Unit 01 PDF - Chemical Reactions and Equations

CHAPTER 1 CHEMICAL REACTIONS AND EQUATIONS GIST OF THE LESSON 1. Chemical reaction- The processes, in which a substance or substances undergo a chemical change to produce new substance or substances, with entire new properties, are kn...

CHAPTER 1 CHEMICAL REACTIONS AND EQUATIONS GIST OF THE LESSON 1. Chemical reaction- The processes, in which a substance or substances undergo a chemical change to produce new substance or substances, with entire new properties, are known as chemical reactions. 2. Signs of a chemical reaction- These factors denote that a chemical reaction has taken place- change of state of substance, change of colour of substance, change in temperature, evolution of gas and formation of precipitate. 3. Chemical Equation: The representation of chemical reaction by means of symbols of substances in the form of formulae is called chemical equation. E.g. - H2 + O2 → H2O 4. Balanced Chemical Equation: A balanced chemical equation has number atoms of each element equal on both left and right sides of the reaction. According to Law of Conservation of Mass, mass can neither be created nor destroyed in a chemical reaction. To obey this law, the total mass of elements present in reactants must be equal to the total mass of elements presents in products. TYPES OF CHEMICAL REACTIONS- 1. Combination reactions - Reactions in which two or more reactants combine to form one product is called Combination Reactions. 2. Decomposition reactions- Reactions in which one compound decomposes into two or more compounds or elements are known as Decomposition Reaction. 3. Displacement reactions- The chemical reaction in which a more reactive element displaces a less reactive element from a compound is known as a displacement reaction. 4. Double displacement reactions- Reactions in which ions are exchanged between two reactants forming new compounds are called Double Displacement Reactions. 5. Precipitation reactions- The reaction in which precipitate is formed by the mixing of the aqueous solution of two salts is called precipitation reaction. 6. Exothermic reactions- Reactions which produce energy are called exothermic reaction. Most of the decomposition reactions are exothermic. 7. Endothermic reactions- Reactions which absorb energy are called endothermic reaction. Most of the combination reactions are endothermic. 8. Oxidation: Addition of oxygen or non-metallic element or removal of hydrogen or metallic element from a compound is known as oxidation. 9. Reduction: Addition of hydrogen or metallic element or removal of oxygen or non-metallic element from a compound is called reduction. 10.Redox reactions -A chemical reactions where oxidation and reduction both take place simultaneously are also known as redox reaction. CuO + H2 → Cu + H2O 11.Neutralization Reaction: The reaction in which an acid reacts with a base to form salt and water by an exchange of ions is called Neutralization Reaction. Example: NaOH + HCl→ NaCl + H2O FLOWCHART 5 MIND MAPPING SHORT NOTES Physical changes Processes in which no new chemical substances are formed are called physical changes. Eg – 1. Melting of Ice 2. Evaporation of water. 3. Dissolution of sugar in water. All these processes do not lead to the formation of new chemical substances. For example, Ice, water, and water vapour are chemically same. i.e., water (H2O). Solution of sugar in water still remains the same chemical substances, that is , sugar and water. Chemical changes Processes in which the original substances lose their nature and identity and form new chemical substances with different properties are called chemical changes. Eg – 1. Rusting of iron articles especially in the rainy season. 2. Souring of milk in summer 3. Burning of coke in air. In all these processes, the original substances lose their nature and identity, and form new chemical substances. For example – Rust is chemically a different compound than the original substance, iron. Similarly, we cannot drink milk which has turned sour because the properties of milk have changed to form some new substances. The process involving a chemical change is called a chemical reaction. 