hardness of water. (i) Calcium chloride (CaCl 2 ) (ii) Magnesium chloride (MgCl 2 ) (iii) Calcium sulphate (CaSO 4 ) (iv) Magnesium sulphate (MgSO 4 ). Q. 8. How does water become temporarily hard in nature? When rainwater flows over the surface of the earth, carbon dioxide dissolves in it. It reacts with the rocks containing calcium carbonate or magnesium carbonate to form the respective hydrogen carbonates. Thus, the water becomes temporary hard. Ca O + CO 2 Ca(H Mg O + CO 2 Mg(H Q. 9. How can you remove temporary hardness of water? Temporary hardness of water can be removed by boiling. When hardwater is boiled, calcium hydrogen carbonate and magnesium hydrogen carbonate decompose to form the respective insoluble carbonates which can be filtered out and the water becomes soft. Q. 10. How can you remove permanent hardness in water? Permanent hardness can be removed by treating the water with sodium carbonate when insoluble metal carbonates are precipitated which can be filtered out and the water becomes soft. CaCl 2 Ca + 2NaCl MgSO 4 Mg SO 4 2 2

3 MgCl 2 Mg + 2NaCl CaSO 4 Ca SO 4 Q. 11. Explain briefly how surface water gets polluted by defecation of humans and animals. In rural areas, people and animals defecate anywhere and everywhere. When it rains, the excreta of humans and animals is washed away into streams and rivers and causes pollution. Q. 12. Why is hard water disadvantageous? Disadvantages of hard water are (i) It is unfit for drinking purposes, as it upsets the stomach. (ii) It is unfit for washing clothes or bathing, as it forms the sticky scum a rather than lather. (iii) It is unfit for dyeing clothes as the colour tends to fade. Q. 13. Why is acid added to water before electrolysis? Acid is added to water before electrolysis because pure water is a bad conductor of electricity. Q. 14. How does the anomalous expansion of water help aquatic organisms in cold climates? On cooling below 4 C upto 0 C, water expands instead of contracting. This is called anomalous expansion of water. Ice floats on the surface of water and forms an insulating layer on the water which allows fishes and aquatic animals to survive below. Q. 15. What do you mean by water pollution? What are its causes and how can water pollution be controlled? Addition of undesirable substances in fresh water bodies is called water pollution. Causes of water pollution: (i) Defecation of living beings in open fields. (ii) Industries and their effluents and agricultural operations. (iii) Thermal pollution. Control of Water Pollution: (i) Pollution caused due to defecation can be solved by using pit latrines and by setting up biogas plants. (ii) Pollution of water, due to agricultural operations can be minimised by using minimum amount of artificial fertilisers, pesticides, insecticides and herbicides. 3

4 Exercises A. Tick ( ) the most appropriate answer. 1. Well water is an example of (a) rainwater (b) river water (c) surface water (d) underground water 2. Temporary hardness is caused due to (a) magnesium bicarbonate only (b) calcium bicarbonate only (c) both magnesium and calcium bicarbonate (d) calcium carbonate only 3. A sample of water, which lathers freely with soap, is called (a) distilled water (b) soft water (c) potable water (d) filtered water 4. One of the causes of water pollution is that farmers use (a) excessive manure (b) excessive fertiliser (c) excessive water (d) excessive seed 5. Temporary hardness of water can be removed by (a) decantation (b) filtration (c) boiling (d) treating with common salt 6. The specific heat capacity of water is (a) 4.2 J/kg C (b) 4200 J/kg C (c400 J/kg C (d.4 J/kg C 7. The percentage of water in human body is (a0% (b) 70% (c) 10% (d) 65% 8. Permanent hardness of water is due to the presence of (a) calcium chloride (b) calcium bicarbonate (c) calcium sulphide (d) calcium carbonate 9. Life originally evolved (a) in water (b) on land (c) in air (d) in laboratory 10. Water is purified by adding gas (a) bromine (b) hydrogen (c) sulphur dioxide (d) chlorine 1. (d. (c) 3. (b) 4. (b) 5. (c) 6. (a) 7. (b) 8. (a) 9. (a) 10. (d). B. Fill in the blank spaces. 1. Carbon dioxide dissolves in water to form A sample of water which freely lathers with soap solution, is called A sample of water which starts lathering with soap solution on boiling, is called temporary Water pipes burst in cold countries in winter due to... expansion of water gram of ice on melting forms 1 g of water and absorbs... of heat energy. 6. Use of artificial... by farmers causes pollution of water. 4 4

