Chemical Reactions and Equations: 64 Ques and Ans

Chemical Reactions and Equations question answer are provided in this post including NCERT questions also.

1. Why should a magnesium ribbon be cleaned before burning in air ?

Answer: Magnesium gets covered with a layer of magnesium oxide when kept in air for a long time. This layer hinders the burning of magnesium. Hence, it is to be cleaned before burning.

2. Write the balanced equation for the following chemical reactions.
   (i) Hydrogen + Chlorine → Hydrogen chloride
   (ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
   (iii) Sodium + Water → Sodium hydroxide + Hydrogen

Answer: (i) H₂ + Cl₂ → 2HCl
(ii) 3 BaCl₂ + Al₂(SO₄)₃ → BaSO₄ + 2 AlCl₃
(iii) 2Na + 2H₂O → 2NaOH + H₂↑

3. Write a balanced chemical equation with state symbols for the following reactions :
   (i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
   (ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.

Answer: (i) BaCl₂ (aq) + Na₂SO₄ (aq) → BaSO₄(s) + 2NaCl (aq)
(ii) NaOH (aq) + HCl(aq) → NaCl(aq) + H₂O(l)

4. A solution of a substance ‘X’ is used for white washing.
   (i) Name the substance ‘X’ and write its formula.
   (ii) Write the reaction of the substance ‘X’ named in (i) above with water.

Answer: (i) The substance whose solution in water is used for white washing is calcium oxide (or quick lime). Its formula is CaO.

(ii) CaO (s) + H₂O (l) → Ca(OH)₂ (aq)

Ca(OH)₂ is Calcium hydroxide.

5. Why is the amount of gas collected in one of the test tubes in text book Activity 1.7 (i.e., electrolysis of water) double of the amount collected in the other? Name this gas.

Answer: In Activity 1.7, water is electrolysed to give H₂ gas at one electrode and O₂ gas at the other electrode.
2H₂O(l) → 2H₂(g) + O₂(g)
Thus two molecules of water on electrolysis give two molecules of hydrogen gas and one molecule of oxygen gas or in other words the amount of hydrogen gas collected would be double than that of oxygen gas.

6. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
   OR
   An iron nail is dipped in the solution of copper sulphate for about 30 minutes. State the change in colour observed. Give reason for the change.

Answer: When an iron nail is dipped in copper sulphate solution, the displacement reaction takes place. The colour of copper sulphate solution fades due to the formation of light green solution of iron sulphate.

Fe (s) + CuSO₄ (aq) → FeSO₄ (aq) + Cu (s)

7. Give an example of a double displacement reaction other than the one given in Activity 1.10 (NCERT Text Book).

Answer: Sodium hydroxide and hydrochloric acid react to form sodium chloride and water.
NaOH (aq) + HCl (aq) → NaCl (aq) + H₂O (l)

8. Identify the substances that are oxidised and the substances which are reduced in the following reactions.
   (i) 4Na(s) + O₂(g) → 2Na₂O(s)
   (ii) CuO (s) + H₂(g) → Cu (s) + H₂O(l)

Answer: (i) Substances oxidised is Na as it gains oxygen and oxygen is reduced.
(ii) Substances reduced is Cu as hydrogen is oxidised as it gains oxygen.

9. Which of the statements about the reaction below are incorrect ?
   2 PbO(s) + C(s) → 2Pb (s) + CO₂(g)
   (a) Lead is getting reduced.
   (b) Carbon dioxide is getting oxidised.
   (c) Carbon is getting oxidised.
   (d) Lead oxide is getting reduced.

(i) (a) and (b)
(ii) (a) and (c)
(iii) (a), (b) and (c)
(iv) All

Answer: (i) (a) and (b)

PbO lose oxygen to form Pb, which indicates PbO get reduced to Pb.

C gain oxygen to form CO₂, which indicates C get oxidised to CO₂.

