Class 10 Science Chapter 1 notes with question answer

Class 10 Science Chapter1 Chemical Reactions and Equations notes of all topics with detailed explanation. You can prepare your Board examination from here because The notes is written by our Expert Theacher with lots of experience. We provide Solution of NCERT question Answer and important intext question

Chemical Reactions and Equations

Physical  changes : Th­e changes which are accompanied with change in physical properties of the substances but no new substance is formed are called physical changes e.g., melting of ice, boiling of water, etc.

 Chemical changes : ­The changes in which the original substances lose their nature and identity to form new chemical substances with different properties are called chemical changes e.g., burning of candle, cooking food, etc

Chemical reactions : Th­e process involving a chemical change is known as a chemical reaction. A chemical reaction formed by two types of substances one is Reactant and another is Product.

Reactant : The substances are initially taken in a chemical reaction are called reactants.

Product: The chemical substances which are formed during a chemical reaction are called products

The chemical reaction is a process in which breaking of chemical bonds (present in the reactant molecules) and making of new chemical bonds (in the product molecules) occur  e.g., burning of magnesium ribbon in air.

Mg + O2 → MgO

Characteristics of chemical reactions

The following observations help us to determine whether a chemical reaction has taken place or not

Change in state : Certain chemical reactions are accompanied with the change of state e.g., when a mixture of hydrogen gas and oxygen gas is ignited with an electric spark at room temperature, liquid water is formed.

Change in colour : Certain chemical reactions are accompanied with the change of colour e.g., when red lead oxide is heated yellow lead monoxide is formed.
Evolution of a gas : Some chemical reactions are accompanied with the evolution of a gas e.g., reaction between a metal (like zinc, magnesium or iron) and dilute sulphuric acid produces hydrogen gas.

Change in temperature : Some chemical reactions occur with change in heat energy or with change in temperature. – Reactions which result in rise in temperature i.e. in which heat is evolved are called exothermic reactions.

C6H12O6(aq) + 6O2(g)→ 6CO2(g) + 6H2O(l) + Heat 
Reactions which result in fall in temperature i.e. in which heat is absorbed are called endothermic reactions.
CaCO + Heat 3( )s→ CaO + ( )s g CO2( )

Introduction Chemical equation

A method of representing a chemical reaction in terms of symbols and formulae of the substances participated is known as chemical equation. ­ere are two ways to represent a chemical equation :

In terms of words : When a chemical equation is written in terms of words, it is called a word equation. ­e chemical reaction between granulated zinc and hydrochloric acid can be written in terms of words as

Zinc + Hydrochloric acid→ Zinc chloride + Hydrogen

In terms of symbols and formulae : Chemical reaction between granulated zinc and hydrochloric acid can be written in terms of symbols and formulae as

Zn + 2HCl →ZnCl2 + H2

 Conventions used in chemical equations : – ­The reactants are written on the le hand side along with plus (+) sign between them. – Similarly, products are written on the right hand side along with plus (+) sign between them. – An arrow (→ ) separates the reactants from the products. – ­e arrowhead points towards the products and tells about the direction of the reaction.

Balanced and unbalanced chemical equations : X Balanced chemical equation : ­e equation which contains an equal number of atoms of each element on both sides of the arrow is called a balanced chemical equation. 2Mg + O2→ 2MgO

A balanced chemical equation must obeys the law of conservation of mass. X Unbalanced chemical equation : ­e equation in which the number of atoms of each element on both sides of the arrow is not equal. H2 + O2→ H2O

Balancing a chemical equation

Balancing of a chemical equation means to equalise the number of atoms of each element on both sides of the equation. Consider the following steps for balancing the chemical equation : For example, iron reacts with water (steam) to form iron(II, III) oxide and hydrogen.

Step-I : Write the chemical equation in the form of a word equation. Keep the reactants on the lehand side and the products on the right hand side. Separate them by an arrow (→ ) with head pointing from the reactants to products. Iron + Steam→ Iron (II, III) oxide + Hydrogen 
Step-II : Write down the symbols and formulae of the various reactants and products which gives skeletal chemical equation. Fe + H2O →Fe3O4 + H2
Step-III : Listing number of atoms of di‑erent elements.
ElementsNumber of atoms on LHSNumber of atoms on RHS
Fe13
H22
O14
Step-IV : Select the compound with maximum number of atoms to start balancing. In that compound, balance the element with maximum number of atoms (e.g., oxygen in the given equation). It may be a reactant or a product

