Chapter – 1

Chemical Reactions and Equations

In this post we have given the detailed notes of class 10 Science Chapter 1 (Chemical Reactions and Equations) in English. These notes are useful for the students who are going to appear in class 10 board exams.

Board	CBSE Board, UP Board, JAC Board, HBSE Board, UBSE Board, PSEB Board, RBSE Board, MPBSE Board
Textbook	NCERT
Class	Class 10
Subject	Science
Chapter no.	Chapter 1
Chapter Name	Chemical Reactions and Equations
Category	Class 10 Science Notes in English
Medium	English

Class 10 Science Chapter 1 Chemical Reactions and Equations in English

Chapter 1: Chemical Reactions and Equations

What is a Chemical Reaction? 🧪

Whenever a substance undergoes a change, that change can be of two types. The first is a physical change, in which the substance can be brought back to its original state after the change. The second is a chemical change, in which the chemical properties of the substance are altered, and the substance cannot be brought back to its original state. 🔄
In a chemical reaction, one or more substances interact with each other and get transformed into one or more new substances with different chemical properties. The substances that take part in a chemical reaction are called reactants, and the new substances formed as a result of the reaction are called products.

2H₂ + O₂ → 2H₂O

Here, hydrogen and oxygen take part in a chemical reaction and form a new substance with new properties, which is water (H₂O).
In other words, we can say that a chemical change is also called a chemical reaction. Many such changes keep happening around us all the time. ⚗️

Chemical Change

A change that alters the chemical properties of a substance, and after which the substance cannot be brought back to its original state, is called a chemical change.

Properties of a Chemical Change

In this type of change, the chemical properties of the substance generally change.
After a chemical change, the substance cannot be obtained back in its previous state.
It is a permanent change.
A new substance is formed after a chemical change.
Examples: milk turning into curd, rusting of iron, etc.
Rusting caused by contact with air.
Fermentation of grape juice.
Cooking of food.
Digestion of food in our body.
The respiration that we carry out.

How to Identify a Chemical Reaction? 🔍

A chemical reaction can be identified by certain observable signs which tell us that a reaction has taken place, such as:

Change in the state of a substance
Change in colour
Evolution of a gas
Change in temperature

Chemical Equations

When a magnesium ribbon is burnt in air, it gets converted into magnesium oxide. This is a statement describing a chemical reaction, but it can also be written in the following way:

Magnesium + Oxygen → Magnesium oxide

(Reactant) (Product)

Note: Writing an equation in this manner is called a word equation. There is another way of writing a chemical reaction:

Mg + O₂ → MgO

This is the symbolic way of writing a chemical equation. Every chemical equation has two parts:

Reactants: The substances that take part in a reaction are called reactants. For example, in the above equation, magnesium and oxygen take part in the reaction, so both are reactants.
Products: The new substances formed during a reaction are called products. For example, MgO is a product formed from Mg and O₂.

Writing Chemical Equations

A chemical equation represents a chemical reaction. In a chemical equation, the reactants are written on the left side of the arrow with a (+) sign between them, and similarly, the products are written on the right side of the arrow with a (+) sign between them.

Skeletal Chemical Equations

Mg + O₂ → MgO

If we observe this equation and count the number of atoms on the left and right sides of the arrow, the number of atoms of each element is not equal on both sides. The number of oxygen atoms is slightly unbalanced — there are two oxygen atoms on the left side but only one on the right side.

Thus, an unbalanced chemical equation is called a skeletal equation.

Balancing skeletal chemical equations: Chemical equations are balanced in order to satisfy the law of conservation of mass. The number of atoms of every element is made equal on both the left and right side of the arrow, so that the atoms of each element are equal on both sides.

Types of Chemical Reactions

In chemical reactions, new substances are formed by the making and breaking of bonds between molecules. For example, when water molecules break, oxygen and hydrogen are produced, whereas when a bond forms between carbon and oxygen, carbon dioxide is obtained.

Chemical Reactions Are of the Following Types 📋

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

1. Combination Reaction 🔗

A reaction in which two or more reactants combine to form a single product is called a combination reaction. The general formula for this reaction is: A + B → AB

CaO(s) + H₂O(l) → Ca(OH)₂(aq)

(Calcium oxide) (Water) (Calcium hydroxide)

(Lime) (Slaked lime)

Comparing this with the definition of a chemical equation, we see that calcium oxide and water, which are two reactants, form a single product, calcium hydroxide.

