CBSE Chemistry Class 10: Chapter 1 Notes, PYQs, and Sample Papers, Study notes of Chemistry

Download CBSE Chemistry Class 10: Chapter 1 Notes, PYQs, and Sample Papers and more Study notes Chemistry in PDF only on Docsity! CHAPTER 1 Chemical Equations Types of Chemical Reactions 20 18 T 16 14 "' C: 0 12 :.:; "' QI :, 10 C' -0 ... 8 QI .D E 6 :, z 4 2 0 1.1 Weightage X,,act ,~ Topic 1.2 is highly scoring topic. Chemical Reactions and Equations - - - -- 1.3 Have You Observed the Effects of Oxidation Reactions in Everyday Life? 1.2 Topics~ 1.3 •► Maximum total weightage is of Topic 1.2 Types of Chemical Reactions. •> Maximum MCQ, SA I, SA 11 and LA type questions were asked from Topic 1.2 Types of Chemical Reactions. x. (!li)C!l3™® y ■ MCQ VSA ■ SAi ■ SA Ii LA ~ Physical changes : Those changes which are accompanied with change in physical properties of the substances but no new substance is formed are called physical changes, e.g., melting of ice, boiling of water, et c. s.. Chemical changes : Those changes in which the original substances lose their nature and identity t o form new chemical substances w ith different properties are called chemical changes, e.g., burn ing of coa l, cooking food, etc. Chemical reactions : The process involving a chemical change is known as a chemical reaction. The chemical substances taken init ially are called reactants and t he chemical substances which are formed during a chemical reaction are ca lled produ~ts. Thus, chemical react ion is a process in wh ich breaking of chemical bonds (present in the reactant molecules) and making of new chemical bonds (in the product molecules) occur, e.g., burning of magnesium ribbon in air. 2Mg + 0 2 ~ 2Mg0 ..___....., Reactants Product Characteristics of chemical reactions : The fol lowing observations help us to determine whether a chemical reaction has taken place or not. 2 t ·n chemical reactions are . tate . Cer a1 h ► Change m_ s "th t he change of state, e.g. , w ~n a accompanied WI nd oxygen gas is ignited mixture of hy~rogenkga:r:om temperature, liquid with an electric spar a water is formed. . electric spark ) 2H20 (/) 2H2(g) + 02(g) room temp. Water Hydrogen Oxygen . . I . Certain chemica l reactions are Ch nge m co our . h ► a . d ·th the change of colour, e.g., w en accompani~ w_i h ted yellow lead monoxide is red lead oxide is ea , formed. 2Pb30 4(sl ~ 6Pb0(sl + Oz(gJi Lead tetraoxide Lead monoxide Oxygen (red colour) (yellow colour) ► Evolution of a gas : Some chemical reactions are accompanied with the evoluti_on of a ga:, e.g., reaction between a metal (like zinc, magnesium or iron) and dilute sulphuric acid produces hydrogen gas. i Zn(sl + H2S04 (dil.) ~ ~nS04(aqJ + Hz(gJ Zinc Sulphuric acid Zinc sulphate Hydrogen ► Change in temperature: Some chemical reactions occur with change in heat energy or with change in temperature. - Reactions which result in rise in temperature i.e., in which heat is evolved are called exothermic reactions e.g., C6H120 6(aq) + 602(g) ~ 6C02(g) + 6H20(1) Glucose + Heat - Reactions which result in fall in temperature i.e., in which heat is absorbed are called endothermic reactions e.g. , CaC03(s) + Heat ~ CaO(sl + C02(g) Lime stone Quick lime Carbon or lime dioxide ~ Chemical equation : A method of representing a chemical reaction in terms of words or symbols and form~lae of the substances participating in the chemical reaction is known as chemical equation. There are two ways to represent a chemical equation : ► In terms of word . Wh h . . . . s . en a c em1cal equation is wn~ten in terms of words, it is called a word equation. The chemical reaction between granulated zin d h writt . c an ydrochloric acid can be . en in terms of words as : ~nc + Hydrochloric acid ~ Reactants ~ inc ch loride+ Hydrogen ► In t Products . erms of symb I reaction bet o s and formulae : Chemica l Ween g 1 hydrochlo · • ranu ated zinc and ric acid can b . symbols and f e written in t erms of ormulae as : Zn + 2HCI ~ ZnCl 2 + H2 Reactants Products ► Conventions used in chemical equations : - The reactants are written on the left h side along with plus ( +) sign between the and rn. Similarly, products are written on the r· h hand side along w ith plus (+) sign betw'!e t them. n An arrow (~) separates t he reactants from the products. The arrowhead points towards t he product and tells about the direction of the reaction.s ~ Balanced and unbalanced chemical equations : ► Balanced chemical equation : The equation which contains an equal number of atoms of each element on both sides of the arrow is called a balanced chemical equation. 2Mg + 0 2 ~ 2Mg0 A balanced chemical equation must obey the law of conservation of mass. ► Unbalanced chemical equation : The equation in which the number of atoms of each element on both sides of the arrow are not equal is called an unbalanced chemical equation. H2 +02~H20 ~ Balancing a chemical equation : Balancing of a chemical equation means to equalise the number of atoms of each element on both sides of the equation. Consider the following steps for balancing the chemical equation : For example, iron reacts with water (steam) to form iron(II, 111) oxide and hydrogen. Step-I : Write the chemical equation in the form of a word equation. Keep the reactants on the left hand side and the products on the right hand side. Separate them by an arrow(~) with head pointing from the reactants to products. Iron+ Steam~ Iron (11 , Ill) oxide+ Hydrogen Step-II : Write down the symbols and formulae of the various reactants and products which gives skeletal chemical equation. Fe+ H20~ Fe30 4 + H2 Step-Ill: List number of atoms of different elements. -Elements Number of atoms Number of atoms onLHS onRHS_ Fe 1 3 -H 2 2 -0 1 4 Step-IV : Select the compound with maximum number of atoms t o st art balancing. It may be a reactant 0~ ~ product. In t hat compound, balance the element wit maximum number of atoms (e.g., oxygen in the given On••-.