• Chemical Reaction : –Whenever a chemical change occurs we can say that a chemical

reaction has taken place. eg – Food gets digested in our body, Rusting of iron.
Chemical Equation :–A chemical reaction can be expressed symbolically by using
chemical equation.
• Mg + O2 MgO
During a physical change, only the state of the substance changes, but not its chemical
composition.
a physical change is a temporary change, which can be reversed by changing the physical
experimental conditions.
At the same time no net energy is absorbed or given out.
During a chemical change, the state as well as the chemical composition of a substance
changes. Furthermore,
a chemical change is a permanent change, which cannot be reversed by changing the
physical experimental conditions. At the same time, either energy is absorbed or given out.
A chemical change can be confirmed by any or all of the following observations:
• change in state
• Formation of precipitate
• change in color
• change in temperature
• evolution of gas
1. change in state
Eg Melting of wax during burning of thread of candle.
2. Formation of precipitate (a solid product which separates out from solution during a
chemical reaction)
H2SO4 + BaCl2 BaSO4 (white ppt)
Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) + 2KNO3(aq)
(Yellow ppt)

3.change in color
Pot. Dichromate(Orange) + SO2 green colour
KMnO4 + Citric acid colour less
(Purple colour)
4.change in temperature:
Endothermic: decomposition of calcium carbonate, photosynthesis, electrolysis of water
KClO3 KCl + O2
Exothermic: Respiration, combustion, burning of magnesium ribbon. Dilution of acids &
bases.
5.evolution of gas
H2SO4 + Zn ZnSO4 + H2
In a chemical reaction, the total mass of the reactants should be equal to the total mass
of the products. This means that the total number of atoms of each element should be
equal on both sides of a chemical equation. Such an equation is called a balanced
chemical equation,
we need to balance a chemical equation to follow the law of conservation of mass.
Steps to balance a chemical equation are:
Step I: To balance a chemical equation, first draw boxes around each formula. Do not change
anything inside the boxes while balancing the equation.
Step II: List the number of atoms of different elements present in the unbalanced equation
Step III: It is often convenient to start balancing with the compound that contains the
maximum number of atoms. It may be a reactant or a product. In that compound, select the
element which has the maximum number of atoms.
Step IV: Finally, to check the correctness of the balanced equation, we count atoms of each
element on both sides of the equation.
Chemical reactions are mainly of five types.
1. Combination reactions
2. Decomposition reactions
3. Displacement reactions
4. Double displacement reactions
5. Oxidation and reduction reactions
1.COMBINATION REACTION: A reaction in which a single product is formed from two
or more reactants is known as a combination reaction.
CaO + H2O Ca (OH)2
2. Decomposition reaction – the reaction in which compound split into two or more simpler
substances.
It require energy in the form of heat, light, or electricity to decompose the compound.
These reactions can be classified into three types, depending on the source of energy for the
reaction.
a) Decomposition by heat or thermal decomposition
FeSO4 Fe2O3 + SO2 + SO3
(Ferrous sulphate) Ferric oxide
Green colour (brown colour)
Pb(NO3)2 PbO + NO2 + O2
Colourless (yellow) (Brown)
KClO3 KCl + O2
b) Decomposition by electricity or electrolysis
c) Decomposition by light or photolysis- You will see that white silver chloride turns grey in
sunlight. This is due to the decomposition of silver chloride into silver and chlorine by light.
2 AgCl Ag + Cl2
White grey/black
Silver bromide also behaves in the same way.
The above reactions are used in black and white photography.
DISPLACEMENT REACTIONS: a more reactive metal replaces a less reactive metal from
the latter’s salt.
Displacement reactions are of two types:
1.Single Displacement Reactions
CuSO4 + Zn ZnSO4 + Cu
CuSO4 + Mg MgSO4 + Cu
CuCl2 + Pb PbCl2 + Cu
(green) (colourless) (brown)
2.Double Displacement Reactions A reactions in which there is an exchange of ions
between the reactants are called double displacement reactions.
For example-Formation of barium sulphate and sodium chloride from sodium sulphate
and barium chloride.
QUESTIONS-ANS
1. Translate the following statements into chemical equations and then balance them.
(a) Hydrogen gas combines with nitrogen to form ammonia.
(b) Hydrogen sulphide gas burns in air to give water and sulpur dioxide.
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a
precipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
(a) 3H2 (g) + N2 (g) → 2NH3 (g)
(b) H2S (g) + 3O2 (g) → SO2 (g) + 2H2O(l)
(c) 3BaCl2 (aq) + Al2(SO4)3 (aq) → 2AlCl3 (aq) + 3BaSO4 ↓(s)
(d) 2K (s) + 2H2O (l) → 2KOH (aq) + H2 (g)
6. Balance the following chemical equations :
(a) HNO3 + Ca (OH)2 → Ca (NO3)2 + H2O
(b) NaOH + H2SO4 → Na2SO4 + H2O
(c) NaCl + AgNO3 → AgCl + NaNO3
(d) BaCl2 + H2SO4 → BaSO4 + HCl
Answer:
(a) 2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O
(b) 2NaOH + H2SO4 → Na2SO4 + 2H2O
(c) NaCl + AgNO3 → AgCl + NaNO3
(d) BaCl2 + H2SO4 → BaSO4 + 2HCl

