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ICSE CHEMISTRY GIVE REASONS

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CHEMISTRY GIVE REASONS CHAPTER 7 METALLURGY Q1. Nitrogen is used to preserve food Due to its inertness. Q2. Group 1 elements are called Alkali metals As they react with water to form their hydroxides which are strong alkalis. Q3. Hydrogen is placed in Group 1 As it has 1 electron. Q4. Most elements do not occur in free state As they are reactive. Q5. Alkali metals are called soft Since they can be cut with a knife. Q6. Alkali metals when freshly cut, they show typical silvery white metallic lustre but soon they turn dull [tarnish] Because of their reaction with air. Q7. Elements of Group 2 are called Alkali Earth metals Because their oxides occur in the earth s crust and their hydroxides are alkalies. Q8. Metals form +ve ion [cations] Because of loss of electrons. Q9. Non-metals form ve ions Because of gain of electrons which have a ve charge. Q10. Potassium is kept at the top of the activity series of metals. Because it is the most reactive metal; the arrangement of metals in the activity series is in the decreasing order of their reactivity. Q11. Hydrogen, though a non-metal has been included in the activity series Because it can form a positive ion. It would occupy the position based on the formation of the positive ion. Q12. The reaction of dilute acids with lead is very slow. Because the salts formed are sparingly soluble in dilute acids. During the reaction, the chloride or sulphate gets coated on the metal and prevents the reaction to proceed further. Q13. Oxides of K, Na, Ca, Mg, Al cannot be reduced by heating. As they are stable to heat and so can be reduced only by electrolysis. Q14. Reactive metals like sodium and potassium are kept in kerosene oil As they react with moisture and air Q14. After tarnishing further oxidation stops. Tarnishing is the oxidation of the metallic surfaces other than iron. The oxide so formed does not flake off, but sticks to the metal surface. Thus after tarnishing further oxidation stops. Q15. Iron corrodes fast in sea water Dissolved salts in water act as electrolyte and enhance the rate of corrosion Q16. Galvanisation [coating of zinc] is preferred to tinning in the prevention of rusting of iron. The presence of a metal which is less reactive than iron increases the rate of rusting of iron, since it will favour the more reactive metal like iron to lose electrons and get oxidised. Therefore, galvanisation is preferred. Q17. Painting of an iron object prevents rusting. Prevents rusting by preventing the iron from coming in contact with atmospheric reagents. Q18. Sometimes corrosion of metals is an advantage. Explain with an example On exposure to air, the surface of metals like aluminium and zinc forms layers of their oxides which are very sticky and impervious in nature, and hence act as a protective layer which protects the metal from further damage. Q19. Sulphide ores are concentrated by froth flotation Because the ores are lighter than their impurities Q20. Oxides of highly active metals like K, Na, Ca, Mg and Al cannot be reduced by carbon or carbon monoxide or hydrogen As they have great affinity towards oxygen Q21. Highly reactive metals can be used as reducing agents Because they can displace metals of low reactivity from their compounds. Q22. Liquation is the process used to refine metals like lead and tin Due to their low melting points. They are heated on a sloping hearth of a furnace. The molten or fused metal flows away leaving behind the impurities. Q23. Electrolytic reduction was chosen as the method for reducing alumina. Because it cannot be reduced by reducing agents like carbon, carbon monoxide or hydrogen. Q24. ANSWER THE FOLLOWING WITH REFERENCE TO HALL-HEROULTS S PROCESS Qa. Aluminium is discharged at cathode and oxygen at anode when +ve ions formed are Na+, Ca+2, Al+3 and ve ions formed are F-, O-2 Due to preferential discharge Qb. The anode has got to be replaced from time to time As it gets oxidised by the oxygen evolved at the anode. Qc. Cryolite and Flurospar are used in the molten electrolyte mixture. Acts as a solvent for the electrolyte mixture and Increases its conductivity since pure alumina is almost a non-conductor of electricity. Qd. Powdered coke is sprinkled over the surface of the electrolyte mixture Reduces heat loss by radiation and Prevents the burning of anode Q25. Water has no action on aluminium Due to layer of oxide on it. Q26. The reaction of aluminium with acids is slow in the beginning but after some time, reacts at an appreciable speed. Because the protective layer of aluminium oxide on the surface of Aluminium resists the attack of the acid. However, after some time, the oxide layer gets dissolved in the acid and the clean surface of aluminium reacts with the acid at an appreciable speed. Q27. Dil. Or Conc. Nitric