Oswaal CBSE Class 10 Science Question Bank 2023-24 Book
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CHAPTER-1
CHEMICAL REACTIONS AND EQUATIONS
Chemical Reaction and Equations
Topic-1 Concepts Covered Chemical reaction and examples, Skeletal and bal-
anced chemical equation, Steps to balance a chemical equation.
Revision Notes
A chemical reaction
A chemical reaction is a process in which the original substance(s) loses its nature and identity and forms new
substance(s) with different properties.
Breaking of the chemical bonds and formation of new chemical bonds is responsible for the occurrence of a
chemical reaction.
The substances which take part in a chemical reaction are called Reactants.
The substances which are formed in a chemical reaction are called Products.
Examples of chemical reaction:
(i) Digestion of food
(ii) Respiration
(iii) Rusting of iron
(iv) Burning of magnesium ribbon
(v) Formation of curd
A chemical reaction can be identified by either of the following observations:
S. No. Characteristics Examples
1. Change in state The combustion reaction of candle wax is characterized by a change in state
from solid to liquid and gas.
2. Change in colour The chemical reaction between citric acid and purple coloured potassium
permanganate solution is characterized by a change in colour from purple
to colourless.
3. Evolution of gas The chemical reaction between zinc and dilute sulphuric acid is characterized
by hydrogen gas. Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g)
4. Change in temperature The reaction between quicklime and water to form slaked lime is
characterized by an increase in temperature.
5. Formation of a When an aqueous solution of sodium sulphate is mixed with the aqueous
precipitate solution of barium chloride, barium sulphate comes in the form of white
precipitate
Na2SO4 (aq)+BaCl2(aq) → BaSO4(↓)+2NaCl(aq)
Chemical equations
A chemical equation is the symbolic representation of a chemical reaction in the form of symbols and formulae.
It is a way to represent the chemical reaction in a concise and informative way.
For example,
Magnesium + Oxygen → Magnesium oxide
(Reactants) (Product)
This equation is called word equation.
The word equation can be written into chemical equation by writing symbols and formulae of the substance in
place of their name.
2Mg + O2 → 2MgO
Writing a chemical equation
(i) The symbols of elements and the formulae of reacting substances (reactants) are written on the left hand side
of the equation, with a plus (+) sign between them.
(ii) The symbols and formulae of the substances formed (products) are written on the right hand side of the
equation, with a plus sign (+) between them.
(iii) An arrow sign (→) is put between the reactants and the products.
(iv) The physical states of the reactants and products are also mentioned in a chemical equation.
Skeletal chemical equation: A chemical equation which simply represents the symbols and formulas of reactants
and products taking part in the reaction is known as skeletal chemical equation for a reaction.
For example: For the burning of Magnesium in the air, Mg + O2 → MgO is the skeletal equation.
Balanced Equation: The equation in which atoms of various elements on both sides of a chemical equation
are equal in accordance with the law of conservation of mass.
The example of balanced chemical equation :
340 atm
(i) CO(g) + 2H2 (g)
→ CH3OH(l)
sunlight
C6H12O6(aq) + 6O2 (g)
(ii) 6CO2(g) + 6H2O(l) chlorophyll
Glucose
The process of equalizing the atoms of various elements both on either sides of an equation is called the balancing
of chemical equation. This is known as hit and trial method. Let us understand this with the help of an example
given below:
Key Word
Law of conservation of mass: It states that, "The matter can neither be created nor destroyed in a chemical
reaction. " OR "
the total mass of reactants = total mass of products".
Example 1
Balancing a chemical equation :
Step 1. Write the chemical equation and draw boxes around each formula.
Fe + H2O → Fe3O4 + H2
Step 2. Count the number of atoms of each element on both the sides of the arrow :
No. of atoms at No. of atoms at
Element
reactant side product side
1. Fe 1 3
2. H 2 2
3. O 1 4
Step 3. Equalize the number of the atoms of element which has the maximum number of atoms (oxygen).
Fe + 4H2O → Fe3O4 + H2
Step 4. Try to equalize all the atoms of elements on reactant and product side by adding coefficient in front of it.
3Fe + 4H2O → Fe3O4 + 4H2
Now, all the atoms of elements are equal on both sides.
Step 5. Write the physical states of reactants and products.
3Fe (s) + 4H2O (g) → Fe3O4 (s) + 4H2 (g)
Solid state = (s), Liquid state = (l), Gaseous state = (g), Aqueous state = (aq)
Step 6. Write necessary conditions of temperature, pressure or catalyst at above or below the arrow.
Types of Chemical Reactions
Topic-2 Concepts Covered Combination reaction, Decomposition reaction,
Displacement reaction, Double displacement reaction, Redox reaction,
Oxidation and reduction reaction, Exothermic and endothermic reaction.