10. Fe₂O₃+ 2Al → Al₂O₃+ 2Fe
    The above reaction is an example of a
    (a) combination reaction
    (b) double displacement reaction
    (c) decomposition reaction
    (d) displacement reaction

Answer: (d) Displacement reaction.

Al being more reactive than Fe, displaces Fe from Fe₂O₃ to form Al₂O₃.

11. What happens when dilute hydrochloric acid is added to iron filings? Tick the correct answer :
    (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: (a) Hydrogen gas and iron chloride are produced.

Fe + HCl → FeCl₂ + H₂

12. What is a balanced chemical equation ? Why should chemical equations be balanced?

Answer: A balanced chemical equation has an equal number of atoms of different elements in the reactants and products.
The chemical equations should be balanced to satisfy the law of conservation of mass.

13. Translate the following statements into chemical equations and then balance them.
    (a) Hydrogen gas combines with nitrogen to form ammonia.
    (b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
    (c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
    (d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

Answer: (a) 3H₂ (g) + N₂ (g) → 2NH₃ (g)
(b) H₂S (g) + 3O₂ (g) → SO₂ (g) + 2H₂O(l)
(c) 3BaCl₂ (aq) + Al₂(SO₄)₃ (aq) → 2AlCl₃ (aq) + 3BaSO₄ ↓(s)
(d) 2K (s) + 2H₂O (l) → 2KOH (aq) + H₂ (g)

14. Balance the following chemical equations :
    (a) HNO₃+ Ca (OH)₂→ Ca (NO₃)₂ + H₂O
    (b) NaOH + H₂SO₄ → Na₂SO₄ + H₂O
    (c) NaCl + AgNO₃ → AgCl + NaNO₃
    (d) BaCl₂ + H₂SO₄ → BaSO₄ + HCl

Answer: (a) 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O
(b) 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O
(c) NaCl + AgNO₃ → AgCl + NaNO₃
(d) BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl

15. Write the balanced chemical equations 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 + 2 Ag
(c) 2Al + 3 CuCl₂ → 2AlCl₃ + 3 Cu
(d) BaCl₂ + K₂SO₄ → BaSO₄ + 2KCl

16. Write the balanced chemical equation for the following and identify the type of reaction in each case :
    (a) Potassium bromide (aq) + Barium iodide (aq) → Potassium iodide (aq) + Barium
    (b) Zinc carbonate(s) → Zinc oxide (s) + Carbon dioxide (g) bromide(s)
    (c) Hydrogen (g) + Chloride (g) → Hydrogen chloride (g)
    (d) Magnesium (s) + Hydrochloric acid (aq) → Magnesium chloride (aq) + Hydrogen (g)

Answer: (a) 2KBr (aq) + Bal₂(aq) → 2Kl(aq) + BaBr₂(s)
Type : Double displacement reaction

(b) ZnCO₃ (s) → ZnO (s) + CO₂ (g)
Type : Decomposition reaction

(c) H₂ (g) + Cl₂ (g) → 2HCl(g)
Type : Combination reaction

(d) Mg (s) + 2HCl (aq) → MgCl₂ (aq) + H₂ (g)
Type : Displacement reaction

17. What does one mean by exothermic and endothermic reactions ? Give examples.

Answer: Exothermic reactions – Those reactions in which heat is evolved are known as exothermic reactions. An exothermic reaction is indicated by writing “+ Heat” on the products side of an equation or “- Heat” on the reactant side of an equation.
Example :
(i) C (s) + O₂ (g) → CO₂ (g) + Heat
(ii) N₂ (g) + 3H₂ (g) → 2NH3 (g) + Heat

or N₂ (g) + 3H₂ (g) – Heat → 2NH3 (g)

Endothermic reactions- Those reactions in which heat is absorbed are known as endothermic reactions. An endothermic reaction is usually indicated by writing “- Heat” on the product side of a chemical equation or “+ Heat” on reactant side of an equation.
Examples :
(i) C (s) + 2S (s) → CS₂ (l) – Heat
(ii) N₂ (g) + O₂ (g) → 2NO(g) – Heat

or N₂ (g) + O₂ (g) + Heat → 2NO(g)

18. Why is respiration considered an exothermic reaction ? Explain.

Answer: Respiration is an exothermic process because during respiration glucose combines with oxygen in the cells of our body to form carbon dioxide and water along with the production of energy.

