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Revision Notes for Class 10 Science
Chapter 1 – Chemical Reactions and Equations

Chemical Change: A change that results in the formation of one or more new compounds
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Observations in a Chemical Reaction
In a chemical reaction or chemical change, the following observations can be made,
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Formation of new substances
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Changes in energy
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•

Change in temperature
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Formation of a precipitate
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Cooking of food

B
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Heating of Lead nitrate

D
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Ripening of fruit
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a
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It demonstrates that one molecule of water is
made up of two hydrogen atoms and one oxygen atom
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One molecule of sulphuric acid has the formula H2SO4
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Chemical Equations - The chemical equation of the reaction is the representation of a
chemical change in terms of symbols and formulae of the reactants and products
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Step I: Writing Skeletal Equation
A word-equation depicts the transformation of reactants into products by placing an arrow
between them ( → )
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"
The reactants are on the left (LHS) of the arrow, while the products are on the right (RHS)
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A balanced account of a chemical transaction is a complete chemical equation, which
symbolically depicts the reactants, products, and their physical states
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Step II: Balancing of Chemical Equation
A balanced chemical equation is one in which the number of atoms of each element is equal
on both sides of the equation
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Equations must be balanced at all times
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If there are 4 H2O hydrogen atoms, then
4  2 = 8 hydrogen atoms are present
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There must be an even number in front of HCl on the reactant side
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) As a result, there will be an even amount of chlorine atoms (i
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, KCl=1
, MnCl2 = 2 , Cl2 = 2
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e
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However, the quantity of chlorine atoms in the
product is unusual
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Let's alter it to the
simplest even number we can think of, which is 2
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 2KMnO4 + HCl → 2KCl + MnCl2 + H2O + Cl2

In 2 KMnO4 , there are 2 K, 2 Mn, and 8 O
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 2KMnO4 + HCl → 2KCl + 2MnCl2 + 8H2O + Cl2

If there are 8 H2O on the product side, there should be 16 H (8 x 2) on the reactants side as
well
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On the left hand side, there are 16 Cl
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So, 10
more Cl atoms are to be accounted for
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 2KMnO4 + 16HCl → 2KCl + 2MnCl2 + 8H2O + 5Cl2

The Hit and Trial approach is a technique for balancing chemical equations
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Let the number of molecules on each side be,
aKMnO4+ bHCl → cKCl + dMnCl2 + eH2O + fCl2
Now comparing LHS and RHS,
K: a = c
Mn: a = d
O: 4a = e
H: b = 2e
Cl: b = c + 2d + 2f
Let us take a = 2, then
c=d=a=2
b = 2e = 8a = 16
So, e = 8
So, 16 = 2 + 2(2) + 2(f)

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f=5
Thus, the required equation is: 2KMnO4+ 16HCl → 2KCl + 2MnCl2 + 8H2O + 5Cl2
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Various reactants and products names

B
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The number of moles of the reactants and products in relation to each other

D
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Volumes of gaseous reactants and products in relation to each other

In short – the Steps Involved in Balancing a Chemical Equation can be Listed As:
Step 1: Write the skeleton equation correctly
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Step 3: Start by balancing elements that only appear once on each side of the arrow
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Step 4: The Elementary substances are balanced last
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Types of Reactions
A
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Types of Combination reactions:
1
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2H 2(g)

→ O2(g) + 2H 2 O( l )

Hydrogen

Oxygen

Water

2
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2CO(g) + O2(g) → 2CO2(g)

3
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NH3(g) +
Ammonia
Colourless

HCl(g)

→

Hydrogen chloride
Colourless

NH 4Cl(s)
Ammonium chloride
White

B
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A decomposition reaction occurs when a complex is broken down into two or more simple
components using heat or electricity
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Electrolysis is the process of decomposing a substance by putting an electric current through
it
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1
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Thermal decomposition
2HgO(s) ⎯⎯⎯⎯⎯⎯⎯
→