acid does not attack the metal aluminium It renders aluminium passive due to the formation of an oxide film on its surface. Q28. Aluminium is used in alloys. Because it is STRONG, LIGHT, and CORROSION RESISTANT metal Q29. Aluminium is used in the form of thin foils for packaging Due to its HIGH MALLEABILITY Q30. Al is use for making explosives like AMMONAL Due to its strong affinity for oxygen Q31. Al is used for making mirrors It s an excellent reflector. Q32. Aluminium is used in painting electric and telegraphic poles Because the poles are made of Iron, therefore to prevent it from rusting. Q33. Ships are made of alloys of Aluminium Because it s unaffected by sea water. Q34. It s used in Aluminothermy Because Al is a good reducing agent. Q35. Al is used for reducing organic compounds Since Aluminium amalgams liberate hydrogen when treated with water. Q36. Mercury is commonly stored in iron bottles Because Mercury does not form an amalgam with Iron. CHAPTER 8 STUDY OF COMPOUNDS HYDROGEN CHLORIDE *With reference to the laboratory preparation of HCl gas, answer the following: Q1. Sodium chloride preferred It is cheap. Q2. Conc. Nitric acid is not used It is volatile and may volatize out along with HCl. Q3. Drying agents like phosphorus pentoxide and quicklime cannot be used. Since they react with it. Q4. HCl gas is not collected over water Its highly soluble in water. Q5. When HCl gas is exposed to air it gives white fumes Due to the formation of HCl acid on reacting with atmospheric water vapour. Q6. In the laboratory preparation of HCl acid, funnel arrangement is used It prevents or minimizes back-suction of water; Provides large surface area for absorption of HCl gas. Q7. HCl acid is used to pickle steel As it dissolves the oxides. Q8. Dil. HCl is prescribed to patients with decreased activity of their gastric juices It helps in the digestion of proteins, and also in destroying microorganisms that enter the alimentary canal along with the food. CHAPTER 9 STUDY OF COMPOUNDS- AMMONIA Q1. Ammonia and ammonium compounds do not occur as minerals They are highly soluble in water. *With reference to the laboratory preparation of ammonia from ammonium chloride, answer the following: Q2. The flask is kept tilted So that the water formed in the reaction does not trickle back into the heated test tube and thus break it. Q3. Other drying agents like conc. sulphuric acid and phosphorus pentoxide not used for drying ammonia As ammonia being basic reacts with them. Q4. Ammonia not collected over water Its soluble in water. Q5. Ammonium nitrate is not used It is explosive in nature and it decomposes forming nitrous oxide and water vapour. Q6. Finely divided catalyst is used in Haber s process It has more surface area and hence increases the efficiency of the catalyst. Q7. A promoter is used To increase the efficiency of the catalyst. Q8. The purification of Nitrogen and Hydrogen is necessary in Haber s process As impurities like carbon dioxide, carbon monoxide, and traces of sulphur compounds poisons the catalyst. Q9. A bottle of liquor Ammonia should be opened very carefully, only after cooling it in ice or cold water There is high pressure inside the bottle of ammonia and on cooling, the pressure drops. This prevents the sudden flushing out of the gas. Q1o. Dry ammonia is neutral even in liquefied form It s a covalent molecule. Q11. In the catalytic oxidation of Ammonia, the platinum [catalyst] continues to glow even after the heating is discontinued The reaction is exothermic. Q12. Liquid Ammonia is a good refrigerant Anhydrous ammonia is a clear, colourless liquid under pressure. It evaporates rapidly and produces cooling effect. *In the manufacture of Nitric acid, Q13. Higher ratio of air is used Because oxygen is only 1/5th in air and its required in all the three chambers. Q14. Quartz is used in the absorption tower Its acid resistant and when packed in layers it helps in dissolving nitrogen dioxide uniformly in water. CHAPTER 10 STUDY OF COMPOUNDS- NITRIC ACID Q1. The acid was formerly called aqua Fortis It means strong water. It was so called because it reacts with nearly all metals. It can even dissolve silver which does not dissolve in other acids. Q2. Pure acid is colourless but the acid obtained in the laboratory is slightly yellow The yellow colour is due to dissolution of reddish brown coloured nitrogen dioxide gas in the acid. Its produced due to the thermal decomposition of a portion of nitric acid. *With reference to the laboratory preparation of Nitric acid, answer the questions that follow: Q3. All glass apparatus I used Nitric acid vapours attack rubber and cork. Q4. Conc. HCl is not used in place of Conc. H2SO4 HCl is volatile and hence nitric acid vapours will carry HCl vapours. Q5. The temperature of the reaction should not exceed 200 C Because sodium sulphate formed at higher temperature forms a hard crust which sticks