C₆H₁₂O₆ (aq) + 6O₂(g) → 6CO₂ (g) + 6H₂O (l) + Energy

19. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.

Answer: In a decomposition reaction, a single compound breaks down to produce two or more simpler substances.
For example: 2H₂O (l) + electricity → 2H₂ (g) + O₂ (g)

While, in a combination reaction, two or more substances simply combine to form a new substance.
For example: 2H₂ (g) + O₂ (g) → 2H₂O (l)

20. Write one equation each for the decomposition reactions where energy is supplied in the form of heat, light or electricity.
    OR
    Decomposition reactions require energy either in the form of heat or light or electricity for breaking down the reactants. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light and electricity.

Answer: The decomposition reactions, i.e., thermal decomposition, photolytic decomposition and electrolytic decomposition reactions respectively, are as follows:

21. What is the difference between displacement and double displacement reactions? Write equations for these reactions.

Answer: In displacement reactions, a more reactive metal displaces a less reactive metal from its solution. For example,
Fe(s) + CuSO₄ (aq) → Cu (s) + FeSO₄ (aq)
This is a displacement reaction where iron displaces copper from its solution.
In double displacement reactions, two reactants in solution exchange their ions. For example,
AgNO₃ (aq) + NaCl (aq) → AgCl (s) + NaNO₃ (aq)
This is a double displacement reaction where silver nitrate and sodium chloride exchange Cl^– and NO₃^– ions between them.

22. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.

Answer: 2AgNO₃ (aq) + Cu (s) → Cu(NO₃)₂ (aq) + 2Ag (s)

23. What do you mean by a precipitation reaction ? Explain by giving examples.

Answer: A reaction in which an insoluble solid called precipitate is formed that separates from the solution is called a precipitation reaction.
Example : When a solution of iron (III) chloride and ammonium hydroxide are mixed, a brown precipitate of iron (III) hydroxide is formed.

FeCl₃ (aq) + 3NH₄OH (aq) → Fe(OH)₃ (s) + 3NH₄Cl

24. Explain the following in terms of gain or loss of oxygen with two examples each:
    (a) Oxidation and
    (b) Reduction.

Answer: (a) Oxidation : The addition of oxygen to a substance is called oxidation.
Example :
(i) S (s) + O₂ (g) → SO₂ (g) (Addition of oxygen to sulphur)
(ii) 2Mg (s) + O₂ (g) → 2MgO (s) (Addition of oxygen to magnesium)

(b) Reduction : The removal of oxygen from a substance is called reduction.
Example: (i) CuO + H₂ → Cu + H₂O
Here, copper oxide is being reduced to copper because oxygen gets removed from copper oxide.

(ii) ZnO + C → Zn + CO
Here, zinc oxide is being reduced to zinc because oxygen gets removed from zinc oxide.

25. A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.

Answer: Element ‘X’ is copper (Cu).
The black coloured compound is copper oxide (CuO). The reaction involved is:

2Cu + O₂ → 2CuO

26. Why do we apply paint on iron articles?

Answer: Paint does not allow iron articles to come in contact with air, water and saves iron articles from damage due to rusting.

27. Oil and fat containing food items are flushed with nitrogen. Why?

Answer: To keep food items fresh and save from getting oxidised, food items are flushed with nitrogen.

28. Explain the following terms with one example each (a) Corrosion, (b) Rancidity.

Answer: (a) Corrosion : It is the process in which metals are eaten up gradually by the action of air, moisture or a chemical (such as an acid) on their surface.
Example : When iron is exposed to moist air for a long period of time, its surface acquires a coating of a brown, flaky substance called rust. Rust is mainly hydrated iron (III) oxide [Fe₂O₃.xH₂O].