6 Characteristics of Chemical Reaction When a chemical reaction takes place, some changes are observed. The easily observable changes that take place in a chemical reaction are called the characteristics of the chemical reaction. These changes help us to check that a chemical reaction has taken place. Some important characteristics of chemical reactions are given below: 1. Formation of a precipitate: Some chemical reactions are accompanied by the formation of a precipitate. A precipitate is a solid substance formed on mixing two substances. For example, (i) In the reaction between lead nitrate and potassium iodide solutions, a yellow precipitate of lead iodide appears. (ii) When silver nitrate solution is mixed with sodium chloride solution, a white precipitate of silver chloride is obtained, or when barium chloride solution is mixed with sodium sulphate solution, a white precipitate of barium sulphate is obtained. 2. Evolution of gas: Some chemical reactions are accompanied by the evolution of gas. For example, (i) In the reaction between zinc and dilute sulphuric acid or hydrochloric acid, hydrogen gas is evolved. (ii) When calcium carbonate is heated, or calcium carbonate reacts with dilute hydrochloric acid, carbon dioxide is evolved. 3. Change in colour: Some chemical reactions are accompanied by change in color. For example, (i) Rust is brown in colour whereas iron is greyish black in colour. (ii) Lead nitrate and potassium iodide solutions are colourless, but on mixing the two, the precipitate of lead iodide formed is yellow in color. 4. Change in state: Some chemical reactions are accompanied by change in state. For example, (i) Solid wax (in the form of candle) burns to form water vapour and carbon dioxide which are gaseous. (ii) Petrol, which is a liquid, burns to form water vapour and carbon dioxide which are gaseous. 5. Change in temperature: Some chemical reactions are accompanied by change in temperature, i.e., rise or fall of temperature. Reactions which are accompanied by rise in temperature are those in which heat is involved. Such reactions are called exothermic reactions. For example, (i) When zinc pieces react with sulphuric acid in a beaker or a flask, it is found to be warm. This can be seen by touching the beaker or the flask. (ii) When water is added into quick lime (CaO), taken in a beaker, slaked lime Ca (OH)2, is formed and the beaker is found to be quite hot. Reactions which are accompanied by fall in temperature are those in which heat is absorbed. Such reactions are called endothermic reactions. For example, when barium hydroxide, Ba(OH)2, is added into ammonium chloride, NH4Cl, taken in a test tube, and mixed with a glass rod, then on touching the bottom of the test tube, it is found to be cooler. This is due to the following endothermic reaction taking place: Barium Hydroxide + Ammonium chloride Barium chloride + Ammonium hydroxide. Reactions involve several changes simultaneously. It is important to know that in several reactions; more than one change may be observed. i.e., they may possess more than one characteristic. For example, (i) When zinc reacts with dilute sulphuric acid, a gas i.e. (H2) is evolved and a rise in temperature takes place. (ii) When lead nitrate and potassium iodide solutions are mixed, a precipitate of lead iodide is formed. Also, change in colour takes place because the solutions taken are colourless while the precipitate formed is yellow. (iii) If hydrochloric acid is added into solid calcium carbonate taken in a beaker, solid disappears to give solution, i.e., change of state takes place. Also, gas (carbon dioxide) is evolved and rise in temperature takes place. 