&.. ~ - \ C Combination reactions : Two or more substances combine to form single substance. e.g. , 2Mg(s) + 02(g) ➔ 2MgO(s) Magnesium Oxygen Magnesium oxide Element+ Compound e.g. , 0 2(g) + 2S02(gl ➔ 2S03(gl Oxygen Sulphur Sulphur dioxide trioxide Decomposition reactions : It is just the opposite of combination reaction. Displacement reactions : The chemical reactions in which more reactive element takes the place ofless reactive 1 element in a compound. Zn(s) + CuS04(aq) Zinc Copper(![) sulphate ➔ ZnS04(aq) + Cu(s) Zinc (II ) sulphate Copper Redox reactions : Copper deposits The reactions in which oxidation (gain of O) and reduction (loss of 0) occur simultaneously are called redox reactions. Reduction e.g., CuO + H2 ➔ Cu + Hp I -1' Oxidation Exothermic and endothermic reactions: Exothermic with liberation of heat. e.g., CrsJ + Oz(g) ➔ C02(g) + heat Endothermic with absorption of heat. e.g., N2cgJ + 0 2(gJ +heat ➔ 2NO(g) J Element + Element Compound + Compound e.g., C(sl + 0 2(g) ➔ C02(g) Carbon Oxygen Carbon e.g. , CaO(s) +C02(g) ➔ CaC0 3(s) Calcium Carbon Calcium dioxide oxide di oXJde carbonate ! ... ~:i,· ~i-:~~ ~~~~~~r(ii,· ~~d~ -i~ -h~~t~d ... ·: _ : strongly, it decomposes to give the : ,-=---- ----------.:...... i.· element mercury and oxygen. \ Thermal decomposition : M Mercury(II) erE Jl;xygen: Decomposition reactions which take oxide ~ =-- · gas : place by absorption of heat. I ~ I .;_ ______________ ;;;J_ ... . .. ... . .. . ..... . ... . ...... -~ .. . ..... . : Photodecomposition : Decomposition reactions which take place by absorption of light e.g., 2AgBr(s) Sunlight ) 2Ag(s)+ Brz(I) ---.._.,.._;;~=====-- S_il_ve_r_br_o_m_id_e_ Silver Bromine ;J Electrolytic decomposition : Decomposition reactions which take place when electric current is passed. e.g., lH O Electricity lH + O 2 (/) 2(g) 2(g) Water Hydrogen Oxygen Grey magnesium . r S~gle displacement reactions: The chemical (Mg) metal : I reactions in which one element takes the place ~ ~ Solution : __l,,, becomes : of another element in a compound. I ~ very . e.g., Mg(s) + CuS04(aq) ➔ MgS04(aq) + Cu(s) pale · l Magnesium Copper Magnesium Copper blue . _ sulphate _ _ sulphate ____ _j Blue copper Brown solid . : __ swp_~~~e. sol~ti?.~ ... ~_e,posits . . - - ~ Double displacement reactions : Two n compounds react by exchange of ions to form -two new compounds. e.g.,NaCl caqJ + AgN03(aqJ ➔ AgClcsJ + NaN03(aqJ Sodium Silver Silver Sodium chloride nitrate chloride nitrate - Neutralisation reactions : When an aqueous solution of an acid reacts with a base by I~ exchange of their ions/radicals to form salt and water. e.g., H2S04(aq) + 2NaOH(aq) ➔ Na2S04(aq) + 2H20m Sulphuric acid Sodium hydroxide Sodium sulphate Water ,., Precipitation reactions : The reactions in which one of the products formed is an insoluble ~ ' , substance and is separated out of the solution as solid (precipitate) is called precipitation reaction. It is indicated by downward arrow ( J, ). e.g., Pb(N03h (aql + 2KI(aql ➔ Pbl2(sl J, + 2KN03(aql Lead nitrate Potassium iodide Lead iodide Potassium nitrate Double displacement reactions in which one of the products formed is a precipitate, are also called precipitation reactions. Previous Years' CBSE Board Questions 1.1 Chemical Equations IMi•I . . . t 'th water to form sodium hydroxide 1 Sodium reac s w1 . h' h · d gas The balanced equation w 1c and hy rogen • . . represents the above reaction is (a) Na(s) + 2H20 (I) ➔ 2t;JaOH(aq) + 2H2(g) ---«,) 2Na(sl + 2H20 (ll ➔ 2NaOH(aq) + H2(g) (c) 2Na(s) + 2H20 ui ➔ NaOH(aq) + 2H2(g) (d) 2Na(sl + H20 (1) ➔ 2NaOH(aq) + 2H2(g) ~ (Term/, 2021-22) ~ 2. It is important to balance the chemical equations to satisfy the law of conservation of mass. Which of the following statements of the law is incorrect? (a) The total mass of the elements present in the reactants is equal to the total mass of the elements present in the products. (b) The number of atoms of each element remains the same, before and after a chemical reaction. (c) The chemica l composition of the reactants is the same before and after the reaction. (d) Mass can neither be created nor can it be destroyed in a chemica l reaction. (Term I, 2021-22) @ 3. In which of the following, the identity of initial substance remains unchanged? 4. (a) Curdl ing of milk (b) Formation of crystals by process of crystallisation (c) Fermentation of grapes (d) Digestion of food (2020) Identify 'x', ' y' and 'z' in t he following reaction : 2KCI0 3(xl ~ 2KCl(x) + O2(zl (a) x = gas; y = react ion condition; z = gas (b) x = solid; y = liquid; z = gas (c) x = ~~mber of moles of KCIO3; y = reaction cond1t1on; z = number of molecules of oxygen (d) x = P~~sical state of KCIO3 and KCI ; y = reaction condition, z = physical state of o 2 . (2020) S. Assertion (A) . Foll • . b . · owing is a alanced chemical equat ion for the action of steam on iron . 3Fe + 4H20 ~ Fe O + 4H . Reason (R) . Th 1 3 4 2 good for .h ~ aw of conservation of mass holds ( ) a c em1cal equation a Both (A) and (R) . are true and reason (R) ·1s t he correct expla t' (b) Both (A) and na ion of the assertion (A). th (R) are true, but reason (R) . t e correct expl . 1s no (c) (A) is true, but (:)ni~~~l~;f the assertion (A). (d) (A) is false, but (R) is tr · ue. (2020) Wf1;W (1 mark) 6. 