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## Chapter-1. Chemical Reactions and Equations previous year questions

### Class 10 Science Chapter 1 – 1 Mark Important Question

1. Identify ‘x’, ‘y’ and ‘z’ in the following reaction : 2KClO3(x) y 2KCl(x) + O2(z)

(a) x = gas; y = reaction condition; z = gas

(b) x = solid; y = liquid; z = gas

(c) x = number of moles of KClO3; y = reaction condition; z = number of molecules of oxygen

(d) x = physical state of KClO3 and KCl; y = reaction condition, z = physical state of O2.

(d) x = physical state of KClO3 and KCl; y = reaction condition, z = physical state of O2

2.   Assertion (A): Following is a balanced chemical equation for the action of steam on iron :

3Fe + 4H2O Fe3O4 + 4H2

Reason (R): The law of conservation of mass holds good for a chemical equation.

(a) Both (A) and (R) are true and reason (R) is the correct explanation of the assertion (A).

(b) Both (A) and (R) are true, but reason (R) is not the correct explanation of the assertion (A).

(c) (A) is true, but (R) is false.

(d) (A) is false, but (R) is true

(a) Both (A) and (R) are true and reason (R) is the correct explanation of the assertion (A).

3.    Calcium oxide reacts vigorously with water to produce slaked lime.

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

This reaction can be classified as

(A) Combination reaction

(B) Exothermic reaction

(C) Endothermic reaction

(D) Oxidation reaction

Which of the following is the correct option?

(a) (A) and (C)

(b) (C) and (D)

(c) (A), (C), and (D)

(d) (A) and (B)

(d) (A) and (B)

4. When hydrogen sulphide gas is passed through a blue solution of copper sulphate, a black precipitate of copper sulphide is obtained and the sulphuric acid so formed remains in the solution. The reaction is an example of a :

(a) combination reaction

(b) displacement reaction

(c) decomposition reaction

(d) double displacement reaction.

(d) double displacement reaction.

5. In a double displacement reaction such as the reaction between sodium sulphate solution and barium chloride solution:

(A) exchange of atoms takes place

(B) exchange of ions takes place

(C) a precipitate is produced

(D) an insoluble salt is produced

The correct option is:

(a) (B) and (D)

(b) (A) and (C)

(c) only (B)

(d) (B), (C), and (D)

(d) (B), (C), and (D)

6. In which of the following does the initial substance’s identity remain unchanged?

(a) Curdling of milk

(b) Formation of crystals by process of crystallization

(c) Fermentation of grapes

(d) Digestion of food

(b) Formation of crystals by process of crystallization

### Class 10 Science Chapter 1 – 2 Mark Important Question

7. (a) State the law that is followed by balancing a chemical equation.

(b) Balance the following chemical equation :

Na + H2O → NaOH + H2

(a) Law of conservation of mass is followed for balancing a chemical equation which states that mass can neither be created nor destroyed

in a chemical reaction. That is, the total mass of the elements present in the products of a chemical reaction has to be equal to the total mass of the elements present in the reactants in a balanced equation.

(b) 2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)

8. Study the following equation of a chemical reaction :

H2 + Cl2 → 2HCl

(i) Identify the type of reaction.

(ii) Write a balanced chemical equation of another example of this type of reaction.

(i) Combination reaction.

(ii) Another example of a combination reaction is

2Na(s) + Cl2(g) → 2NaCl(s)

Sodium+Chlorine →Sodium chloride

9. State the type of chemical reactions, represented by the following equations :

(a) A + BC → AC + B

(b) A + B → C

(c) PQ + RS → PS + RQ

(d) A2O3 +2B → B2O3 + 2A

(a) Displacement reaction.

(b) Combination reaction.

(c) Double displacement reaction.

(d) Displacement reaction or redox reaction.

### Class 10 Science Chapter 1 – 1 Mark Important Question

10. Explain the significance of photosynthesis. Write the balanced chemical equation involved in the process.

Photosynthesis means synthesis with the help of light. It is the process that gives life to all living beings. Photosynthesis is a process by which plants utilize carbon dioxide and water in the presence of sunlight

to produce glucose and oxygen.

6CO2 + 12H2O→ C6H12O6 + 6O2 + 6H2O

11. Write balanced chemical equations for the following chemical reactions :

(a) Hydrogen + Chlorine → Hydrogen chloride

(c) Zinc oxide + Carbon → Zinc + Carbon monoxide

(a) H2(g) + Cl2(g) → 2HCl(g)

(b) Pb(s) + CuCl2(aq) → PbCl2(aq)+ Cu(s)

(c) ZnO(s) + C(s) → Zn(s) + CO(g)

12.  1 g of copper powder was taken in a China dish and heated. What change takes place on heating? When hydrogen gas is passed over this heated substance, a visible change is seen in it. Give the chemical equations of reactions, the name, and the color of the products formed in each case.

