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ISC Class XII Prelims 2025 : English Paper 1 (English Language)

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CHAPTER 8 Water ORAL QUESTIONS (Page 109) 1. Solid, liquid, gas 2. 5 O 5 +d +d H H Structure of water molecule 3. Copper sulphate pentahydrate or Copper(II) sulphate penta-hydrate (CuSO4.5H2O) Iron sulphate heptahydrate or Iron (II) sulphate heptahydrate (FeSO4.7H2O) 4. The compounds which do not contain any water of crystallisation are called Anhydrous Salts. 5. The following properties of water make it a universal solvent. Water is a polar molecule. Water has a high dielectric constant. Water can form hydrogen bonds with other polar compounds. Water dissolves many substances either by reacting with them chemically or by forming hydrogen bonds with them. Water is cheap and easily available everywhere. ORAL QUESTIONS (Page 114) 1. Water regulates body temperature by the process of sweating and evaporation. Water inside the bodies of animals/humans dissolves minerals, gases and many products of digestion and carries them at the place of need in the body. 2. Water occurs in nature as solid (ice), liquid (water) and gas (vapour). 3. Various sources of water in nature are oceans, seas, rivers, lakes, ponds, wells and springs. 4. A homogeneous-looking heterogeneous mixture in which particles having size between 1 and 100 nm (nm is the symbol for nanometre : 1 nm = 10 9 m) are dispersed in a continuous medium is called a colloid. ORAL QUESTIONS (Page 115) 1. hydrogen and oxygen 2. H2O 3. Pure water is a poor conductor of electricity. Before electrolysis, it is made conductive by adding a small quantity of an alkali (e.g., sodium hydroxide) or an acid (e.g., sulphuric acid). 4. cathode 25 26 TEACHER RESOURCE MANUAL ORAL QUESTIONS (Page 119) 1. Density of water is maximum at 4 C. 2. Water expands on freezing. 3. Water pipes burst during winters in cold places because water expands on freezing. That is, the density of ice is less than that of water at 0 C. 4. Density of ice is less than water therefore ice floats on water. 5. Water has the highest heat capacity of all liquids. It is due to its high heat capacity that water is used in hot water bottles. 6. Three physical properties of water are as follows: Colour, taste and odour : Pure water is a colourless, odourless and tasteless liquid. The dissolved salts and gases give a taste to water. That is why water from different sources tastes differently. Freezing and melting points : Pure water freezes to ice at 0 C under 1 atmospheric pressure. Ice melts into water at 0 C under 1 atmospheric (or 1 atm) pressure. So, the freezing point of water and the melting point of ice under 1 atmospheric (or 1 atm) pressure is 0 C. Density : Density of water at 0 C is 0.9998 g/mL. Density of water increases to 1 g/mL at 4 C. Above 4 C, the density of water start decreasing with a rise in temperature. Thus at 4 C, the density of water is maximum at 1 g/mL. 7. The melting point of ice is 0 C and boiling point of water is 100 C. 8. Water has no effect on the colours of litmus. 9. (a) 2Na (s) sodium (b) water SO3 (g) + sulphur trioxide (c) 3Fe (s) iron (red hot) room temp. vigorous + 2H2O (l) + 4H2O (g) 2NaOH (aq) + sodium hydroxide H2 (g) hydrogen H2O (l) H2SO4 (aq) water sulphuric acid steam steam Fe3O4 (s) ferrosoferric oxide + 4H2 (g) hydrogen ORAL QUESTIONS (Page 122) 1. Water which does not give lather with soap solution and forms curdy precipitate with it is called hard water. 2. Ca(HCO3)2 boiling calcium bicarbonate (in temporary hard water) Mg(HCO3)2 magnesium bicarbonate (in temporary hard water) 3. Temporary boiling CaCO3 (s) + H2O (l) + calcium cabonate precipitate (can be water MgCO3 (s) + H2O (l) + magnesium carbonate precipitate water CO2 (g) carbon dioxide CO2 (g) ICSE SCIENCE CHEMISTRY 8 27 4. Ca2+ (aq) + Na CO 2 3 in permanent hard water CaCO3 (s) + 2Na+ (aq) sodium carbonate calcium carbonate (precipitate) Mg2+ (aq) + Na2CO3 in permanent hard water go out in soft water MgCO3 (s) + 2Na+(aq) sodium carbonate magnesium carbonate (precipitate) go into soft water I. OBJECTIVE TYPE QUESTIONS A. Choose the correct answer. 