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Question 1: Why should a magnesium ribbon be cleaned before burning in air? Answer: A magnesium ribbon should be cleaned before burning in air because it reacts with carbon dioxide (CO2) present in the air to form a protective and inert layer of magnesium carbonate. This layer is unreactive, so it needs to be removed before burning to ensure a clean and efficient reaction.
Question 2: Write the balanced equations for the following chemical reactions. (i) Hydrogen + chlorine → hydrogen chloride Answer: H2 + Cl2 → 2HCl
(ii) Barium chloride + aluminum sulfate → barium sulfate + aluminum chloride Answer: 3BaCl2 + Al2(SO4)3 → 3BaSO4 + 2AlCl3
(iii) Sodium + water → sodium hydroxide + hydrogen Answer: 2Na + 2H2O → 2NaOH + H2
Question 3: Write a balanced chemical equation with state symbols for the following reactions. (i) Solutions of barium chloride and sodium sulfate in water react to give insoluble barium sulfate and a solution of sodium chloride. Answer: BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaCl(aq)
(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to form sodium chloride solution and water. Answer: NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
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Question 1: A solution of substance X is used for white washing. (i) Name the substance X and write its formula. Answer: Substance X used for white washing is called quicklime, and its chemical formula is CaO.
(ii) Write the reaction of substance X named in (i) with water. Answer: The reaction of quicklime (CaO) with water is as follows: CaO(s) + H2O(l) → Ca(OH)2(aq) + Heat
Question 2: Why is the amount of gas collected in one of the test tubes in activity 1.7 double of the amount collected in another? Name this gas. Answer: The gas collected in activity 1.7 (electrolysis of water) is hydrogen (H2). The amount of hydrogen gas collected is double that of the oxygen gas (O2) because water (H2O) decomposes during electrolysis to form hydrogen and oxygen gases in a ratio of 2:1 by volume.
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Question 1: Why does the color of copper sulfate solution change when an iron nail is dipped in it? Answer: When an iron nail is dipped in a copper sulfate solution, a displacement reaction occurs. Iron, being more reactive than copper, displaces copper from the copper sulfate solution, resulting in the formation of iron sulfate. This change in the composition of the solution causes the color to change from blue (copper sulfate solution) to green (iron sulfate solution).
Question 2: Give an example of a double displacement reaction other than the one given in activity 1.10. Answer: An example of a double displacement reaction is the reaction between silver nitrate (AgNO3) and sodium chloride (NaCl) to form silver chloride (AgCl) and sodium nitrate (NaNO3). AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)
Question 3: Identify the substances that are oxidized and reduced in the following reactions. (i) 4Na + O2 → 2Na2O Answer: In this reaction, sodium (Na) is oxidized because it combines with oxygen (O2) to form sodium oxide (Na2O). Oxygen is reduced because it gains electrons from sodium.
(ii) CuO + H2 → Cu + H2O Answer: In this reaction, copper oxide (CuO) is reduced because it loses oxygen and forms copper (Cu). Hydrogen (H2) is oxidized because it gains oxygen and forms water (H2O).
Question 1: Which of the statements about the reactions below are incorrect? (i) Lead is getting reduced. (ii) CO2 is getting oxidized. (iii) Carbon is getting oxidized. (iv) Lead oxide is getting reduced.
Answer: The incorrect statements are (i) and (ii). In the given reaction, lead oxide (PbO) reacts with carbon (C) to form lead (Pb) and carbon dioxide (CO2). Lead is actually getting oxidized, and carbon dioxide is getting reduced.
Question 2: The reaction Fe2O3 + 2Al -> Al2O3 + 2Fe is an example of which type of reaction? (a) Combination reaction (b) Double displacement reaction (c) Decomposition reaction (d) Displacement reaction
Answer: The given reaction is a displacement reaction. Aluminum (Al) is more reactive than iron (Fe), so it displaces iron from iron(III) oxide (Fe2O3) to form aluminum oxide (Al2O3) and elemental iron (Fe).
Question 3: What happens when dilute HCl is added to iron fillings? (a) Hydrogen gas and iron chloride are produced. (b) Chlorine gas and iron hydroxide are produced. (c) No reaction takes place. (d) Iron salt and water are produced.
Answer: When dilute hydrochloric acid (HCl) is added to iron fillings, hydrogen gas and iron chloride are produced. Iron is more reactive than hydrogen, so it displaces hydrogen from the acid to form iron chloride and liberate hydrogen gas.
Question 4: What is a balanced chemical equation? Why should chemical equations be balanced?
Answer: A balanced chemical equation represents a chemical reaction in which the number of atoms of each element is equal on both sides of the equation. Chemical equations should be balanced for the following reasons:
- To comply with the law of conservation of mass, which states that the total mass of the reactants must be equal to the total mass of the products. Balancing ensures that atoms are neither created nor destroyed during a reaction.
