Chapter 3 class x


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Chapter 3 class x

  1. 1. The easiest way to start grouping substances is bycomparing their physical properties. collect thesamples of following metals – iron, copper,aluminium, magnesium, sodium, lead, zinc andany other metal that is easily available.Metals, in their pure state, have a shiningsurface. This property is called metallic lustre.The metals are generally hard. The hardnessvaries from metal to metal.
  2. 2. Some metals can be beaten into thin sheets. Thisproperty is called malleability. Gold and silverare the most malleable metalsThe ability of metals to be drawn into thin wiresis called ductility. Gold is the most ductile metal.You will be surprised to know that a wire ofabout 2 km length can be drawn from one gramof gold.It is because of their malleability and ductilitythat metals can be given different shapesaccording to our needs.
  3. 3. The above activity shows that metals aregood conductors of heat and have highmelting points. The best conductors of heatare silver and copper. Lead and mercuryare comparatively poor conductors of heat.
  4. 4. Do metals also conduct electricity? Let us find out.You must have seen that the wires that carry currentin your homes have a coating of polyvinylchloride (PVC)or a rubber-like material. Why are electric wires coatedwith such substances?What happens when metals strike a hard surface? Do theyproduce a sound? The metals that produce a sound on striking ahard surface are said to be sonorous. Can you now say why schoolbells are made of metals?
  5. 5. There are very few non-metals as compared tometals. Some of the examples of non-metals arecarbon, sulphur, iodine, oxygen, hydrogen, etc.The non-metals are either solids or gases exceptbromine which is a liquid.
  6. 6. You must have concluded that we cannot group elements accordingto their physical properties alone, as there are many exceptions. Forexample –(i) All metals except mercury exist as solids at roomtemperature. Metals have high melting points but galliumand caesium have very low melting points. These twometals will melt if you keep them on your palm.(ii) Iodine is a non-metal but it is lustrous.(iii) Carbon is a non-metal that can exist in differentforms. Each form is called an allotrope. Diamond, anallotrope of carbon, is the hardest natural substanceknown and has a very high melting and boiling point.Graphite, another allotrope of carbon, is a conductor ofelectricity.(iv) Alkali metals (lithium, sodium, potassium) are so softthat they can be cut with a knife. They have low densitiesand low melting points.
  7. 7. Elements can be more clearly classified asmetals and non-metals on the basis of theirchemical properties.Most non-metals produce acidic oxides whendissolve in water. On the other hand, mostmetals, give rise to basic oxides.
  8. 8. Q1. Give an example of a metal which(i) is a liquid at room temperature.(ii) can be easily cut with a knife.(iii) is the best conductor of heat.(iv) is a poor conductor of heat.Q2. Explain the meanings of malleable andductile.
  9. 9. Magnesium burns in air with a dazzling white flame.Almost all metals combine with oxygen to form metal oxides.For example, when copper is heated in air, it combines withoxygen to form copper(II) oxide, a black oxide.Similarly, aluminium forms aluminium oxide.
  10. 10. We have learnt that metal oxides are basic innature. But some metal oxides, such asaluminium oxide, zinc oxide, etc., show bothacidic as well as basic behaviour. Such metaloxides which react with both acids as well asbases to produce salts and water are known asamphoteric oxides.Aluminium oxide reacts in the following mannerwith acids and bases –
  11. 11. Most metal oxides are insoluble inwater but some of these dissolve inwater to form alkalis. Sodium oxideand potassium oxide dissolve inwater to produce alkalis as follows–
  12. 12. All metals do not react with oxygen at the same rate.Different metals show different reactivities towardsoxygen. Metals such as potassium and sodium react sovigorously that they catch fire if kept in the open. Hence,to protect them and to prevent accidental fires, they arekept immersed in kerosene oil. At ordinary temperature,the surfaces of metals such as magnesium, aluminium,zinc, lead, etc., are covered with a thin layer of oxide.The protective oxide layer prevents the metal fromfurther oxidation. Iron does not burn on heating butiron filings burn vigorously when sprinkled in the flameof the burner. Copper does not burn, but the hot metalis coated with a black coloured layer of copper(II) oxide.Silver and gold do not react with oxygen even at hightemperatures.
