CLASS 10 Chemical REACTIONS and Equations Previous Year Questions

Chap 1 : Chemical Reactions and Equations


1. In electrolysis of water, why is the volume of gas collected over one electrode double that of gas collected over the other electrode.

Ans : [CBSE S.R 2018, CBSE 2009, 2012, 2013]

Water contains hydrogen and oxygen in the ratio of 2 : 1, that is why volume of H2 is double than that of oxygen.

2. What can be seen when a strip of copper metal is placed in a solution of silver nitrate?

Ans : [CBSE 2015]

The solution will become blue, shiny silver metal will get deposited.

Cu(s) + 2AgNO3(aq) $ Cu(NO3)2(aq) + 2Ag(s)

3. State one industrial application of reduction process.

Ans : [CBSE 2015]

It is used in the extraction of metals e.g.,

ZnO(s) + C(s) Heat Zn(s) + CO2(g)

4. Which one of the following is a chemical change? Give reason also.

(a) Burning of wax (b) Melting of wax

Ans : [CBSE 2014]

Burning of gas is a chemical change because new products with new properties will be formed on burning.

5. Which one is a chemical change: Rusting of iron or melting of iron?

Ans : [CBSE 2014]

Rusting of iron is a chemical change.

6. State one basic difference between a physical change and a chemical change.

Ans : [CBSE 2014, 2011]

In a physical change, no new substance is formed. In a chemical change, new substance(s) with new properties is/are formed.

7. Name the oxidising and reducing agent in the following reaction:

CuO + H2 $ Cu + H2O

Ans : [CBSE 2013]

CuO is oxidising agent H2 is reducing agent.

8. Write a complete balanced chemical equation for the following reaction:

Sodium hydroxide + Sulphuric acid $

Ans : [CBSE 2013]

2NaOH + H2SO4 $ Na2SO4 + 2H2O

9. Name and state the law which is kept in mind when we balance chemical equations.

Ans : [CBSE 2012]

Law of conservation of mass. It states Matter can neither be created nor be destroyed. The total mass of reactants must be equal to total mass of products.

10. What is meant by a chemical reaction?

Ans : [CBSE 2011]

Chemical reaction, a process in which one or more substances, the reactants, are converted to one or more different substances, the products. Substances are either chemical elements or compounds. A chemical reaction rearranges the constituent atoms of the reactants to create different substances as products.

11. AgNO3(aq) + NaCl(aq) $ AgCl + NaNO3(aq)

FeS + H2SO4 $ FeSO4 + H2S

Consider the above mentioned two chemical equations with two different kinds of arrows along with the product. What do these two different arrows indicate?

Ans : [CBSE 2011]

shows evolution of gas,represents a precipitate.

12. Hydrogen being a highly inflammable gas and oxygen being a supporter of combustion, yet water, a compound made up of hydrogen and oxygen is used to extinguish fire. Why?

Ans : [CBSE 2011]

H2O is a compound constituted of hydrogen of oxygen elements and being a compound it has different properties as compared to its constituting elements.

13. N2(g) + 3H2(g) $ 2NH3(g), name the type of reaction.

Ans : [CBSE 2011]

Combination reaction.

14. If copper metal is heated over a flame it develops a coating. What is the colour and composition of coating?

Ans : [CBSE 2011]

Black coloured coating is formed. It is a due to formation of copper oxide.

15. Write a balanced chemical equation to represent the following reaction: carbon monoxide reacts with hydrogen gas at 340 atm to form methyl alcohol.

Ans : [CBSE 2011]

CO(g) + 2H2(g) $ CH3OH(l)

16. Which one is a chemical change: fermentation of fruit juice or diluting fruit juice?

Ans : [CBSE 2011]

Fermentation of fruit juice is a chemical change.

17. Is burning of a candle wax a physical or a chemical change?

Ans : [CBSE 2011]

Burning of a candle wax is a chemical change.