7  As several changes are taking place in the different substances present around us, this means that a large number of chemical reactions are occurring in the nature. Chemical Equation There are two ways of representing a chemical reaction as follows: (i) In terms of words (called word equation), For example, the above reaction (i) may be represented in the form of word equation as follows: Zinc + Sulphuric Acid Zinc Sulphate + Hydrogen (Reactants) (Products) Thus, the names of the reactants are written on the left-hand side (LHS) with a plus sign (+) between them. The names of the products are written on the right-hand side (RHS) with a plus sign between them. An arrow (→) is put between the reactants and the products, such that the direction of the arrowhead is from the reactants towards products. (ii) In terms of symbols and formulae (called chemical equation): Even the above method of representing a chemical reaction is not too short. A still shorter and faster method is used by writing symbols and formulae of the different reactants and products in place of their names. Thus in terms of symbols and formulae, the above reaction may be represented as follows: Zn + H2SO4 → ZnSO4 + H2 This is called a chemical equation. In fact, this is the most common method of representing a chemical reaction. Hence, we define a chemical equation as follows: “The short-hand method of representing a chemical reaction in terms of symbols and formulae of the different reactants and products is called a chemical equation. Balancing of a Chemical Equation Balancing of a chemical equation means making the number of atoms of each element equal on both sides of the equation. Steps involved in the balancing of a chemical equation: Step I: To write the word equation-Write down the equation in the word form by writing the names of the reactants on the left side, and those of the products on the right side. This step is not required if the equation is given in terms of symbols and formulas. Step II: To write the skeletal chemical equation- Write down the symbols and formulas of the various reactants and products. This gives us the skeletal chemical equation. Step III: To enclose the formulas in boxes- Enclose the formula of each reactant and product in a box. This is done to remember that during balancing of the chemical equation, the formula of any reactant or product cannot be changed. Step IV: To list the number of atoms of different elements on LHS(Reactants) and RHS (Products) Step V: To select the biggest formula to start balancing- Start the process of balancing by choosing the compound which has the maximum number of atoms, irrespective of the fact whether it is a reactant or a product. Step VI: To start balancing of different elements-Having selected the compound with the biggest formula as above, first balance the element of this compound which has the highest number of atoms. Then balance other elements one by one. To balance the atoms of an element, put a small whole number coefficient before the formula of the compound (or symbol of the element for elementary substances). If selection of the biggest formula appears inconvenient, balance the atoms of that element which occurs at minimum number of places on both sides of the equation. Atoms of the element which occur at maximum places are balanced last of all. Step VII: To check the correctness of the balanced equation- Finally check the correctness of the balanced equation by counting the number of atoms of each element on both sides of the equation. 8 The above method of balancing of chemical equation is called Hit and Trial method as we keep on trying to balance the equation using smallest whole number coefficients. TYPES OF CHEMICAL REACTIONS Chemical reactions can be classified in following types: (i) Combination Reaction: Reactions in which two or more reactants combine to form one product is called Combination Reactions. (A + B → AB) e.g.: 2 Mg(s) + O2(g) → 2MgO(s) C (s) + O2(g) → CO2(g) (ii) Decomposition Reaction: Reactions in which one compound decomposes into two or more compounds or elements are known as Decomposition Reaction. A decomposition reaction is just opposite of combination reaction. (AB → A + B) Types of decomposition: 1. Thermal Decomposition: The decomposition of a substance on heating is known as Thermal Decomposition. Example i) ii) 2. Electrolytic Decomposition: Reactions in which compounds decompose into simpler compounds because of passing of electricity, are known as Electrolytic Decomposition. This is also known as Electrolysis. Example: When electricity is passed in water, it decomposes into hydrogen and oxygen. 