7. What is a ba lanced chemical equation? ( 2021 CJ Translate t he following st atement into a bal h . I . anced c em1ca equation : " Barium chloride reacts with aluminium sulphat . I . . hi 'd e to give a umm1um c on e and barium sulphate:· (2019) HifiiM (2 marks) -8. Give the chemical name of the reactants as well as the products of the following chemical equation: HNO3 + Ca(OH)2 ~ Ca(NO3)2 + H2O (2021 C) 4itiil (3 marks) 9. Explain the significance of photosynthesis. Write the balanced chemical equation involved in the process. (Board Term/, 2017)C!J 10. Write balanced chemical equations for the following chemical reactions : (a) Hydrogen+ Chlorine ➔ Hydrogen chloride (NCERT lntext) (b) Lead + Copper chloride ➔ Lead chloride+ Copper (c) Zinc oxide+ Carbon ➔ Zinc+ Carbon monoxide (Board Term I, 2014) 1.2 Types of Chemical Reactions 1Mti01 11. When aqueous solutions of potassium iodide and lead nitrate are mixed, an insoluble substance separates out. The chemical equation for the reaction involved is (a) Kl+ PbNO3 ---➔ Pbl + KNO3 (b) 2KI + Pb(NO3)2 ---➔ Pbl2 + 2KNO3 (c) Kl+ Pb(NO3) 2 ---➔ Pbl+ KNO3 (d) Kl+ PbNO3 ---➔ Pbl2 + KNO3 (2023)@ 12. A metal ribbon 'X' burns in oxygen w ith a dazzling white flame forming a whit e ash 'Y. The correct description of X, Y and the type of reaction is (a) X = Ca; Y = CaO; Type of reaction = Decomposition (b) X = Mg; Y = MgO; Type of reaction = Combination (c) X = Al ; Y = Al 2 0 3 ; Type of reaction = Thermal decomposition (d) X = Zn; Y= ZnO; Type of reaction = Endot hermic (2023) 1~ The balanced chemical equation showing reaction between quick lime and water is (a) 2Ca0 + H20 ~ 2CaOH + H2 + Heat (b) CaO + H20 ~ Ca(OH)2 + H2 + Heat (c) CaO + H20 ~ Ca(OH) 2 + Heat (d) 2Ca0 + 3H20 ~ 2Ca(OH)3 + 0 2 + Heat (2023) 14. Study the following chemical reaction: 2Na(sl + 2H20 ul ~ 2NaOH(aqJ + H2(sli The reducing agent in this reaction is (a) Na (b) H20 (c) NaOH (d) H2 (2023) 15. Assertion (A) : In the following reaction ZnO+ C~Zn + CO ZnO undergoes reduct ion. Reason (R) : Carbon is a reducing agent that reduces ZnO to Zn. (a) Both Assertion (A) and Reason (R) are true and Reason (R) is t he cor rect explanation of the Assert ion (A) (b) Both Assert ion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assert ion (A) (c) Assertion (A) is true, but Reason (R) is False. (d) Assert ion (A) is false, but Reason (R) is true. (2023) ~ 16. Assertion (A) : Reaction of quick lime with water is an exothermic reaction. Reason (R) : Quickl ime reacts vigorously with water releasing a large amount of heat. (a) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A) (b) Both Assert ion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A) (c) Assert ion (A) is true, but Reason (R) is False. (d) Assert ion (A) is false, but Reason (R) is true. (2023) 17. A student took sodium sulphate solution in a test tube and added barium chloride solution to it. He observed that an insoluble substance has formed. The colour and molecu lar formula of the insoluble substance is 18. (a) grey, Ba2S04 (c) white, BaS04 (b) yellow, Ba(SO 4)2 (d) pink, Ba504 (Term /, 2021-22)~ C6H1206(aq) + 602(sl ➔ 6C02(sl + 6H20 (1) The above reaction is a/an (a) displacement reaction (b) endothermic reaction (c) exothermic reaction (d) neutralisation react ion. (Term I, 2021-22)@ 19. Which of the following statement about the reaction given below are correct? Mn02 + 4HCI ➔ MnCl2 + 2H20 + Cl2 (i) HCI is oxidized to Cl2. (ii) Mn02 is reduced to MnCl2. (iii) MnCl2 acts as an oxidizing agent. (iv) HCI acts as an oxidizing agent. (a) (ii), (iii) and (iv) (b) (i) , (ii) and (iii) (c) (i) and (ii) only (d) (iii) and (iv) only (Term/, 2021-22) 20. Which one of the fo llowing reactions is catogorised as t hermal decomposition reaction? (a) 2H20 ul ➔ 2H2(gl + 0 2(gl (b) 2AgBr(sl ➔ 2Ag(s) + Br2(gl (c) 2AgCl (s) ➔ 2Ag(sl + Cl2(g) (d) CaC03(s) ➔ CaO(s) + C02(g) (Term/, 2021-22) ~ 21. Assertion (A) Burning of natural gas is an endothermic process. Reason (R) : Methane gas combines with oxygen to produce carbon dioxide and water. (a) Both (A) and (R) are true and (R) is the correct explanation of (A). (b) Both (A) and (R) are true but (R) is not the correct explanation of (A) . (c) (A) is true, but (R) is false. (d) (A) is false, but (R) is true. (Term I, 2021-22) 22. Consider the following processes I. Dilution of sulphuric acid 11. Sublimation of dry ice 111. Condensation of water vapours IV. Dissolution of ammonium chloride in water The endothermic process(es) is/are (a) I and Ill (b) II only (c) 111 only (d) 11 and IV (Term/, 2021-22) ~ 23. When lead nitrate powder is heated in boiling tube, we observe (a) brown fumes of nitrogen dioxide (b) brown fumes of lead oxide (c) yellow fumes of nitrogen dioxide (d) brown fumes of nitric oxide. (Term/, 2021-22) @ 24. Assertion (A) : Silver salts are used in black and white photography. Reason (R) : Silver salts do not decompose in the presence of light. (a) Both (A) and (R) are true and (R) is the correct explanation of (A). (b) Bot h (A) and (R) are true but (R) is not the correct explanation of (A). (c) (A) is true, but (R) is false. (d) (A) is false, but (R) is true. (Term/, 2021-22) 25. Calcium oxide reacts vigorously with water to produce slaked lime. CaO(sl + H20 ul ~ Ca(OH)2(aq) MtiA (5 m~~s) __ - t· n be a redox a displacement reac ,o 55. (a) Can . I . with the help of an example. reaction? Exp arn of chemical reaction in the (b) Write the type following : (i) Reaction betwieen an acid and a basle2017) . f . (Board Term , (ii) Rusting o iron. Have You Observed the Effects of 1.3 Oxidation Reactions in Everyday Life? IMi•i 56. Copper utensils slowly lose their shiny brown surface and gain a green coat on prolonged exposure to atmospheric air. This is due to the formation of a coating of (a) copper sulphate (b) copper carbonate (c) cupric oxide (d) cuprous oxide. 