  13. 13. Sodium is the most reactive of the samplesof metals taken here. The reaction ofmagnesium is less vigorous implying that itis not as reactive as sodium. But burning inoxygen does not help us to decide about thereactivity of zinc, iron, copper or lead.
  14. 14. Metals react with water and produce a metaloxide and hydrogen gas. Metal oxides that aresoluble in water dissolve in it to further formmetal hydroxide. But all metals do not react withwater.Metals like potassium and sodium react violentlywith cold water. In case of sodium andpotassium, the reaction is so violent andexothermic that the evolved hydrogenimmediately catches fire.
  15. 15. The reaction of calcium with water is less violent. The heatevolved is not sufficient for the hydrogen to catch fire.Calcium starts floating because the bubbles of hydrogengas formed stick to the surface of the metal.Magnesium does not react with cold water. It reacts withhot water to form magnesium hydroxide and hydrogen.It also starts floating due to the bubbles of hydrogen gassticking to its surface.Metals like aluminium, iron and zinc do not react eitherwith cold or hot water. But they react with steam to formthe metal oxide and hydrogen.Metals such as lead, copper, silver and gold do not react with water at all.
  16. 16. Metals react with acids to give a saltand hydrogen gas.Hydrogen gas is not evolved when a metalreacts with nitric acid. It is because HNO3 isa strong oxidising agent. It oxidises the H2produced to water and itself gets reduced toany of the nitrogen oxides (N2O, NO, NO2).But magnesium (Mg) and manganese (Mn)react with very dilute HNO3 to evolve H2 gas.
  17. 17. The rate of formation of bubbles was the fastestin the case of magnesium. The reaction was alsothe most exothermic in this case. The reactivitydecreases in the order Mg > Al > Zn > Fe. In thecase of copper, no bubbles were seen and thetemperature also remained unchanged. Thisshows that copper does not react with dilute HCl.
  18. 18. Reactive metals can displace less reactive metalsfrom their compounds in solution or molten form.All metals are not equally reactive. We checkedthe reactivity of various metals with oxygen,water and acids. But all metals do not react withthese reagents. So we were not able to put all themetal samples we had collected in decreasingorder of their reactivity. Displacement reactionsstudied in Chapter 1 give better evidence aboutthe reactivity of metals. It is simple and easy ifmetal A displaces metal B from its solution, it ismore reactive than B.Which metal, copper or iron, is more reactive according to your observations.
  19. 19. The reactivity series is a list of metals arranged in theorder of their decreasing activities. After performingdisplacement experiments, the following series, knownas the reactivity or activity series has been developed.
  20. 20. We learnt that noble gases, which have acompletely filled valence shell, show littlechemical activity. We, therefore, explain thereactivity of elements as a tendency toattain a completely filled valence shell.
  21. 21. sodium atom has one electron in its outermost shell. If itloses the electron from its M shell then its L shellnow becomes the outermost shell and that has a stableoctet. The nucleus of this atom still has 11 protons butthe number of electrons has become 10, so there is anet positive charge giving us a sodium cation Na+. Onthe other hand chlorine has seven electrons in itsoutermost shell and it requires one more electron tocomplete its octet. If sodium and chlorine were to react,the electron lost by sodium could be taken up bychlorine. After gaining an electron, the chlorine atomgets a unit negative charge, because its nucleus has 17protons and there are 18 electrons in its K, L and Mshells. This gives us a chloride anion C1–. So both theseelements can have a give-and-take relation betweenthem as follows
  22. 22. Sodium and chloride ions, being oppositely charged,attract each other and are held by strong electrostaticforces of attraction to exist as sodium chloride (NaCl). Itshould be noted that sodium chloride does not exist asmolecules but aggregates of oppositely charged ions.
  23. 23. Let us see the formation of one more ionic compound,magnesium chloride.The compounds formed in this manner by the transfer ofelectrons from a metal to a non-metal are known as ioniccompounds or electrovalent compounds. Can you namethe cation and anion present in MgCl ?