1. (a) 2. (b) 3. (b) 4. (b) 5. (a) 6. (c) 7. (d) B. State True or False. Also correct the false statements. 1. False 2. False 3. True 4. True 8. (a) 9. (c) 10. (b) 5. False C. Fill in the blanks. 1. Water is an essential component of all living beings . 2. Seeds need water for germination . 3. Pure water boils at 100 C under normal pressure. 4. Water helps in transporting people and goods from one place to another. 5. When water is electrolysed, the gas produced at cathode is hydrogen . D. Assertion-Reason 1. (d) 2. (a) 3. (b) 4. (a) 5. (a) II. SHORT ANSWER TYPE QUESTIONS 1. Why is natural water not considered safe for drinking ? Ans. The natural water is not considered safe for drinking because it is contaminated with many impurities that come naturally and in the form of different chemicals from industries. Due to these reasons, we should avoid drinking natural water. 2. What is the importance of water for humans, animals and plants ? Ans. Water is very important for humans, animals and plants in the following ways : (i) Water regulates body temperature by the process of sweating and evaporation. (ii) Water inside the bodies of plants, animals and humans transport minerals, gases and many products of digestion and carries them at the place of need in the body. (iii) Water provides a medium for all biochemical reactions inside our bodies to take place. (iv) Water is also needed by plants for their growth and carrying out photosynthesis. 3. Draw a labelled diagram of the apparatus used for the electrolysis of water. Ans. Glass tubes Oxygen Hydrogen Electrodes Acidulated water Electrolysis of water 28 TEACHER RESOURCE MANUAL 4. Why do ice cubes float on the surface of water ? Give reason. Ans. In ice, water molecules form a three dimensional structure with large open space between them. Ice occupies more volume than an equal mass of water. That is why ice is lighter than water and floats over it. 5. What is meant by the water of crystallisation ? Ans. The water molecules present in the crystal lattice of the substance (per molecule of the substance) is called its water of crystallisation. Different salts contain different number of water molecules as the water of crystallisation. 6. Give one example of hydrated salt. Ans. Sodium carbonate decahydrate or washing soda (Na2CO3.10H2O) is an example of hydrated salt. 7. How is the temporary hardness of water removed ? Ans. Temporary hardness of water can easily be removed by boiling. On boiling, calcium and magnesium bicarbonates decompose to give the precipitate of calcium and magnesium carbonates. The precipitates are removed by filtration. Ca(HCO3)2 boiling calcium bicarbonate (in temporary hard water) Mg(HCO3)2 CaCO3 (s) + H2O (l) + calcium cabonate precipitate (can be water MgCO3 (s) + H2O (l) + boiling magnesium bicarbonate (in temporary hard water) magnesium carbonate precipitate water CO2 (g) carbon dioxide CO2 (g) carbon dioxide 8. Why can t we use hardwater for washing our clothes ? Ans. Soap is the sodium salt of stearic acid (C17H35COO Na, sodium stearate). When soap is added to hard water, it reacts with the calcium and magnesium ions present in hard water to form insoluble calcium or magnesium stearate (white and curdy precipitate). CaCl2 + 2C17H35COONa 2NaCl + MgCl2 + 2C17H35COONa 2NaCl + (C17H35COO)2Mg calcium chloride sodium stearate magnesium chloride sodium stearate C17H35COO)2Ca calcium stearate magnesium stearate As a result, soap will not produce lather with water until all the calcium and magnesium ions have precipitated. Thus, a part of the soap is wasted, and washing becomes wasteful. This is also the reason why hard water is not fit for washing (laundry) purposes. III. LONG ANSWER TYPE QUESTIONS 1. List five uses of water. Ans. The uses of water