Chemical Reactions
and Equations
Class X – Science Chapter 1 Chemistry

Chemical Reactions
• The breaking and making of bonds between different
atoms to produce new substances is called
Chemical Reaction.
• There are two type of changes namely physical and
chemical changes.
o Melting of Ice (Physical Change)
o Rusting of Iron (Chemical Change)
o Formation of Curd (Chemical change)

Physical change Vs Chemical Change
• Any change that involves chemical changes is called
Chemical reaction.
Physical Change Chemical Change
Change is temporary Change is permanent
No new substance is formed.
There is change only in
physical properties.
New substances are formed
with entirely different physical
and chemical properties
Change can be reversed by
simple physical methods
Change cannot be reversed by
simple physical methods
Energy may or may not be
released or absorbed
Energy is released or absorbed
during chemical change

Chemical Equations
• The representation of chemical reactions in the form
of formulae of reactants and products separated by
an arrow mark is called Chemical Equation.
o Word Equation
Magnesium + Oxygen Magnesium Oxide
(Reactants) (Product)
LHS RHS
o Chemical Equation
Mg + O2 MgO
(Reactants) (Product)
LHS RHS

Balanced Equation
• The same number of atoms of each element on the
LHS (i.e. reactants) and RHS (i.e. products) of the
equation.
o Chemical Equation – Example 1
Mg + O2 MgO
LHS RHS
Mg  1
O  2
Mg  1
O  1
LHS RHS
Mg  1
O  2 x 1
Mg  1
O  1 x 2
Mg + O2 2MgO

Balancing of Equations
Mg + O2 2MgO
LHS RHS
Mg  1
O  2
Mg  2
O  2
LHS RHS
Mg  1 x 2
O  2
Mg  2 x 1
O  2
2 Mg + O2 2MgO

Balancing of Equations
o Chemical Equation – Example 2
Fe + H2O Fe3O4 + H2
LHS RHS
Fe  1
H  2
O  1
Fe  3
H  2
O  4
LHS RHS
Fe  1 x 3 = 3
H  2
O  1
Fe  3 x 1 = 3
H  2
O  4
3Fe + H2O Fe3O4 + H2

Balancing of Equations
3Fe + H2O Fe3O4 + H2
LHS RHS
Fe  3
H  8
O  4
Fe  3
H  2
O  4
LHS RHS
Fe  3
H  2
O  1 x 4 = 4
Fe  3
H  2
O  4 x 1 = 4
3Fe + 4H2O Fe3O4 + H2

Balancing of Equations
LHS RHS
Fe  3
H  8
O  4
Fe  3
H  8
O  4
Thus the equation is balanced!!!!
LHS RHS
Fe  3
H  8
O  4
Fe  3
H  2 x 4 = 8
O  4
3Fe + 4H2O Fe3O4 + 4H2

Informative Equation
• Informative Equation is the representation of
physical states using symbols like
Physical State Representation
Solid s
Liquid l
Gas g / ↑
Water soluble solution aq
Precipitate ↓
Reaction Conditions such as
Temperature, Pressure, Catalyst
Indicated above or
below the arrow
Heat Δ

Informative Equation
• Examples:
3Fe(s) + 4H2O (l) Fe3O4(s) + 4H2↑
CO (g) + 2H2 (g) 340 atm CH3OH (l)
6CO2 (aq) + 12H2O (l) Sunlight C6H12O6 (aq) + 6O2 (aq) + 6H2O (l)
Chlorophyll

Type of Reactions
• Combination
• Decomposition
• Displacement
• Double displacement
• Oxidation
• Reduction
• Redox Reaction
• Precipitation
• Exothermic reaction
• Endothermic reaction

Combination Reaction
• A reaction in which two or more reactants combine
to form a single product is called chemical
combination.
• Burning of coal:
C (s) + O2 (g) CO2 (g)
(Carbon) (Oxygen) (Carbon dioxide)
• Formation of Water from H2(g) and O2 (g)
2 H2 (g) + O2 (g) 2H2O (l)
(Hydrogen) (Oxygen) (Water)
• Formation of slaked lime:
CaO (s) + H2O(l) Ca(OH)2 (aq)
(Quick Lime) (Water) (Slaked Lime – Calcium Hydroxide)