- Balancing provides information about the physical state of reactants and products (solid, liquid, gas, or aqueous).
- Balanced equations allow us to determine the heat changes (endothermic or exothermic) that occur during a chemical reaction.
Question 5: Translate the following statements into chemical equations and then balance them. (a) Hydrogen gas combines with nitrogen to form ammonia. (b) Hydrogen sulfide gas burns in air to give water and sulfur dioxide. (c) Barium chloride reacts with aluminum sulfate to give aluminum chloride and a precipitate of barium sulfate. (d) Potassium metal reacts with water, giving potassium hydroxide and hydrogen gas.
Answer: (a) The balanced chemical equation for the first statement is: N2 + 3H2 -> 2NH3
(b) The balanced chemical equation for the second statement is: 2H2S + 3O2 -> 2H2O + 2SO2
(c) The balanced chemical equation for the third statement is: 3BaCl2 + Al2(SO4)3 -> 2AlCl3 + 3BaSO4
(d) The balanced chemical equation for the fourth statement is: 2K + 2H2O -> 2KOH + H2
Question 7. Write the balanced chemical equation for the following reactions: (a) Calcium hydroxide + carbon dioxide → calcium carbonate + water (b) Zinc + silver nitrate → zinc nitrate + silver (c) Aluminium + copper chloride → aluminium chloride + copper (d) Barium chloride + potassium sulphate → barium sulphate + potassium chloride
Answer: (a) Ca(OH)2 + CO2 → CaCO3 + H2O (b) Zn + 2AgNO3 → Zn(NO3)2 + 2Ag (c) 2Al + 3CuCl2 → 2AlCl3 + 3Cu (d) BaCl2 + K2SO4 → BaSO4 + 2KCl
Question 8. Write the balanced chemical equation for the following and identify the type of chemical reaction: (a) Potassium bromide (aq) + barium iodide (aq) → potassium iodide (aq) + barium bromide (s) (b) Zinc carbonate (s) → zinc oxide (s) + carbon dioxide (g) (c) Hydrogen (g) + chlorine (g) → hydrogen chloride (g) (d) Magnesium (s) + hydrochloric acid (aq) → magnesium chloride (aq) + hydrogen (g)
Answer: (a) 2KBr (aq) + BaI2 (aq) → 2KI (aq) + BaBr2 (s) (Double displacement reaction) (b) ZnCO3 (s) → ZnO (s) + CO2 (g) (Decomposition reaction) (c) H2 (g) + Cl2 (g) → 2HCl (g) (Combination reaction) (d) Mg (s) + 2HCl (aq) → MgCl2 (aq) + H2 (g) (Displacement reaction)
Question 9. What does one mean by exothermic and endothermic reactions? Give equations.
Answer: Exothermic reactions release heat energy, while endothermic reactions absorb heat energy.
Examples: Exothermic reaction: CH4 + 2O2 → CO2 + 2H2O + Heat
Endothermic reaction: 2HgO + Heat → 2Hg + O2
Question 10. Why is respiration considered an exothermic reaction? Explain.
Answer: Respiration is considered an exothermic reaction because it releases heat energy. When we consume food, such as fats, proteins, and carbohydrates, during the digestion process, these complex nutrients are broken down into simpler forms like glucose and fatty acids through the action of enzymes. This breakdown reaction releases heat. During respiration, the glucose and fatty acids formed through digestion are oxidized by the oxygen we breathe, resulting in the production of carbon dioxide (CO2) and water (H2O) along with the release of heat energy. Since heat is released, respiration is classified as an exothermic reaction.
Equation: C6H12O6 + 6O2 → 6CO2 + 6H2O + Heat
Glucose (from food) (Chemical Respiration)
Question 11. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Answer: Decomposition reactions are called the opposite of combination reactions because they involve the splitting of one substance into two or more simpler substances, whereas combination reactions involve the combination of two or more substances to form a new compound.
Example equations: Combination reaction: CaO + CO2 → CaCO3
2H2 + O2 → 2H2O
Decomposition reaction: CaCO3 → CaO + CO2
2FeSO4 → Fe2O3 + SO2 + SO3
Question 12. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light, or electricity.
Answer: (i) CaCO3 (s) → CaO (s) + CO2 (g) + heat
(ii) 2AgBr (s) → 2Ag (s) + Br2 (g) + light
(iii) 2H2O (l) → 2H2 (g) + O2 (g) + electricity
Question 13. What is the difference between displacement and double displacement reactions? Write equations.