1.1 Chemical Equations ■Mi-I (2020C) 1. Which of the following correctly represents a balanced chemical equation ? (a) Fe(s) + 4H2O(g)---) Fe3O4(s) + 4H2(g) (b) 3Fe(s) + 4H2O(gl---) Fe3O4{s) + 4H2(g) (c) 3Fe{s) + H2O(g) ---) Fe3O4(s) + H2(g) (d) 3Fe(s) + 4H2O(g)---) Fe3O4(sl + H2(gJ (Term/, 2021-22) (!ej 2. Why is it important to balance a skeletal chemical equation? (a) To verify law of conservation of energy. (b) To verify the law of coinstant proportion. (c) To verify the law of conservation of mass. (d) To verify the law of conservation of momentum. (Term I, 2021-22) (]J 1·2 Types of Chemical Reactions IMi•W 3. In the redox reaction Mn0 2 + 4HCI ---) MnCl2 + 2H O + Cl (a) Mn0 2 is reduced to Mn~I & HC21. "d• H O 2 1s ox, ,zed to 2 WitiiM (3 marks) What happens when food materia ls containing fats 57. b and oils are left for a long time? List two o servable changes and suggest thr~e ways by which this phenomenon can be prevented. (2020)~ (a) In t he following reaction : 58. H 0 MnO2 + 4HCI--) MnCl2 + 2 2 + Cl2 Identify the oxidant and reductant. (b) Give reasons : (i) Antioxidants are added to foods conta ining fats and oils. (ii) White silver chloride turns grey in sunlight. (2019 C) (i) 59. Why is respiration considered as an exothermic reaction? (NCERn (ii) Write chemical name and the formu la of 4. the brown gas produced during thermal decomposition of lead nitrate. (iii) Why do chips manufactures flush bags of chips with gas such as nitrogen? (Board Term I, 2015) J (b) MnO2 is reduced to MnCl2 & HCI is oxid ized to Cl2 (c) MnO2 is oxidized to MnCl2 & HCI is reduced to Cl2 (d) MnO2 is oxidized to MnCl2 & HCI is reduced to H20 (2022-23) ,___ __ Magnesium ri bbon Burner _ ___,'&P 1 --....~--- Watch glass ~~~~~-;:r------::::or- Magnesium oxide r----------< Which of the following is the correct observation of the reaction shown in the above set up? (a) Brown powder of M agnesium oxide is formed. (b) Colourless gas w hich turns lime water milky is evolved. (c) Magnesium ribbon burns with brill iant white light. (d) Reddish brown gas with a smell of burning Sulphur has evolved. (2022-231 LO ts ile iis its FJ lie IT) of ra1 ~~ I to to to 13) ss m of I. , is ite Chemical Reactions and Equations Assertion (A): Silver bromide decomposition is used in black and wh ite photography. Reason (R) : Light provides energy for this exothermic reaction. (a) Both A and R are true and R is the correct explanation of A. (b) Both A and R are true and R is not the correct explanation of A. (c) A is true but R is false. (d) A is false but R is true. (2022-23) ~ 6_ Reema took 5 ml of lead nitrate solution in a beaker and added approximately 4 ml of potassium iodide solution to it. What would she observe? 7. 8. 9. (a) The solution turned red. (b) Yellow precipitate was formed. (c) White precipitate was formed. (d) The reaction mixture became hot. (Term I, 2021 -22) In the reaction of iron with copper sulphate solution : CuS04 +Fe ➔ Cu+ FeS04 Which option in the given table correctly represents the substance oxidised and the reducing agent? (a) (b) (c) (d) Substance Oxidized Fe Fe Cu CuS04 Reducing Agent Fe FeS04 Fe Fe (Term I, 2021-22) (Aii: The chemical reaction between copper and oxygen can be categorized as (a) displacement reaction (b) decomposition reaction (c) combination reaction (d) double displacement reaction. (Term I, 2021-22) Limestone ~eated ) X + CO 2 tep 1 l+ H20 Step2 Slaked lime Identify the correct option from the given table which represents the type of reactions occurring in step 1 and step 2. (a) (b) (c) (d) I Endothermic X ✓ ✓ X Exothermic ✓ X ✓ X (Term I, 2021-22) ~ 11 10. Assertion (A) : Decomposition of vegetable matter into compost is an endothermic reaction. Reason (R) : Decomposition reaction involves breakdown of a single reactant into simpler products. (a) Both A and R are true and R is the correct explanation of A. (b) Both A and R are true and R is not the correct explanation of A. (c) A is true but R is false. (d) A is false but R is true. (Term I, 2021 -22) 11. Assertion (A) : After white washing the walls, a shiny white finish on walls is obtained after two to three days. Reason (R) : Calcium oxide reacts with carbon dioxide to form calcium hydrogen carbonate which gives shiny white finish. (a) Both A and R are true, and R is correct explanation of the assertion. (b) Both A and R are true, but R is not the correct explanation of the assertion. (c) A is true, but R is false. (d) A is false, but R is true. (2020-21) A\i;W 11 mark) 12. List any two observations when ferrous sulphate is heated in a dry test tube. (2020-21) 13. Identify the products formed when 1 ml of dil. hydrochloric acid is added to 1 g of sodium metal. (2020-21) 41411 13 marks) 14. (i) 0 + @ .... @ + 0 (ii) @ + @ .... @ + @ Identify the types of reaction mentioned above in (i) and (ii). Give one example for each type in the form of a balanced chemical equation. (2022-23) ~ 15. (a) Which of the following reactions is/are an endothermic reaction(s) where decomposition also happens? • Respiration • Heating of lead nitrate • Decomposition of organic matter • Electrolysis of acidified water (b) Silver chloride when kept in the open turns grey. Illustrate this w ith a ba lanced chemical equation. (2020-21) (ill _ ______ JD~e!t~a~il~e~d~ f~=l;~••)~\._1.1 .. • .. ~ .. • .. ~.. ►... -r ..e._ _______ ____ IIDPIMl·P&Miti = 1. (b) : The balanced equation is 2Na(s) + 2H20(I) ~ 2Na0H(aq) + H2(g) Concept Applied (@1 :, Use the law of conservation of mass. 14. {a) : Oxidation 2Na + 2H20 ~ 2NaOH + H2 I 1' Reduction Here, Na is oxidised to NaOH and H20 is reduced to H Na acts as a reducing agent and H20 acts as an oxidisin~ agent. 2. (c) : A balanced equation follows law of conservation of mass that means the total mass of reactants is equal to the total mass of products but the chemical composition of reactants does not remain same before and after the reaction. 