Class 10th Science NCERT book

Chapter 1 - Chemical Reactions and Equations

What is Chemical change?

Chemical change is a change in which one or more new substances are formed.

In a chemical change—

• New substances are formed.

• Energy changes are involved. 

• There is a change in mass during the reaction.

• Permanent change takes place. 

Examples –

❖ Cooking of food

❖ Rusting of iron

❖ Heating of Lead nitrate

❖ Souring of milk

❖ Ripening of fruit.

Rusting of iron is a chemical change because

❖ A new substance iron oxide is formed.

❖ The change is permanent; the article has got a rust layer (which may only peal off).

❖ There is an increase in mass when rust forms.

❖ An energy change has taken place (which may not be visible).

Chemical changes are also known as chemical reactions.

Exothermic Reaction - A chemical reaction which is accompanied by evolution of heat energy 

is known as exothermic reaction.

 

The amount of heat (energy) produced is written along with the products. This indicates that heat 

is given out.

 

Endothermic Reactions - A chemical reaction which is accompanied by absorption of heat 

energy is known as exothermic reaction.

The amount of heat (energy) produced is written along with the reactants. This indicates that heat is absorbed

What is Chemical Formula?

Chemical Formulae - The chemical formula of a substance is the symbolic representation of the actual number of atoms present in one molecule of that substance.

 

(a) Formula of one molecule of water is H2O. It shows that one molecule of water is made up 

of 2 atoms of hydrogen and one atom of oxygen. 

(b) Formula of one molecule of sulphuric acid is H2SO4. It shows that one molecule of 

sulphuric acid is made up of 2 atoms of hydrogen, 1 atom of Sulphur and 4 atoms of 

oxygen. 

What is Chemical Equation?

Chemical Equations - Representation of a chemical change in terms of symbols and formulae of the reactants and products is known as chemical equation of the reaction.

Steps for writing Chemical Equation

Step I: Writing skeletal Equation

A word-equation shows change of reactants to products through an arrow placed between them (

). The arrow may be read as "to yield" or "to form" or "to give" and shows the direction of the 

reaction.

The reactants are placed on the left hand side (LHS) of the arrow and the products on the right 

hand side (RHS). The different reactants as well as products are connected by a plus sign (+). 

A complete chemical equation represents the reactants, products and their physical states 

symbolically and is a balanced account of a chemical transaction. 

Step II: Balancing of Chemical Equation

An equation in which number of atoms of each element is equal on both the sides of the equation 

is known as balanced chemical equation.

A chemical equation is balanced so that the numbers of atoms of each type involved in a 

chemical reaction are the same on the reactant and product sides of the equation.

Equations must always be balanced. 

Whenever H2O is present on any side, the number of hydrogen on both the sides should be an 

even number (2 atoms of hydrogen in water). If there are 4H2O, then 4 × 2 = 8 hydrogen atoms. 

If there are 7H2O, then 7 × 2 = 14 hydrogen atoms i.e., all are even numbers.



On the reactant side, there must be an even number in front of HCl. (What that number is, we 

will find out later). As a result, the number of chlorine atoms will also be even. But on the 

product side, the number of chlorine atoms is odd (i.e., KCl = 1, MnCl2= 2, Cl2 = 2. i.e., 1 + 2 + 

2 = 5). The only odd number of chlorine atoms is in KCl. Let us change it into the simplest even 

number possible i.e., 2.

Since number of K atoms in 2 KCl = 2, place 2 in front of KMnO4 to balance K atom. 

In 2 KMnO4, there are 2 K, 2 Mn, and 8 O. So add these numbers in front of K, Mn and O, (K is 

already done). 

If there are 8 H2O on the product side, there should be 16 H (8 x 2) on the reactants side as well. 

Now the only unbalanced one is  Chlorine. On the left hand side, there are 16 Cl. On the right 

hand side, firstly, there are 2 Cl in 2 KCl + 4 Cl in 2 MnCl2, making total of 6(2 + 4). 

Subsequently 10 more Cl atoms are to be accounted for. So place 5 in front of Cl2 to make it 10 

(5 x 2). 

This type of balancing the chemical equations is known as the Hit and trial method.

Information conveyed by a chemical equation

1. Names of various reactants and products

2. Formulae of reactants and products

3. Relative number of moles of the reactants and products

4. Relative masses of reactants and products

5. Relative volumes of gaseous reactants and products

Limitation of a Chemical Equation

• It does not mention the state of the substances. Accordingly, the following symbols should be 

added to make it informative: (s) for solid, (l) for liquid, (aq) if the reactant or product is 

present as a solution in water, (g) for gas and (vap) for vapour.