9 3. Photochemical Decomposition: Reactions in which a compound decomposes because of sunlight are known as photolysis or photo chemical decomposition reaction. Example: When silver chloride is put in sunlight, it decomposes into silver metal and chlorine gas. Photographic paper has a coat of silver chloride, which turns into grey when exposed to sunlight. It happens because silver chloride is colourless while silver is a grey metal. (iii) Displacement Reaction: The chemical reaction in which a more reactive element displaces a less reactive element from a compound is known as a displacement reaction (A + BC → AC + B) Displacement reaction takes place only when ‘A’ is more reactive than B. Eg: Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g) Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s) (iv) Double Displacement Reaction: Reactions in which ions are exchanged between two reactants forming new compounds are called Double Displacement Reactions. (AB + CD →AC + BD) eg: i) When the solution of barium chloride reacts with the solution of sodium sulphate, white precipitate of barium sulphate is formed along with sodium chloride. BaCl2(aq) + Na2SO4(aq) → BaSO4(s) (Precipitate) + 2NaCl(aq) ii) When sodium hydroxide (a base) reacts with hydrochloric acid, sodium chloride and water are formed. NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) Note: Double Displacement Reaction, in which precipitate is formed, is also known as precipitation reaction. Neutralisation reactions are also examples of double displacement reaction. Precipitation Reaction: The reaction in which precipitate is formed by the mixing of the aqueous solution of two salts is called precipitation reaction. 10 Example: AgNO3 + NaCl → AgCl + NaNO3 precipitate (v) Neutralization Reaction: The reaction in which an acid reacts with a base to form salt and water by an exchange of ions is called Neutralization Reaction. Example: NaOH + HCl→ NaCl + H2O (vi) Redox reactions Oxidation: Addition of oxygen or removal of hydrogen from a compound is known as oxidation. Elements or compounds in which oxygen or non-metallic element is added or hydrogen or metallic element is removed are called to be oxidized. Reduction: Addition of hydrogen or removal of oxygen from a compound is called Reduction. The compound or element which undergoes reduction is called to be reduced. Oxidizing agent: The substance which gives oxygen for oxidation is called an Oxidizing agent. The substance which removes hydrogen is also called an Oxidizing agent. Reducing agent: The substance which gives hydrogen for reduction is called a reducing agent. The substance which removes oxygen is also called a reducing agent. The reaction in which oxidation and reduction both take place simultaneously is called Redox reaction. When copper oxide is heated with hydrogen, then copper metal and water are formed. CuO + H2 → Cu + H2O (i) In this reaction, CuO is changing into Cu. Oxygen is being removed from copper oxide. 11 Removal of oxygen from a substance is called reduction, so copper oxide is being reduced to copper. (ii) In this reaction, H2 is changing to H2O. Oxygen is being added to hydrogen. Addition of oxygen to a substance is called oxidation, so hydrogen is being oxidised to water. The substance which gets oxidised is the reducing agent. The substance which gets reduced is the oxidizing agent. (vii) Exothermic and Endothermic Reactions: Exothermic Reaction: Reaction in which energy is produced is called Exothermic Reaction. Most of the decomposition reactions are exothermic. Example: (i)Respiration is an exothermic reaction in which energy is released. (ii)When quick lime (CaO) is added to water, it releases heat energy. CaO(s) + H2O(l) ⟶ Ca(OH)2(aq) + Heat Calcium oxide Water Calcium hydroxide Endothermic Reaction: A chemical reaction in which heat energy is absorbed is called Endothermic Reaction. Example: Decomposition of calcium carbonate. SECTION A MULTIPLE CHOICE QUESTIONS 