Science – CH 1 Chemical Reactions and Equations Class 10 NCERT Solution | Best Explanation


In this chapter, students are introduced to the world of chemical reactions, emphasizing their importance in everyday life. The chapter begins by defining chemical reactions and discussing various types of reactions. It delves into the concept of chemical equations as a means to represent and balance reactions, adhering to the law of conservation of mass.

  • Key Topics:
  • NCERT Solutions
  • Importance
  1. Chemical Reactions:
    • Understanding the concept of chemical reactions and how substances transform into new substances with different properties.
  2. Types of Chemical Reactions:
    • Exploring different types of reactions, such as combination, decomposition, displacement, and double displacement reactions.
  3. Chemical Equations:
    • Introduction to chemical equations as a symbolic representation of chemical reactions, involving reactants and products.
  4. Balancing Chemical Equations:
    • The importance of balancing chemical equations to ensure the conservation of mass and understanding the stoichiometry of reactions.
  5. Examples and Practice Problems:
    • Application of concepts through various examples and exercises to reinforce the understanding of chemical reactions and equations.
  6. Real-Life Applications:
    • Illustrating the relevance of chemical reactions in daily life, from the food we eat to the combustion processes.
  7. Exothermic and Endothermic Reactions:
    • Distinguishing between exothermic and endothermic reactions and understanding their energy changes.

The NCERT Solutions for Class 10 Science Chapter 1 provide detailed explanations and step-by-step solutions to exercises and problems presented in the NCERT textbook. These solutions help students grasp the concepts effectively and enhance their problem-solving skills

Understanding chemical reactions lays the foundation for advanced studies in chemistry. This chapter not only prepares students for future scientific pursuits but also fosters an appreciation for the chemical processes occurring in the world around them.

By the end of the chapter, students should have a clear understanding of the principles of chemical reactions, the significance of balanced equations, and the ability to represent and interpret various chemical processes.


1. Which of the statements about the reaction below are incorrect?
2PbO(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 the above


(i) (a) and (b)

Explanation: (a) Because Oxygen is being removed and (b) Because the removed oxygen from Lead is added to the elemental Carbon.


The answer is 4. Displacement reaction.

Explanation: The Oxygen from the Ferrous oxide is getting displaced to the Aluminium metal to form Aluminium Oxide. In this reaction, Aluminum is a more reactive metal than Fe. Therefore, Al will displace Fe from its oxide. This type of chemical reaction, called displacement reaction, is in which one of the elements displaces another. Here, less reactive metal is displaced by more reactive metal. Since one-time displacement is occurring, it is called a single displacement reaction.

3.What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer.
1.Hydrogen gas and Iron chloride are produced.
2.Chlorine gas and Iron hydroxide are produced.
3.No reaction takes place.
4.Iron salt and water are produced.


  1. Hydrogen gas and Iron chloride are produced.

Explanation: The Chlorine from Hydrogen chloride is displaced by the Iron fillings to undergo the following reaction.

2HCl + Fe → FeCl+ H2

4. What is a balanced chemical equation? Why should a chemical equation be balanced?


A balanced equation is the one in which number of different atoms on both the reactant and product sides are equal. Balancing chemical equations is necessary for the reaction should obey The Law of Conservation of mass. Balancing the chemical equation has no defined method and is purely a trial-and-error attempt.

5.  Translate the following statements into chemical equations and balance them.
(a) Hydrogen gas combines with nitrogen to form ammonia.
(b) Hydrogen sulphide gas burns in the 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.


(a) Unbalanced: H+ N→ NH3

Balanced: 3H+ N→ 2NH3

(b) Unbalanced: H2S + O→ H2O + SO2

Balanced: 2H2S + 3O→ 2H2O + 2SO2

(c)  Unbalanced:

BaCl+ Al2(SO4)→ AlCl+ BaSO4

Balanced: 3BaCl+ Al2(SO4)→ 2AlCl+ 3BaSO4

(d) Unbalanced: K + H2O → KOH + H2

Balanced:  2K + 2H2O → 2KOH + H2

6. 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


(a) 2HNO+ Ca(OH)→ Ca(NO3)+ 2H2O

(b) 2NaOH + H2SO→ Na2SO+ 2H2

(c) NaCl + AgNO→ AgCl + NaNO3

(d) BaCl+ H2SO→ BaSO+ 2HCl

7. Write the balanced chemical equation for the following reactions.
Calcium hydroxide + Carbon dioxide —-> Calcium carbonate + Water
Zinc + Silver nitrate —-> Zinc nitrate + Silver
Aluminium + Copper chloride —-> Aluminium chloride + Copper
Barium chloride + Potassium sulphate —-> Barium sulphate + Potassium chloride


2Ca(OH)+ 2CO→ 2CaCO+ 2H2O

Zn + 2AgNO→ Zn(NO3)+ 2Ag

2Al + 3CuCl→ 2AlCl+ 3Cu

BaCl+ K2SO→ BaSO+ 2KCl

8. Write a balanced chemical equation for the following and identify the type of reaction of each case.
KBr + BaI→ KI + BaBr2
ZnCO→ ZnO + CO2
H+ Cl → HCl
Mg + HCl → MgCl+ H2


2KBr + BaI→ 2KI + BaBr2 (Double Displacement Reaction)

ZnCO→ ZnO + CO2 (Decomposition Reaction)

H2 + Cl → 2HCl (Combination Reaction)

Mg + 2HCl → MgCl+ H2 (Displacement Reaction)

9. What is meant by exothermic and endothermic reactions? Give examples.


An endothermic reaction occurs when energy is absorbed from the surroundings in the form of heat (For example, Photosynthesis, melting of ice, evaporation). Conversely, an exothermic reaction is one in which energy is released from the system into the surroundings (Examples: Explosions, concrete setting, nuclear fission and fusion).