 Example: CaCO3 (s)+ HCl (aq) CaCl2 (s) + H2O (l) + CO2(g)

• The reaction may or may not be complete. An equation does not reveal this. 

• It does not give any information regarding the speed of the reaction. 

• It does not give the concentration of the substances. In some cases, terms like diluted (dil) 

and concentrated (conc) may be added. 

• It does not give the conditions of temperature, pressure, catalyst, etc. This is overcome by 

mentioning these above or below the arrow. e.g.

• It does not give any idea about color changes, which has to be mentioned separately. 

• It does not give any indication regarding the production or absorption of heat. This is 

mentioned separately.

C + O2 CO2 + Heat

2C + O2 2CO + Heat 

• Some reactions are reversible. They are represented by or 

• Chemical reactions that proceed with evolution of heat energy, that is, in which heat is given 

out along with the product, are called exothermic reactions.

• Chemical reactions that proceed with the absorption of heat energy are called endothermic reactions.

Balancing of simple Chemical Equations - The number of atoms of each element should 

remain same before and after the reaction.

Balancing: To make the number of atoms of all the elements equal on both the sides in a skeletal equation. A simple equation is balanced by Hit and trial method.

Steps involved in balancing a Chemical Equation:

Step 1: Write the correct skeleton equation.

Step 2: Start with the compound that has the maximum atoms or maximum kinds of atoms and 

the atoms present in it are balanced first.

Step 3: Balance elements that appear only once on each side of the arrow first. Then balance 

elements that appear more than once on a side.

Step 4: Elementary substances are balanced last of all.

Step 5: If required the whole equation is multiplied by some suitable number in order to make all the coefficients whole numbers

.

Balancing of Ionic Equations:

A balanced ionic equation must satisfy mass as well as charge balance.

Calculations based on Chemical Equations:

We can get a lot of quantitative information from a chemical reaction. Number of moles of 

reactants and products can be calculated from a chemical reaction.

Types of Reactions

Combination or Synthesis Reactions: 

The reactions in which two or more substances combine to form a single new substance.

Types of Combination reactions:

1. Combination of two elements to form a compound

• Burning of hydrogen in air or oxygen to produce water. 

 

2. Combination Reactions involving an Element and a Compound

• Burning of carbon monoxide in oxygen to form carbon dioxide.

2CO (g) + O2(g) 2CO2 (g) 

3. Combination Reactions involving Two Compounds

• Combination of ammonia and hydrogen chloride to produce ammonium chloride. 

 

Decomposition reactions: are opposite to combination reactions. In a decomposition reaction a 

compound breaks down into two or more simple substances by the application of heat or 

electricity. 

When a substance decomposes due to heat it is called thermal decomposition, while 

decomposition due to electricity, is called electrolytic decomposition. 

Electrolysis: The decomposition of a substance by passing electric current through it is called 

electrolysis.

Photolysis: The decomposition of a compound with light is called photolysis.

1. Mercuric oxide, when heated, undergoes thermal decomposition, to give mercury and oxygen.

2. Similarly, if blue crystals of copper nitrate are heated, they undergo thermal decomposition to 

 give black colored copper oxide, reddish brown fumes of nitrogen dioxide, and a colorless 

 gas of oxygen.

 3. When water taken in an electrolytic cell, is acidified with a small quantity of sulphuric acid 

 and a direct current passed through it undergoes electrolytic decomposition to yield hydrogen 

 and oxygen. 

 2𝐻2𝑂(𝐼)

𝐸𝑙𝑒𝑐𝑡𝑟𝑜𝑙𝑦𝑡𝑖𝑐 𝐷𝑒𝑐𝑜𝑚𝑝𝑜𝑠𝑖𝑡𝑖𝑜𝑛

→ 2𝐻2(𝑔) + 𝑂2(𝑔)

 Water Hydrogen Oxygen

4. If an electric current is passed through molten lead bromide, it decomposes to give lead and 

 bromine. 

Activity Series: The activity series of metals is a list of metals in the order of their decreasing 

chemical activity.

A substitution or displacement reaction is a chemical change in which atoms of one element 

replace the atoms of another element from the molecules of a compound. Elements which are 

higher in the Activity Series displace those elements which are placed below them. More electro 

positive elements displace lesser electropositive elements. Conversely, higher electro negative 

elements will displace lesser electro negative elements. For e.g., 

The iodine so liberated, dissolves in the chloroform, giving it a purple color. 