Question 7
Write the balanced chemical equations for the following reactions :
(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
(b) Zinc + Silver nitrate → Zinc nitrate + Silver
(c) Aluminium + Copper chloride → Aluminium chloride + Copper
(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride
Answer:
(a) Ca (OH)2 + CO2 → CaCO3 + H2O
(b) Zn + 2AgNO3 → Zn(NO3)2 + 2 Ag
(c) 2Al + 3 CuCl2 → 2AlCl3 + 3 Cu
(d) BaCl2 + K2SO4 → BaSO4 + 2KCl

Question 8
Write the balanced chemical equation for the following and identify the type of reaction in
each case :
(a) Potassium bromide (aq) + Barium iodide (aq) → Potassium iodide (aq) + Barium
(b) Zinc carbonate(s) → Zinc oxide (s) + Carbon dioxide (g) bromide(s)
(c) Hydrogen (g) + Chloride (g) → Hydrogen chloride (g)
(d) Magnesium (s) + Hydrochloric acid (aq) → Magnesium chloride (aq) + Hydrogen (g)
Answer:
(a) 2KBr (aq) + Bal2(aq) → 2Kl(aq) + BaBr2(s)
Type : Double displacement reaction
(b) ZnCO3 (s) → ZnO (s) + CO2 (g)
Type : Decomposition reaction
(c) H2 (g) + Cl2 (g) → 2HCl(g)
Type : Combination reaction
(d) Mg (s) + 2HCl (aq) → MgCl2 (aq) + H2 (g)
Type : Displacement reaction
Question 9
What does one mean by exothermic and endothermic reactions? Give examples.
Answer:
Exothermic reactions: Those reactions in which heat is evolved are known as exothermic
reactions. An exothermic reaction is indicated by writing “+ Heat”on the products side of an
equation.
Example :
(i) C (s) + O2 (g) → CO2 (g) + Heat
(ii) N2 (g) + 3H2 (g) → 2NH3 (g) + Heat
Endothermic reactions : Those reactions in which heat is absorbed are known as
endothermic reactions. An endothermic reaction is usually indicated by writing “Heat” on the
product side of a chemical equation.
Examples :
(i) C (s) + 2S (s) → CS2 (l) – Heat
(ii) N2 (g) + O2 (g) → 2NO(g) – Heat
Question 10
Why is respiration considered an exothermic reaction? Explain.
Answer:
Respiration is an exothermic process because during respiration glucose combines with
oxygen in the cells of our body to form carbon dioxide and water along with the production
of energy.

Question 11
Why are decomposition reactions called the opposite of combination reactions? Write
equations for these reactions.
Answer:
In a decomposition reaction, a single compound breaks down to produce two or more simpler
substances.
For example:

While, in a combination reaction, two or more substances simply combine to form a new
substance.
For example:

Question 12
Write one equation each for the decomposition reactions where energy is supplied in the
form of heat, light or electricity.
OR
Decomposition reactions require energy either in the form of heat or light or electricity
for breaking down the reactants. Write one equation each for decomposition reactions
where energy is supplied in the form of heat, light and electricity. [CBSE 2015 (Delhi)]
Answer:

Question 13
What is the difference between displacement and double displacement reactions? Write
equations for these reactions.
Answer:
In displacement reactions, a more reactive metal displaces a less reactive metal from its
solution. For example,
Fe(s) + CuSO4(aq) → Cu(s) + FeSO4(aq)
In double displacement reactions, two reactants in solution exchange their ions. For example,
AgNO3(aq) + NaCl (aq) → AgCl(s) + NaNO3 (aq)

Question 14
In the refining of silver, the recovery of silver from silver nitrate solution involved
displacement by copper metal. Write down the reaction involved.
Answer:

Question 15
What do you mean by a precipitation reaction ? Explain by giving examples.
Answer:
A reaction in which an insoluble solid called precipitate is formed that separates from the
solution is called a precipitation reaction.
Example : When a solution of iron (III) chloride and ammonium hydroxide are mixed, a
brown precipitate of iron (III) hydroxide is formed.