NCERT Class 10 Science Chapter 1 Notes Chemical Reactions and Equations- Download PDF Notes

NCERT Notes Class 10 Chapter 1: Download PDF

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NCERT Notes Class 10 Chapter 1

Chemical Reactions and Equations Class 10 Science notes introduces the basics of chemical changes, how to represent them using balanced chemical equations, and the different types of chemical reactions, like combination, decomposition, displacement, and redox reactions.

Chemical Reactions and Equations

This chapter discusses in detail the balanced and unbalanced chemical reactions, along with explaining the process of balancing these reactions. This chapter discusses the significance of chemical equations and their representation using various symbols. Understanding chemical reactions is important for grasping the fundamental concept for science students.

Chemical Equations

The change of one chemical substance into another is known as a chemical reaction. Rusting of iron, curdling of milk, and respiration are examples of chemical reactions. The formed substance in a chemical reaction is completely different from its original substance; this process is an indication of a complete chemical reaction. Also, a chemical reaction is a process in which the rearrangement of atoms takes place, which leads to the formation of new compounds. To strengthen your understanding and practise more problems, you can also refer to NCERT Solutions for Class 10 Science Chapter 1

$\underset{\text { (Reactants) }}{\text { Magnesium }+ \text { Oxygen }} \rightarrow \underset{\text { (Product) }}{\text { Magnesium oxide }}$

Reactants-

The one that is taking part in a chemical reaction is a reactant.

Products-

The new substance that is formed after a chemical reaction is called a product.

Example-

Magnesium burns in the air to form magnesium oxide is a type of chemical reaction.

$\mathrm{Mg}+\mathrm{O}_2$ $\xrightarrow{\text { Heat }}$$ 2 \mathrm{MgO}(\mathrm{s})$

In this reaction, the reactants are magnesium and oxygen while the new substance MgO is the product.

Writing a Chemical Equation

For representing a chemical reaction some simple and formulas can be used and this representation is called a chemical equation. For example, when A and B are a reactant and C and D are the products formed after a reaction the following representation can be used to represent reaction.

$A+B \rightarrow C+D$

• When hydrogen undergoes a reaction with oxygen water is formed. In which the hydrogen and oxygen is the reactant and the water obtained is the product can be represented as

$\mathrm{H}_2+\mathrm{O}_2 \rightarrow \mathrm{H}_2 \mathrm{O}$

• So with the help of a chemical equation we can represent it in a concise and informative way. It can also be divided into two types of balance the chemical equation and unbalanced chemical equation.

Characteristics of Chemical Reaction

Evolution of gas: In some reactions, there is an evolution of gas for example the reaction between zinc and dilute sulphuric acid is followed by the evolution of hydrogen gas. It can be represented as

$\mathrm{Zn}+\mathrm{H}_2 \mathrm{SO}_4 \rightarrow \mathrm{ZnSO}_4+\mathrm{H}_2$

Change in color: In some other reactions, there may be a change in color. For example, when citric acid and purple color potassium permanganate solution undergo a chemical reaction there is a change in color from purple to colorless. And also the chemical reaction between sulfur dioxide and acidified potassium dichromate solution is also followed by a change in color from orange to green.

Change in-state: For some chemical reactions, there is a change in the state of the substance. For example, when candle wax undergoes a combustion reaction there is a change of state from solid to liquid and gas. And the gas formed in this type of reaction is carbon dioxide.

Change in temperature: For some reactions, there is a change in temperature that is a rise in temperature or a fall in temperature occurs. For example, the reaction between quicklime and water to form slaked lime is followed by a rise in temperature. And also the reaction taking place between zinc granules and dilute sulphuric acid is also followed by a rise in temperature.

Formation of the precipitate: In the reaction between a sulphuric acid and barium chloride solution, it occurs the formation of a white precipitate of barium sulfate.

$\mathrm{BaCl}_2+\mathrm{H}_2 \mathrm{SO}_4 \rightarrow \mathrm{BaSO}_4(\mathrm{~s})+2 \mathrm{HCl}$

Unbalanced Chemical Equation

When the number of atoms of each element in the reactant and product side is not equal it is called an unbalanced chemical equation.

$\mathrm{Fe}+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{Fe}_3 \mathrm{O}_4+\mathrm{H}_2$

In this reaction the number of atoms of the element iron is not equal and also oxygen is also not equal. So we need to balance that unbalanced equation to its balanced form.

In this chemical equation, 3 iron is formed at the product side so we need to multiply iron with 3 on the reactant side. And also the oxygen atom formed at the product is four so the water is also multiplied by 4. So to balance both sides hydrogen evolved should also be multiplied by 4.