Answer: I. Displacement reaction: CuSO4 (aq) + Fe (s) → FeSO4 (aq) + Cu (s)
In this reaction, iron (Fe) is more reactive than copper (Cu), so it displaces copper from copper sulfate (CuSO4), forming iron sulfate (FeSO4) and freeing copper. The color of the solution changes from blue to green. This is a single displacement reaction.
II. Double displacement reaction: AgNO3 (aq) + NaCl (aq) → AgCl (s) + NaNO3 (aq)
In this reaction, displacement occurs twice, and there is a mutual exchange of ions between silver nitrate (AgNO3) and sodium chloride (NaCl). This results in the formation of silver chloride (AgCl) as a precipitate, and sodium nitrate (NaNO3) remains in the solution. It is a double displacement reaction.
AgNO3 + NaCl → AgCl ↓ + NaNO3
Question 14. In the refining of silver, the recovery of silver from silver nitrate solution involves displacement by copper metal. Write down the reaction involved.
Answer: The chemical equation for the displacement reaction is: Cu (s) + 2AgNO3 (aq) → Cu(NO3)2 (aq) + 2Ag (s)
Copper (Cu) + Silver nitrate (AgNO3) → Copper nitrate (Cu(NO3)2) + Silver (Ag)
Question 15. What do you mean by a precipitation reaction? Explain by giving examples.
Answer: A precipitation reaction occurs when two solutions containing soluble reactants are mixed, resulting in the formation of an insoluble product called a precipitate. The precipitate separates out from the solution. These reactions are characterized by the formation of a solid product.
Examples: AgNO3 (aq) + NaCl (aq) → AgCl (s) + NaNO3 (aq)
Na2SO4 (aq) + BaCl2 (aq) → BaSO4 (s) + 2NaCl (aq)
In the first example, the combination of silver nitrate (AgNO3) and sodium chloride (NaCl) forms silver chloride (AgCl) as a precipitate, which appears as a solid in the solution. In the second example, the combination of sodium sulfate (Na2SO4) and barium chloride (BaCl2) forms barium sulfate (BaSO4) as a precipitate.
Question 16. Explain the following in terms of the gain or loss of oxygen with two examples each. (a) Oxidation (b) Reduction
Answer: (a) Oxidation: Oxidation refers to the process in which a chemical substance gains oxygen.
- C + O2 → CO2 In this reaction, carbon (C) combines with oxygen (O2) to form carbon dioxide (CO2). Carbon undergoes oxidation as it gains oxygen.
- 2Mg + O2 → 2MgO Here, magnesium (Mg) reacts with oxygen (O2) to produce magnesium oxide (MgO). Magnesium is oxidized as it gains oxygen.
(b) Reduction: Reduction is the process in which a chemical substance loses oxygen.
- ZnO + C → Zn + CO In this reaction, zinc oxide (ZnO) reacts with carbon (C) to form zinc (Zn) and carbon monoxide (CO). Zinc undergoes reduction as it loses oxygen.
- PbO + C → Pb + CO Lead oxide (PbO) reacts with carbon (C) to yield lead (Pb) and carbon monoxide (CO). Lead is reduced as it loses oxygen.
Question 17. A shiny brown-colored element ‘X’ on heating in air becomes black in color. Name the element ‘X’ and the black-colored compound formed.
Answer: The shiny brown-colored element ‘X’ is copper. When heated in air, it turns black due to the formation of black-colored copper oxide (CuO).
Equation: 2Cu + O2 → 2CuO Copper + Oxygen → Copper oxide (black)
Question 18. Why do we apply paint on iron articles?
Answer: We apply paint on iron articles to prevent their corrosion. Corrosion is the process of slow degradation or eating up of metals when exposed to moist air. When iron comes in contact with moisture and air, it undergoes oxidation, leading to the formation of iron oxide (rust). By applying paint, the iron surface is protected from direct exposure to moist air, cutting off contact and preventing rusting.
Question 19. Oil and fat-containing food items are flushed with nitrogen. Why?
Answer: Oil and fat-containing food items are flushed with nitrogen to prevent rancidity. When these food items are exposed to oxygen in the air, they undergo oxidation and become rancid, resulting in changes in taste and smell. Nitrogen, being an inert gas, helps to create a nitrogen-rich environment that reduces the presence of oxygen, thus inhibiting oxidation and preventing rancidity. Flushing food items with nitrogen gas helps to maintain their freshness and quality. For example, “Lays” chips are flushed with nitrogen gas to prevent rancidity.
Question 20. Explain the following terms with one example of each. (a) Corrosion (b) Rancidity
Answer: (a) Corrosion: Corrosion refers to the process of slow degradation or eating up of metals when exposed to moist air. It occurs when a metal is attacked by substances present in its surroundings, such as moisture and acids.
