🧪 Chemical Reactions and Equations – Class 10 Science Notes (Simplified)

Chemical Reactions and Equations Class 10 Notes

Chemical Reactions and Equations is a key chapter in Class 10 Science that helps you understand how substances interact and transform in daily life. With these simplified notes, you'll easily grasp core concepts like types of reactions and how to balance equations. To make your revision even more convenient, a downloadable PDF version is also available for future reference and exam prep.

Subject	Science (Chemistry)
Class	10
Board	CBSE
Chapter No.	1
Chapter Name	Chemical Reactions and Equations
Type	Notes
Session	2025-26

Whenever a chemical change occurs, we can say that a chemical reaction has taken place.

Activity 1.1:

Magnesium ribbon is cleaned by rubbing it with sandpaper before burning.
Magnesium reacts with atmospheric oxygen to develop a layer of magnesium oxide. This layer does not allow the underlying magnesium to undergo combustion.
Magnesium ribbon burns with a dazzling white flame and changes into a white powder. This powder is magnesium oxide.
It is formed due to the reaction between magnesium and oxygen present in the air.
2Mg + O2 → 2MgO

Activity 1.2:

Lead nitrate reacts with Potassium Iodide to form Lead Iodide (yellow precipitate) and Potassium nitrate.
Pb(NO3)2 + 2KI → PbI2 (yellow ppt)+ 2KNO3

Activity 1.3:

Formation of hydrogen gas by the action of dilute sulphuric acid on zinc.
Zn + 2HCl → ZnCl2 + H2

Chemical Equations

Balanced Chemical Equation:

Definition: A balanced chemical equation is an equation in which the number of atoms of various elements is equal on both sides of the equation.

Reason for balancing: An equation should be balanced due to the law of conservation of mass 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.

Writing Symbols of Physical States:

States	Symbol
gaseous	g
liquid	l
aqueous	aq
solid	s

Sometimes the reaction conditions, such as temperature, pressure, catalyst, etc., for the reaction are indicated above and/or below the arrow in the equation. For example:

Types of Chemical Reactions

Combination Reaction
Decomposition Reaction
Displacement Reaction
Double Displacement Reaction
Oxidation and Reduction

Some other types:

Exothermic reaction
Endothermic reaction
Precipitation reaction
Redox reaction

Combination Reaction

Definition: A reaction in which a single product is formed from two or more reactants is known as a combination reaction.

Examples:

i. Calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide) releasing a large amount of heat.

reaction between calcium oxide and water

A solution of slaked lime (calcium hydroxide) is used for whitewashing walls. It reacts slowly with the carbon dioxide in the air to form a thin layer of calcium carbonate on the walls.

calcium hydroxide reacts with carbon dioxide in air to form calcium carbonate

ii. Burning of coal: C + O2 → CO2

iii. Formation of water from H2(g) and O2(g): 2H2 (g) + O2 (g) → 2H2O (l)

Exothermic Reaction:

Definition: Reactions in which heat is released along with the formation of products are called exothermic chemical reactions.

Examples:

i. Burning of natural gas: CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (g)

ii. Respiration: Carbohydrates are broken down to form glucose. This glucose combines with oxygen in the cells of our body and provides energy. The special name of this reaction is respiration.

iii. The decomposition of vegetable matter into compost.

Decomposition Reaction

Definition: The reaction in which a single reactant breaks down to give simpler products is called a decomposition reaction.

Examples:

i. Ferrous sulphate crystals (FeSO4.7H2O) lose water when heated and the color of the crystals changes. It then decomposes to ferric oxide (Fe2O3), sulphur dioxide (SO2), and sulphur trioxide (SO3). Ferric oxide is a solid, while SO2 and SO3 are gases.

ii. Decomposition of calcium carbonate to calcium oxide (quick lime) and carbon dioxide on heating is an important decomposition reaction.

When a decomposition reaction is carried out by heating, it is called thermal decomposition.

iii. Heating of lead nitrate. You will observe the emission of brown fumes. These fumes are of nitrogen dioxide (NO2).

iv. Electrolysis of water is a decomposition reaction. The mole ratio of hydrogen and oxygen gases liberated during the electrolysis of water is 2:1.

