CBSE Class 10 Science Chemical Reactions and Equations VBQs Set 04

ASSERTION AND REASON QUESTIONS

Direction: In the following Questions, the Assertion and Reason have been put forward. Read the statements carefully and choose the correct alternative from the following:
(a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.
(b) The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.
(c) Assertion is true but the Reason is false.
(d) The statement of the Assertion is false but the Reason is true.

Question. Assertion: Burning of Natural gas is an endothermic process.
Reason: Methane gas combines with oxygen to produce carbon dioxide and water.
(a) a
(b) b
(c) c
(d) d
Answer: (d) The statement of the Assertion is false but the Reason is true.

Question. Assertion: AgBr is used on photographic and X-ray film.
Reason: AgBr is photosensitive and changes to Ag and bromine in presence of sunlight and undergoes decomposition reaction.
(a) a
(b) b
(c) c
(d) d
Answer: (a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.

Question. Assertion: Magnesium ribbon keeps on burning in atmosphere of nitrogen.
Reason: Magnesium reacts with nitrogen to form magnesium nitrides and this reaction is combination reaction.
(a) a
(b) b
(c) c
(d) d
Answer: (a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.

Question. Assertion: Zinc reacts with sulphuric acid to form zinc sulphate and hydrogen gas and it is displacement reaction.
Reason: Zinc reacts with oxygen to form zinc oxide.
(a) a
(b) b
(c) c
(d) d
Answer: (b) The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.

Question. Assertion: \( MnO_2 + 4HCl \longrightarrow MnCl_2 + Cl_2 + 2H_2O \) is redox reaction.
Reason: \( MnO_2 \) oxidises HCl to \( Cl_2 \) and gets reduced to \( MnCl_2 \).
(a) a
(b) b
(c) c
(d) d
Answer: (a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.

Question. Assertion: A lead nitrate on thermal decomposition gives lead oxide, brown coloured nitrogen dioxide and oxygen gas.
Reason: Lead nitrate reacts with potassium iodide to form yellow ppt of lead iodide and the reaction is double displacement as well as precipitation reaction.
(a) a
(b) b
(c) c
(d) d
Answer: (b) The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.

Question. Assertion: Following is a balanced chemical equation for the action of steam on iron.
\( 3Fe + 4H_2O \longrightarrow Fe_3O_4 + 4H_2 \)
Reason: The law of conservation of mass holds good for a chemical equation.
(a) a
(b) b
(c) c
(d) d
Answer: (a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.

Question. Assertion: A reaction in which a substance is decomposed into two or more simpler products is known as decomposition reaction.
Reason: The decomposition of a substance is impossible without supplying energy.
(a) a
(b) b
(c) c
(d) d
Answer: (a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.

Question. Assertion: \( PbO_2 + 4HCl \longrightarrow PbCl_2 + Cl_2 + 2H_2O \) is redox reaction.
Reason: \( PbO_2 \) oxidise HCl to \( Cl_2 \) and gets reduced to \( PbCl_2 \).
(a) a
(b) b
(c) c
(d) d
Answer: (a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.

Question. Assertion: After white washing the walls, a shiny white finish on the walls is obtained after two to three days.
Reason: Calcium oxide reacts with carbon deoxide to form calcium hydrogen carbonate which gives shiny finish.
(a) a
(b) b
(c) c
(d) d
Answer: (c) Assertion is true but the Reason is false.

Question. Assertion: A reaction in which a substance is decomposed into two or more substances is known as decomposition reaction.
Reason: The decomposition of a substance is impossible without supplying energy.
(a) a
(b) b
(c) c
(d) d
Answer: (a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.

Question. Assertion: Combustion reaction is also called exothermic oxidation reaction.
Reason: In a combustion reaction \( O_2 \) is added and heat is released.
(a) a
(b) b
(c) c
(d) d
Answer: (a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.

Question. Assertion: Silver bromide decomposition is used in black and white photography.
Reason: Light provides energy for this exothermic reaction.
(a) a
(b) b
(c) c
(d) d
Answer: (c) Assertion is true but the Reason is false.