Example: The black coating on silver and the green coating on copper are examples of corrosion. Silver develops a black coat when exposed to hydrogen sulfide in the air, resulting in the formation of silver sulfide.
2Ag + H2S → Ag2S + H2
(b) Rancidity: Rancidity occurs when fats and oils undergo oxidation, leading to changes in their smell and taste. It is commonly observed in food items containing fats and oils.
Example: When fats and oils in food products become rancid, their smell and taste get altered. To prevent rancidity, antioxidants are added to foods containing fats and oils. Storing food in airtight containers also helps slow down oxidation. Nitrogen gas flushing is done for chips to prevent rancidity caused by oxidation.
Selected NCERT Exemplar Problem
Question 1. Which of the following is not a physical change? (a) Boiling of water to give water vapor (b) Melting of ice to give water (c) Dissolution of salt in water (d) Combustion of liquefied petroleum gas (LPG)
Answer: (d) Because it is an irreversible reaction.
Question 2. The following reaction is an example of: 4NH3 + 5O2 → 4NO + 6H2O (i) Displacement reaction (ii) Combination reaction (iii) Redox reaction (iv) Neutralization reaction
Answer: (iii) and (iv) because NH3 is undergoing oxidation, O2 is undergoing reduction, so it is a redox reaction, and since NH3 is basic and O2 is acidic, it is also a neutralization reaction.
Question 3. Which of the following statements about the given reaction are correct? 3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g) (i) Iron metal is getting oxidized (ii) Water is getting reduced (iii) Water is acting as a reducing agent (iv) Water is acting as an oxidizing agent
Answer: (i), (ii), and (iv) because Fe is gaining oxygen to give Fe3O4, H2O is losing oxygen to give H2, and anything that undergoes reduction is a good oxidizing agent.
Question 4. Which of the following are exothermic processes? (i) Reaction of water with quicklime (ii) Dilution of an acid (iii) Evaporation of water (iv) Sublimation of camphor (crystals)
Answer: (i) and (ii) because in both reactions heat is given out.
Question 5. Three beakers labeled as A, B, and C each containing 25 mL of water were taken. A small amount of NaOH, anhydrous CuSO4, and NaCl were added to beakers A, B, and C, respectively. It was observed that there was an increase in the temperature of the solutions contained in beakers A and B, whereas in the case of beaker C, the temperature of the solution falls. Which one of the following statement(s) is(are) correct? (i) In beakers A and B, exothermic processes have occurred. (ii) In beakers A and B, endothermic processes have occurred. (iii) In beaker C, an exothermic process has occurred. (iv) In beaker C, an endothermic process has occurred.
Answer: (i) only because in the case of beakers A and B, heat is given out, resulting in an increase in temperature, indicating an exothermic reaction. In beaker C, heat is absorbed from water, causing a decrease in temperature, indicating an endothermic process.
Question 6. A dilute ferrous sulfate solution was gradually added to the beaker containing acidified permanganate solution. The light purple color of the solution fades and finally disappears. Which of the following is the correct explanation for the observation? (a) KMnO4 is an oxidizing agent, it oxidizes FeSO4 (b) FeSO4 acts as an oxidizing agent and oxidizes KMnO4 (c) The color disappears due to dilution, no reaction is involved (d) KMnO4 is an unstable compound and decomposes in the presence of FeSO4 to a colorless compound
Answer: (a) KMnO4 is an oxidizing agent. It oxidizes FeSO4. The reaction can be represented as: 8H2SO4 + 10FeSO4 + 2KMnO4 → 5Fe2(SO4)3 + K2SO4 + 2MnSO4 + 8H2O
Question 7. Which of the following is a double displacement reaction? (i) Pb + CuCl2 → PbCl2 + Cu (ii) Na2SO4 + BaCl2 → BaSO4 + 2NaCl (iii) C + O2 → CO2 (iv) CH4 + 2O2 → CO2 + 2H2O
Answer: (ii) only, because an exchange of ions (both cation and anion) occurs in the reaction: Na2SO4 + BaCl2 → BaSO4 + 2NaCl
Question 8. Which of the following statements is true about the exposure of silver chloride to sunlight? (i) The formation of silver occurs by the decomposition of silver chloride. (ii) Silver chloride undergoes sublimation. (iii) Chlorine gas decomposes from silver chloride. (iv) Silver chloride undergoes oxidation.
Answer: (i) only, because when exposed to sunlight, silver chloride decomposes to form silver. The reaction can be represented as: 2AgCl(s) → 2Ag(s) + Cl2(g)
Question 9. What is true about the slaking of lime and the resulting solution? (i) It is an endothermic reaction. (ii) It is an exothermic reaction. (iii) The pH of the resulting solution will be more than seven. (iv) The pH of the resulting solution will be less than seven.