Decomposition Reaction
• A reaction in which a single reactant breaks down to form
two or more products is known as decomposition
reaction.
• Three types of decomposition are thermal
decomposition, electrolytic decomposition and photolytic
decomposition.
• Thermal decomposition – Decomposition reaction taking place
in the presence of heat
2FeSO4 (s) Heat Fe2O3 + SO2 (g) + SO3 (g)
(Ferrous Sulphate) (Ferric Oxide) (Sulphur (Sulphur
dioxide) trioxide)
2CaCO3 (s) Heat CaO (s) + CO2 (g)
(Limestone) (Quick Lime) (Carbon dioxide)
2 Pb(NO3)2
Heat 2PbO + 4NO2 (g) + O2 (g)
(Lead Nitrate) (Lead Oxide0 (Nitrogen (Oxygen)
dioxide)

Decomposition Reaction
• Photolytic Decomposition – Decomposition reaction
taking place in the presence of light.
2AgCl (s) Sunlight 2Ag (s) + Cl2 (g)
(Silver Chloride) (Silver) (Chlorine)
2AgBr (s) Sunlight 2Ag (s) + Br2 (g)
(Silver Bromide) (Silver) (Bromine)
• Electrolytic Decomposition – Decomposition
reaction taking place in the presence of electricity.
2H2O (l) Electrolysis 2H2 (g) + O2 (g)
(Water) (Hydrogen) (Oxygen)

Displacement Reaction
• When a more reactive element displaces less reactive
element from its compound it is called displacement
reaction.
Fe(s) + CuSO4 (aq) FeSO4 (aq) + Cu(s)
(Iron) (Copper Sulphate) (Iron Sulphate) (Copper)
Zn(s) + CuSO4 (aq) ZnSO4 (aq) + Cu(s)
(Zinc) (Copper Sulphate) (Zinc Sulphate) (Copper)
Pb(s) + CuCl2 (aq) PbCl2 (aq) + Cu(s)
(Lead) (Copper Chloride) (Lead Chloride) (Copper)

Double Displacement Reaction
• The reaction in which two different ions or group of
atoms in the reactant molecules are displaced it is
called as double displacement reaction.
Na2SO4 (aq) + BaCl2 (aq) BaSO4 (s) + 2NaCl (aq)
(Sodium (Barium (Barium (Sodium
Sulphate) Chloride) Sulphate) Chloride)

Oxidation and Reduction
• The process of addition of oxygen to a substance or
removal of hydrogen from a substance is called
oxidation.
2Cu (s) + O2 (g) Heat 2CuO (s)
(Copper) (Oxygen) (Copper Oxide)
• The process of removal of oxygen from a substance
or addition of hydrogen to a substance is called
Reduction.
2CuO (s) + H2 (g) Heat Cu (s) + H2O (l)
(Copper Oxide) (Hydrogen) (Copper) (Water)

Redox Reaction
• The reactions in which oxidation and reduction takes
place simultaneously is called redox reaction.

Precipitation Reaction
• Chemical reaction which involves formation of water
insoluble precipitation is called precipitation
reaction.
Na2SO4 (aq) + BaCl2 (aq) BaSO4 (s) + 2NaCl (aq)
(Sodium (Barium (Barium (Sodium
Sulphate) Chloride) Sulphate) Chloride)

Exothermic Reaction
• Reactions that involve evolution of heat are called
exothermic reactions.
• Burning of Natural Gas
CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (g) + Energy
(Natural (Oxygen) (Carbon (Water)
gas) dioxide)
• Decomposition of vegetable into compost is also an
example of an exothermic reaction.