Cathode: Hydrogen; anode: Oxygen.

Hydrogen is collected in double the amount of oxygen because water is formed by the chemical combination of hydrogen and oxygen in the ratio 2:1 by volume, so it decomposes in the same ratio.

2H2O(l) → 2H2(g) + O2(g).

v. White silver chloride turns grey in sunlight.

2AgCl (s) → 2Ag (s) + Cl2 (g)

The application of this reaction is in black-and-white photography.

Endothermic reaction:

Reactions in which energy (either in the form of heat, light, or electricity is absorbed are known as endothermic reactions.

Displacement Reaction

Definition: The reaction in which a more reactive element displaces a less reactive element from its salt solution is called displacement reaction.

Examples:

i. Iron displaces copper, from copper sulphate solution. Iron nail become brownish in color and the blue colour of the copper sulphate solution fades.

Fe + CuSO4 → FeSO4 + Cu

ii. Zn (s) + CuSO4 (aq) → ZnSO4 (aq) + Cu (s)

iii. Pb(s) + CuCl2(aq) → PbCl2(aq) + Cu(s)

Double Displacement Reaction

Definition: Reactions in which there is an exchange of ions between the reactants are called double displacement reactions.

Examples:

i. Na2SO4 + BaCl2 → BaSO4 (ppt) + 2NaCl

You will observe that a white substance (BaSO4), which is insoluble in water, is formed. This insoluble substance formed is known as a precipitate. Any reaction that produces a precipitate can be called a precipitation reaction.

ii. When potassium iodide solution is added to a solution of lead nitrate in a test tube, a yellow color precipitate is formed. The compound precipitated is Lead Iodide.

Pb(NO3)2 + 2KI → PbI2 + 2KNO3

Oxidation and Reduction

Definition: If a substance gains oxygen or loses hydrogen during a reaction, it is said to be oxidized. If a substance loses oxygen or gains hydrogen during a reaction, it is said to be reduced.

Examples:

i. Oxidation of copper to copper oxide: When we heat copper powder, the surface of copper powder becomes coated with black copper(II) oxide. This is because oxygen is added to copper and copper oxide is formed.

2Cu + O2 → 2CuO

ii. Reduction of copper oxide to copper: If hydrogen gas is passed over this heated material (CuO), the black coating on the surface turns brown as the reverse reaction takes place and copper is obtained.

CuO + H2 → Cu + H2O

Redox Reactions

Definition: The reactions in which one reactant gets oxidized while the other gets reduced are called oxidation-reduction reactions or redox reactions.

Note: The substance which gets oxidized is the reducing agent and the substance which get reduced is the oxidising agent.

Examples:

ii. ZnO + C → + Zn + CO

Carbon is oxidized to CO and ZnO is reduced to Zn. [Here, Carbon is the reducing agent and ZnO is the Oxidizing agent.]

iii. MnO2 + 4HCl → MnCl2 + 2H2O + Cl2

HCl is getting oxidized to Cl2 while MnO2 is getting reduced to MnCl2. [Here, HCl is the reducing agent and MnO2 is the oxidizing agent.]

Effects of Oxidation and Reduction in everyday life

i. Corrosion:

When a metal is attacked by substances around it such as moisture, acids, etc., it is said to corrode and this process is called corrosion.

Examples:

Rusting of iron
The black coating on silver
The green coating on copper

Disadvantages of Corrosion:

Corrosion causes damage to car bodies, bridges, iron railings, ships, and to all objects made of metals, especially those of iron.
Every year an enormous amount of money is spent to replace damaged iron.

ii. Rancidity

When fats and oils are oxidized, they become rancid, and their smell and taste change. Usually, substances that prevent oxidation (antioxidants) are added to foods containing fats and oil. Keeping food in air-tight containers helps to slow down oxidation.

Chips manufacturers usually flush bags of chips with a gas such as nitrogen to prevent the chips from getting oxidized.

🧪 Class 10 Science – Chapter 1: Chemical Reactions and Equations

✅ NCERT In-Text Questions & Back Exercise Answers

📘 In-Text Questions – Page 1

Q1. Why should a magnesium ribbon be cleaned before burning in air?
Answer:
Magnesium ribbon is cleaned to remove the oxide layer (magnesium oxide) formed on its surface due to exposure to air. This helps the ribbon to burn easily with a bright flame.