Question. Electrolysis of water is a decomposition reaction. The molar ratio of hydrogen and oxygen gases liberated during electrolysis of water is
(a) 1 : 1
(b) 2 : 1
(c) 4 : 1
(d) 1 : 2
Answer: (b) 2 : 1

Question. Which of the following is (are) an endothermic process(es)?
(i) Dilution of sulphuric acid
(ii) Sublimation of dry ice
(iii) Condensation of water vapours
(iv) Evaporation of water
(a) (i) and (iii)
(b) (ii) only
(c) (iii) only
(d) (ii) and (iv)
Answer: (d) (ii) and (iv)

Question. Which of the following gases can be used for storage of fresh sample of an oil for a long time?
(a) Carbon dioxide or oxygen
(b) Nitrogen or oxygen
(c) Carbon dioxide or helium
(d) Helium or nitrogen
Answer: (d) Helium or nitrogen

Question. The following reaction is used for the preparation of oxygen gas in the laboratory
\[ 2KClO_{3}(s) \xrightarrow[Catalyst]{Heat} 2KCl (s) + 3O_{2}(g) \]
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 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

Question. Which one of the following processes involve 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 presence of air at high temperature
Answer: (d) Heating copper wire in presence of air at high temperature

Question. In which of the following chemical equations, the abbreviations represent the correct states of the reactants and products involved at reaction temperature?
(a) \( 2H_{2}(l) + O_{2}(l) \longrightarrow 2H_{2}O(g) \)
(b) \( 2H_{2}(g) + O_{2}(l) \longrightarrow 2H_{2}O(l) \)
(c) \( 2H_{2}(g) + O_{2}(g) \longrightarrow 2H_{2}O(l) \)
(d) \( 2H_{2}(g) + O_{2}(g) \longrightarrow 2H_{2}O(g) \)
Answer: (c) \( 2H_{2}(g) + O_{2}(g) \longrightarrow 2H_{2}O(l) \)

Question. Three beakers labelled as A, B and C each containing 25 ml of water were taken. A small amount of \( NaOH \), anhydrous \( CuSO_{4} \) and \( NaCl \) were added to the 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 case of beaker C, the temperature of the solution falls. Which one of the following statements(s) is (are) correct?
(i) In beakers A and B, exothermic process has occurred.
(ii) In beakers A and B, endothermic process has occurred.
(iii) In beaker C exothermic process has occurred.
(iv) In beaker C endothermic process has occurred.
(a) (i) only
(b) (ii) only
(c) (i) and (iv)
(d) (ii) and (iii)
Answer: (c) (i) and (iv)

Question. A dilute ferrous sulphate solution was gradually added to the beaker containing acidified potassium permanganate solution. The light purple colour of the solution fades and finally disappears. Which of the following is the correct explanation for the observation?
(a) \( KMnO_{4} \) is an oxidising agent, it oxidises \( FeSO_{4} \)
(b) \( FeSO_{4} \) acts as an oxidising agent and oxidises \( KMnO_{4} \)
(c) The colour disappears due to dilution : no reaction is involved
(d) \( KMnO_{4} \) is an unstable compound and decomposes in presence of \( FeSO_{4} \) to a colourless compound.
Answer: (a) \( KMnO_{4} \) is an oxidising agent, it oxidises \( FeSO_{4} \)

Question. Which among the following statement(s) is/(are) true? Exposure of silver chloride to sunlight for a long duration turns grey due to
(i) the formation of silver by decomposition of silver chloride
(ii) sublimation of silver chloride
(iii) decomposition of chlorine gas from silver chloride
(iv) oxidation of silver chloride
(a) (i) only
(b) (i) and (iii)
(c) (ii) and (iii)
(d) (iv) only
Answer: (a) (i) only

Question. Solid calcium oxide reacts vigorously with water to form calcium hydroxide accompanied by liberation of heat. This process is called slaking of lime. Calcium hydroxide dissolves in water to form its solution called lime water. Which among the following is (are) true about slaking of lime and the solution formed?
(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
(a) (i) and (ii)
(b) (ii) and (iii)
(c) (i) and (iv)
(d) (iii) and (iv)
Answer: (b) (ii) and (iii)

Question. 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
Answer: (b) Lead acetate