Calcium hydroxide Ca(OH)₂: Calcium hydroxide is used for whitewashing walls. This is a combination reaction, and when walls are whitewashed with calcium hydroxide, it reacts with the CO₂ present in air to form a thin layer of calcium carbonate. Water (H₂O) is also formed in this reaction, which evaporates. This is why the wall shines after two or three days — it is due to the formation of this thin layer of calcium carbonate. The equation for this process is as follows:

Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

(Calcium hydroxide) (Calcium carbonate)

Let’s Look at Some Other Combination Reactions

Burning of coal: C(s) + O₂(g) → CO₂(g)
Formation of water: 2H₂(g) + O₂(g) → 2H₂O(l)
Formation of sulphur dioxide: S(s) + O₂(g) → SO₂(g)
Rusting of iron (formation of ferrous oxide): 3Fe(s) + 2O₂(g) → Fe₃O₄(s)

Types of Chemical Reactions Based on Heat

A. Exothermic Reactions: Reactions in which heat is released during the reaction are called exothermic reactions. 🔥 Examples:

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)
Respiration is also an example of an exothermic reaction, in which cells release heat during respiration.
Decomposition of vegetable waste, dried grass, or leaves of trees into compost is also exothermic.

B. Endothermic Reactions: Reactions in which heat is absorbed are called endothermic reactions. ❄️ Example:

Ba(OH)₂ + 2NH₄Cl → BaCl₂ + 2NH₄OH

2. Decomposition Reaction ✂️

Reactions in which a single reactant breaks down to form two or more products are called decomposition reactions. Decomposition reactions are of three types:

Thermal decomposition: In this type, decomposition takes place with the help of heat. Example:

CaCO₃(s) → CaO(s) + CO₂(g)

Electrolytic decomposition: In this type, heat is provided in the form of electricity. Example:

2H₂O(l) → 2H₂(g) + O₂(g)

Photolytic decomposition: When heat for decomposition is provided in the form of light. Example:

2AgCl(s) → 2Ag(s) + Cl₂(g)

3. Displacement Reaction ⚡

Reactions in which a more reactive substance displaces a less reactive substance from its compound are called displacement reactions.

Example 1:

Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

(Copper sulphate) (Ferrous sulphate)

Here iron is more reactive than copper, so it displaces the less reactive copper from its compound, copper sulphate. In this reaction, the colour of copper sulphate is blue, but as soon as iron nails are put into the solution, the colour of the solution changes from blue to brownish due to the displacement of copper.

Example 2:

Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)

(Copper sulphate) (Zinc sulphate)

Example 3:

Pb(s) + CuCl₂(aq) → PbCl₂(aq) + Cu(s)

(Copper chloride) (Lead chloride)

In examples 2 and 3, both zinc and lead displace copper from its compound in the reaction. This happens because copper is less reactive than both zinc and lead.

4. Double Displacement Reaction 🔄

A reaction in which an exchange of ions takes place between the reactants is called a double displacement reaction.
The general formula for a double displacement reaction is: AB + CD → AD + CB

Examples:

Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl
NaOH + H₂SO₄ → Na₂SO₄ + H₂O
NaCl + AgNO₃ → AgCl + NaNO₃
BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl
BaCl₂ + K₂SO₄ → BaSO₄ + 2KCl

5. Oxidation and Reduction Reaction ⚖️

Oxidation: When there is an addition of oxygen or a removal of hydrogen (or both) from a substance, it is called oxidation.

Examples of oxidation:

Addition of Oxygen —

(i) C + O₂ → CO₂

[Carbon gains oxygen and is oxidised to carbon dioxide.]

(ii) 4P + 5O₂ → 2P₂O₅

[Phosphorus gains oxygen and is oxidised to phosphorus pentoxide.]

(iii) 2Cu + O₂ → 2CuO

[Here copper gains oxygen and is oxidised to copper oxide.]

Removal of Hydrogen:

[When a compound loses hydrogen atoms, it is also said to be oxidised, for example, when hydrogen sulphide or methane loses hydrogen atoms during a reaction, the substance is oxidised.]

Reduction reaction: When there is an addition of hydrogen or a removal of oxygen (or both) from a substance, it is called reduction. Example of reduction:

CuO + H₂ → Cu + H₂O

Sometimes both these reactions take place together.

Redox Reaction

A reaction in which one reactant gets oxidised while another gets reduced during the reaction is called a redox reaction.
When oxidation and reduction take place together in a reaction, it is called a redox reaction. Example:

ZnO + C → Zn + CO

Here, in the same reaction, both oxidation and reduction take place, hence it is a redox reaction.

Oxidising agent: The substance that gives oxygen for oxidation, or removes hydrogen for reduction, is called an oxidising agent.

Reducing agent: The substance responsible for the removal of oxygen, or which gives hydrogen for reduction, is called a reducing agent. Example:

CuO + H₂ → Cu + H₂O

In the above example, CuO (copper oxide) is reduced to copper, so CuO (copper oxide) is the substance getting reduced. Since CuO (copper oxide) provides oxygen for the oxidation of hydrogen, copper oxide is the oxidising agent.
H₂ (hydrogen) is oxidised to water (H₂O), and hence it is responsible for the removal of oxygen from CuO (copper oxide). So, H₂ (hydrogen) is the reducing agent.