Exothermic Reaction
• Respiration is an exothermic process.
• The food that we intake is broken down into simpler
substances such as carbohydrates, protein, fats
during digestion.
• Carbohydrates thus absorbed are broken down to
glucose.
• Glucose combines with oxygen in the cells of our
body and provides energy. This reaction is called
as respiration
C6H12O6 (aq) + 6O2 (aq) CO2 (g) + 2H2O (g) + Energy
(Glucose) (Oxygen) (Carbon (Water)
dioxide)
•

Endothermic Reaction
• Reactions which occur by the absorption of heat are
called endothermic reaction.
2FeSO4 (s) Heat Fe2O3 + SO2 (g) + SO3 (g)
(Ferrous Sulphate) (Ferric Oxide) (Sulphur (Sulphur
dioxide) trioxide)
2CaCO3 (s) Heat CaO (s) + CO2 (g)
(Limestone) (Quick Lime) (Carbon dioxide)

Rancidity
• The process of slow oxidation of oil and fat present in the
food materials resulting in the change of smell and taste
in them is called rancidity.
• Rancidity of food can be prevented
o By keeping food materials in air tight container.
o Refrigeration of cooked food at low temperature
o Packing of food items like potato chips in packs containing
nitrogen gas instead of air. It prevents oils and fats from
reacting with air.
o Avoid keeping the cooked food and food materials in direct
contact with air.
o By adding anti-oxidants e.g. BHA (Butrylated Hydroxy Anisole).

Corrosion
• The phenomenon in which metals are slowly eaten
away by the reaction of air, water, chemicals present
in the atmosphere is called Corrosion.
o E.g. Black coating on Silver and Green coating on Copper
• Rusting of Iron is due to corrosion. It can be
prevented:
o By Applying paint on Iron articles, rusting (Corrosion) can
be prevented.
o Paint does not allow oxygen and moisture to come in
contact with the surface of Iron.

Activity – 1.1
• What happens when heat is applied on Magnesium
Ribbon by burning it on Spirit Lamp?
Magnesium burns with dazzling light and substance formed is
Magnesium oxide.
2Mg (s) + O2 (g) 2 MgO (s)
Inference:
o It changes state from hard metal to Powder.
o Endothermic reaction.
o It is a combination reaction.

Activity – 1.2
• What do you observe when Lead Nitrate reacts with
Potassium Iodide solution?
Lead Nitrate reacts with potassium iodide to form Lead iodide
which is insoluble in water and yellow in color.
Pb(NO3)2 (aq) + 2KI (aq) PbI2 (s) ↓ + KNO3(aq)
(Yellow
Precipitate)
Inference:
o It changes state from liquid to solid.
o Double Displacement Reaction.
o Precipitation reaction.

Activity – 1.3
• Which gas will evolve when Zinc metal reacts with dilute
Hydrochloric acid? How will you identify the gas.
o When Zinc reacts with dilute Hydrochloric acid, Zinc chloride is
formed and bubbles of hydrogen gas are observed.
o Conical flask becomes hot.
Zn (s) + 2HCl (aq) ZnCl2 (aq) + H2 (g) + Δ
(Dilute)
Inference:
o Evolution of Hydrogen gas (evolves with a pop sound upon a
burning candle)
o Exothermic reaction.
o Oxidation reaction as Hydrogen is removed.
o It is also a Displacement Reaction.

Activity – 1.4a
• What happens when quick lime is mixed with water?
o Calcium Oxide (quick lime) reacts vigorously with water to
produce slaked lime (calcium hydroxide) releasing large
amount of heat.
CaO (s) + H2O (l) Ca(OH)2 (aq) + Δ
(Quick Lime) (Water) (Slaked Lime – Calcium Hydroxide)
Inference:
o Combination reaction.
o Exothermic reaction.

Activity – 1.4b
• What happens when Calcium Hydroxide reacts with
Carbon dioxide?
o Calcium Hydroxide reacts with Carbon dioxide to form
Calcium Carbonate and Water.
Ca(OH)2 (s) + CO2 (g) CaCO3 (s) + H2O (g)
Inference:
o This is a common reaction that occurs during Whitewash.
Lime water (Calcium Hydroxide) reacts with Carbon dioxide
in atmosphere to form thick coat of Calcium Carbonate.