  24. 24. You may have observed the following general properties for ioniccompounds—(i) Physical nature: Ionic compounds are solids and are somewhat hardbecause of the strong force of attraction between the positive and negativeions. These compounds are generally brittle and break into pieces whenpressure is applied.(ii) Melting and Boiling points: Ionic compounds have high melting andboiling points. This is because a considerable amount of energy is requiredto break the strong inter-ionic attraction.(iii) Solubility: Electrovalent compounds are generally soluble in water andinsoluble in solvents such as kerosene, petrol, etc.(iv) Conduction of Electricity: The conduction of electricity through asolution involves the movement of charged particles. A solution of an ioniccompound in water contains ions, which move to the opposite electrodeswhen electricity is passed through the solution. Ionic compounds in thesolid state do not conduct electricity because movement of ions in the solidis not possible due to their rigid structure. But ionic compounds conductelectricity in the molten state. This is possible in the molten state since theelectrostatic forces of attraction between the oppositely charged ions areovercome due to the heat. Thus, the ions move freely and conductelectricity.
  25. 25. Q1. (i) Write the electron-dot structures forsodium, oxygen and magnesium.(ii) Show the formation of Na O and MgO by thetransfer of electrons.(iii) What are the ions present in thesecompounds?Q2. Why do ionic compounds have high meltingpoints?
  26. 26. The earth’s crust is the major source ofmetals. Seawater also contains somesoluble salts such as sodium chloride,magnesium chloride, etc. The elements orcompounds, which occur naturally in theearth’s crust, are known as minerals. Atsome places, minerals contain a very highpercentage of a particular metal and themetal can be profitably extracted from it.These minerals are called ores.
  27. 27. Having the knowledge of reactivity series of metals, we caneasily understand how a metal is extracted from its ore. Somemetals are found in the earth’s crust in the free state. Some arefound in the form of their compounds. The metals at the bottomof the activity series are the least reactive. They are often foundin a free state. For example, gold, silver, platinum and copperare found in the free state. Copper and silver are also found inthe combined state as their sulphide or oxide ores. The metals atthe top of the activity series (K, Na, Ca, Mg and Al) are soreactive that they are never found in nature as free elements.The metals in the middle of the activity series (Zn, Fe, Pb, etc.)are moderately reactive. They are found in the earth’s crustmainly as oxides, sulphides or carbonates. we will find that theores of many metals are oxides. This is because oxygen is avery reactive element and is very abundant on the earth.
  28. 28. Thus on the basis of reactivity, wecan group the metals into thefollowing three categories–(i) Metals of low reactivity;(ii) Metals of medium reactivity;(iii) Metals of high reactivity.Different techniques are to beused for obtaining the metalsfalling in each category.Several steps are involved in theextraction of pure metal fromores. A summary of these steps isgiven in next section. Each step isexplained in detail in the furthernext sections.
  29. 29. Ores mined from the earth are usuallycontaminated with large amounts of impuritiessuch as soil, sand, etc., called gangue. Theimpurities must be removed from the ore priorto the extraction of the metal. The processesused for removing the gangue from the ore arebased on the differences between the physicalor chemical properties of the gangue and theore. Different separation techniques areaccordingly employed.
  30. 30. Metals low in the activity series are very unreactive. Theoxides of these metals can be reduced to metals byheating alone. For example, cinnabar (HgS) is an ore ofmercury. When it is heated in air, it is first convertedinto mercuric oxide (HgO). Mercuric oxide is thenreduced to mercury on further heating.Similarly, copper which is found as Cu S in nature can beobtainedfrom its ore by just heating in air.
  31. 31. The metals in the middle of the activity series such asiron, zinc, lead, copper, etc., are moderately reactive.These are usually present as sulphides or carbonates innature. It is easier to obtain a metal from its oxide, ascompared to its sulphides and carbonates. Therefore,prior to reduction, the metal sulphides and carbonatesmust be converted into metal oxides. The sulphide oresare converted into oxides by heating strongly in thepresence of excess air. This process is known asroasting.The carbonate ores are changed into oxides by heatingstrongly in limited air. This process is known as
  32. 32. The chemical reaction that takes place duringroasting and calcination of zinc ores can beshown as follows –The metal oxides are then reduced to thecorresponding metals by using suitable reducingagents such as carbon. For example, when zincoxide is heated with carbon, it is reduced tometallic zinc.