18. Write a balanced equation for the chemical reaction that can be characterised as precipitation reaction.

Ans : [CBSE 2011]

AgNO3(aq) + NaCl(aq) $ AgCl(s) + NaNO3(aq)

It is a precipitation reaction.

19. State the main difference between endothermic reaction and an exothermic reaction.

Ans : [CBSE 2011]

In endothermic reaction, heat is absorbed. In exothermic reaction, heat is evolved.

20. What happens chemically when quick lime is added to water filled in a bucket?

Ans : [CBSE 2010, 2008]

Calcium hydroxide (Slaked lime) is formed with evolution of heat and hissing sound.

CaO(s) + H2O(l) $ Ca(OH)2(aq)

21. Define oxidation and reduction.

Ans : [CBSE 2011, 2010]

Oxidation is a process in which oxygen is added or loss of electrons take place. Reduction is a process in which hydrogen is added or gain of electrons takes place.

22. Give an example of double displacement reaction (only with complete balanced equation).

Ans : [CBSE 2011, 2010]

BaCl2(aq) + H2SO4 $ BaSO4(s) + 2HCl(aq)

It is a double displacement reaction.

23. On what basis is a chemical reaction balanced?

Ans : [CBSE 2010, 2008]

Chemical equation is balanced on the basis of law of conservation of mass.

24. What change in colour is observed when white silver chloride is left exposed to sun¬light? State the type of chemical reaction in this change.

Ans : [CBSE 2010, 2009]

Grey coloured silver metal is formed and pungent smelling chlorine gas is evolved.

2AgCl(s) sunlight 2Ag(s) + Cl2(g)

It is photochemical decomposition reaction.

25. Write a balanced chemical equation for the reaction between sodium chloride and silver nitrate indicating the physical state of the reactants and the products.

Ans : [CBSE 2010]

AgNO(aq) + NaCl(aq) $ AgCl(s) + NaNO(aq) Heat

26. Complete and balance the following equation:

Fe2O3 + Al $

Ans : [CBSE 2010]

Fe2O3 + 2Al $ 2Fe + Al2O3

27. Balance the following chemical equation:

Pb(NO3)2 $ PbO + NO2 + O2

Ans : [CBSE 2009]

2Pb(NO3)2 Heat 2PbO + 4NO2 + O2

28. Identity the type of reaction in the following example:

Na2SO4(aq) + BaCl2(aq) $ BaSO4(s) + 2NaCl(ag)

Ans : [CBSE 2008(C)]

Double displacement reaction.

29. Identify the type of reaction in the following example:

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

Ans : [CBSE 2008(C)]

Displacement reaction.

30. Identify the type of reaction in the following example:

2H2(g) + O2(g) $ 2H2O(l)

Ans : [CBSE 2008(C)]

Combination reaction.

31. Balance the given chemical equation:

Al(s) + CuCl2(aq) $ AlCl3(aq) + Cu(s)

Ans : [CBSE 2008(C)]

2Al(s) + 3CuCl2(aq) $ 2AlCl3(aq) + 3Cu(s)

32. Balance the given chemical equation:

FeSO4(s) Heat Fe2O3(s) + SO2(g) + SO3(g)

Ans : [CBSE 2008]

2FeSO4(s) Heat Fe2O3(s) + SO2(g) + SO3(g)

33. Balance the following chemical equation:

Fe(s) + H2O(g) $ Fe3O4(s) + H2(g)

Ans :

3Fe(s) + 4H2O(g) $ Fe3O4(s) + 4H2(g)

34. On adding dilute hydrochloric acid to copper oxide powder, the solution formed is blue green. Predict the new compound formed which imparts a blue green colour to the solution.

Ans : [CBSE 2008]

CuO + 2HCl $CuCl2 + 2H2O

Copper chloride solution imparts blue green colour to the solution.

35. Why is respiration considered as exothermic process?

Ans : [CBSE 2008]

Respiration is an exothermic process because energy is given out in respiration.


36. Write the essential condition for the following reaction

to take place: 2AgBr $ 2Ag + Br2 Write application of this reaction.