📘 In-Text Questions – Page 6

Q1. A solution of a substance ‘X’ is used for whitewashing. What is ‘X’? Write the chemical reaction of ‘X’ with water.
Answer:
‘X’ is calcium oxide (quick lime).
When it reacts with water, it forms calcium hydroxide, which is used for whitewashing.

$\text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2$

Q2. Why does the whitewashed wall turn shiny after two to three days?
Answer:
Calcium hydroxide reacts with carbon dioxide in the air to form calcium carbonate, which gives a shiny finish.

$\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}$

Q3. Why is the amount of gas collected in one of the test tubes in electrolysis of water double the amount collected in the other? Name the gas.
Answer:
Water is made of two hydrogen atoms and one oxygen atom. During electrolysis, hydrogen gas is produced in double the amount as oxygen.
Gas collected in double amount = Hydrogen (H₂)

📘 In-Text Questions – Page 10

Q1. Why does the color of copper sulfate solution change when an iron nail is dipped in it?
Answer:
Iron is more reactive than copper and displaces it from copper sulfate solution.
Reaction:
$\text{Fe} + \text{CuSO}_4 \rightarrow \text{FeSO}_4 + \text{Cu}$
The blue color fades and a brown deposit of copper forms on the nail.

Q2. Give an example of a double displacement reaction other than mentioned in the textbook.
Answer:
Example:
$\text{AgNO}_3 + \text{NaCl} \rightarrow \text{AgCl} \downarrow + \text{NaNO}_3$
Here, silver chloride (AgCl) is a white precipitate.

Q3. Identify the type of reaction and write a balanced equation:
(a) Potassium bromide reacts with barium iodate
(b) Zinc reacts with silver nitrate

Answer:
(a) Double displacement reaction
$2KBr + Ba(IO_3)_2 \rightarrow BaBr_2 + 2KIO_3$

(b) Displacement reaction
$Zn + 2AgNO_3 \rightarrow Zn(NO_3)_2 + 2Ag$

📘 Back Exercise Questions – Page 13

Q1. Why do we balance a chemical equation?
Answer:
To satisfy the Law of Conservation of Mass, which states that mass can neither be created nor destroyed in a chemical reaction.

Q2. Write the balanced chemical equation for the following reactions:
(a) Hydrogen + Chlorine → Hydrogen chloride
(b) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
(c) Sodium + Water → Sodium hydroxide + Hydrogen

Answer:
(a) $H_2 + Cl_2 \rightarrow 2HCl$
(b) $3BaCl_2 + Al_2(SO_4)_3 \rightarrow 3BaSO_4 + 2AlCl_3$
(c) $2Na + 2H_2O \rightarrow 2NaOH + H_2$

Q3. Identify the type of reactions:
(a) $NaOH + HCl \rightarrow NaCl + H_2O$
(b) $2Pb(NO_3)_2 \xrightarrow{heat} 2PbO + 4NO_2 + O_2$
(c) $C + O_2 \rightarrow CO_2$
(d) $Zn + CuSO_4 \rightarrow ZnSO_4 + Cu$
(e) $BaCl_2 + Na_2SO_4 \rightarrow BaSO_4 + 2NaCl$

Answer:
(a) Neutralization reaction
(b) Decomposition reaction
(c) Combination reaction
(d) Displacement reaction
(e) Double displacement reaction

Q4. What is the difference between displacement and double displacement reactions? Write examples.

Answer:

Displacement Reaction	Double Displacement Reaction
One element replaces another	Exchange of ions between two compounds
Example: $Zn + CuSO_4 \rightarrow ZnSO_4 + Cu$	Example: $AgNO_3 + NaCl \rightarrow AgCl + NaNO_3$

Q5. What happens when dilute hydrochloric acid is added to iron filings? Write the reaction.
Answer:
Hydrogen gas is released and iron chloride is formed.
$Fe + 2HCl \rightarrow FeCl_2 + H_2 \uparrow$

Q6. What happens when zinc granules are added to dilute sulfuric acid?
Answer:
Hydrogen gas is evolved and zinc sulfate is formed.
$Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2 \uparrow$