  33. 33. Obtaining metals from their compounds is areduction process. Besides using carbon (coke)to reduce metal oxides to metals, sometimesdisplacement reactions can also be used. Thehighly reactive metals such as sodium, calcium,aluminium, etc., are used as reducing agentsbecause they can displace metals of lowerreactivity from their compounds. For example,when manganese dioxide is heated withaluminium powder, the following reaction takesplace –
  34. 34. These displacement reactions are highlyexothermic. The amount of heat evolved isso large that the metals are produced in themolten state. In fact, the reaction of iron(III)oxide (Fe O ) with aluminium is used to joinrailway tracks or cracked machine parts.This reaction is known as the thermitreaction.
  35. 35. The metals high up in the reactivity series are veryreactive. They cannot be obtained from their compoundsby heating with carbon. For example, carbon cannotreduce the oxides of sodium, magnesium, calcium,aluminium, etc., to the respective metals. This isbecause these metals have more affinity for oxygen thancarbon. These metals are obtained by electrolyticreduction. For example, sodium, magnesium andcalcium are obtained by the electrolysis of their moltenchlorides. The metalsare deposited at the cathode (the negatively chargedelectrode), whereas, chlorine is liberated at the anode
  36. 36. The reactions are –Similarly, aluminium is obtained by the electrolyticreduction of aluminium oxide.
  37. 37. The metals produced by various reductionprocesses described above are not verypure. They contain impurities, which mustbe removed to obtain pure metals. The mostwidely used method for refining impuremetals is electrolytic refining.
  38. 38. Electrolytic Refining: Many metals, such ascopper, zinc, tin, nickel, silver, gold, etc., arerefined electrolytically. In this process, the impuremetal is made the anode and a thin strip of puremetal is made the cathode. A solution of the metalsalt is used as an electrolyte. On passing thecurrent through the electrolyte, the pure metalfrom the anode dissolves into the electrolyte. Anequivalent amount of pure metal from theelectrolyte is deposited on the cathode. Thesoluble impurities go into the solution, whereas,the insoluble impurities settle down at the bottomof the anode and are known as anode mud.
  39. 39. 1. Define the following terms.(i) Mineral (ii) Ore (iii) Gangue2. Name two metals which are found in nature inthe free state.3. What chemical process is used for obtaininga metal from its oxide?
  40. 40. 􀂄 Silver articles become black after some timewhen exposed to air. This is because it reactswith sulphur in the air to form a coating of silversulphide. 􀂄 Copper reacts with moist carbon dioxide inthe air and slowly loses its shiny brown surfaceand gains a green coat. This green substance iscopper carbonate. 􀂄 Iron when exposed to moist air for a long timeacquires a coating of a brown flaky substancecalled rust.
  41. 41. The rusting of iron can be prevented bypainting, oiling, greasing, galvanising,chrome plating, anodising or making alloys.Galvanisation is a method of protectingsteel and iron from rusting by coating themwith a thin layer of zinc. The galvanisedarticle is protected against rusting even ifthe zinc coating is broken.
  42. 42. Alloying is a very good method of improving the properties ofa metal. We can get the desired properties by this method.For example, iron is the most widely used metal. But it isnever used in its pure state. This is because pure iron is verysoft and stretches easily when hot. But, if it is mixed with asmall amount of carbon (about 0.05 %), it becomes hard andstrong. When iron is mixed with nickel and chromium, we getstainless steel, which is hard and does not rust. Thus, if iron ismixed with some other substance, its properties change. Infact, the properties of any metal can be changed if it is mixedwith some other substance. The substance added may be ametal or a non-metal. An alloy is a homogeneous mixture oftwo or more metals, or a metal and a non metal. It is preparedby first melting the primary metal, and then, dissolving theother elements in it in definite proportions. It is then cooled toroom temperature.
  43. 43. If one of the metals is mercury, then the alloy is knownas an amalgam. The electrical conductivity and meltingpoint of an alloy is less than that of pure metals. Forexample, brass, an alloy of copper andzinc (Cu and Zn), and bronze, an alloy of copper and tin(Cu and Sn), are not good conductors of electricitywhereas copper is used for making electrical circuits.Solder, an alloy of lead and tin (Pb and Sn), has a lowmelting point and is used for welding electrical wirestogether.