$3 \mathrm{Fe}+4 \mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{Fe}_3 \mathrm{O}_4+4 \mathrm{H}_2$

So the unbalanced equation becomes a balanced equation. In the representation of the chemical equation, the state can also be mentioned that is whether it is in the gaseous form or liquid form or solid form can be given by descending with brackets.

Balanced Chemical Equation

When the number of atoms of each element in a chemical reaction is equal on both sides, it is called a balanced chemical equation.

$\mathrm{Zn}+\mathrm{H}_2 \mathrm{SO}_4 \rightarrow \mathrm{ZnSO}_4+\mathrm{H}_2$

Lets briefly discuss the steps of balancing a chemical reaction,

Step 1: Write the unbalanced chemical equation
Start with the correct chemical formulas of reactants and products.
Example:
Unbalanced equation:

$\mathrm{Fe}+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{Fe}_3 \mathrm{O}_4+\mathrm{H}_2$
Step 2: List the number of atoms of each element
Count atoms of each element on both reactant and product sides.

Step 3: Start balancing with the element that appears least frequently
Start with elements that appear in fewer compounds, excluding hydrogen and oxygen initially.

In the example above, balance Fe first:

$3 \mathrm{Fe}+4 \mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{Fe}_3 \mathrm{O}_4+ \mathrm{H}_2$

Step 4: Balance hydrogen and oxygen atoms at the end
Hydrogen and oxygen are often present in multiple compounds. Balance them after metals and other elements.

$\mathrm{Fe}+4 \mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{Fe}_3 \mathrm{O}_4+\mathrm{H}_2$

Step 5: Double-check atom count
Make sure the number of atoms of each element is equal on both sides.

Step 6: Confirm it's the simplest ratio
Coefficients must be the lowest possible whole numbers.

The final Balanced equation will be, $3 \mathrm{Fe}+4 \mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{Fe}_3 \mathrm{O}_4+4 \mathrm{H}_2$

Types of Chemical Reactions

Reactions are classified into different types based on the changes in reactants and products. Understanding these types helps in mastering important concepts for exams. You can also download Class 10 Science Chapter 1 notes PDF for revision and exam preparation.

Combination Reaction

The chemical reaction in which two or more reactants are combined to form another product is a combination reaction. That is,

$A+B \rightarrow A B$

Magnesium reacts with oxygen to form magnesium oxide is a typical example of this reaction.

2Mg+O₂ $\xrightarrow{\text { Heat }}$2MgO(s)

Decomposition reaction

When one compound decomposes into two or more compounds, it is called a decomposition reaction. That is,

AB→ A+B

A typical example of a decomposition reaction is the formation of calcium oxide and carbon dioxide by the decomposition of calcium carbonate.

$\mathrm{CaCO}_3(\mathrm{~s}) \xrightarrow{\text { Heat }} \mathrm{CaO}(\mathrm{s})+\mathrm{CO}_2(\mathrm{~g})$

Thermal decomposition is when a substance undergoes a decomposition reaction by heating it is thermal decomposition.

$\mathrm{CaCO}_3(\mathrm{~s}) \xrightarrow{\text { Heat }} \mathrm{CaO}(\mathrm{s})+\mathrm{CO}_2(\mathrm{~g})$

Electrolytic decomposition is the reaction signature of a compound decomposes into another component by the supply of electricity is electrolytic decomposition. Electrolysis is a type of electrolytic decomposition.

For example, when electricity is passed in water it will decompose into oxygen and hydrogen. That is,

2H₂O→2H₂+O₂

White silver chloride turns grey in sunlight. This is due to the decomposition of silver chloride into silver and chlorine by light.

$2 \mathrm{AgCl}(\mathrm{s}) \xrightarrow{\text { Sunlight }} 2 \mathrm{Ag}(\mathrm{s})+\mathrm{Cl}_2(\mathrm{~g})$

Silver bromide also behaves in the same way.

$
2 \mathrm{AgBr}(\mathrm{~s}) \xrightarrow{\text { Sunlight }} 2 \mathrm{Ag}(\mathrm{~s})+\mathrm{Br}_0(\mathrm{~g})
$

Photolysis or Photodecomposition

The decomposition reaction performed in the presence of sunlight is known as photolysis. For example, when silver chloride is put in the sunlight it will decompose to produce the metal silver and chlorine gas. The reaction is,

2AgCl → 2Ag+Cl₂

Displacement Reaction

A chemical reaction in which a reactive element is replaced with a less reactive element is a displacement reaction. It can be represented as,

A+BC→ AC+B

The reaction taking place between Zinc and hydrochloric acid is an example of a displacement reaction.