When the copper powder is heated in a China dish, the reddish brown surface of copper powder becomes coated with a black substance which is copper oxide.

2Cu(s) + O2(g) → 2CuO(s)

When hydrogen gas is passed over CuO, the black coating on the surface turned reddish brown due to the formation of Cu.

CuO(s) + H2(g) → Cu(s) + H2O

13. A compound ‘A’ is used in the manufacture of cement. When dissolved in water, it evolves a large amount of heat and forms compound ‘B’.

(i) Identify A and B.

(ii) Write the chemical equation for the reaction of A with water.

(iii) List two types of reaction in which this reaction may be classified.

(i) A is calcium oxide, CaO which is used in

the manufacturing of cement.

B is calcium hydroxide Ca(OH)2.

(ii) CaO(s) + H2O(l) → Ca(OH)2(s)

(iii) The given reaction is a combination reaction.

Example : NH3(g) + HCl(g) → NH4Cl(s)

2NO(g) + O2(g) → 2NO2(g)

14. Mention the reason the colour changes observe when :

(i) silver chloride is exposed to sunlight.

(ii) copper powder is strongly heated in the presence of oxygen.

(iii) a piece of zinc is dropped in copper sulphate solution.

(i) When white silver chloride is left exposed to sunlight, its colour changes to grey as it decomposes to silver in the presence of sunlight.

2AgCl(s) → 2Ag(s) + Cl(g)

This type of reaction is called a photodecomposition reaction.

(ii) When copper powder is strongly heated in presence of oxygen, the reddish brown surface of copper powder becomes coated with a black

substance which is copper oxide.

2Cu(s) + O2(g) → 2CuO(s)

(iii) When a piece of zinc is dropped in copper sulphate solution, then the blue colour of copper sulphate fades gradually due to the formation of colourless zinc sulphate solution, and reddish brown copper metal gets deposited on zinc piece.

CuSO4(aq) + Zn(s) → ZnSO4(aq) + Cu(s)

15. Lead nitrate solution is added to a test tube containing potassium iodide solution.

(a) Write the name and colour of the

compound precipitated.

(b) Write the balanced chemical equation for the reaction involved.

(a) When lead nitrate is added to potassium iodide then a yellow precipitate of lead iodide is formed along with potassium nitrate.

(b) Balanced chemical reaction is as follows :

Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) ↓ + 2KNO3(aq)

(c) This type of reaction is called precipitation reaction in which one of the products formed is an insoluble substance or this is also called a double displacement reaction.

16. 2 g of ferrous sulphate crystals are heated in a dry boiling tube.

(a) List any two observations.

(b) Name the type of chemical reaction taking place.

(c) Write the balanced chemical equation for the reaction and name the products formed.

(a) Ferrous sulphate crystals (FeSO4.7H2O) lose water when heated and the colour of the crystals changes. It then decomposes to ferric oxide (Fe2O3), sulphur dioxide (SO2), and sulphur trioxide (SO3) with a smell of burning sulphur.

(b) This is a thermal decomposition reaction.

(c) 2FeSO4(s) → Fe2O3(s) + SO2(g) + SO3(g)

17. You might have noted that when copper powder is heated in a China dish, the reddish brown surface of copper powder becomes coated with a black substance.

(a) Why has this black substance formed?

(b) What is the black substance?

(c) Write the chemical equation of the reaction that takes place.

(d) How can the black coating on the surface be turned reddish brown?

(a) The black substance is formed because copper combines with oxygen.

(b) The black substance is copper oxide (CuO).

(c) 2Cu + O2 → 2CuO

(d) The black coating on the surface can be turned reddish brown by passing hydrogen gas over the hot copper oxide.

CuO + H2 → Cu + H2O

18. 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.

Decomposition reaction involving absorption of heat :

ZnCO3(s) →ZnO(s) + CO2(g)

Decomposition reaction involving absorption of light :

2H2O2(l) → 2H2O(l) + O2(g)

Hydrogen peroxide Water Oxygen

Decomposition reaction involving absorption of electrical energy :

2Al2O3(l) → 4Al(l) + 3O2(g)

19. Take 3 g of barium hydroxide in a test tube, now add about 2 g of ammonium chloride and mix the contents with the help of a glass rod.