Question. Which of the following are combination reactions?
(i) \( 2KClO_{3} \xrightarrow{Heat} 2KCl + 3O_{2} \)
(ii) \( MgO + H_{2}O \longrightarrow Mg(OH)_{2} \)
(iii) \( 4Al + 3O_{2} \longrightarrow 2Al_{2}O_{3} \)
(iv) \( Zn + FeSO_{4} \longrightarrow ZnSO_{4} + Fe \)
(a) (i) and (iii)
(b) (iii) and (iv)
(c) (ii) and (iv)
(d) (ii) and (iii)
Answer: (d) (ii) and (iii)

Question. Write the balanced chemical equation for the following equations for the following reaction 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.
(b) Sodium hydroxide solution is treated with acetic acid to form sodium acetate and water.
(c) Ethanol is warmed with ethanoic acid to form ethyl acetate in presence of conc. \( H_{2}SO_{4} \).
(d) Ethene is burnt in presence of oxygen to form carbon dioxide, water and releases heat and light.

Answer: (a) \( N_{2}(g) + 3H_{2}(g) \xrightarrow{Catalyst, 773 K} 2NH_{3}(g) \)
Combination reaction
(b) \( NaOH + CH_{3}COOH \longrightarrow CH_{3}COONa + H_{2}O \)
Double displacement reaction/Neutralisation reaction.
(c) \( C_{2}H_{5}OH + CH_{3}COOH \xrightarrow{conc. H_{2}SO_{4}} CH_{3}COOC_{2}H_{5} + H_{2}O \)
Double displacement reaction/Esterification.
(d) \( C_{2}H_{4} + 3O_{2} \longrightarrow 2CO_{2} + 2H_{2}O + \text{heat} + \text{light} \)
Redox reaction/combustion reaction.

Question. Write the balanced chemical equations for the following reactions and identify the type of reaction in each case.
(a) Thermite reaction, iron (III) oxide reacts with aluminium and gives molten iron and aluminium oxide.
(b) Magnesium ribbon is burnt in an atmosphere of nitrogen gas to form solid magnesium nitride.
(c) Chlorine gas is passed in an aqueous potassium iodide solution to form potassium chloride solution and solid iodine.
(d) Ethanol is burnt in air to form carbon dioxide, water and releases heat.

Answer: (a) \( Fe_{2}O_{3}(s) + 2Al(s) \longrightarrow Al_{2}O_{3}(s) + 2Fe(l) + \text{heat} \)
It is a displacement reaction because Al is displacing Fe from \( Fe_{2}O_{3} \).
(b) \( 3Mg(s) + N_{2}(g) \xrightarrow{Burning} Mg_{3}N_{2}(s) \)
It is a combination reaction as magnesium on burning reacts with \( N_{2} \) to form magnesium nitride.
(c) \( Cl_{2}(g) + 2KI(aq) \longrightarrow 2KCl(aq) + I_{2}(aq) \)
It is a displacement reaction because chlorine is displacing iodine from potassium iodide to form potassium chloride and solid iodine.
(d) \( C_{2}H_{5}OH(l) + 3O_{2}(g) \longrightarrow 2CO_{2}(g) + 3H_{2}O(l) + \text{heat} \)
It is oxidation reaction. It is also called combustion reaction.

Question. Complete the missing components/variables given as x and y in the following reactions:
(a) \( Pb(NO_{3})_{2}(aq) + 2KI(aq) \longrightarrow PbI_{2}(x) + 2KNO_{3}(y) \)
(b) \( Cu(s) + 2AgNO_{3}(aq) \longrightarrow Cu(NO_{3})_{2}(aq) + x \)
(c) \( Zn(s) + H_{2}SO_{4}(aq) \longrightarrow ZnSO_{4}(x) + H_{2}(y) \)
(d) \( CaCO_{3}(s) \xrightarrow{x} CaO(s) + CO_{2}(g) \)

Answer: (a) \( Pb(NO_{3})_{2}(aq) + 2KI(aq) \longrightarrow PbI_{2}(s) + 2KNO_{3}(aq) \) therefore ‘x’ is (s), ‘y’ is (aq).
(b) \( Cu(s) + 2AgNO_{3}(aq) \longrightarrow Cu(NO_{3})_{2}(aq) + 2Ag(s) \) therefore ‘x’ is Ag(s).
(c) \( Zn(s) + H_{2}SO_{4}(aq) \longrightarrow ZnSO_{4}(aq) + H_{2}(g) \) therefore ‘x’ is (aq), ‘y’ is (g).
(d) \( CaCO_{3}(s) \xrightarrow{heat} CaO(s) + CO_{2}(g) \) ‘x’ is ‘heat’.