Answer: (ii) and (iii), because the slaking of lime is an exothermic reaction that releases heat. The resulting compound, calcium hydroxide (Ca(OH)2), is basic, so the pH of the resulting solution will be more than seven.
Question 10. When barium chloride reacts with ammonium sulphate, what type of reaction occurs? (i) Displacement reaction (ii) Precipitation reaction (iii) Combination reaction (iv) Double displacement reaction
Answer: (ii) only, because the reaction between barium chloride and ammonium sulphate is a precipitation reaction. It forms a white precipitate of barium sulphate. BaCl2 + (NH4)2SO4 → BaSO4 + 2NH4Cl
Question 11. What is the mole ratio of hydrogen and oxygen gases liberated during electrolysis of water? (a) 1 : 1 (b) 2 : 1 (c) 4 : 1 (d) 1 : 2
Answer: (b), because during electrolysis of water, hydrogen and oxygen gases are liberated in the ratio of 2 : 1 by volume. 2H2O → 2H2 + O2
Question 12. Which of the following processes is an endothermic process? (i) Dilution of sulphuric acid (ii) Sublimation of dry ice (iii) Condensation of water vapours (iv) Evaporation of water
Answer: (ii) and (iv), because sublimation of dry ice and evaporation of water are both endothermic processes that absorb heat from their surroundings, resulting in cooling.
Question 13. If lead nitrate is not available, which of the following can be used in place of lead nitrate in a double displacement reaction with aqueous potassium iodide? (a) Lead sulphate (insoluble) (b) Lead acetate (c) Ammonium nitrate (d) Potassium sulphate
Answer: (b), because lead acetate is a soluble salt of lead that can be used in the reaction. The reaction can be represented as: Pb(CH3COO)2 + 2KI → PbI2 + 2CH3COOK (Yellow precipitate)
Question 14. Which of the following gases can be used for long-term storage of fresh oil samples? (a) Carbon dioxide or oxygen (b) Nitrogen or oxygen (c) Carbon dioxide or helium (d) Helium or nitrogen
Answer: (d), because helium and nitrogen gases provide an inert atmosphere and do not react with the oil or fat, ensuring the preservation of the fresh sample.
Question 15. The following reaction is used for the preparation of oxygen gas in the laboratory: 2KClO3(s) → 2KCl(s) + 3O2(g)
Which of the following statement(s) is (are) correct about the reaction? (a) It is a decomposition reaction and endothermic in nature. (b) It is a combination reaction. (c) It is a decomposition reaction and accompanied by the release of heat. (d) It is a photochemical decomposition reaction and exothermic in nature.
Answer: (a) It is a decomposition reaction and endothermic in nature because KClO3 is breaking down into more than one component and requires heat for product formation.
Question 16. Which one of the following processes involves chemical reactions? (a) Storing of oxygen gas under pressure in a gas cylinder. (b) Liquefaction of air. (c) Keeping petrol in a China dish in the open. (d) Heating copper wire in the presence of air at high temperature.
Answer: (d) Because a chemical reaction occurs where a new product with different properties, copper oxide (CuO) in this case, is formed.
Question 17. In which of the following chemical equations do the abbreviations represent the correct states of the reactants and products involved at reaction temperature? (a) 2H2O(l) → 2H2O(l) + O2(g) (b) 2H2O(g) → 2H2O(l) (c) 2H2O(g) → 2H2O(l) (d) 2H2O(g) → 2H2O(g)
Answer: (c) Because the standard state for hydrogen and oxygen is gas, and for water, it is liquid.
Question 18. Which of the following are combination reactions? (i) 2KClO3 → 2KCl + 3O2 (ii) MgO + H2O → Mg(OH)2 (iii) 4Al + 3O2 → 2Al2O3 (iv) Zn + FeSO4 → ZnSO4 + Fe
Answer: (d) (ii) and (iii) only. This is because in combination reactions, two or more components combine to form a single product. The other reactions, (i) and (iv), are not combination reactions. (i) is a decomposition reaction, and (iv) is a displacement reaction.