Summary:

Substance oxidised: H₂ // gains oxygen
Substance reduced: CuO // loses oxygen
Oxidising agent: CuO // provides oxygen for oxidation
Reducing agent: H₂ // responsible for removal of oxygen

Effects of Oxidation Reactions in Everyday Life 🌍

In our daily life, many such reactions keep occurring around us, of which the corrosion of metals and the rancidity of food are common examples that happen due to the effect of oxidation reactions.

1. Corrosion: The process in which the surface of a metal reacts with air, water, and moisture and gets corroded (eaten away) is called corrosion.

Note — Corrosion and rusting are not exactly the same thing; rusting refers specifically to the corrosion of the surface of a metal like iron.

Prevention of corrosion: The following methods are used to prevent corrosion:

Galvanisation (coating with zinc)
Painting the surface of the metal
Applying oil or grease on the surface of the metal

2. Rancidity: When fats and oils present in food undergo aerial oxidation, their taste and smell change and they become foul-smelling. This spoiling of food is called rancidity.

Rancidity is a phenomenon in which fatty or oily food, when kept for a long time, gets oxidised, which changes its taste.

Food prepared in fats or oils, such as vegetables, chips, etc., gets spoiled due to rancidity.
The taste of an oxidised food item changes.
Rancid food is not fit for eating.

Prevention of Rancidity in Fatty and Oily Food

Fatty and oily food items can be prevented from becoming rancid, or the rate of rancidity can be reduced. The following methods are used to prevent it:

Adding antioxidants to fatty and oily food items can prevent them from becoming rancid.
While packaging food items, the oxygen gas in the container is replaced with nitrogen gas, which prevents rancidity.
Storing food items in airtight containers reduces the rate of oxidation, which in turn slows down the rate of rancidity.
Food items are kept away from heat and light to protect them from rancidity.
Food items are kept in a refrigerator to protect them from rancidity and to slow down its rate.

Balancing a Chemical Equation ⚖️

Here, we will balance chemical equations using the observation method, also known as the hit-and-trial method.

Let us take the equation Fe + H₂O → Fe₃O₄ + H₂ as an example.

Steps:

Assume that each formula is enclosed in a box. Write them as boxes as shown below. Remember, no change should be made inside the box itself.

[Fe] + [H₂O] → [Fe₃O₄] + [H₂]

Make a list of the different types of atoms present in the unbalanced equation and count them. In this way:

First, we see which element has the highest number of atoms on either the reactant or the product side. Using this criterion, we find that the compound Fe₃O₄ has the most oxygen atoms — 4 in total.

To balance the number of oxygen atoms, we place a coefficient of 4 with H₂O, writing it as 4H₂O. We then get the equation:

Fe + 4H₂O → Fe₃O₄ + H₂

The next element with the highest number of atoms is Fe, which we balance using the same rule.

Placing a coefficient of 3 with Fe on the reactant side gives 3Fe, so the equation becomes:

3Fe + 4H₂O → Fe₃O₄ + H₂

Finally, we need to balance the hydrogen atoms on both sides. In our new equation, hydrogen is present as 4H₂O on the reactant side and as H₂ on the product side. The reactant side has 4 × 2 = 8 atoms while the product side has only 2 atoms.

Here, instead of using 2 on the left and 8 on the right as before, we get the coefficient by dividing 8 by 2, i.e., (8 ÷ 2) = 4. So, the coefficient will be 4, which when placed with hydrogen on the product side gives 4 × 2 = 8. The equation now becomes:

3Fe + 4H₂O → Fe₃O₄ + 4H₂

Now we can see that this equation is fully balanced.

Example — II

Let us now try to solve a new equation using the observation (hit-and-trial) method.

HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O

(i) Looking at the equation given above, we find that the compound Ca(NO₃)₂ has the highest number of NO₃ groups. To balance it, we have 1 on the LHS and 2 on the RHS, so we write 2HNO₃ on the reactant side.

Nitrogen and oxygen get balanced when we write 2HNO₃ on the reactant side and Ca(NO₃)₂ on the product side, giving us the equation:

2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O

(ii) Calcium is already balanced. Now we only need to balance the hydrogen atoms. On the reactant side, there are a total of 4 hydrogen atoms, while on the product side, there are 2.

The product needs a coefficient of 2 because (4 ÷ 2) = 2, so we get the equation:

2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O

(iii) There is nothing left to do in this equation now. Listing and counting the atoms of the various elements present, we get:

Element Reactant Product

O 8 8

N 2 2

Ca 1 1

H 4 4

This shows us that the equation is now balanced.