Question. Which among the following changes are exothermic or endothermic in nature?
(a) Decomposition of ferrous sulphate.
(b) Dilution of sulphuric acid.
(c) Dissolution of sodium hydroxide in water.
(d) Dissolution of ammonium chloride in water.

Answer: (a) It is endothermic reaction.
(b) It is exothermic process.
(c) It is exothermic process.
(d) It is endothermic process.

Question. Write the balanced chemical equations for the following reactions.
(a) Sodium carbonate on reaction with hydrochloric acid in equal molar concentrations gives sodium chloride and sodium hydrogen carbonate.
(b) Sodium hydrogencarbonate on reaction with hydrochloric acid gives sodium chloride, water and liberates carbon dioxide.
(c) Copper sulphate on treatment with potassium iodide precipitates cuprous iodide (\( Cu_{2}I_{2} \)), liberates iodine gas and also forms potassium sulphate.

Answer: (a) \( Na_{2}CO_{3}(s) + 2HCl(aq) \longrightarrow NaCl(aq) + CO_{2}(g) + H_{2}O(l) \)
(b) \( NaHCO_{3}(s) + HCl(aq) \longrightarrow NaCl(aq) + H_{2}O(l) + CO_{2}(g) \)
(c) \( 2CuSO_{4}(aq) + 4KI(aq) \longrightarrow 2K_{2}SO_{4}(aq) + Cu_{2}I_{2}(s) + I_{2}(s) \)

Question. Identify the reducing agent in the following reactions:
(a) \( 4NH_{3} + 5O_{2} \longrightarrow 4NO + 6H_{2}O \)
(b) \( H_{2}O + F_{2} \longrightarrow HF + HOF \)
(c) \( Fe_{2}O_{3} + 3CO \longrightarrow 2Fe + 3CO_{2} \)
(d) \( 2H_{2} + O_{2} \longrightarrow 2H_{2}O \)

Answer: (a) \( NH_{3} \) is reducing agent.
(b) \( H_{2}O \) is reducing agent.
(c) CO (Carbon monoxide) is reducing agent.
(d) \( H_{2} \) is reducing agent.

Question. Identify the oxidising agent (oxidant) in the following reactions:
(a) \( Pb_{3}O_{4} + 8HCl \longrightarrow 3PbCl_{2} + Cl_{2} + 4H_{2}O \)
(b) \( 2Mg + O_{2} \longrightarrow 2MgO \)
(c) \( CuSO_{4} + Zn \longrightarrow Cu + ZnSO_{4} \)
(d) \( V_{2}O_{5} + 5Ca \longrightarrow 2V + 5CaO \)
(e) \( 3Fe + 4H_{2}O \longrightarrow Fe_{3}O_{4} + 4H_{2} \)
(f) \( CuO + H_{2} \longrightarrow Cu + H_{2}O \)

Answer: (a) \( Pb_{3}O_{4} \) (Red lead). It is also called Sindur used by married ladies. It is an oxidant (oxidising agent).
(b) \( O_{2} \) is oxidising agent.
(c) \( CuSO_{4} \) is oxidising agent.
(d) \( V_{2}O_{5} \) is oxidising agent.
(e) \( H_{2}O \) is oxidising agent.
(f) \( CuO \) is oxidising agent.

Question. A solution of potassium chloride when mixed with silver nitrate solution, an insoluble white substance is formed. Write the chemical reaction involved and also mention the type of the chemical reaction.

Answer: \( KCl(aq) + AgNO_{3}(aq) \longrightarrow AgCl(s) + KNO_{3}(aq) \)
It is a double displacement reaction. It is also a precipitation reaction as \( AgCl \) is a white precipitate.

Question. Why do fireflies glow at night?

Answer: It is because protein present in fireflies undergoes oxidation in presence of air and an enzyme. This chemical reaction involves emission of visible light. Therefore, fireflies glow at night.