Activity – 1.5
• What happens when ferrous sulphate crystals are heated
in a dry boiling tube over a burner?
o The green colour of ferrous sulphate crystals changes to
brownish-black ferric oxide and smell of burning sulphur
dioxide is observed.
2FeSO4 (s) Heat Fe2O3 (s) + SO2 (g) + SO3 (g)
(Ferrous Sulphate) (Ferric Oxide) (Sulphur (Sulphur
Green Brownish-black dioxide) trioxide)
Inference:
o Smell is due to burning Sulphur dioxide.
o Thermal Decomposition reaction.
o Endothermic reaction.

Activity – 1.6
• What happens when Lead Nitrate is heated in a
boiling tube over a burner?
o Pungent smelling, brown fumes are evolved due to NO2 gas
and brownish residue of lead oxide (PbO) is left.
2Pb(NO3)2 (s) Heat PbO (s) + 4NO2 (g) + O2 (g)
(Lead Nitrate) (Lead Oxide) (Nitrogen (Oxygen)
Brownish residue
Inference:
o Pungent smell and Brown fumes are due to Nitrogen dioxide
o Thermal Decomposition reaction.
o Endothermic reaction.

Activity – 1.7
• Electrolysis of water.
o When water is electrolysed, Hydrogen and Oxygen are
emitted.
2H2O (l) Electrolysis 2H2 (g) + O2 (g)
(Water) (Hydrogen) (Oxygen)
Inference:
o Volume of Hydrogen gas is twice the volume of Oxygen.
o One of the gases catches fire and burns with pop sound
indicating that it is Hydrogen.
o In other gas, candle burns brightly indicating that it is
oxygen.
o Electrolytic Decomposition reaction.

Activity – 1.8
• Why do we store Silver chloride in dark coloured
bottles?
o Silver chloride turns grey in sunlight because silver metal is
formed due to photolytic reaction and chlorine gas is
produced.
2AgCl (s) Sunlight 2Ag (s) + Cl2 (g)
Inference:
o Photolytic Decomposition reaction.

Activity – 1.9
• Why does the colour of Copper Sulphate solution
change when an iron nail is dipped in it?
o Iron being more reactive than Copper, displaces it there by
forming a iron sulphate and copper metal.
Fe (s) + CuSO4 (S) FeSO4 (s) + Cu (s)
(Iron) (Copper Sulphate) (Iron Sulphate) Copper
(Blue) (Green)
Inference:
o Color changes from blue to Green due to formation of Iron
sulphate.
o Displacement Reaction.

Activity – 1.10
• Aqueous solution of Sodium Chloride, Sodium Sulphate and
Calcium Chloride are taken in three separate test tubes. Using
aqueous barium chloride how do you identify sodium sulphate?
o When Sodium sulphate reacts with barium chloride, white
precipitate of barium sulphate is formed.
Na2SO4 (aq) + BaCl2 (aq) BaSO4 (s) ↓ + 2NaCl (s)
Inference:
o Sodium Chloride & Calcium Chloride do not react with Barium
Chloride. Hence solution remains clear.
o Upon reaction with Barium Chloride, Sodium Sulphate
produces Barium Sulphate, a white precipitate.
o This is a Double Displacement Reaction.

Activity – 1.11a
• Why does Copper acquire a green coat when exposed to air for
a long time?
o Green coating is formed in copper due to corrosion. Green
coating is a mixture of copper carbonate and copper
hydroxide.
2Cu + H2O + CO2 + O2 Cu(OH)2 + CuCO3
Inference:
o Copper on exposure to air reacts with gases and moisture
in air to form Copper hydroxide and Copper carbonate

Activity – 1.11b
• Why does Copper Powder turn black when heated ?
o The surface of copper powder on heating becomes black as
it reacts with Oxygen to produce Copper oxide.
2 Cu + O2 Heat 2 CuO
Inference:
o Copper Oxide formed due to combination of Copper and
Oxygen is black in color.
o Combination reaction
o Endothermic reaction
o Oxidation reaction.