1. MnO2 + 4HCl →MnCl2 + 2H2O + Cl2 Identify the substance oxidized in the above equation. (a) MnCl2 (b) HCl (c) H2O (d) MnO2 2. Electrolysis of water is a decomposition reaction. The mole ratio of hydrogen and oxygen gases liberated during electrolysis of water is: (a) 1: 1 (b) 2:1 (c) 4:1 (d) 1:2 3. Pb + CuCl2 → PbCl2 + Cu The above reaction is an example of: (a) combination (b) double displacement (c) decomposition (d) displacement 4. When green coloured ferrous sulphate crystals are heated, the colour of the crystal changes because (a) it is decomposed to ferric oxide (b) it loses water of crystallisation (c) it forms SO2 (d) it forms SO3 5. 2HNO3 + Ca (OH)2 → Ca (NO3)2 + 2H2O; is an example of (i) displacement reaction (ii) double displacement reaction (iii) neutralisation reaction (iv) combination reaction. (a) (i) and (ii) (b) (ii) and (iii) (c) (iii) and (iv) (d) (i) and (iv) 12 6. What is observed when a solution of potassium iodide is added to silver nitrate solution? (a) No reaction takes place (b) White precipitate of silver iodide is formed (c) yellow precipitate of Agl is formed (d) Agl is soluble in water. 7. Identify ‘x’, ‘y’ and ‘z’ in the following balanced reaction. x Pb(NO3)2 (s) y PbO+ z NO2 +O2 (a) 2, 4, 2 (b) 2, 2, 4 (c) 2, 4, 4 (d) 4, 2, 2 8. Oxidation involves (i) gain of electron (ii) loss of electron (iii) addition of oxygen or electronegative element (iv) removal of hydrogen or electropositive element (a) (i), (ii), (iii) (b) (ii), (iii), (iv) (c) (i), (iii), (iv) (d) (i), (ii), (iv) 9. Which of the following statements about the given reaction are correct? 3Fe(s) + 4H2O(g) → Fe3O4(g) + 4H2(g) (i) Iron metal is getting oxidised (ii) Water is getting reduced (iii) Water is acting as reducing agent (iv) Water is acting as an oxidising agent (a) (i), (ii) and (iii) (b) (iii) and (iv) (c) (i), (ii) and (iv) (d) (ii) and (iv) 10. Barium chloride on reacting with ammonium sulphate forms barium sulphate and ammonium chloride. Which of the following correctly represents the type of the reaction involved? (i) Displacement reaction (ii) Precipitation reaction (iii) Combination reaction (iv) Double displacement reaction (a) (i) only (b) (ii) only (c) (iv) only (d) (ii) and (iv) 11. The brown gas evolved on heating of copper nitrate is (a) O2 (b) NO2 (c) N2 (d) NO 12. Zinc reacts with silver nitrate to form which compounds? (a) Zn (NO3)2 + Ag (b) ZnNO3 + Ag (c) AgNO3 + Zn (NO3)2 (d) Ag + Zn (NO3)3 13. Which of the reactions is used in black and white photography? (a) Combination Reaction (b) Decomposition Reaction (c) Displacement reaction (d) Oxidation reaction 14. Which option shows oxidation? (a) Zn → Zn 2+ (b) 2H+ → H2 (c) Zn 2+ → Zn (d) H2 → 2H 13 15. A substance X which is a group 2 element is used intensively in the cement industry. This element is present in bones also. On treatment with water, it forms a solution which turns red litmus blue. Element X is (a) Cu (b) Ca (c)Na (d) Al 16. On immersing an iron nail in CuSO4 solution for few minutes (a) no reaction takes place (b) the colour of solution fades away (c) the surface of iron nails acquires a black coating (d) the colour of solution changes to green 17. In which of the following, heat energy will be evolved? (a) Electrolysis of water (b) Dissolution of NH4Cl in water (c) Burning of L.P.G. (d) Decomposition of AgBr in the presence of sunlight 18. A substance ‘X’ is used in white-washing and is obtained by heating limestone in the absence of air. Identify ‘X’. (a) CaOCl2 (b) Ca (OH)2 (c) CaO (d) CaCO3 19. In the double displacement reaction between aqueous potassium iodide and aqueous lead nitrate, a yellow precipitate of lead iodide is formed. While performing the activity if lead nitrate is not available, which of the following can be used in place of lead nitrate? a) Lead sulphate (insoluble) (b) Lead acetate (c) Ammonium nitrate (d) Potassium sulphate 20. When Ag is exposed to air it gets a black coating of (a) AgNO3 (b) Ag2S (c) Ag2O (d) Ag2CO3 MULTIPLE CHOICE QUESTIONS ANSWERS 1.b 2. b 3.d 4. b 5.b 6.c 7.b 8. b 9.c 10. d 11.b 12. a 13. b 14. a 15. b16.d 17.c 18. c 19. b 20. b