Short Answer Type Questions
Question 19. Write the balanced chemical equations for the following reactions and identify the type of reaction in each case. (a) Nitrogen gas is treated with hydrogen gas in the presence of a catalyst at 773 K to form ammonia gas. N2(g) + 3H2(g) → 2NH3(g) (Type of reaction: Combination)
(b) Sodium hydroxide solution is treated with acetic acid to form sodium acetate and water. NaOH(aq) + CH3COOH(aq) → CH3COONa(aq) + H2O(l) (Type of reaction: Neutralization and Double displacement)
(c) Ethanol is warmed with ethanoic acid to form ethyl acetate in the presence of concentrated H2SO4. C2H5OH(l) + CH3COOH(l) → CH3COOC2H5(l) + H2O(l) (Type of reaction: Esterification)
(d) Ethane is burnt in the presence of oxygen to form carbon dioxide, water, and releases heat and light. 2C2H6(g) + 7O2(g) → 4CO2(g) + 6H2O(g) + Energy (Type of reaction: Combustion)
Question 20. Write the balanced chemical equation for the following reactions and identify the type of reaction in each case. (a) In the Thermite reaction, iron (III) oxide reacts with aluminum and gives molten iron and aluminum oxide. Fe2O3(s) + 2Al(s) → 2Fe(l) + Al2O3(s) (Type of reaction: Displacement)
(b) Magnesium ribbon is burnt in an atmosphere of nitrogen gas to form solid magnesium nitride. 3Mg(s) + N2(g) → Mg3N2(s) (Type of reaction: Combination)
(c) Chlorine gas is passed into an aqueous potassium iodide solution to form potassium chloride solution and solid iodine. 2KI(aq) + Cl2(g) → 2KCl(aq) + I2(s) (Type of reaction: Displacement)
(d) Ethanol is burnt in air to form carbon dioxide, water, and releases heat. C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(g) + Energy (Type of reaction: Combustion)
Question 21. Complete the missing components/variables given as X and Y in the following. (a) Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) + 2KNO3(aq) (b) Cu(s) + 2AgNO3(aq) → Cu(NO3)2(aq) + 2Ag(s) (c) Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g) (d) CaCO3(s) → CaO(s) + CO2(g)
Question 22. Which among the following changes are exothermic or endothermic in nature? (a) Decomposition of ferrous sulfate (b) Dilution of sulfuric acid (c) Dissolution of sodium hydroxide in water (d) Dissolution of ammonium chloride in water
Answer: (a) Decomposition of ferrous sulfate is endothermic. (b) Dilution of sulfuric acid is exothermic. (c) Dissolution of sodium hydroxide in water is exothermic. (d) Dissolution of ammonium chloride in water is endothermic.
Question 23. Identify the reducing agent in the following reactions. (a) 4NH3 + 5O2 → 4NO + 6H2O (b) H2 + F2 → 2HF + H2O (c) Fe2O3 + 3CO → 2Fe + 3CO2 (d) 2H2 + O2 → 2H2O
Answer: (a) NH3 is the reducing agent. (b) H2 is the reducing agent. (c) CO is the reducing agent. (d) H2 is the reducing agent.
Question 24. Identify the oxidizing agent (oxidant) in the following reactions and rewrite the chemical equations.
(a) Lead (II) oxide reacts with hydrochloric acid to produce lead chloride, chlorine gas, and water. PbO + 8HCl → 3PbCl2 + Cl2 + 4H2O (Oxidizing agent: HCl)
(b) Magnesium reacts with oxygen to form magnesium oxide. 2Mg + O2 → 2MgO (Oxidizing agent: O2)
(c) Copper (II) sulfate reacts with zinc to produce copper and zinc sulfate. CuSO4 + Zn → Cu + ZnSO4 (Oxidizing agent: CuSO4)
(d) Vanadium (V) oxide reacts with calcium to produce vanadium and calcium oxide. V2O5 + 5Ca → 2V + 5CaO (Oxidizing agent: V2O5)
(e) Iron reacts with water vapor to form iron (III) oxide and hydrogen gas. 3Fe + 4H2O → Fe2O3 + 4H2 (Oxidizing agent: H2O)
(f) Copper (II) oxide reacts with hydrogen gas to produce copper and water. CuO + H2 → Cu + H2O (Oxidizing agent: CuO)
Question 25. Write the balanced chemical equations for the following reactions.
(a) Sodium carbonate reacts with hydrochloric acid to produce sodium chloride and sodium hydrogen carbonate. Na2CO3 + 2HCl → 2NaCl + NaHCO3
(b) Sodium hydrogen carbonate reacts with hydrochloric acid to produce sodium chloride, water, and carbon dioxide. NaHCO3 + HCl → NaCl + H2O + CO2
(c) Copper sulfate reacts with potassium iodide to produce cuprous iodide, iodine gas, and potassium sulfate. CuSO4 + 2KI → CuI + I2 + K2SO4
Question 26. A solution of potassium chloride, when mixed with silver nitrate solution, forms an insoluble white substance. Write the chemical reaction involved and mention the type of chemical reaction.
KCl(aq) + AgNO3(aq) → AgCl(s) + KNO3(aq) (Type of reaction: Double displacement)
Question 27. Ferrous sulfate decomposes with the evolution of a gas that has the characteristic odor of burning sulfur. Write the chemical reaction involved and identify the type of reaction.
2FeSO4(s) → Fe2O3(s) + SO2(g) + SO3(g) (Type of reaction: Decomposition)
Question 28. Why do fireflies glow at night?