4Fe + 3O₂ + 2xH₂O → 2Fe₂O₃.xH₂O

(b) Rancidity : The condition produced by aerial oxidation of fats and oils in foods marked by unpleasant smell and taste is called rancidity.
Rancidity spoils the food materials prepared in fats and oils which have been kept for a considerable time and makes them unfit for eating.
Rancidity can be prevented by adding anti-oxidants to foods containing fats and oils. It can also be prevented by flushing fat and oil containing foods with nitrogen before sealing.

29. Identify the substances that are oxidised and the substances that are reduced in the following reactions.
    (i) 4Na(s) + O₂(g) → 2Na₂O(s)
    (ii) CuO(s) + H₂(g) → Cu(s) + H₂O(l)

Answer: (i) 4Na(s) + O₂ (g) → 2Na₂O(s)

Here oxygen is added to sodium. The addition of oxygen is called oxidation. So the substance that is oxidized is sodium Na. Substance oxidised is Na and reduced is O₂.

(ii) CuO(s) + H₂(g) → Cu(s) + H₂O(l)

In this reaction, copper oxide (CuO) gives the oxygen required for the oxidation of hydrogen; therefore, copper oxide is the oxidizing agent. Hydrogen is responsible for removing oxygen from copper oxide, therefore, hydrogen is the reducing agent here. Substance oxidised is H₂ and reduced is CuO.

30. When crystals of lead nitrate are heated strongly in a dry test tube
    (a) crystals immediately melt
    (b) a brown residue is left
    (c) white fumes appear in the test tube
    (d) a yellow residue is left

Answer: (b) Pungent smelling, brown fumes are evolved due to NO₂ gas and brown coloured residue of lead oxide (PbO) is left.

31. A dilute ferrous sulphate solution was gradually added to the beaker containing acidified permanganate solution. The light purple colour of the solution fades and finally disappears. Which of the following is the correct explanation for the observation? [NCERT Exemplar]
    (a) KMnO₄is an oxidising agent, it oxidises FeSO₄
    (b) FeSO₄acts as an oxidising agent and oxidises KMnO₄
    (c) The colour disappears due to dilution, no reaction is involved
    (d) KMnO₄ is an unstable compound and decomposes in the presence of FeSO₄ to a colourless compound.

Answer: (a) Potassium permanganate (KMnO₄) in the presence of dil. H₂SO₄, i.e. in acidic medium, acts as a strong oxidising agent. In acidic medium, KMnO₄ oxidises ferrous sulphate to ferric sulphate.

2KMnO₄ + 8H₂SO₄ + 10 FeSO₄ → K₂SO₄ + 5 Fe₂(SO₄)₃ + 2 MnSO₄ + 8 H₂O

32. Dilute hydrochloric acid is added to granulated zinc taken in a test tube. The following observations are recorded. Point out the correct observation.
    (a) The surface of metal becomes shining
    (b) The reaction mixture turns milky
    (c) Odour of a pungent smelling gas is recorded
    (d) A colourless and odourless gas is evolved

Answer: (d) Zinc metal reacts with dil. HCl to form zinc chloride and bubbles of colourless and odourless hydrogen gas is evolved.

33. When a magnesium ribbon is burnt in air, the ash formed is
    (a) black
    (b) white
    (c) yellow
    (d) pink

Answer: (b) When a Mg ribbon is burnt in air, the ash formed is of magnesium oxide which is white in colour.
2Mg(s) + O₂(g) → 2MgO(s)

34. Three beakers labelled as A, B and C each containing 25 mL of water were taken. A small amount of NaOH, anhyd. CuSO₄and 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 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, the exothermic process has occurred.
    IV. In beaker C, endothermic process has occurred. [NCERT Exemplar]
    (a) Only I
    (b) Only II
    (c) I and IV
    (d) II and III

Answer: (c) As in case of beakers A and B, heat is given out, so temperature became high, hence it is an exothermic reaction while in beaker C, heat is absorbed from water, so temperature falls, hence it is an endothermic process.