SECTION B ASSERTION- REASON TYPE QUESTIONS DIRECTION: Each of these questions contains an assertion followed by reason. Read them carefully and answer the question on the basis of following options. You have to select the one that best describes the two statements. (a) If both Assertion and Reason are correct and reason is the correct explanation of Assertion. (b) If both Assertion and Reason are correct, but reason is not the correct explanation of Assertion. (c) If Assertion is correct but Reason is incorrect. (d) If Assertion is incorrect but Reason is correct. Q1. Assertion (A): Photosynthesis is considered as an endothermic reaction. Reason (R): Energy gets released in the process of photosynthesis 14 Q2. Assertion (A): Decomposition of vegetable matter into compost is an example of exothermic reactions. Reason (R): Exothermic reaction are those reactions in which heat is evolved. Q3. Assertion (A): When HCl is added to zinc granules, a chemical reaction occurs. Reason (R): Evolution of a gas indicates that the chemical reaction is taking place. Q4. Assertion (A): Calcium carbonate when heated gives calcium oxide and water. Reason (R): On heating calcium carbonate, decomposition reaction takes place. Q5. Assertion (A): Chemical reaction changes the physical and chemical properties of a substance Reason (R): Chemical change involves a change in the chemical composition of matter, and a new substance is formed 6. Assertion (A): AgBr is used in photographic and X-ray film. Reason (R): AgBr is photosensitive and changes to Ag and bromine in presence of sunlight and undergoes decomposition reaction. 7. Assertion (A): Magnesium ribbon keeps on burning in atmosphere of nitrogen. Reason (R): Magnesium reacts with nitrogen to form magnesium nitrides and this reaction is combination reaction. 8. Assertion (A): Zinc reacts with sulphuric acid to form zinc sulphate and hydrogen gas and it is displacement reaction. Reason (R): Zinc reacts with oxygen to form zinc oxide. 9. Assertion (A): MnO2 + 4HCl → MnCl2 + Cl2 + 2H2O is redox reaction. Reason (R): MnO2 oxidises HCl to Cl2 and gets reduced to MnCl2. 10. Assertion (A): White silver chloride turns grey in sunlight. Reason (R): Decomposition of silver chloride in presence of sunlight takes place to form silver metal and chlorine gas. ASSERTION – REASON ANSWER KEY Q NO 1 2 3 4 5 6 7 8 9 10 ANS C A A A A A A B A A SECTION C CASE STUDY QUESTIONS 1.A chemical reaction is a representation of chemical change in terms of symbols and formulae of reactants and products. There are various types of chemical reactions like combination, decomposition, displacement, double displacement, oxidation and reduction reactions. Reactions in which heat is released along with the formation of products are called exothermic chemical reactions. All combustion reactions are exothermic reactions. (i) The massive force that pushes the rocket forward through space is generated due to the (a) combination reaction (b) decomposition reaction (c) displacement reaction (d) double displacement reaction Answer: (b) The massive force that pushes the rocket forward through space is generated due to the decomposition reaction. Hydrogen peroxide decomposes and provides it with a considerable reaction force thrust. (ii) A white salt on heating decomposes to give brown fumes and yellow residue is left behind. The yellow residue left is of 15 (a) lead nitrate (b) nitrogen oxide (c) lead oxide (d) oxygen gas Answer: (c) Lead nitrate decomposes to give brown fumes of nitrogen dioxide gas and yellow residue of lead oxide is left behind. (iii) Which of the following reactions represents a combination reaction? (a) CaO (s) + H2O (l) → Ca (OH)2 (aq) (b) CaCO3 (s) → CaO (s) + CO2(g) (c) Zn(s) + CuSO4 (aq) → ZnSO4 (aq) + Cu(s) (d) 2FeSO4(s) → Fe2O3 (s) +SO2(g) + SO3(g) Answer: (a) A reaction in which two or more reactants combine to form a single product is known as a combination reaction. (iv) Complete the following statements by choosing correct type of reaction for X and Y. Statement 1: The heating of lead nitrate is an example of ‘X’ reaction. Statement 2: The burning of magnesium is an example of ‘Y’ reaction. (a)X-Combination,Y-Decomposition (b)X-Decomposition,Y-Combination (c)X-Combination,Y-Displacement (d) X- Displacement, Y-Decomposition Answer: (b) Heating of lead nitrate to form nitrogen dioxide and lead oxide is an example of thermal decomposition reaction and the burning of magnesium ribbon in the air to form magnesium oxide is an example of combination reaction. 