15. {a) : The reaction in which oxygen is added or hydrogen is removed or loss of electrons takes place is called an oxidation reaction. In the reaction, Reduction I ~ 3. (b): Formation of crystals is a physical change while ZnO + C--> Zn+ CO ./ 1' others are chemical change. 4 . (d) : 2KCI03(s) Heat) 2KCl(s) + 302(9) 5. (a) : A balanced chemical equation must obey the law of conservation of mass. 6. The equation which contains an equal number of atoms of each element on both sides of the arrow is called a balanced chemical equation. 7. 3BaCl2 + Al2(S04)3 ~ 2AICl3 + 3BaS0 4 8. Reactants:- Nitric acid, calcium hydroxide (slaked lime) Products:- Calcium nitrate, water ~- Ph?tosynthesis means synthesis with the help of light. It 1s the process that gives life to all living beings. Photosynthesis is a process by which plants utilize carbon dioxide and water in the presence of sunlight to produce glucose and oxygen. 6CO + 12H O Sunlight 2 2 Chlorophyll ) C6H1206 + 602 + 6H20 Glucose 10· (a) H2(g) + Cl2(g) ~ 2HCl(s) (b) Pb(sJ + CuCl2(aqJ ~ PbCl2(aq)+ Cu() (c) ZnO(sJ + C(s) ~ Zn(sJ + CO s 11. (b) : Pb(NO ) 2 (g) 3 2(aqJ + Kl(aq) ~ Pbl2 ( ) J, + 2KNO 5 3(aq) Yellow ppt. 12. (b) : Magnesium metal M (X) . dazzling white flame f .' g _burns in oxygen with a ormmg a white ash, MgO (Y) 2Mg+Q 2M . This is a t 2 ~ gO ype of combination rea f !J ( ) . . . C Ion. . c . Quick lime reacts . produce calcium hydro 'd (vigorously with water to I x, e slaked lim ) arge amount of heat (ex th . e and release a Oxidation (i) Carbon is getting oxidised to carbon monoxide. {ii) Zinc oxide is getting reduced to zinc. Carbon is a reducing agent that reduces ZnO to Zn. 16. {a) : Reaction of quick lime {CaO) with water is an exothermic reaction because CaO reacts vigorously with water releasing a large amount of heat. CaO(s) + H20(1) ---. Ca(OH)2(aq) + Heat 17. (c) : The reaction between sodium sulphate and barium chloride is an example of double displacement reaction or precipitation reaction in which white ppt. of Ba504 is obtained. Na2S04 + BaCl2~ 2NaCI + BaS0 4 J. (White ppt.) 1~. {c) : In the process of respiration, glucose combines with oxyg~n in cells of our body and provides energy. Thus, respiration is an exothermic process. C6H1206(aq) + 602(gJ ~ 6C02(g) + 6H20(I) + Energy Reduced I w 19. {c) : Mn02 + 4HCI ~ MnCI + 2H O Cl l 2 2 + 2 1' Oxidised As_ hrdrogen is removed from HCI to give Cl HCI is being ox1d1sed wherea . 2, M Cl h s oxygen is removed from Mn02 to give ox~· ~· ence Mn02 is reduced. Here Mn02 acts as an isrng agent !nd HCI acts as a reducing agent. Answer Tips G ~ Anoxidisingag t· th it It . . en is e substance which gets reduced se a_nd ox1d1ses other substance whereas a reducing agent is the subst h. reduces th h ance w ich gets oxidised itself and e ot er substance. 0 erm,c reaction). CaO(sJ + H20 --) C (0 (Quick lime) m ( a H)2(aq) + heat Slaked lime) 20. (d) : 2H2 o Elect ric current UJ 2H2(s) + 0 2(s) 2 sunli'ght (Electrolytic decomposition reaction! AgCl(sJ ➔ 2Ag(sJ + Cl2(s) (Photo-decomposit ion) ISS lQ ' the IS in :tion ; are 'hich dide long tion Jble ,ent t, it iced ,er he hen vith thiS Chemical Reactions and Equations 4q (a) Ferrous sulphate crystals (FeSO4.7H2O) lose water when heated and the colour of the crystals changes. It then decomposes to ferric oxide (Fe2O3), sulphur dioxide (SO2) and sulphur trioxide (SO3) with a smell of burning sulphur. (b) Th is is a thermal decomposition reaction. (c) 2FeSO4(s) ~ Fe2O3(s) + SO2(g) + SO3(g) Ferrous sulphate Ferric oxide Sulphur Sulphur dioxide trioxide Commonly Made Mistake A 2+ d f . . . F 3+ :> Ferrous ion is Fe an emc 10n 1s e . 49. (a) The black substance is formed because copper combines with oxygen. {b) The black substance is copper oxide (CuO). (c) 2Cu(s) + O2(g) ~ 2CuO(s) Copper Oxygen Copper(ll)oxide (Reddish brown) (From air) (Black) (d) The black coating on the surface can be turned reddish brown by passing hydrogen gas over the hot copper oxide. CuO(sl+H2(gl~ Cu(sl +H2O(gl (Black) (Reddish) brown 50. Decomposition reaction involving absorption of heat: t,,. ZnCO3(s) ~ ZnO(sl + CO2(g) Zinc carbonate Zinc oxide Carbon dioxide Decomposition reaction involving absorption of light: 2H O Light > 2H O 0 2 2(/) ---7 2 (/) + 2(g) Hydrogen peroxide Water Oxygen Decomposition reaction involving absorption of electrical energy : 2AI O Electric current ) 4AI(/) 2 3(/) Alumina Aluminium + 3O2(g) Oxygen 51. (i) When barium hydroxide is added into ammonium chloride, the bottom of test tube is found to be cooler. (i i) It is an endothermk reaction. Endothermic reactions involve the formation of chemical bonds through the absorption of heat from the surroundings, thereby creating a cooling effect. (iii) Ba(OH)2 + 2NH4CI ~ BaCl2 + 2NH4OH 52. (a) AgNO3(aqJ + KCl (aqJ ~ AgCl(sl + KNO3(aq) Silver Potassium Silver Potassium nitrate chloride chloride nitrate (white, insoluble) It is a double displacement reaction. (b) Ferrous su lphate crystals (FeSO4.7H2O) lose water when heated and the colour of the crystals changes. It t hen decomposes to ferric oxide (Fe2O 3 ) , sulphur dioxide (SO2) and sulphur trioxide (SO3) with a smell of burning sulphur. 15 This is a thermal decomposition react ion. Heat 2FeSO4(sl ~ Fe2O3(sl + SO2(gl + SO3(g) Ferrous Ferric Sulphur Sulphur sulphate oxide dioxide trioxide 53. (i) Combination reaction. (ii) Precipitation reaction or double displacement reaction. (iii) Thermal decomposition reaction. 54. Those reactions in which addition of hydrogen to a substance or removal of oxygen from a substance take place are called reduction reactions. Oxidation I .;i,, (a) Fe2O3 + 2AI ~ Al2O3 + 2Fe Reduction Fe2O3 is getting reduced to Fe and Al is getting oxidised to Al20 3. Oxidation I -sl, (b) 2PbO + C ~ 2Pb + CO2 Reduction PbO is reduced to Pb and C is oxidised to CO2. s Answer Tips #' !