Revision Notes
Types of Chemical Reactions
I. Combination Reaction: The reaction in which two or more reactants combine to form a single product.
e.g., (i) Burning of coal
C(s) + O2(g) → CO2(g)
(ii) Formation of water
, Oswaal CBSE Revision Notes Chapterwise & Topicwise, SCIENCE, Class-X 3
2H2(g) + O2(g) → 2H2O(l)
(iii) CaO(s) + H2O(l) → Ca(OH)2 (aq) + Heat
(Quick lime) (Slaked lime)
Exothermic Reactions: Reaction in which heat is released along with formation of products.
e.g., (i) Burning of natural gas.
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) + Heat
(ii) Respiration is also an exothermic reaction.
C6H12O6(aq) + 6O2(g) → 6CO2(aq) + 6H2O(l) + energy
(Glucose)
II. Decomposition Reaction:
The reaction in which a compound splits into two or more simpler substances is called decomposition reaction.
A→B+C
(a) Thermal decomposition: When decomposition is carried out by heating.
e.g., (i) 2FeSO4(s) Heat
CaO(s) + CO2(g)
(Lime stone) (Quick lime)
(b) Electrolytic Decomposition: When decomposition is carried out by passing electricity.
Electric
e.g., 2H2O(l) current
2H2(g) + O2(g)
(c) Photolytic Decomposition: When decomposition is carried out in presence of sunlight.
Sunlight
e.g., (i) 2AgCl(s)
2Ag(s) + Cl2(g)
Sunlight
(ii) 2AgBr(s)
2Ag(s) + Br2(g)
Endothermic Reaction: The reactions which require energy in the form of heat, light or electricity to break reactants
are called endothermic reactions.
III. Displacement Reaction: The chemical reactions in which more reactive element displaces less reactive element
from its salt solution.
e.g., (i) Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
(Iron) (Copper sulphate) (Ferrous sulphate) (Copper)
The iron nail becomes brownish in colour by deposition of Cu and blue colour of CuSO4 changes into dirty
green colour due to formation of FeSO4.
(ii) Zinc displaces copper forming zinc sulphate. Zn is more reactive than copper.
Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
(Zinc Sulphate)
IV. Double Displacement Reaction: A reaction in which new compounds are formed by mutual exchange of ions
between two compounds.
Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq)
(Sodium sulphate) (Barium chloride) (Barium sulphate) (Sodium chloride)
White precipitate of BaSO4 is formed, so it is also called precipitation reaction.
V. Oxidation and Reduction:
Oxidation: Loss of electrons
Reduction: Gain of electrons
Mnemonics
Concept: Types of decomposition reaction
Mnemonics: PET
Interpretations:
Photolytic reaction, Electrolytic reaction, Thermal reaction
Concept: Oxidation and reduction reaction
Mnemonics: OIL RIG
Interpretations:
Oxidation Is Loss of electrons, Reduction Is Gain of electrons
Concept: Types of chemical reactions
Mnemonics: ROC.D3
Interpretations:
Reduction, Oxidation, Combination, Decomposition, Displacement, Double Displacement
Oxidation: It is a process of gaining oxygen during a reaction by an atom, molecule or ion.
2Cu + O2 Heat
2CuO
Reduction: It is the gain of electrons or a decrease in the oxidation state of an atom by another atom, an ion or a
molecule.
CuO + H2 → Cu + H2O
In this reaction, CuO is reduced to Cu and H2 is oxidised to H2O. In other words, one reactant gets oxidised while
the other gets reduced. Such reactions are called oxidation-reduction reactions or redox reactions.
VI. Important equation
Redox (Oxidation and Reduction) Reaction :
Effects of oxidation reactions in everyday life :
1. Corrosion: Corrosion is a process in which metals are deteriorated by action of air, moisture, chemicals, etc. It is a
redox reaction where metal gets oxidised to metal oxide and oxygen gets reduced to oxide ion.
Examples:
(a) Corrosion of iron is called rusting. Iron objects when left in moist open air for sometime get coated with a
reddish brown powder. The process is known as rusting.
(b) Green coating on Copper articles and black coating on silver ornaments are another example of corrosion.
Effects of corrosion :
(a) Rusting causes damage to ships, car bodies, bridges, railings.
(b) Corrosion is a wasteful process because it leads to wastage of tonnes of various metals every year and lot of
money is spent to repair or replace it.
Prevention of Rusting:
(a) The iron articles should be painted.
(b) The machine parts should be oiled and greased.
(c) Galvanised iron pipes should be used for water supply.
(d) Iron can be coated with chromium to prevent rusting.
2. Rancidity : Rancidity is the process of slow oxidation of oil and fat, present in the food materials resulting in the
production of foul odour and taste in them. When cooked food items are placed for a long time, they become
rancid and unsuitable for the consumption.
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