10. Why is respiration considered to be an exothermic reaction?


For the survival of life, we require energy. We obtain this energy from the food we eat. The food molecules, through the process of digestion, is broken down into simpler molecule like glucose. These substances come in contact with the Oxygen present in our body cells to form Carbon dioxide and water along with a certain amount of energy (Respiration process). Since the energy is in the form of heat (that maintains our body temperature), respiration is considered to be an exothermic reaction. The reaction taking place is:

C6H12O+ 6O→ 6CO+ 6H2O + Energy

11. Why are decomposition reactions called the opposite of Combination reactions? Write equations for decomposition reactions.


Combination reaction is said to be the reaction between two or more molecules to form a larger molecule; whereas the decomposition reaction is defined as the splitting of larger molecules into two or more smaller molecules. This essentially explains that the decomposition reaction is the opposite of the combination reaction.

In most cases, the decomposition reaction is endothermic since the heat from the surrounding or induced heat is used to break the bonds of the larger molecule. A few examples of decomposition reactions are

ZnCO3 → ZnO + CO2

CaCO3 + Energy → CaO + CO2

2HgO → 2Hg + O2

12. Write one equation each for decomposition reactions in which energy is supplied in the form of heat, light or electricity.


(a) Thermal decomposition reaction (Thermolysis)

Decomposition of potassium chlorate: When heated strongly, potassium chlorate decomposes into potassium chloride and oxygen. This reaction is used for the preparation of oxygen.

2KClO+ Heat → 2KCl + 3O2

(b) Electrolytic decomposition reaction (Electrolysis)

Decomposition of sodium chloride: On passing electricity through molten sodium chloride, it decomposes into sodium and chlorine.

Description: NCERT Solution Class 10 Science Chapter 1-1

(c) Photodecomposition reaction (Photolysis)

Decomposition of hydrogen peroxide: In the presence of light, hydrogen peroxide decomposes into water and oxygen.

Description: NCERT Solution Class 10 Science Chapter 1-2

13.  What is the difference between displacement and double displacement reactions? Write relevant equations for the above.


A displacement reaction is one when a more reactive substance displaces a less reactive one from its salt solution, whereas a double displacement reaction is one where a mutual exchange of ions happens between two compounds.

In a displacement reaction, only a single displacement takes place, whereas in the double displacement reaction, as the name suggests, two displacement takes place between the molecules.


Displacement reaction

Mg + 2HCl → MgCl+ H2

Double displacement reaction

2KBr + BaI2 → 2KI + BaBr2

14. In the refining of Silver, the recovery of silver from Silver nitrate solution involves displacement reaction by Copper metal. Write down the reaction involved.


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

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


When two solutions containing soluble salts are combined, a double displacement reaction takes place in which the ions are exchanged between the compounds. When one of such compounds formed is in solid form (that is insoluble in aqua), then it settles down at the bottom of the container. This solid is known as the precipitate and the respective reaction is termed as the precipitation reaction. A few examples of precipitation reactions are

CdSO4(aq) + K2S(aq) → CdS(s) + K2SO4(aq)

2NaOH(aq) + MgCl2(aq) → 2NaCl(aq) + Mg(OH)2(s)

16. Explain the following in terms of the gain of oxygen with two examples each.

(a) Oxidation

(b) Reduction


(a) In a chemical reaction, when the oxygen is added to the element to form its respective oxide it is the element being oxidised. Example:

4Na(s) + O2(g) → 2Na2O(s)

H2S + O→ H2O + SO2

 (b) In a chemical reaction, when the oxygen is removed from the compound, then it is said to be reduced. Example:

CuO(s) + H2(g) → Cu(s) + H2O(l)

2HgO → 2Hg + O2

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


The shiny brown-coloured element is Copper metal (Cu). When the metal is heated in air, it reacts with atmospheric oxygen to form copper oxide. Hence, the black-coloured compound is copper oxide.

2Cu(s) + O2(g) → 2CuO(s)

18) Why do we apply paint on iron articles?


Iron articles are painted to prevent them from rusting. When left unpainted, the metal surface comes in contact with the atmospheric oxygen and in the presence of moisture it from Iron(III) oxide. But if painted, the surface does not come in contact with moisture and air, thus prevents Rusting.

19) Oil and Fat containing food items are flushed with Nitrogen. Why?


The main purpose of flushing Nitrogen into food packets that contain oil and fat items is to prevent Rancidity which occurs when the oil or fat reacts with the oxygen letting out an unpleasant smell and taste. Therefore, by flushing Nitrogen, an unreactive surrounding is created, thus preventing rancidity.

20) Explain the following terms with one example each.

(a) Corrosion

(b) Rancidity


(a) Corrosion is a process where a refined metal is oxidised by atmospheric oxygen to form a more stable compound, such as oxides. The metal gradually degrades during the corrosion process. Rusting of iron is a good example of corrosion where the iron is converted to Iron oxide. Millions of dollars are spent annually in preventing rusting from bridges and other monuments.

 (b) The condition produced by the aerial oxidation of the oil and fat present in the food material that produces an unpleasant taste and smell. The rancidity is retarded when the food is kept inside the refrigerator since the low temperature does not promote the oxidation reaction.

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