Zn+2HCl→ ZnCl₂+H₂

Precipitation Reaction

The reaction in which a precipitate is formed after the chemical reaction is called a precipitation reaction. An example for it is,

$\mathrm{AgNO}_3+\mathrm{NaCl} \rightarrow \mathrm{AgCl}(\mathrm{s})+\mathrm{NaNO}_3$

AgCl is the precipitate.

Neutralisation Reaction

A chemical reaction in which acid and a base undergo a reaction to form a neutral product that is water is called a neutralisation reaction.

NaOH+HCl→ NaCl+H₂O

Oxidation Reaction

The removal of hydrogen or metallic element and the addition of oxygen or nonmetallic element from a compound is known as oxidation. The element in which the nonmetallic element or oxygen is added and hydrogen or metallic element is removed is said to be oxidized.

Example: Rusting iron.

Reduction Reaction

Removal of oxygen or nonmetallic elements and addition of hydrogen or metallic elements from any compound is called reduction. And the element in which it will undergo A reduction process is said to be reduced.

Redox Reaction

When the process of reduction and oxidation takes place simultaneously, it is said to be a redox reaction. And the substance which removes hydrogen or gives oxygen is said to be an oxidizing agent. And the one which gives hydrogen or removes oxygen is said to be a reducing agent.

$\mathrm{CuO}+\mathrm{H}_2 \rightarrow \mathrm{Cu}+\mathrm{H}_2 \mathrm{O}$

In this reaction oxygen is removed from CuO therefore it is said to be a reduction process. While oxygen is added to H₂ so it is an oxidation process.

Exothermic Reaction

The reaction in which energy is produced is called an exothermic reaction. And the energy is produced in the form of heat.

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

Endothermic Reaction

When heat energy is absorbed or consumed to undergo a chemical reaction it is an endothermic reaction.

CaCO₃ $\xrightarrow{\text { Heat }}$ CaO+CO₂

Corrosion

Have you noticed the colour of the coating formed on copper and silver? When a metal is attacked by substances surrounding it, such as moisture and acids, it is said to corrode, and this process is called corrosion. The black coating on silver and the green coating on copper are other examples of corrosion.

Rancidity

Have you ever tasted or smelled the fat/oil-containing food materials left for a long time? When fats and oils are oxidised, they become rancid and their smell and taste change. Usually, substances which prevent oxidation (antioxidants) are added to foods containing fats and oils. Keeping food in air-tight containers helps to slow down oxidation.

Chemical Reactions and Equations: Previous Year Question and Answer

Students can practice previous year questions from NCERT Class 10 Science Chapter 1 Chemical Reactions and Equations to help students understand the exam pattern and focus on important topics. These solved questions on chemical reactions and equations strengthen concepts and improve problem-solving skills for CBSE Board exam preparation.

Question: The white precipitates formed In a chemical reaction between sulphuric acid and barium chloride solution is of:

(1) Sulphur

(2) Barium sulphate

(3) Chlorine

(4) Silver chloride

Answer: White precipitates formed In a chemical reaction between sulphuric acid and barium chloride solution is of Barium sulphate.

Hence, the answer is the option (2).

Question: In the standard notation for a voltaic cell, the double vertical line "||" represents:

(1) salt bridge

(2) gas electrode

(3) a wire (metal) connection

(4) a phase boundary

Answer: In the standard notation for a voltaic cell, the double vertical line "||" represents salt bridge.

Hence, the answer is option (1).

Question: Dry HCl gas does not change the colour of dry litmus paper. Why?

Answer: 1. Dry HCl gas does not change the colour of dry litmus paper because it does not release hydrogen ions
$\left(\mathrm{H}^{+}\right)$in the absence of water.
2. Acids show their acidic properties only in aqueous (water) solution, where they ionise to give $\mathrm{H}^{+}$ions.
Since there is no water in dry HCl gas or on dry litmus paper, no $\mathrm{H}^{+}$ions are produced, and thus, no colour change is observed.

CBSE Class 10 Science Chapter-wise Notes

In addition to NCERT Class 10 Science Chapter 1 Notes Chemical Reactions and Equations, students can refer to the NCERT notes of other Class 10 chapters provided below.

NCERT Solutions

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NCERT Books and Syllabus

Students can refer to the links given below to access NCERT Books and Syllabus