> Gain of O or loss of H : Oxidation Loss ofO or gain ofH: Reduction Oxidation I -sl, 55. (a) ZnO+C ~Zn+CO I 1' Reduction Here, carbon is oxidised to CO and ZnO is reduced to Zn. Thus, the above reaction is a displacement reaction as well as redox reaction. (b) (i) Neutralisation reaction (ii) Oxidation reaction. 56. (b) :2Cu(sl + CO2(g) + O2(gl + H2Oul ~ Air Moisture CuCO3-Cu(OH}2 Basic copper carbonate (green) 57. Food materials containing fats and oils change their taste and smell due to a process called rancidity. Rancidity is a process in which air reacts with fats and oils which changes the smell and taste of food. Methods of prevention : Vacuum packing, refrigeration of food materials, placing of food materials away from direct sunlight. Oxidation 58. (a) MnO2 + 4HCI ----"7 MnCl2 + 2H2O + Cl2 Oxidant : MnO2 Reductant : HCI Reduction 16 (b) C) Slow oxidation of oils and fats present in food mate:ials resulting in compounds with unpleasant smell is known as rancidity. . Antioxidants are added to foods con:aining fats _and oils to prevent the oxidation of fats and oils and thus increase the shelf life of food. (ii) 2AgCl(sJ -----7 2Ag + Cl2 White Grey It undergoes photochemical decomposition re_action. Silver ch loride turns grey in sunlight to form silver metal. 59. (i) The glucose produced in our body during digestion combines with oxygen in the cells o! our b~dy and provides energy. The special name of this reaction is respiration. Thus respiration is an exothermic process because energy is produced during this process. C6H12O6(aqJ + 6O2(gJ ---) 6CO2(g) + 6H2Ou) + Energy (ii) 2Pb(NO3)2 Heat ) 2PbO(s) + 4NO(g) + O2(gl Lead Lead Nitrogen Oxygen nitrate monoxide dioxide (Brown fumes) Brown gas evolved is nitrogen dioxide (NO2). (iii) Chips manufacturers usually flush bags of chips with gas such as nitrogen because atmospheric oxygen can react with chips which may cause change in colour, change in taste. So to cut the contact between air and the chips, nitrogen gas is used which prevents the chips from getting oxidised. , , .. uestions 1. (b) : 3Fe(s) + 4H2O(gl ~ Fe3O4(sl + 4H2(gl (0.80) 2. (c) : The chemical equation in which the number of atoms of each element on both the sides are equal is called the balanced chemical equation. A chemical equation should be balanced because the total mass of elements present in the product of a chemical reaction has to be equal to the total mass of elements present in the reactants, i.e., the law of conservation of mass must hold good for the reaction. (0.80) 3. (b): MnO2 is reduced to MnCl2 as the oxidation state of Mn reduces from +4 to +2 while HCI is oxidized to Cl as the oxidation state of Cl changes from -1 to O. (1) 4. (c) : Magnesium ribbon burns with brilliant white light. (1) 5 ( ) . 2A B sunlight · c · g r(s) ) 2Ag(s) + Br 2(g) !his reaction is used in black and white photography. This 1s an endothermic reaction as it requires energy in the form of light. (1) 6. (b) : Yellow precipitate is formed. Pb(NO3)2 + 2KI ➔ PbI2l + 2KNO3 (0.80) Yellow ppt. 7. mtm CBSE Champion Science Clas s 10 (a) : Here Fe is oxidized and acts as a reducing a &ent 8. (c) : Combination reaction. (0.8oi Heat 2Cu + 0 2 ) 2CuO (O ( ) Heat '80) 9. c : CaCO3 ) CaO + CO2; Endothermic rea t· c ion CaO + H2O ➔ Ca(OH)2 +Heat; Exothermic reaction 10. (d) : Decomposition of vegetable compost is an exothermic reaction. (0.80) matter into (0.80) 11. (c) : Assertion is true, but reason is false. Ca(OH)2(aql + CO2(gl ➔ CaCO3(sl + H2Ou1 Calcium hydroxide reacts with carbon dioxide in air to form calcium carbonate which gives shiny white finish. (l) Heat 12. 2FeSO4 ) Fe2O3(sl + SO2(sl + so 3 \sl Ferrous Ferric oxide Sulphur Sulphur sulphate (Reddish (Green) brown) dioxide (Smell of burning sulphur) trioxide (i) Green coloured ferrous sulphate decomposes to reddish brown ferric oxide. (ii) Colourless gas with choking smell is evolved. (1) 13. 2Na + 2HCI ~ 2NaCI + H2(sl Sodium Hydrochloric Sodium Hydrogen metal acid chloride Sodium chloride and hydrogen gas are formed. (1) 14. (i) Displacement Reaction Fe(s) + CuSO 4(aq) ➔ FeSO 4 (aq) + Cu(s) Zn(s) + Cu5O4(aq) ➔ Zn5O4(aq) + Cu(s) Pb(sl + CuCl2(aql ➔ PbCl2(aq) + Cu(sl (1.5) (Any one of the reaction or other displacement reaction.) (ii) Double displacement Reaction Na25O4 (aq) + BaCl2(aq) ➔ BaSO4(s) + 2NaCl (aq) (Any one of the reaction or other double displacement reaction.) (1.5) 15. (a) Heating of lead nitrate and electrolysis of acidified water involves decomposition process and these are endothermic reactions as heat is absorbed in these reactions. 2Pb(NO3)2 heat ) 2PbO(sl + 4NO2(gl Lead Lead Nitrogen nitrate monoxide dioxide + O2(g) Oxygen (Brown fumes) 2H2O(/) electric ) 2H + 0 current 2(8') 2(8') (2) (b) 2AgCl(s) Sunlight ) 2Ag(sl + Cl2(g) (1) (Grey) Silver chloride turns grey in sunlight to form silver metal. _ ___________ _i_ __ _ ass lQ ~&ent to.soi :a.so) 1ction 0.80) into 0.80) 1ir to h. (1) 03(g) lphur oxide s to (1) (1) 1.5) Jn.) lent 1.5) ; of and d in (2) (1) al. Self Assessment L----- -------------- 2. @ase Based Questions (4 or 5 marks) 1. Read the passage given below and answer the following questions: In decomposition reactions, a single reactant breaks down to form two or more products. Decomposition reaction is opposite to combination reaction. Thermal decomposition reactions use the energy in form of heat for decomposition of reactants. Electrolytic decomposition reactions involve the use of electrical energy for the decomposition of reactant molecules. Photolysis or photochemical decomposition involves the use of light energy for the purpose of decomposition. (i) Give one example of decomposition