Fe + 4H2O→ Fe3O4 + H2

Step-V : To balance the atoms of an element, put a whole number coecient before the formula of the compound. If selection of the biggest formula appears inconvenient, balance the atoms of that element which occurs at minimum number of places on both sides of the equation. Atoms of the element which occur at maximum places are balanced at last

To balance H-atoms on both sides :

Fe + 4H2O→ Fe3O4 + 4H2

To balance Fe-atoms on both sides : So, the equation would be

3Fe + 4H2O→ Fe3O4 + 4H2

Step VI : For checking the correct balanced equation, we count atoms of each element on both sides of the equation.
ElementsNumber of atoms on LHSNumber of atoms on RHS
Fe33
H88
O44

As the number of atoms of each element on both sides of the equation are equal, the equation is balanced. 3Fe(s) + 4H2O(g) →Fe3O4(s) + 4H2(g

Types of chemical reactions

As we know, in chemical reactions, bonds present in reactants break and new bonds form in the products. ­is exchange of species can take place in a number of ways resulting in di‑erent types of reactions, which can be explained as follows :

Combination reactions : ­The reactions in which two or more substances combine to form a single substance under suitable conditions. Examples : – Combustion of coal C(s) + O2(g)→ CO2(g) – Combination of nitric oxide with oxygen to form nitrogen dioxide. 2NO(g) + O2(g) → 2NO2(g) Nitric oxide Nitrogen dioxide (brown gas) – Combination of ammonia with hydrogen chloride gas to form a white solid mass of ammonium chloride. NH3(g) + HCl(g)→ NH4Cl(s) Ammonium chloride (white)

 Decomposition reactions : The reactions in which a single substance breaks down to give two or more smaller substances under suitable conditions. ­ree types of decomposition reactions are as follows : – The reactions occur in presence of heat.

2FeSO4( )s heat → Fe2O3 ( )s + SO2( )g ­ + SO3( )g ­

Displacement reactions : ­The chemical reactions in which one element takes the position of another element present in the compound. Examples : Zn(s) + CuSO4(aq)→ ZnSO4(aq) + Cu(s)

Double displacement reactions : The reactions in which two compounds react by exchange of ions to form two new compounds, are called double displacement reactions. Example : AgNO + NaCl → AgCl( )s + NaNO3( )

­There are two types of double displacement reactions : – Precipitation reactions : ­ose reactions in which aqueous solution of two compounds on mixing react to form an insoluble compound which further separates out as a precipitate are called precipitation reactions. Examples : FeCl2(aq) + 2NaOH(aq)→ Fe(OH)2(s)↓ + 2NaCl(aq) Ferrous hydroxide (dirty green ppt.) Na2SO4(aq) + BaCl2(aq)→ BaSO4(s) + 2NaCl(aq) (Barium sulphate) (white ppt.)

Neutralisation reactions

The reactions of acids and bases in which product formed is neutral to litmus are known as neutralisation reactions.

Examples : NaOH(aq) + HCl(aq)→ NaCl(aq) + H2O

CuO(s) + 2HCl(aq)→ CuCl2(aq) + H2O

Redox reactions : Reactions in which oxidation and reduction take place simultaneously are called redox reactions. –

Oxidation : The reactions in which the addition of oxygen to a substance or removal of hydrogen from a substance takes place are called oxidation reactions. On the other hand, the substance which either gives oxygen or removes hydrogen in an oxidation reaction is known as an oxidising agent. –

Reduction : The reactions in which addition of hydrogen to a substance or removal of oxygen from a substance takes place are called reduction reactions. On the other hand, the substance which either gives hydrogen or removes oxygen in a reduction reaction is known as reducing agent. Examples :

Effect of oxidation reactions in everyday life

As oxygen is the most essential element for sustaining life, it is involved in variety of reactions which has wide range of e‑ects on our daily life. ­The two effects are discussed below :

 Corrosion : It is a process in which metals are decayed gradually by the action of air, moisture and acids on their surface. Basically, it is caused by oxidation of metals by oxygen present in the air. Example : Rusting of iron

Corrosion causes damage to car bodies, iron railings, ships and to all objects made up of metals, specially those of iron. –

Prevention of corrosion : Corrosion can be prevented by coating the surface by a layer of another metal which does not corrode e.g., coating of iron with zinc. and also by coating surface with grease, paint or oil, etc.

 Rancidity : ­The slow oxidation of oils and fats present in food materials resulting in compounds with unpleasant smell is known as rancidity. Vacuum packing, refrigeration of food materials, placing of food materials away from direct sunlight will slow down the process of rancidity

Leave a Comment