Many changes take place in our surroundings
Cooking of food, burning of fuel are the
reactions in which substances are totally
transformed. Such transformations are called
The event in which substance gets transformed
permanently (Chemical changes) is called
The things we use daily are formed because of
e.g : The painted walls , things made from plastics
To represent chemical reaction they are written
in the form of equation
e.g When Calcium Oxide reacts with water, it produces a
single compound, Calcium hydroxide and energy is
• This reaction is represented in equation form as under.
• CaO + H2O Ca(OH)2 + energy
Calcium Oxide Water Calcium Hydroxide
CaO + H2O Ca(OH)2
• In a chemical reaction, reactants are written
on left hand side of the arrow whereas
products are written on right hand side of
• The substances taking part in a chemical
reaction are called reactants and the
substances formed during the reaction are
Types of Chemical reactions
• Combination reactions
• Reactants two or more than two
• Product only one
• e.g : C + O2 CO2
• Reactant only one,
• Products more than one
• e.g : 2H2 0 2H2 + O2
• Two or more reactants and products
• One constituent from the reactant displaces
another constituent and separates it to form
• e.g : Zn + 2HCl ZnCl2 + H2
Oxidation and reduction
• Oxidation- Oxygen combines with an element
or a compound.
e.g : C + O2 CO2
• Reduction - A reaction in which hydrogen
combines with an element or compound or a
reaction in which oxygen is lost from a
Cl2 + H2 2HCl
The speed of chemical reactions
Some chemical reactions are fast whereas some
reactions takes place slowly.
e.g : Ripening of fruits, rusting of iron(Slow), Burning of
Paper, Lightening of fire crackers(Fast)
Various factors such as the size of the reactant
particles, their temperature and concentration and
the presence of catalysts affect the speed of
Catalyst doesn’t take part in chemical reaction,
it’s mere presence speeds up a chemical
e.g: Manganese dioxide, Raney nickel
In some chemical reactions, heat is given out.
e.g : Burning of Magnesium in air.
In some chemical reactions heat is absorbed.
e.g: dissolving of urea in water
Following changes are observed during a
-Change in the state of the
- Change in temperature
- Change in colour
- Generation of gas
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