Now touch the test tube from the outside.

(i) What do you feel on touching the test tube?

(ii) State the inference about the type of reaction that occurred.

(iii) Write the balanced chemical equation of the reaction involved.

(i) When barium hydroxide is added into ammonium chloride, the bottom of the test tube is found to be cooler.

(ii) It is an endothermic reaction.

(iii) Ba(OH)2 + 2NH4Cl → BaCl2 + 2NH4OH

20. Name the type of chemical reaction represented by the following equation :

(i) CaO + H2O → Ca(OH)2

(ii) 3BaCl2 + Al2(SO4)3 → 2AlCl3 + 3BaSO4

(iii) 2FeSO4 → Fe2O3 + SO2 + SO3

(i) Combination reaction.

(ii) Precipitation reaction or double displacement reaction.

(iii) Thermal decomposition reaction.

21. What is a reduction reaction?

Identify the substances that are oxidized and the substances that are reduced in the following reactions.

(a) Fe2O3 + 2Al → Al2O3 + 2Fe

(b) 2PbO + C → 2Pb + CO2

Those reactions in which the addition of hydrogen to a substance or removal of oxygen from a substance take place are called reduction reactions.

(a) Fe2O3 + 2Al → Al2O3 + 2Fe

Fe2O3 is getting reduced to Fe and Al is getting oxidized to Al2O3.

(b) 2PbO+ C → 2Pb + CO2

PbO is reduced to Pb and C is oxidized to CO2.

22. What happens when food materials containing fats and oils are left for a long time? List two observable changes and suggest three ways by which this phenomenon can be prevented.

Food materials containing fats and oils change their taste and smell due to a process called rancidity. Rancidity is a process in which air reacts with fats and oils which changes the smell and taste of food.

Methods of prevention :

The vacuum packing, refrigeration of food materials, and placing of food materials, away from direct sunlight.

23. (i) Why is respiration considered an exothermic reaction?

(ii) Write the chemical name and the formula of the brown gas produced during the thermal decomposition of lead nitrate.

(iii) Why do chips manufacturers flush bags of chips with a gas such as nitrogen?

(i) The glucose produced in our body during digestion combines with oxygen in the cells of our body and provides energy. The special name of this reaction is respiration. Thus respiration is an exothermic process because energy is produced during this process.

C6H12O6(aq) + 6O2(g) → 6CO2(g)

+ 6H2O(l) + Energy

(ii) 2Pb(NO3)2 → 2PbO(s) + 4NO(g) + O2(g)

Brown gas evolved is nitrogen dioxide (NO2).

(iii) Chips manufacturers usually flush bags of chips with a gas such as nitrogen because atmospheric oxygen can react with chips which may cause colour changes and changes in taste. So to cut the contact between air and the chips, nitrogen  gas is used which do prevent its oxidation.

### Class 10 Science Chapter 1 – 5 Mark Important Question

24. (a) Can a displacement reaction be a redox reaction? Explain with the help of an example.

(b) Write the type of chemical reaction in the following :

(i) Reaction between an acid and a base

(ii) Rusting of iron.

(a) Consider the following displacement reaction :

Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)

Here, Zn has changed into ZnSO4 (i.e., Zn2+ ions) by losing electrons. Hence, Zn has been oxidized. CuSO4 (i.e., Cu2+) has changed into Cu by the gain of electrons. Hence, CuSO4 has been reduced. Thus,

the above reaction is a displacement reaction as well as a redox reaction.

(b) (i) Neutralisation reaction

(ii) Oxidation reaction.

25. Mention the type of chemical reaction that takes place when :

(i) a magnesium ribbon is burnt in the air.

(ii) limestone is heated.

(iii) silver bromide is exposed to sunlight.

(iv) electricity is passed through acidified water.

(v) ammonia and hydrogen chloride are mixed.

Write the chemical equation for each reaction.

(i) 2Mg(s) + O2(g) → 2MgO(s)

This is a combination reaction.

(ii) CaCO3(s) → CaO(s) + CO2(g)

This is a thermal decomposition reaction.

(iii) 2AgBr(s) → 2Ag(s) + Br2(g)

This is a photodecomposition reaction.

(iv) 2H2O(l) → 2H2(g) + O2(g)

This is an electrolytic decomposition reaction.

(v) NH3(g) + HCl(g) → NH4Cl(s)

This is a combination reaction.

## CONCLUSION – Class 10 science chapter 1 important question

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