reaction. (1 mark) (ii) 2Pb(N03)2 ~ 2Pb0 + nA + 0 2 Assuming n as coefficient and A as product, what is nA in the given reaction? (1 mark) (iii) Amino acid is formed by the decomposition of which component of our diet? Is decomposition reaction exothermic or endothermic? (2 marks) OR What happens when AgCI is exposed to sunlight? What type of chemical reaction takes place when electricity is passed through water? Read the passage given below and answer the following questions : Oxidation has damaging effect on metals as well as on food. The damaging effect of oxidation on metal is studied as corrosion and that on food is studied as rancidity. The phenomenon due to which metals are slowly eaten away by the reaction of air, water and chemicals present in atmosphere, is called corrosion. For example, iron articles are shiny when new, but get coated with a reddish brown powder when left for sometime. This process is known as rusting of iron. Rancidity is the process of slow oxidation of oil and fat (which are volatile in nature) present in the food materials resulting in the change of smell and taste in them. (i) Rancid ity can be prevented by {a) adding antioxidants (b) packaging oily food in nitrogen gas (c) both (a) and (b) (d) none of these. (ii) Combination of phosphorus and oxygen is an example of (a) oxidation (b) reduction (c) decomposition (d) none of these. (iii) A science teacher wrote the following statements about rancidity : (I) When fats and oils are reduced, they become rancid. (11) In chips packet, rancidity is prevented by oxygen. (111) Rancidity is prevented by adding antioxidants. Select the correct option. (a) (I) only (b) (11) and (111) only (c) (Ill) only (d} (I), (II) and (111) (iv) Two statements are given below regarding rusting of iron. (I) The rusting of iron is a redox reaction and 3+ 602-reaction occurs as, 4Fe + 302 ~ 4Fe + (11) The metallic iron is oxidised to Fe2 + and 02 is reduced to 0 2-. Select the correct statement(s). (a) I only (b) 11 only (c) Both I and 11 (d) None of these (v) Which of the following measures can be adopted to prevent or slow down rancidity? (I) Food materials should be packed in air tight container. (11) Food should be refrigerated. (Ill) Food materials and cooked food should be kept away from direct sunlight. (a) Only II and 111 (b) Only I and 111 (c) Only I and II (d) I, II and 111 (A & R Questions (1 mark) In the following questions a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following. (a) Assertion and reason both are correct statements and reason is correct explanation for assertion. (b) Assertion and reason both are correct statements but reason is not correct explanation for assertion. (c) Assertion is correct statement but reason is wrong statement. (d) Assertion is wrong statement but reason is correct statement. 3. Assertion : Zinc reacts with sulphuric acid to form zinc sulphate and hydrogen gas and it is a displacement reaction. Reason : Zinc reacts with oxygen to form zinc oxide. 4 . Assertion : In a balanced chemica l equation, total mass of the reactants is equal to the total mass of the products. Reason : Mass can neither be created nor destroyed during a chemical change. 20 not be equal on the two sides of the equation is called a skeletal (or unbalanced) equation. ~ Ba CBSE Champion Scien ce Class 1 to undergo reduction. Hhence,hoxidation anct reduN,• O lways take place toget er. T ese reactions '- lion a are c 1 redox reactions. a led 19_ (a) Molecular equation : CuCl2(aq) + Fe(s) -----7 FeCl2(aq) + Cu(s) 12. (i) Painting the iron article. (ii) Greasing or oiling the iron article. Both cover the surface of iron and cut off direct contact of air and moisture with the iron surface. 13. We apply paint on iron articles so as to preven~ it from rusting. When the surface of iron is coated with paint, its surface does not come in contact with oxygen and moisture, therefore, rusting does not take place. , Total ionic equation : 14. In the process of respiration, glucose combines with oxygen in the cells of our body and provides energy. Thus, respiration is an exothermic process. C6H1z06(aql + 602(8)---"7 6C02(sl + 6H20w + Energy oxidation 15. (i) I heat '1, 4Na(s) + 02(s) ~ 2Na20 (s) I t reduction Sodium is oxidised and 0 2 is reduced. oxidation I '1.- ( ·, ·1) C O H heat) C H 0 U (s) + 2(s) U(s) + 2 (n I t reduction H2 is oxidised and CuO is reduced. 16. (i) The substance X is calcium oxide (also called quick lime). Its formula is Cao. (ii) CaO(s) + H20(n ~ Ca(OHh(aq) (X) Calcium hydroxide 17. The reaction between copper(II) sulphate and iron is represented as Cu SO 4(aq) + Fe(s) ---'7 Fe SO 4(aq) + Cu(s) Blue Light green Brown In this displacement reaction, a more reactive element iron displaces another less reactive element from its compound copper(l 1) sulphate. The brown copper metal gets deposited on the iron nails. The colour of the solution changes from blue to light green due to the formation of ferrous sulphate (having Fe2 + ions). OR Calcium hydroxide (or slaked lime) is formed accompanied by a hissing sound. So much heat is evolved during the reaction that the reaction mixture starts boiling. The chemical equation for the reaction is : CaO(s) + H20 (aq)---'7 Ca(OHh(aq) + heat Quick lime Slaked lime (Cu2( + l + 2Cl(aql) + Fe(sl -----7 (Fe2~ql + 2Cl(aq)) + Cu ~ w Net ionic equation : Cu2~q) + Fe(s) -----7 Fe2~q) + Cu(sl For (b) and (c) no reaction takes place. 20 (i) In the first reaction, potassium bromides 1 . • . . . 0 ut1on reacts with chlorine solution to form potassium chi . . S . th' . onde Solution and bromine. o, in 1s reaction, chlor'i . . . . ne is displacing bromine from potassium bromide to f potassium chloride and bromine. Thus, it is a displace;~ reaction. (ii) In the second reaction, iron combines with sulphur to form iron(II) sulphide. So, it is a combination reaction. 