Question. Grapes hanging on the plant do not ferment but after being plucked from the plant can be fermented. Under what conditions do these grapes ferment? Is it a chemical or a physical change?

Answer: Grapes when attached to plants are living and therefore they have their immune system due to which they cannot get fermented. When microbes attack plucked grapes in absence of air, they undergo fermentation to form alcohol. This is a chemical process.

Question. During the reaction of some metals with dilute hydrochloric acid, following observations were made.
(a) Silver metal does not show any change.
(b) The temperature of the reaction mixture rises when aluminium (Al) is added.
(c) The reaction of sodium metal is found to be highly explosive.
(d) Some bubbles of a gas are seen when lead (Pb) is reacted with the acid.
Explain these observations giving suitable reasons.

Answer: (a) It is because silver is less reactive than hydrogen. It cannot displace hydrogen from dilute acid.
(b) It is because the reaction is exothermic.
(c) It is because sodium is highly reactive and forms hydrogen gas in the presence of moisture (\( H_{2}O \)) which catches fire as the reaction is highly exothermic and \( H_{2} \) is highly inflammable.
(d) It is due to formation of hydrogen gas. The reaction becomes slow after some time as \( PbCl_{2}(s) \) covers Pb metal.
\( Pb(s) + 2HCl(dil.) \longrightarrow PbCl_{2}(s) + H_{2}(g) \)

Question. A substance ‘X’, which is an oxide of 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. Identify ‘X’ and also write the chemical reactions involved.

Answer: The substance ‘X’ is calcium oxide (\( CaO \)), element is calcium. Calcium oxide is used in cement industry. Calcium is present in bones also in form of calcium phosphate. Calcium oxide dissolves in water forming alkali which turns red litmus blue.
\( CaO(s) + H_{2}O(l) \longrightarrow Ca(OH)_{2}(aq) \)
Calcium oxide + Water \(\longrightarrow\) Calcium hydroxide (Lime water)

Question. Write a balanced chemical equation for each of the following reactions and also 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 on heating with carbon monoxide 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) \( Pb(COOCH_{3})_{2}(aq) + 2HCl(dil.) \longrightarrow PbCl_{2}(s) + 2CH_{3}COOH(l) \);
Double displacement reaction
(b) \( 2Na(s) + 2C_{2}H_{5}OH(l) \longrightarrow 2C_{2}H_{5}ONa(l) + H_{2}(g) \);
Displacement reaction
(c) \( Fe_{2}O_{3}(s) + 3CO(g) \longrightarrow 2Fe(s) + 3CO_{2}(g) \);
Redox reaction
(d) \( 2H_{2}S(s) + 3O_{2}(g) \longrightarrow 2H_{2}O(l) + 2SO_{2}(g) \);
Redox reaction

Question. Why do we store silver chloride in dark coloured bottles?

Answer: It is because silver chloride decomposes to silver and chlorine gas in presence of sunlight.
\( 2AgCl(s) \xrightarrow{Sunlight} 2Ag(s) + Cl_{2}(g) \)

Question. Balance the following chemical equations and identify the type of chemical reaction.
(a) \( Mg(s) + Cl_{2}(g) \longrightarrow MgCl_{2}(s) \)
(b) \( HgO(s) \xrightarrow{Heat} Hg(l) + O_{2}(g) \)
(c) \( Na(s) + S(s) \xrightarrow{Fuse} Na_{2}S(s) \)
(d) \( TiCl_{4}(l) + Mg(s) \longrightarrow Ti(s) + MgCl_{2}(s) \)
(e) \( CaO(s) + SiO_{2}(s) \longrightarrow CaSiO_{3}(s) \)
(f) \( H_{2}O_{2}(l) \xrightarrow{U.V.} H_{2}O(l) + O_{2}(g) \)

Answer: (a) \( Mg(s) + Cl_{2}(g) \longrightarrow MgCl_{2}(s) \);
Combination reaction
(b) \( 2HgO(s) \longrightarrow 2Hg(l) + O_{2}(g) \);
Decomposition reaction
(c) \( 2Na(s) + S(s) \xrightarrow{Fuse} Na_{2}S(s) \);
Combination reaction
(d) \( TiCl_{4}(l) + 2Mg(s) \longrightarrow Ti(s) + 2MgCl_{2}(s) \);
Displacement reaction
(e) \( CaO(s) + SiO_{2}(s) \longrightarrow CaSiO_{3}(s) \);
Combination reaction
(f) \( 2H_{2}O_{2}(l) \xrightarrow{U.V.} 2H_{2}O(l) + O_{2}(g) \);
Decomposition reaction