35. Which of the following will be required to identify the gas evolved when dilute hydrochloric acid reacts with zinc metal?
    (a) Red litmus paper
    (b) pH paper
    (c) Lime water
    (d) A burning splinter

Answer: (d) On reacting with dil. HCl, zinc metal forms zinc chloride and hydrogen gas is evolved. Presence of hydrogen gas can be checked by a burning splinter because H₄ gas burnt in a splinter with a pop sound.

36. On immersing an iron nail in CuSO₄ solution for few minutes, you will observe that
    (a) no reaction takes place
    (b) the colour of solution fades away
    (c) the surface of iron nails acquire a black coating
    (d) the colour of solution changes to green

Answer: (b) Fe(s) + CuSO₄ (aq) → FeSO₄ (aq) + Cu(s)
Fe is present above in the reactivity series of metals. Hence, Fe displaces Cu from CuSO₄ solution. And the colour of the solution fades away. This is an example of displacement reaction.

37. What happens when ferrous sulphate crystals are heated? [CCE 2014]
    (a) A gas having the smell of burning sulphur is evolved
    (b) No gas is evolved
    (c) Brown coloured gas is evolved
    (d) Colourless and odourless gas is evolved

Answer: (a) The green colour of ferrous sulphate crystals changes to brownish black ferric oxide and smell of burning sulphur is evolved due to SO₂ and SO₃.

38. The colour of the precipitate formed when barium chloride solution is mixed with sodium sulphate solution is:
    (a) blue
    (b) black
    (c) white
    (d) green

Answer: (c) This is an example of a double displacement reaction and a white precipitate of barium sulphate is formed.

BaCl₂ (aq) +Na₂SO₄ (aq) → BaSO₄ (s)+ 2 NaCl (aq)

39. How the colour changes when the gases after thermal decomposition of ferrous sulphate come in contact with an acidified solution of potassium dichromate?
    (a) Green to orange
    (b) Red to colorless
    (c) Orange to green
    (d) Blue to green

Answer: (c) The color changes from orange to green due to the formation of iron (III) sulphate.

40. Why precipitates are formed in some of the reactions especially during double displacement reactions? Give an example of precipitation reaction.

Answer: Double displacement reaction occur in aqueous solution, and the combination of cation and anion occur forming a compound which is insoluble in water. That is why always precipitation reaction occur.

BaCl₂ (aq) +Na₂SO₄ (aq) → BaSO₄ (s)+ 2 NaCl (aq)

41. Why does corrosion takes place?

Answer: Corrosion take place due to gradual attack of air and moisture on the substance.

42. Why food containing fats and oils should be kept in air tight containers?

Answer: Rancidity is a process of slow oxidation of oil and fat present in food material resulting in the change of smell and taste. To prevent the oxidation of food containing fats and oils, so, it should be kept in air tight containers.

43. Why chips manufactures usually flush bags with gas such as nitrogen?

Answer: While packaging of chips they are flushed or surrounded with an inert gas such as nitrogen to prevent its contact with oxygen of air. This is done to avoid rancidity of fats and oils.

44. What is balanced chemical equation? Why should chemical equations be balanced?

Answer: A balanced chemical reaction is an equation that has equal numbers of each type of atom on both sides of the arrow. 

A chemical equation is a written symbolic representation of a chemical reaction. The reactant chemical(s) are given on the left-hand side and the product chemical(s) on the right-hand side. 

The chemical equation is balanced in order to obey law of conservation of mass which states that atoms can neither be created nor destroyed in a chemical reaction, so the number of atoms that are present in the reactants has to balance the number of atoms that are present in the products.

45. Why do we apply paint on iron articles?

Answer: The iron articles can be protected from rusting by applying paint on them so that the iron surface does not come in contact with air or oxygen and moisture which cause rusting.