5 7. Boiling hard water with... removes permanent hardness. 8. Alum helps in the rapid... of suspended particles in river water is a process for sterilisation of water by addition of chlorine gas. 10. The boiling point of water is 100 C at... pressure. 11. Solubility of potassium nitrate in water... with increase in temperature. 12. The freezing point of water corresponds to the melting point of Dissolved carbon dioxide in water is used for... by aquatic plants. 14. Water has... density and... volume at 4 C. 15. Freezing mixture contains... and is the purest form of natural water. 17. Boiling removes... hardness of water. 18. Water changes white anhydrous copper sulphate carbonic acid 2. soft water 3. hard water 4. anomalous calories 6. fertilisers 7. washing soda 8. settling 9. Chlorination 10. normal 11. increases 12. ice 13. photosynthesis 14. maximum, minimum 15. salt, water 16. Rainwater 17. temporary 18. blue C. Statements given below are incorrect. Write the correct statements. 1. The density of pure water at 4 C is g cm 3. The density of pure water at 4 C is 1 g/cm Hot water bottles are used in winter, because water has the highest density. Hot water bottles are used in winter because water has high specific heat. 3. Water pipes burst in winter, because pipes contract at low temperature. Water pipes burst in winter because water expands at low temperatures. 4. Well water is an example of surface water. Well water is an example of underground water. 5. Potable water must be clear and free from disease causing bacteria only. Potable water must be clear and free from disease causing bacteria and suspended impurities. 6. Seawater contains 1.5% of common salt. Seawater contains 2.5% of common salt. 7. Presence of magnesium chloride in water makes it temporarily hard. Presence of magnesium chloride in water makes it permanently hard. 8. Distilled water is a form of natural water. Rainwater is a form of natural water. 9. Increase in pressure on the surface of water decreases the boiling point of water. Increase in pressure on the surface of water increases the boiling point of water. 5

6 10. The density of water at 0 C is more than the density of water at 4 C. The density of water at 0 C is less than the density of water at 4 C. 11. Temporary hardness in water is due to presence of calcium sulphate in water. Permanent hardness in water is due to presence of calcium sulphate in water. 12. Solubility of gases in water is inversely proportional to the pressure of water. Solubility of gases in water is directly proportional to the pressure of water. D. Write true or false in front of the statements given below: 1. Pure water has a pleasant taste. 2. Ice floats over water as its density is less than that of water. 3. Plants do not need water during photosynthesis. 4. Seawater contains 3.5% of common salt. 5. Dissolved salts in potable water provide only taste, but are useless for living beings. 6. Dysentery and cholera are caused by drinking polluted water. 7. Magnesium bicarbonate in water makes it permanently hard. 8. Hard water wastes soap during washing of clothes. 9. Washing soda removes permanent hardness when added to water. 10. Always add water to acid. 11. You are not advised to bring inflammable liquids such as alcohol, spirit and ether near a candle flame. 12. Do not heat beakers or evaporating dish directly over the flame. 13. Never interchange the stoppers of reagent bottles. 14. Do not play with chemicals. 1. F 2. T 3. F 4. F 5. F 6. T 7. F 8. T 9. T 10. F 11. T 12. T 13. T 14. T E. Name the following. 1. The scientist who showed that water is a compound of hydrogen and oxygen. 2. The type of water which is not potable since it is free from dissolved salts. 3. The state of water which is neither solid nor liquid. 4. A gas dissolved in water whose percentage is higher than that in ordinary air. 5. The substance which is not a magnesium salt and causes temporary hardness in water. 6. The apparatus in which impure water or seawater is heated, followed by condensation of the vapours resulting in formation of pure water. 7. A substance which, when added to water, changes its boiling point. 8. Two sources of soft water. 6 6

7 9. The approximate percentage of water in the human body. 1. Henry Cavendish 2. Distilled water 3. Vapour (clouds) 4. Oxygen 5. Calcium bicarbonate 6. Distillation apparatus 7. Sugar 8. Rainwater and distilled water 9. 70% F. Answer the following by selecting the correct word/s in each case. 1. Ice at 0 C has a lower/higher density than water at 4 C and hence sinks/floats on water. 2. Natural water is a poor/good conductor of heat and a poor/good conductor of electricity. 3. Alcohol is miscible/immiscible in water, while kerosene is miscible/ immiscible in water. 4. Temporary hard water contains soluble calcium and magnesium chlorides/bicarbonates/sulphates. 5. Solubility of gases in water is directly/inversely proportional to the temperature of water. 1. lower, floats 2. good, good 3. miscible, immiscible 4. bicarbonates 5. inversely G. Complete the following equation. boil 1. Ca(H + CO 2 O 2. CaSO 4 + Ca SO CO 2 + Sunlight Chlorophyll C 6 H 12 O 6 + 6O 2 4. CaOCl 2 O Ca(OH + 1. Ca 2. Na H 2 O 4. Cl 2 H. Give a suitable word for the following sentences. 1. Water fit for human consumption. 2. Clean and pure water but unfit for human consumption. 3. Water containing more than 2% of dissolved salts and unfit for human consumption. 4. A chemical which softens permanent hard water. 5. Water found in wells. 6. A solid state of water having density less than 1 gcm A chemical used in excess by farmers, which causes water pollution. 8. A gas dissolved in water, which acts as food for the water plants. 1. Potable 2. Distilled water 3. Saline water 4. Sodium carbonate 5. Underground water 6. Ice 7. Fertiliser 8. Carbon dioxide **** 7

6 WATER. I. Tick ( ) the most appropriate answer. 1. River water is a (a) mixture (b) solution (c) compound (d) element 2.

6 WATER. I. Tick ( ) the most appropriate answer. 1. River water is a (a) mixture (b) solution (c) compound (d) element 2. 6 WATER I. Tick ( ) the most appropriate answer. 1. River water is a (a) mixture (b) solution (c) compound (d) element 2. Potable water is (a) transparent (b) transparent and colourless (c) transparent,