21. (i) Oxidation is a reaction which involves addition or gain of oxygen. For example, 4Na + 0 2 -----7 2Na20 ; Heat 0 2Cu+02 ~ 2Cu (ii) Reduction is a reaction which involves loss or removal of oxygen. For example, Heat CuO + H2 ~ Cu + H20 ZnO + C ~ Zn + CO OR The chemical reactions in which one element takes the place of another element is called displacement reaction. For example: Zinc displaces copper from copper sulphate solution: Zn(s) + Cu504(aq) ~ ZnS04(aq) + Cu(s) Double displacement· reactions are those in which two compounds react to form two other compounds by mutual exchange of atoms or group of atoms. For example, in the reaction between barium chloride and sulphuric acid. barium chloride exchanges its Cl- ions with sot ions 01 H2S04. BaCl2(aq) + H2504(aq) ~ Ba504(sl + 2HCl(aql 22. (a) BaCl2(aql + Cu504(a l ~ BaS04(slt + cuCl210i1 I · q · ·tat1on tis an example of a double displacement and precipi reaction. 18. Oxidation or reduction cannot take place alone. This is because if one substance loses oxygen, i.e., undergoes reduction, there must be another substance to take up this oxygen, i.e., to undergo oxidation. Similarly, if one substance loses hydrogen, i.e. , undergoes oxidation, there : must be another substance to take up this hydrogen, i.e., '. (b) to the The surface of copper becomes black due formation of black copper oxide. 2Cu (s) + 02(8) ~ 2CuO(s) . From air Copper oxide (Black) ·d tiO' Smee, copper is oxidised to copper oxide, it is an oxi 3 . r re cf Th' · • react10 a ion. 1s 1s also an example of combination - - ·--------------~--,---------------------------------, Chemical Reactions and Equations (c) In this reaction, ferrous sulphate crysta ls (Fe5O4.7H2O) decompose to form brown solid, ferric oxide (Fe2o3) and gases having odour of burning sulphur (502 and 503) are produced. 2Fe5O4.7H2O(sl Heat) Fe2O3(s) + SO2(g) + SO3(sl + 7H2O(ll Ferrous sulphate Ferric Sulphur Sulphur (Green) oxide dioxide trioxide (Brown) Th is is an example of decomposit ion reaction. (d) Iron nails displace copper from copper sulphate solution wh ich deposits on it as reddish brown layer and also t he solution becomes green due to the formation of ferrous sulphate and blue colour of copper sulphate fades away. Fe(s) + CuSO4(aq) ~ FeSO4(aq) + Cu(s) Iron Copper Ferrous sulphate Copper su lphate (Blue) (green) This is an example of displacement reaction. (e) Quick lime (CaO) reacts with water forming slaked lime, Ca(OHh releasing a large amount of heat. CaO(s) + H2Oui ~ Ca(OHh(aq) + Heat Quick lime Water Calcium hydroxide (Slaked lime) This is an example of combination and exothermic reaction. 23. (a) Ba(s) + 12(s) ~ Bal2(s) (b) HBr(aq) + RbOH(aql ~ RbBr(aq) + H2Oui (c) Ca(s) + 2HNO3(aq) ~ Ca(NO3h(aq) + H2(g) (d) C10Ha(s) + 1202(8) ~ 10CO2(sl + 4H2Oui (e) 2Al(OHlJ(s) ~ Al2O3(s) + 3H2O(g) 24. (a) In a chemica l change, the products are quite different from the reactants. Therefore, it cannot be normally reversed. (b) A chemical equation has to be balanced to meet the requirements of the law of conservation of mass. According to the law, the total mass of the reacting species taking part in the reaction is the same as that of the products formed. Since, there is a direct re lationship between the mass of the different species and t heir number, it is always essential to balance a chemical equation. (c) Because they differ in t he arrangement of carbon atoms present and have d ifferent shapes, that is why they evolve different amount of heat. CGraphite + 0 2(g) ~ C02(g) + 393.5 kJ CDiamond + 0 2(g) ~ CO2(g) + 395.4 kJ 21 (d) The milkiness in lime water is due to the formation of calcium carbonate which is in the form of suspended particles. When carbon gas is passed in excess, t he milkiness disappears due to t he formation of soluble calcium hydrogen carbonate. CaCO3(sl + CO2(s) + H2O(aq) ~ Ca(HCO3)2(aql (White ppt.) Soluble (e) Presence of 0 2 and water (moisture) are two essential conditions for rusting. 25. (a) Corrosion : It is the process of deterioration of a metal as a result of its reaction with air or water present in environment. Therefore, when metals are exposed to atmosphere, they react with air or water present in the environment and form undesirable compounds on their surfaces. This process is called corrosion. Almost all metals except noble metals such as gold, platinum and palladium get corroded. For example, when iron is exposed to moisture for a long time, its surface acquires coating of a brown flaky substance called rust. Similarly, surface of copper acquires a green coating of basic copper carbonate, etc. (b) Rancidity : When fats and oils are exposed to air, they get oxidised and become rancid and their smell and colour change. This phenomenon is called rancidity. To prevent rancidity of food materials containing fats and oils, certain substances (called antioxidants) are added which prevent oxidation and hence rancidity. Some food materials are stored in air tight containers or the bags containing the food materials (such as chips) are flushed with inert gas such as nitrogen to prevent them from getting oxidised. OR The metals zinc and copper are not ionized or dissociated in contact with H2O. Both CuSO4 and ZnSO4 are soluble salts, and so they are written in ionic form. Molecular equation : CuSO4(aq) + Zn(s) ~ Cu(s) + ZnSO4(aq) Total ionic equation : 2+ 2- [ 2+ 2- [Cu (aq) + S04(aq)] + Zn(s) ~ Cu(s) + Zn (aq) + SO4 (aq)] . . . 2+ 2+ Net ionic equation : Cu (aq) + Zn(s) ~ Cu(s) + Zn (aq) In this displacement reaction, t he more active metal , zinc, displaces the ions of t he less active met al, copper, from its sa lt solut ion.