Ans : [CBSE 2016]

The reaction will take place in presence of sunlight. This reaction is used in black and white photography.

37. (a) Complete the following equation for the chemical reaction:

FeSO4(s) Heat Fe2O3 + ___ +___

(b) What happens when water is added to quicklime (CaO)? Write the chemical equation.

Ans : [CBSE 2016]

(a) 2FeSO4(s) Heat Fe2O3(s) + SO2(g) + SO3(g)

(b) Slaked lime Ca(OH)2 is formed. Hissing sound and lot of heat is also produced: CaO(s) + H2O(l) $ Ca(OH)2(aq)

38. Write balanced chemical equation for the following reactions:

a. Hydrogen sulphide burns in air to give water and sulphur dioxide.

b. Barium chloride reacts in aqueous solution with zinc sulphate to give zinc chloride and barium sulphate.

Ans : [CBSE 2016]

a. 2H2S(g) + 3O2(g) $ 2H2O(l) + 2SO2(g)

b. BaCl(aq) + ZnSO(aq) $ BaSO4(s) + ZnCl2(aq)

39. List two observations that are noticed when an iron nail is put inside copper sulphate solution. Write the chemical equation for the reaction that occurs.

Ans : [CBSE 2016]

The blue coloured solution will become pale green. Reddish brown metal will get deposited.

40. Name the reducing agent in the following reaction: 3MnO2 + 4A1 $ 3Mn + Al2O3 State which is more reactive, Mn or Al and Why?

Ans : [CBSE 2016, 2015]

Al is the reducing agent. Al is more reactive than Mn.

Reason: It is because Al is displacing Mn from MnO2

41. “We need to balance a skeleton chemical equation”. Give reason to justify the statement.

Ans : [CBSE 2015]

We must balance a skeletal equation so as to ensure that the reaction follows ‘Law of conservation of mass’. The total mass of reactants must be equal to the total mass of products, that is why all reactions should he balanced.

42. A metal is treated with dilute H2SO4. The gas evolved is collected by the method as shown in figure.

Answer the following questions:

a. Name the gas liberated.

b. Name the method used for collection of gas.

c. Is the gas soluble or insoluble in water?

d. Is the gas lighter or heavier than air?

Ans : [CBSE 2015, CBSE 2010]

a. The gas liberated is H2

b. It is collected by downward displacement of water.

c. The gas is insoluble in water.

d. Hydrogen gas is lighter than air.

43. Giving an example list two important information which makes a chemical equation more useful (informative).

Ans : [CBSE 2015]

a. It should include physical states of reactants and products.

b. It should specify conditions under which reaction takes place e.g.,

2Na(s) + 2H2O(l) $ 2NaOH(aq) + H2(g) (Cold) H2(g) + Cl2(g) sunlight 2HCl(g)

44. Consider the following chemical equation: X + Barium chloride $ Y + Sodium chloride (White ppt) Identify (a) X and Y (b) The type of reaction.

Ans : [CBSE 2015]

(a) X is silver nitrate, Y is silver chloride.

(b) The reaction is an example of double displacement (precipitation) reaction.

45. (a) Write a balanced chemical equation for the process of photosynthesis.

(b) When do desert plant take up carbon dioxide and perform photosynthesis.

Ans : [CBSE 2015]

a. 6CO2(g) + 6H2O(l) chlorophyll + sunlight C6H12O6(aq) + 6O2(g)

b. Desert plant take up carbon dioxide and perform photosynthesis at night.

46. Two reactions are given below:

a. 2KI + Cl2 $ 2KCl + I2

b. 2K + Cl2 $ 2KCl

Identify the type of reaction, giving justification in each case.

Ans : [CBSE 2015]

a. Displacement reaction because Cl2 is displacing I2 from KI solution.

b. Combination reaction because K reacts with Cl2 to form potassium chloride.

47. On heating copper powder in air, the surface of copper powder becomes coated with black CuO. How can this black coating be converted into brown copper? Write chemical equation for the reaction that occurs during the colour change.