to the walls of the retort and is difficult to remove although the yield of HNO3 is higher. Q6. Nitric acid is normally stored in coloured bottles In the presence of sunlight, nitric acid decomposes to give yellow solution due to the formation of nitrogen dioxide. Q7. Metals like Iron, aluminium, cobalt and nickel become passive [inert] when treated with pure concentrated nitric acid Due to the formation of extremely thin layer of insoluble metallic oxide [passivity] which stops the reaction. Q8. Dil. Nitric acid is generally considered a typical acid except for its reaction with metals Since it does not liberate hydrogen. Q9. Nitric acid is used to etch designs on copper and brassware It acts as a solvent for a large number of metals except noble metals. Q10. It is used to purify gold Gold may contain Cu, Ag, Zn, Pb etc., as impurities which dissolve in Nitric acid. *With reference to Brown Ring test, answer the following questions: Q11. A freshly prepared ferrous sulphate solution is used Because on exposure to the atmosphere, it is oxidised to ferric sulphate which will not give the brown ring. Q12. The brown ring of nitroso ferrous sulphate is formed at the junction of the two liquids The conc. sulphuric acid being heavier settles down, and the ferrous sulphate layer remains above it resulting in the formation of the brown ring at the junction. Q13. The brown ring of nitroso ferrous sulphate decomposes on disturbing the test tube The heat evolved decomposes the unstable brown ring. CHAPTER 11 SULPHURIC ACID Q1. It is called the KING OF CHEMICALS There is no other manufactured compound which is used by such a large number of key industries. Q2. It is known as OIL OF VITRIOL In the later Middle Ages, it was obtained as an oily viscous liquid by heating crystals of green vitriol. *With reference to the contact process, answer the following questions: Q3. The purification of gases is done Because it contains various impurities which must be removed, otherwise the catalyst loses its efficiency. Q4. Sulphur trioxide is not directly absorbed by water Because it does not dissolve in water satisfactorily and it gives a lot of heat and forms misty droplets of sulphuric acid. Q5. High pressure is a favourable condition High pressure favours the reaction because the product formed has less volume than reactant. But the towers which are able to withstand high pressure are difficult to build. Hence the pressure of 1-2 atmosphere is used. Q6. Excess of oxygen is a favourable condition This increases the production of sulphur trioxide. Q7. Vanadium Pentoxide is used as a catalyst Platinum is more efficient than vanadium pentoxide but platinum is more expensive and also it easily gets poisoned by impurities like arsenic [III] oxide. Q8. Pure [100%] sulphuric acid is not an acid As it does not ionise. Q9. Sulphuric acid shows acidic properties Sulphuric acid when dissolved in water forms hydronium ion. Q10. It is a dibasic acid Because it ionises in two stages. Q11. Water is never poured on acid to dilute it As a large amount of heat is evolved which changes poured water to steam. The steam so formed causes spurting of acid which can cause burn injuries, so dilution is done by pouring acid on a given amount of water in a controlled manner by continuous stirring, else acid being heavier will settle down. The evolved heat is dissipated in the water itself and hence the spurting of acid is minimized. Q12. Conc. sulphuric acid is a non-volatile acid Since it has a high boiling point. Q13. It is used in the preparation of volatile acids Because it is a non-volatile acid. It prepares volatile acids from their salts by double decomposition. Q14. Conc. sulphuric acid is an oxidising agent Because on thermal decomposition, it yields nascent oxygen [O]. Q15. Sulphur dioxide gas is both an oxidising as well as reducing agents In the presence of water, sulphur dioxide is usually a reducing agent. If reacted with a stronger reducing agent, it acts as an oxidising agent. Q16. Sulphur dioxide is a temporary bleaching agent Due to its reducing nature in presence of moisture. Q17. Sulphur dioxide is used to bleach only delicate materials As it is not a strong bleaching agent. Q18. Sulphuric acid is a dehydrating agent It has a great affinity for water. Q19. Conc. Sulphuric acid reacts with skin to give blisters Because of removal of water, and chars the skin black. Q20. Conc. Sulphuric acid removes water of crystallisation from salts Due to its dehydrating property. Q21. Sulphuric acid is used in the extraction of metals Its reaction with metallic compounds gives sulphates which on electrolysis give the metal in the pure form. Q22. Sulphuric acid is used to pickle metals Remove metallic impurities like oxides and carbonates from the surface of metals before galvanising. GIVE REASONS PREPARED BY MRS. MERCY JOSE

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