are as follows : (i) Water is used in cooking. (ii) Water is used for drinking by almost all living organisms. (iii) Water is used in industries for various purposes. (iv) Water is used for cleaning clothes, utensils etc. (v) Water is used in large quantities for growing crops. ICSE SCIENCE CHEMISTRY 8 29 2. What are the characteristics of suspension? Ans. Characteristics of suspensions are: Visibility : The particles in a suspension can be seen with naked eyes or under a simple microscope. Particle size : In a suspension, the size of the particles is of the order of 10 7 m or larger. Sedimentation : The particles in a suspension have a tendency to settle down. Very fine particles, however, remain suspended in the medium. Separation by filtration : Larger particles in a suspension can be separated from the liquid or air by filtration. 3. How will you find out if the given sample of water is soft or hard ? How is hard water softened ? Ans. Hard water does not form lather with the soap solution. Hard water can be softened by the following methods : (i) By boiling : By this method, temporary hardness can be removed. (ii) By adding washing soda : Calcium and magnesium ions present in hard water react with sodium carbonate to produce insoluble carbonates. These insoluble substances are removed by filtration. (iii) Ion-exchange resin method : Hardness of water can also be removed by passing hard water through a column packed with ion-exchange resins. The ion-exchange resins exchange calcium and magnesium ions (Ca2+ and Mg2+ ) present in hard water for sodium and potassium ion (Na+ and K+). Ion-exchange method can remove both the types of hardness of water. 4. What are colloids ? Mention three characteristics of colloids. Ans. A homogeneous-looking heterogeneous mixture in which particles having size between 1 and 100 nm are dispersed in a continuous medium is called a colloid. Colloids are commonly called sols. Milk, Office paste (gum), Blood, Toothpaste, Jelly, etc., are some examples of colloids. The three characteristics of colloids are as follows : (i) Colloid particles are large enough to cause scattering of light. (ii) The constituents of a colloid cannot be separated by simple physical methods like filtration. (iii) In a colloid, the dispersed particles can be seen only with a powerful microscope. 5. How does water react with the following: (a) K2O (b) CaO (c) CO2 (d) Fe (e) Zn Write balanced chemical equation for the reactions. Ans. (a) K 2O (s) + Potassium oxide (b) (c) CaO (s) 3Fe (s) Iron (Red hot) 2KOH (aq) Potassium hydroxide H 2O (l) Ca(OH)2 (aq) Calcium oxide (Lime) Carbon dioxide (d) Water + CO2 (g) H 2 O (l) Water + Calcium hydroxide (Limewater) H 2O (l) Water + 4H2O (g) Steam steam H2CO3 (aq) Carbonic acid Fe3O4 (s) Ferrosoferric oxide + 4H2 (g) Hydrogen 30 TEACHER RESOURCE MANUAL (e) Zn (s) Zinc powder + boiling H2O (l) Boiling water ZnO (s) Zinc oxide + H2 (g) Hydrogen 6. Describe the characteristic properties of colloids. Ans. Some characteristic properties of a colloid are described below : Heterogeneous nature. A colloid (or sol) is heterogeneous in nature. The particles in a colloid can be seen only with a powerful microscope. Particle size. The size of particles in a colloid lies between 10 9 m and 10 7 m. Separation of colloidal particles by filtration. Colloidal particles can easily pass through the pores of a filter paper. Therefore, colloidal particles cannot be separated by ordinary filtration. Brownian movement of colloidal particles. The zig-zag motion of particles in a colloid is called Brownian motion. Light scattering by colloidal particles. Colloids scatter a light beam passing through them. The scattering of light by the particles in a colloid is called tyndall effect. The Tyndall effect is shown in the figure. IV. CONCEPT BASED QUESTIONS 1. (c) 2. (b) 3. (b) 4. (d) ACTIVITIES AND PROJECTS Do yourself GROUP DISCUSSION Do yourself 5. (b)

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