Ans : [CBSE 2015]

2Cu + O2 Heat 2CuO(Black) Copper oxide on heating with H2 will change back to reddish brown copper metal.

CuO(s) + H2(g) Heat Cu(s) + H2O(g)

48. What is observed when a solution of potassium iodide is added to a solution of lead nitrate? Name the type of reaction. Write a balanced chemical equation to represent the above chemical reaction.

Ans : [CBSE 2014, 2013]

Yellow precipitate is formed due to formation of lead iodide. It is a precipitation as well as double displacement reaction.

50. Give one example of each:

a. Chemical reaction showing evolution of a gas.

b. Change in colour of a substance during chemicalreaction.

Ans : [CBSE 2013]

a. 2K(s) + 2H2O(l) $ 2KOH(aq) + H2(g)

b. FeSO4+ 7H2O(aq) Heat FeSO4(s) + 7H2O(l) (Pale green) (Dirty white)

51. Translate the following statement into chemical equation and then balance it. “A metal in the form of ribbon burns with a dazzling white flame and changes into white powder.”

Ans : [CBSE 2013]

2Mg(s) + O2(g) Burning 2MgO(s) + Light + Heat

52. It has been found that marble of Taj is getting corroded due to development of industrial areas around it. Explain this fact giving a chemical equation.

Ans : [CBSE 2013]

Taj is made up of CaCO3 which reacts with acid formed by pollution of SO2(g) and NO2 from Mathura refinery and other industries. CaCO3 + 2H2SO4 $ CaSO4 + CO2 + 2H2O

CaCO3 + 2HNO3 $ Ca(NO3)2 + H2O + CO2

53. Consider the chemical equations given below and answer the questions which follow:

(i) CuO + H2 Heat Cu + H2O

(ii) ZnO + C Heat Zn + CO

a. Name the substances that are oxidised and reduced respectively in each case,

b. Identify the reducing agent in each case.

Ans : [CBSE 2013]

a. H2 is getting oxidised to H2O, CuO getting reduced to Cu.

b. H2 is reducing agent in (i) reaction whereas C is reducing agent in second reaction.

54. What is a redox reaction? Identify the substances oxidised and the substance reduced in the following reactions:

a. MnO2 + 4HCl $ MnCl2 + Cl2 + H2O

b. CuO + H2 $ Cu + H2O

Ans : [CBSE 2012]

Redox reaction is a reaction in which oxidation and reduction takes place simultaneously.

c. HCl is the substance oxidised, MnO2 is the substance getting reduced.

d. H2 is getting oxidised, CuO is getting reduced.

55. Write balanced chemical equations for the following reactions:

a. Silver bromide on exposure to sunlight decomposes into silver and bromine.

b. Sodium metal reacts with water to form sodium hydroxide and hydrogen gas.

Ans : [CBSE 2012]

a. 2AgBr(s) sunlight 2Ag(s)+ Br2(g)

b. 2Na(s) + 2H2O(l) $ 2NaOH(l) + H2(g)

56. Identify the type of reaction from the following equations:

a. CH4 + 2O2 $ CO2 + 2H2O

b. Pb(NO3)2 + 2KI $ PbI2 + 2KNO3

c. CaO + H2O $ Ca(OH)2.

4 CuSO4 + Zn $ ZnSO + Cu

Ans : [CBSE 2012]

a. Oxidation reaction

b. Double displacement reaction

c. Combination reaction

d. Displacement reaction

57. Write balanced equation for the reaction between Mg and hydrochloric acid. Name the product obtained, identify the type of reaction.

Ans : [CBSE 20i2]

Mg(s) + 2HCl(dil) $ MgCl2(aq) + H2(g)

Magnesium chloride and hydrogen gas are formed in this reaction. It is a displacement reaction.

58. a. What is colour of ferrous sulphate crystals? How does this colour change after heating?

b. Name the products formed on strongly heating ferrous sulphate crystals. What type of chemical reaction occurs in this change?

Ans : [CBSE 2012.2009]

a.FeSO H2O $

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