46. Why surface of copper powder becomes black when heated slowly in a china dish?

Answer: When the copper powder is heated in a china dish the copper powder surface becomes coated with black colour substance due to the formation of copper oxide by surface oxidation. The black colour is due to the formation of this copper (II) oxide as the copper comes in contact with the air.

2Cu + O₂ → 2CuO

47. Why corrosion is considered as a serious problem?

Answer: Corrosion of iron occurs when it is exposed to oxygen and with the presence of water, it creates a red iron oxide commonly called rust.

Corrosion of iron is a serious problem as it negatively affects iron’s desirable properties, which loses its strength and finally becomes unfit for further use. This leads to the damage and wastages of tons of iron.

48. Why most of the reactions are categorized as redox reactions?

Answer: Most of the reactions are categorised as redox, because they undergo both oxidation and reduction. 
Cl₂ + 2 KI → 2 KCl + I₂

49. Why aqueous solution of copper sulphate should not be stored in iron container?

Answer: More reactive metal displace less reactive metal. Copper sulphate is not kept in iron vessel because iron being more reactive than copper displaces it form copper sulphate forming ferrous sulphate and copper. in short,displacement reaction will take place.

CuSO₄ (aq) + Fe (s) → FeSO₄ (aq) + Cu (s)

50. Why most of the reactions are exothermic in nature?

Answer: Most of the reactions occur by releasing energy. That means all reactions occur by breaking of bonds in reactant and formation of product. The breaking of bond needs more enthalpy than the formation of products. Many reactions like oxidation, combustion all occur by liberation of energy. Moreover the exothermic reactions are always spontaneous and the product produced are stable.
Ex. CH_​4 + 2O₂ → CO₂ + 2 H₂O

51. Is oxidation an exothermic or an endothermic reaction? Give a reason in support of your answer.

Answer: Oxidation reactions are exothermic. 

For example, the burning of hydrogen is an oxidation process in which hydrogen combines with oxygen to form water.

2H₂​ (g) + O₂​ (g) → 2H₂​O (l) 

52. What is advantage and disadvantage of corrosion?

Answer: Advantage: Sometimes when metal is exposed to moisture it forms an oxide layer which protects the metal from being further corroded by corrosion. This process is called anodizing. Ex., Sacrificial anode as zinc which protects the underlying copper or iron.
Disadvantage: The metal slowly starts corroded and its strength decreases and the lustrous appearance of the metal will be lost.

53. (i) What is corrosion? What are the conditions favorable for corrosion to takes place?

(ii) What is the chemical formula of rust?

Answer: (i) Corrosion is a natural process of wasting of metal due to reaction of the metals with environmental factors like moisture, oxygen etc to form corresponding metallic compounds. The metal undergoing corrosion losses its strength, appearance and its efficiency.

Corrosion generally takes place under following conditions:

1. Exposure of metals to humid climatic conditions for a long period of time
2. Exposure of metals to reactive gases and liquids for long period of time.
3. Environmental condition of the place where metal is placed like temperature of the surrounding, moisture content in the air, etc.

(ii) The chemical formula for rust is Fe₂O₃.nH₂O(s), where n is the water of crystallization. The rust is formed by the following equation:

4Fe + 6H₂O + 3O₂ → 4Fe(OH)₃

The iron hydroxide formed dehydrates to produce Fe₂O₃.nH₂O(s).

54. What happens chemically when quick lime is added to water?

Answer: When quicklime is added to water, it forms slaked lime along with the evolution of heat. There will be a rise in the temperature of the bucket. Calcium oxide reacts with water to form calcium hydroxide, also called slaked lime.

This is highly exothermic reaction.

CaO + H₂O → Ca(OH)₂

55. Identify the substance oxidised and reduced in the following chemical equations:

(i) MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂

(ii) 2H₂S + SO₂ → 2H₂O + 3S

(iii) Fe₂O₃ + 2Al → Al₂O₃ + 2Fe

(iv) H₂S + Cl₂ → 2HCl + S

(v) Zn + CuSO₄ → Cu + ZnSO₄

Answer: (i) MnO₂ reduced to MnCl₂ because of loss of oxygen and HCl oxidised to Cl₂ because of loss of hydrogen.