Fireflies glow at night through a process called bioluminescence. They possess phosphorus compounds in their bodies that emit heat and light.
Question 29. Grapes hanging on the plant do not ferment, but after being plucked from the plant, they can be fermented. Under what conditions do these grapes ferment? Is it a chemical or a physical change?
Grapes ferment under anaerobic conditions, which means in the absence of air. This is a chemical change because new products are formed during the fermentation process.
Question 30. Classify the following changes as physical or chemical changes.
(a) Evaporation of petrol – Physical change (b) Burning of Liquefied Petroleum Gas (LPG) – Chemical change (c) Heating of an iron rod to red hot – Physical change (d) Curdling milk – Chemical change (e) Sublimation of solid ammonium chloride – Physical change
Question 31: During the reaction between certain metals and dilute hydrochloric acid, the following observations were made: (a) Silver metal remains unchanged. (b) The temperature of the reaction mixture increases when aluminium (Al) is added. (c) Sodium metal reacts explosively. (d) Lead (Pb) produces gas bubbles when reacted with the acid.
Explain these observations and provide suitable reasons.
Answer: (a) Silver does not undergo any change because it is less reactive than hydrogen and cannot displace it when reacted with acid. (b) The rise in temperature is due to the exothermic nature of the reaction between aluminium and hydrochloric acid, resulting in the release of heat. (c) Sodium is a highly reactive metal, which leads to a highly explosive reaction with hydrochloric acid. (d) Lead reacts with dilute hydrochloric acid to produce hydrogen gas. The formation of gas bubbles is an indication of this reaction: Pb + 2HCl → PbCl2 + H2
Question 32: A substance X, which is an oxide of a group 2 element, is extensively used in the cement industry. This element is also present in bones. When X is treated with water, it forms a solution that turns red litmus paper blue. Identify X and provide the chemical reactions involved.
Answer: X is calcium oxide (CaO). When calcium oxide reacts with water, it forms calcium hydroxide (Ca(OH)2), which is a basic compound. CaO + H2O → Ca(OH)2
Question 33: Write balanced chemical equations for the following reactions and classify them. (a) Lead acetate solution is treated with dilute hydrochloric acid to form lead chloride and acetic acid solution. (b) A piece of sodium metal is added to absolute ethanol to form sodium ethoxide and hydrogen gas. (c) Iron (III) oxide, when heated with carbon monoxide gas, reacts to form solid iron and liberates carbon dioxide gas. (d) Hydrogen sulphide gas reacts with oxygen gas to form solid sulphur and liquid water.
Answer: (a) Pb(CH3COO)2 + 2HCl (dil) → PbCl2 + 2CH3COOH This is a double displacement reaction and a precipitation reaction. (b) 2C2H5OH + 2Na → 2C2H5ONa + H2 This is a displacement reaction. (c) Fe2O3 + 3CO → 2Fe + 3CO2 This is a redox reaction. (d) 2H2S + O2 → 2S + 2H2O This is an oxidation reaction.
Question 34: Why is silver chloride stored in dark-colored bottles?
Answer: Silver chloride is stored in dark-colored bottles because it turns grey when exposed to sunlight. This is due to the decomposition of silver chloride into silver and chlorine by light: 2AgCl (sunlight) → 2Ag + Cl2 Hence, storing it in dark-colored bottles helps prevent this decomposition.
Question 35: Balance the following chemical equations and identify the type of chemical reaction. (a) Mg + Cl2 → MgCl2 (b) HgO → Hg + O2 (c) Na + S → Na2S (d) TiCl4 + 2Mg → Ti + 2MgCl2 (e) CaO + SiO2 → CaSiO3 (f) H2O2 → H2O + O2
Answer: (a) 2Mg + Cl2 → 2MgCl2 (Combination reaction) (b) 2HgO → 2Hg + O2 (Decomposition reaction) (c) 2Na + S → Na2S (Combination reaction) (d) TiCl4 + 2Mg → Ti + 2MgCl2 (Displacement reaction) (e) CaO + SiO2 → CaSiO3 (Combination reaction) (f) 2H2O2 → 2H2O + O2 (Decomposition reaction)
Question 36: When a magnesium ribbon is burned in oxygen, it forms a white compound X accompanied by the emission of light. If the burning ribbon is placed in an atmosphere of nitrogen, it continues to burn and forms a compound Y. (a) Write the chemical formulas of X and Y. (b) Write a balanced chemical equation for the dissolution of X in water.
Answer: (a) X = Magnesium oxide (MgO); Y = Magnesium nitride (Mg3N2) (b) MgO + H2O → Mg(OH)2 (Magnesium hydroxide)
Question 37: Zinc liberates hydrogen gas when reacted with dilute hydrochloric acid, whereas copper does not. Explain why.