Question. A magnesium ribbon is burnt in oxygen to give a white compound ‘X’ accompanied by emission of light. If the burning ribbon is now placed in an atmosphere of nitrogen, it continues to burn and forms a compound ‘Y’.
(a) Write the chemical formulae of ‘X’ and ‘Y’.
(b) Write a balanced chemical equation, when X is dissolved in water.

Answer: (a) \( 2Mg(s) + O_{2}(g) \xrightarrow{Burning} 2MgO(s) + \text{Light} \)
Formula of ‘X’ is \( MgO \).
\( 3Mg(s) + N_{2}(g) \longrightarrow Mg_{3}N_{2}(s) \)
Formula of ‘Y’ is \( Mg_{3}N_{2} \).
(b) \( MgO(s) + H_{2}O(l) \longrightarrow Mg(OH)_{2}(aq) \)
Magnesium oxide (X) + Water \(\longrightarrow\) Magnesium hydroxide

Question. Zinc liberated hydrogen gas when reacted with dilute hydrochloric acid, whereas copper does not. Explain why?

Answer: \( Zn(s) + 2HCl(dil.) \longrightarrow ZnCl_{2}(aq) + H_{2}(g) \)
Zinc is more reactive than \( H_{2} \), therefore, displace \( H_{2} \) from \( dil. HCl \). Copper is less reactive than hydrogen, therefore, it does not liberate \( H_{2}(g) \) from dilute acid.

Question. A silver article generally turns black when kept in the open for a few days. The article when rubbed with toothpaste again starts shining.
(a) Why do silver articles turn black when kept in the open for a few days? Name the phenomenon involved.
(b) Name the black substance formed and give its chemical formula.

Answer: (a) Silver reacts with \( H_{2}S \) gas present in atmosphere to form a black compound \( Ag_{2}S \) (silver sulphide) on its surface. This phenomenon is called corrosion.
(b) \( Ag_{2}S \) (silver sulphide) is a black coloured solid.

Question. On heating blue coloured powder of copper (II) nitrate in boiling tube, copper oxide (black), oxygen gas and a brown gas ‘X’ is formed.
(a) Write a balanced chemical equation of the reaction.
(b) Identify the brown gas ‘X’ evolved.
(c) Identify the type of reaction.
(d) What could be the pH range of aqueous solution of the gas ‘X’?

Answer: (a) \( 2Cu(NO_{3})_{2}(s) \xrightarrow{heat} 2CuO(s) + 4NO_{2}(g) + O_{2}(g) \)
Copper(II) nitrate (Blue) \(\longrightarrow\) Copper oxide (Black) + Nitrogen dioxide 'X' (Brown) + Oxygen
(b) The brown gas ‘X’ is nitrogen dioxide (\( NO_{2} \)).
(c) The reaction is a thermal decomposition reaction.
(d) The gas ‘X’ is acidic in nature because it is a non-metallic oxide. Its aqueous solution has pH less than 7.

Question. What happens when a piece of
(a) zinc metal is added to copper sulphate solution?
(b) aluminium metal is added to dilute hydrochloric acid?
(c) silver metal is added to copper sulphate solution?
Also, write the balanced chemical equation if the reaction occurs.

Answer: (a) The solution will become colourless due to formation of zinc sulphate and reddish brown copper metal will get deposited.
\( Zn(s) + CuSO_{4}(aq) \longrightarrow ZnSO_{4}(aq) + Cu(s) \)
Zinc (Grey) + Copper sulphate (Blue) \(\longrightarrow\) Zinc sulphate (Colourless) + Copper (Reddish brown)
(b) Hydrogen gas and aluminium chloride solution will be formed.
\( 2Al(s) + 6HCl(dil.) \longrightarrow 2AlCl_{3}(aq) + 3H_{2}(g) \)
(c) No reaction will take place because silver is less reactive than copper, it cannot displace copper from copper sulphate.