(ii) SO₂ reduced to S because of loss of oxygen and H₂S oxidised to H₂O because of gain of oxygen.

(iii) Fe₂O₃ reduced to Fe because of loss of oxygen and Al oxidised to Al₂O₃ because of gain of oxygen.

(iv) Cl₂ reduced to HCl because of gain of hydrogen and H₂S oxidised to S because of loss of hydrogen.

(v) CuSO₄ reduced to Cu because of loss of oxygen and Zn oxidised to ZnSO₄ because of gain of oxygen.

56. On adding dil. HCl to copper oxide, the solution formed is blue-green. Predict the new compound formed which imparts a blue-green colour to the solution.

Answer: (a)  The new compound formed which imparts a blue-green colour to the solution is Copper (II) chloride, CuCl₂.

CuO + 2HCl → CuCl₂ + H₂O

57. What are the various observations which help us to determine whether a chemical reaction has taken place or not?

Answer: A chemical reaction can be observed with the help of any of the following observation:

(i) Change in colour

(ii) Change of state

(iii) Evolution of a gas

(iv) Change in temperature

(v) Formation of a precipitate

58. What is the utility of photochemical decomposition reaction?

Answer: This reaction is used in black and white photography.

59. Consider the given redox reaction: A³⁺ + B → B³⁺ + A

Identify the substance being oxidised, the substance being reduced and the reducing agent.

Answer: A³⁺ gain electrons to form A while B lose electron to form B³⁺.

So, B oxidised to B³⁺, A³⁺ reduced to A and B acts as reducing agent.

60. What changes can be observed when:

(i) Crystals of FeSO₄ are heated.

(ii) Lead nitrate is heated strongly.

(iii) Potassium chlorate is heated in the presence of manganese oxide.

(iv) Limestone is heated strongly.

Answer: (i) Decomposition reaction takes place. Change in colour and evolution of gas is observed.

Colour of crystals of FeSO₄ is green which changes to reddish brown, i.e., Fe₂O₃. And two different gases evolve, i.e., SO₂ and SO₃.

(ii) Evoloution of gas is observed. It decomposes into lead(II) oxide,nitrogen dioxide gas and oxygen gas.

(iii) Evolution of gas is observed. It decomposes into potassium chloride and oxygen gas.

(iv) Change in temperature and evolution of gas is observed. It decomposes to give calcium oxide and carbon dioxide with large amount of heat evolution.

61. Give the significance of s, l, g and  aq in a chemical equation.

Answer: s represents solid, l represents liquid, g represents gas and aq represents aqueous solution (i.e., solution in water).

62. Write chemical equations for the following reactions:

(i) Zinc metal reacts with hydrochloric acid to produce an aqueous solution of zinc chloride and hydrogen gas.

(ii) When solid mercury(II) oxide is heated, liquid mercury and oxygen gas is produced.

Answer: (i) Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)

(ii) 2HgO (s) + Heat → 2Hg(l) + O₂(g)

63. How are the following informations indicated in a chemical equation?

(i) Evolution of heat energy.

(ii) A solution made in water

(iii) Evolution of a gas

(iv) Formation of a precipitate.

Answer: (i) Evolution of heat energy is indicated by writing +Q kJ or +Q kcal along with the products.

(ii) A solution made in water is indicated by writing aq along with the formula of a substance.

(iii) Evolution of a gas is indicated by an arrow pointing upwards, along with formula of gas.

(iv) Formation of a precipitate is indicated by putting an arrow pointing downwards, along with the formula of a compound.

64. Why do gold and silver not corrode in moist air?

Answer: This is because they are not attacked by air and moisture, as they are least reactive in nature.

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Read More: Class 10 Science Chapter 1 Chemical Reactions and Equations

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