Double Displacement Reactions / Metathesis reactions: The reactions in which two 

compounds react to form two different compounds by mutual exchange of ions.

Types of Double displacement reactions:

1. Precipitation Reaction

2. Neutralization Reactions

1. Precipitation 

In all the above reactions a white substance, which is insoluble in water, is formed. This 

insoluble substance formed is known as a precipitate. A reaction that produces a precipitate is 

called a precipitation reaction. 

2. Neutralization

Neutralization is a type of double displacement reaction, in which, the reactants are a base and an 

acid, and the products are salt and water. The positive charge of the hydrogen ion of the acid, and 

the negative charge of hydroxyl ions or oxide ions of the base, lose their electrical charge, and 

become covalent molecule of water. 

A neutralization reaction is basically a reaction between H+

and OH-

ions i.e., 

Oxidation - Reduction Reactions/Redox Reactions 

Classical Concept of Oxidation and Reduction

"Oxidation is a reaction in which oxygen is added or hydrogen is removed from a substance." 

Addition of Oxygen - Oxygen adds on to magnesium to become magnesium oxide.

Removal of Hydrogen - Hydrogen is removed from hydro iodic acid to liberate free iodine. 

"Reduction is a reaction in which oxygen is removed from a substance or hydrogen is added to a 

substance." 

Removal of Oxygen - Oxygen is removed from copper oxide to form copper metal. 

Addition of Hydrogen - Hydrogen adds to chlorine to form hydrogen chloride gas. 

Oxidation and reduction reactions may occur simultaneously, these reactions as "redox" 

reactions.

Example of Redox Reaction

Example of Non-redox Reaction 

Redox reactions are the reactions in which oxidation and reduction takes place simultaneously.

Oxidizing and Reducing Agents

Oxidizing Agent: A substance that brings about oxidation.

Reducing Agents: A substance that brings about reduction.

Another definition of Oxidation and Reduction

Oxidation: The process of addition of electronegative element or radical or removal of 

electropositive element or radical.

Reduction: The process of addition of electropositive element or radical or removal of 

electronegative element or radical.

Electronic Concept of Oxidation and Reduction

Oxidation: The process in which there is loss of electrons.

Reduction: The process in which there is gain of electrons.

The Effects of Oxidation Reactions in Every Day Life 

Corrosion 

Many metals are chemically active elements and get easily affected by substances like moisture, 

air, acids, etc. One must have observed iron articles that are shiny when new, but get coated with 

a reddish brown powder when left for some time. This process is commonly known as rusting of 

iron. The problem with iron (as well as many other metals) is that oxidation takes place and the 

oxide formed does not firmly adhere to the surface of the metal causing it to flake off easily. This 

eventually causes structural weakness and disintegration of the metal. 

Hence metal is attacked by substances around it, it is said to corrode and this process is called 

corrosion. Corrosion causes deterioration of essential properties in a material. 

What happens to copper vessels or artifacts when exposed to air and water? They slowly get 

tarnished by acquiring a thin green oxide layer. Similarly, silver quickly acquires a thin black 

oxide coating in moist air. The heaviest metal lead also tarnishes in moist weather. The black 

coating on silver and the green coating on copper are examples of corrosion in which the oxides 

formed strongly bond to the surface of the metal, preventing the surface from further exposure to 

oxygen and consequently slowing down corrosion. 

Rancidity 

Have you ever tasted or smelt the fat/oil containing food materials left for a long time? This 

unpleasant change in the flavor and odour of a food is called rancidity. The most important cause 

of rancidity is the deterioration in fats and fatty foods because of oxidation process. When an 

oxygen atom replaces hydrogen atom in the fatty acid molecule it destabilizes the molecule. 

Factors which accelerate fat oxidation include, salt, light, water, bacteria, moulds trace metals 

(iron, zinc, etc.). 

Usually substances which retard fat oxidation or rancidity are called antioxidants (such as BHT, 

BHA, vitamin E, and vitamin C, and spices such as sage and rosemary). These are added to 

foods containing fats and oil to prevent such spoiling. Keeping food in air tight containers or air 

tight wrapping also helps to slow down oxidation. 

Some high fat foods such as potato chips are packaged in materials that protect them from light 

and oxygen and the containers are flooded with nitrogen to further exclude oxygen. At times, to

avoid the presence of oxygen altogether, vacuum packaging is used in some processed foodstuff.