Q7. What is rancidity? How can it be prevented?
Answer:
Rancidity is the oxidation of fats and oils that leads to a bad smell and taste.
Prevention:

Storing in airtight containers
Refrigeration
Using antioxidants like BHT
Flushing packets with nitrogen gas

🧪 Class 10 Science Chapter 1 – Chemical Reactions and Equations

📚 CBSE Past Year Questions with Answers (Marks-wise)

🔹 1-MARK QUESTIONS

Q1. Why should a magnesium ribbon be cleaned before burning in air?
📌 CBSE 2015
Answer:
Magnesium ribbon is cleaned to remove the oxide layer formed on its surface so that it can burn easily.

Q2. Write the balanced chemical equation for the reaction of quicklime with water.
📌 CBSE 2016
Answer:
$\text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2$

Q3. Define a combination reaction with one example.
📌 CBSE 2020
Answer:
In a combination reaction, two or more substances combine to form a single product.
Example:
$\text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2$

🔸 2-MARK QUESTIONS

Q4. Why does the color of copper sulphate solution change when an iron nail is dipped in it? Write the balanced chemical equation for this reaction.
📌 CBSE 2018
Answer:
Iron is more reactive than copper, so it displaces copper from copper sulphate solution.
Reaction:
$\text{Fe} + \text{CuSO}_4 \rightarrow \text{FeSO}_4 + \text{Cu}$
The blue color fades and brown copper gets deposited.

Q5. What is meant by rancidity? Mention one method to prevent it.
📌 CBSE 2017
Answer:
Rancidity is the spoilage of food containing fat/oil due to oxidation, leading to bad taste and smell.
Prevention: Add antioxidants or store in airtight containers.

🔶 3-MARK QUESTIONS

Q6. Write balanced chemical equations for the following:
(i) Hydrogen combines with nitrogen to form ammonia
(ii) Calcium carbonate decomposes into calcium oxide and carbon dioxide
(iii) Zinc reacts with dilute hydrochloric acid
📌 CBSE 2019
Answer:
(i) $N_2 + 3H_2 \rightarrow 2NH_3$
(ii) $CaCO_3 \rightarrow CaO + CO_2$
(iii) $Zn + 2HCl \rightarrow ZnCl_2 + H_2$

Q7. What is a redox reaction? Identify the substances oxidized and reduced in the following reaction:
$CuO + H_2 \rightarrow Cu + H_2O$
📌 CBSE 2018
Answer:
A redox reaction is a chemical reaction in which both oxidation and reduction take place.

Oxidized: $H_2$ (to $H_2O$ )
Reduced: $CuO$ (to $Cu$ )

🔴 5-MARK QUESTIONS

Q8. Define the following with one example each:
(a) Combination reaction
(b) Decomposition reaction
(c) Displacement reaction
(d) Double displacement reaction
(e) Redox reaction
📌 CBSE 2020
Answer:

(a) Combination reaction – Two or more substances combine to form a single product.
Example: $CaO + H_2O \rightarrow Ca(OH)_2$

(b) Decomposition reaction – A single compound breaks down into simpler products.
Example: $2H_2O \xrightarrow{electricity} 2H_2 + O_2$

(c) Displacement reaction – A more reactive element displaces a less reactive one.
Example: $Zn + CuSO_4 \rightarrow ZnSO_4 + Cu$

(d) Double displacement reaction – Exchange of ions between two compounds.
Example: $Na_2SO_4 + BaCl_2 \rightarrow BaSO_4 + 2NaCl$

(e) Redox reaction – Reaction involving oxidation and reduction.
Example: $CuO + H_2 \rightarrow Cu + H_2O$

Q9. What is meant by the chemical equation? Write the steps for balancing the following equation:
$Fe + H_2O \rightarrow Fe_3O_4 + H_2$
📌 CBSE 2021
Answer:
A chemical equation is the symbolic representation of a chemical reaction using chemical formulas.

Balancing steps:

Unbalanced: $Fe + H_2O \rightarrow Fe_3O_4 + H_2$

Balance Fe: $3Fe$
Balance H and O: $4H_2O$

Balanced Equation:
$3 F e + 4 H_{2} O \to F e_{3} O_{4} + 4 H_{2}$

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