2.A reaction in which two or more reactants combine to form a single product is called a combination reaction. For example, calcium oxide reacts vigorously with water to form calcium hydroxide. The reaction is highly exothermic in nature, as lots of heat is produced during the reaction. CaO(s) + H2O(l) ⟶ Ca(OH)2(aq) + Energy Calcium oxide Water Calcium hydroxide Solution of Ca (OH)2 is used to white wash the walls. Calcium hydroxide reacts slowly with carbon dioxide in air to form a thin layer of calcium carbonate on the wall which gives a shiny appearance to wall. Calcium carbonate will form after two or three days of whitewash. (i) What is the chemical name of quick lime? (a) Calcium oxide (b) Calcium carbonate (c) Calcium hydroxide (d) Carbon dioxide (ii) When carbon dioxide is passed through lime water, (a) calcium hydroxide is formed (b) white precipitate of CaO is formed (c) lime water turns milky (d) colour of lime water becomes green. Answer: c (iii) Following observations are observed when calcium oxide reacts vigorously with water. Identify the incorrect observations (I) It is an endothermic reaction (II) Slaked lime is produced. 16 (III) Quick lime is produced. (IV) It is an exothermic reaction. (V) It is a combination reaction (a) (I) and (II) (b) (III) and (IV) (c) (I) and (III) (d) (II), (IV) and (V) Answer: c (iv) Quick lime combines vigorously with water to form (A) which reacts slowly with the carbon dioxide in air to form (B) Identify the compounds(A) and (B) (A) (B) (a) Calcium carbonate Calcium hydroxide (b) Calcium hydroxide Calcium carbonate (c) Calcium Calcium bicarbonate (d) Calcium bicarbonate Calcium Answer : b 3.Reactions in which one element takes place of another element in a compound, are known as displacement reactions. In general, more reactive elements displaces a less reactive element from its compound. In all single displacement reactions, only one element displaces another element from its compound. The single displacement reactions are, however, written as just displacement reactions. The displacement reaction between iron (III) oxide and powdered aluminium produces so much heat that iron metal obtained is in molten form. (i) Copper displaces which of the following metals from its salt solution? (a) ZnSO4 (b) FeSO4 (c) AgNO3 (d) NiSO4 Answer: c (ii) When zinc reacts with dilute sulphuric acid, the gas evolved is (a) red in colour and have a sweet smelling (b) green in colour and have a foul smell (c) colourless, odourless and burns with a pop sound (d) colourless, pungent smelling and burns with a pop sound Answer: c (iii) When dry hydrogen is passed over a heated oxide of metal X using the apparatus shown below, a reddish-brown residue is obtained The reddish -brown residue could be (a) copper (b) lead (c) silver (d) zinc Answer: a (iv) Which of the following reactions is a displacement reaction? (a) CaO + H2O⟶ Ca (OH)2 (b) MgCO3⟶ Mg+CO2 (c) Mg+CuSO4⟶ MgSO4 + Cu (d) H2 + Cl2⟶ 2HCl Answer: c (v) When dilute hydrochloric acid is added to granulated zinc placed in a test tube, the observation made is (a) the surface of the metal turns shining (b) the reaction mixture turns milky (c) greenish yellow gas is evolved (d) the colourless and odourless gas evolves with a pop sound. Answer: d 4.Those reactions in which two compounds react by an exchange of ions to form two new compounds are called double displacement reactions. A double displacement reaction usually occurs in solution and one of the products, being insoluble, precipitate out (separates as a solid). Any reaction in which an insoluble solid (called precipitate) is formed that separates from the solution is called a precipitation reaction. The reaction in 17