Mercuric oxide
Red

2Hg (l) +

Mercury
Silvery liquid

O 2(g) 
Oxygen

2
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Thermal decomposition
2Cu(NO3 )2(s) ⎯⎯⎯⎯⎯⎯⎯
→

2CuO(s) +

4NO2(g) + O2(g)

Copper nitrate

Copper

Nitrogen

Blue

oxide

dioxide

Black

Oxygen

Reddishbrown

3
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Electrolytic Decomposition
2H 2O(l) ⎯⎯⎯⎯⎯⎯⎯⎯
→

Water

2H 2(g) + O2(g)
Hydrogen

Oxygen

4
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Electrolytic decomposition
PbBr2 ⎯⎯⎯⎯⎯⎯⎯
⎯
→

Lead bromide
(molten)

Pb(s) +
Lead

Br2(l)
Bromine

Activity Series: The metal activity series is a list of metals arranged in decreasing chemical
activity order
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A Substitution or Displacement Reaction occurs when the atoms of one element
replace the atoms of another element in a compound's molecules
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Electropositive elements that are more abundant displace electronegative elements that are
less abundant
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For e
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,
Fe(s) + 2CuSO 4(aq) →
Iron

Copper
Sulphate
Blue

Iron (II)
Sulphate
Green

Copper
Reddish
Brown

+ H 2SO 4(aq) → MgSO 4(aq) + H 2O(g)

Mg (s)

Magnesium

2KI(aq)

4FeSO 4(aq) + Cu (s) 

+

Potassium
Iodide

Sulphuric
acid

Magnesium
Sulphate

Water

Cl2(g) → 2KCl(aq) + I 2(s)
Chlorine

Potassium
Chloride

Iodine

The iodine that has been freed dissolves in the chloroform, turning it purple
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Double Displacement Reactions / Metathesis Reactions: The mutual exchange of ions
in which two chemicals react to generate two distinct molecules
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Precipitation
In all of the following processes, a white material is generated that is insoluble in water
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A precipitation reaction is a reaction that
results in the formation of a precipitate
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Neutralization
The reactants are a base and an acid, and the products are salt and water
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The positive charge of the acid's hydrogen ion and the negative

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charge of the base's hydroxyl ions or oxide ions lose their electrical charge and form covalent
water molecules
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e
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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
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Addition of Oxygen - Magnesium oxide is formed when oxygen reacts with magnesium
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Removal of Hydrogen - To liberate free iodine, hydrogen is removed from hydro iodic
acid
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Removal of Oxygen - Copper metal is formed by removing oxygen from copper oxide
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Addition of Hydrogen - When hydrogen reacts with chlorine, hydrogen chloride gas is
formed
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Redox reactions occur when oxidation and reduction reactions occur at the same time
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Oxidizing and Reducing Agents
A
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B
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Another Definition of Oxidation and Reduction
1
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2
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Electronic Concept of Oxidation and Reduction
1
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2
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The Effects of Oxidation Reactions on Everyday Life
Corrosion
Many metals are chemically active elements that are easily affected by moisture, oxygen,
acids, and other things
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The rusting of iron is the common name
for this phenomenon
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This eventually
leads to structural weakening and metal breakdown
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When copper vessels or artefacts are exposed to air and water, what happens? They
gradually tarnish as a thin layer of green oxide forms on them
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In wet weather, even the heaviest metal
lead tarnishes
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Rancidity
Have you ever tasted or smelled fat/oil-containing food materials that have been left out for
an extended period of time? Rancidity refers to an undesirable change in a food's flavour and
odour
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When an oxygen atom replaces a hydrogen atom in a fatty acid
molecule, the molecule becomes destabilised
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Antioxidants are chemicals that prevent fat from oxidising or becoming rancid (such as
BHT, BHA, vitamin E, and vitamin C, and spices such as sage and rosemary)
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Keeping food in airtight containers
or airtight wrapping also helps to slow down oxidation
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Vacuum packaging is sometimes employed in processed goods to completely eliminate the
presence of oxygen
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