Answer: Zinc is more reactive than hydrogen, so it can displace hydrogen from hydrochloric acid and form zinc chloride and hydrogen gas. Copper, on the other hand, is less reactive than hydrogen and cannot displace it from the acid. Therefore, copper does not liberate hydrogen gas in this reaction.
Question 38: Silver articles generally turn black when kept in the open for a few days. When rubbed with toothpaste, they regain their shine. (a) Why do silver articles turn black when exposed to air? Name the phenomenon involved. (b) Name the black substance formed and provide its chemical formula.
Answer: (a) Silver articles turn black when exposed to air due to the reaction of silver with hydrogen sulfide gas (H2S) present in the atmosphere. This phenomenon is known as tarnishing. (b) The black substance formed is silver sulfide (Ag2S).
Long Answer Type Questions
uestion 39: When blue-colored powder of copper (II) nitrate is heated in a boiling tube, copper oxide (black), oxygen gas, and a brown gas X are formed. (a) Write a balanced chemical equation for the reaction. (b) Identify the brown gas X. (c) Determine the type of reaction. (d) What could be the pH range of an aqueous solution of gas X?
Answer: (a) 2Cu(NO3)2(s) → 2CuO(s) + O2(g) + 4NO2(g) (b) The brown gas X is nitrogen dioxide gas (NO2). (c) This is a decomposition reaction. (d) The gas X is acidic, so the pH range of an aqueous solution of gas X would be between 1 to 7.
Question 40: Provide characteristic tests for the following gases. (a) CO2 (b) SO2 (c) O2 (d) H2
Answer: (a) CO2: When CO2 is passed through lime water, it turns milky due to the formation of a white precipitate of calcium carbonate (CaCO3). (b) SO2: It changes an orange solution of acidified potassium dichromate to green. (c) O2: It rekindles a glowing splinter and turns a colorless alkaline solution of pyrogallol to dark brown. (d) H2: It burns with a pale blue flame and produces a popping sound when a burning taper is brought near it.
Question 41: Explain what happens when: (a) Zinc metal is added to copper sulfate solution. (b) Aluminum metal is added to dilute hydrochloric acid. (c) Silver metal is added to copper sulfate solution. Also, write the balanced chemical equations if a reaction occurs.
Answer: (a) Zinc metal displaces copper from copper sulfate solution, resulting in the formation of zinc sulfate and copper metal: Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s) The blue color of the solution fades away due to the displacement reaction. (b) Aluminum metal reacts with dilute hydrochloric acid to produce aluminum chloride and hydrogen gas: 2Al(s) + 6HCl(aq) → 2AlCl3(aq) + 3H2(g) Hydrogen gas is evolved during this reaction. (c) No reaction takes place when silver metal is added to copper sulfate solution: Ag(s) + CuSO4(aq) → No reaction occurs
Question 42: Describe the reactions that occur when zinc granules are treated with dilute solutions of H2SO4, HCl, HNO3, NaCl, and NaOH. Write the chemical equations if a reaction occurs.
Answer: The chemical reactions that take place are:
- Zinc reacts with dilute H2SO4 to form zinc sulfate and hydrogen gas: Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g) Hydrogen gas is evolved.
- Zinc reacts with dilute HCl to form zinc chloride and hydrogen gas: Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g) Hydrogen gas is evolved.
- Zinc reacts with dilute HNO3 to form zinc nitrate, nitric oxide gas, and water: 3Zn(s) + 8HNO3(aq) → 3Zn(NO3)2(aq) + 2NO(g) + 4H2O(l) NO gas is evolved.
- No reaction occurs when zinc reacts with NaCl: Zn(s) + NaCl(aq) → No reaction occurs
- Zinc reacts with NaOH to form sodium zincate and hydrogen gas: Zn(s) + 2NaOH(aq) → Na2ZnO2(aq) + H2(g) Hydrogen gas is evolved.
Question 43: When a drop of barium chloride solution is added to an aqueous solution of sodium sulphite, a white precipitate is obtained. (a) Write a balanced chemical equation for the reaction involved. (b) What other name can be given to this precipitation reaction? (c) Why does the white precipitate disappear when dilute hydrochloric acid is added to the reaction mixture?
Answer: (a) BaCl2(aq) + Na2SO3(aq) → BaSO3(s) + 2NaCl(aq) (White precipitate of barium sulphite is obtained.) (b) This precipitation reaction can also be called a double displacement reaction. (c) When dilute hydrochloric acid is added, it reacts with barium sulphite (BaSO3) to form barium chloride (BaCl2), which is a soluble salt. As a result, the white precipitate disappears because it dissolves in the acid solution.