Oh, Hi did you observe both the
 
reactions? In this reaction like calcium
 
oxide reacts with water to form calcium
 
hydroxide, heat is evolved. And how do
 
we come to know that? Because the beaker
 
is warm. While in this reaction calcium
 
carbonate to decompose absorbs
 
heat. So there will be some reactions in
 
which heat is evolved and in some heat
 
will be absorbed. So the chemical
 
reactions in which heat is evolved is
 
called exothermic reactions and the
 
chemical reaction in which heat is
 
absorbed are called endothermic
 
reaction. EXO means giving up and endo means
 
absorbing . Ok! Now let's concentrate on
 
exothermic reactions. Reactions in which
 
heat is evolved.
 
Let's consider the same example, calcium
 
oxide reacts with water to form calcium
 
hydroxide. Now in this reaction heat is
 
evolved. So we write plus heat after the
 
product. So we write plus heat here
 
after calcuim hydroxide. Let's consider
 
some more examples of exothermic
 
reactions. Natural gas. Natural gas is
 
nothing but methane gas. So burning of
 
natural gas is an exothermic reaction.
 
Methane gas is  written as CH4. We use this
 
natural gas in our cars or in our
 
vehicles as fuel that is a CNG fuel.
 
Compressed Natural gas. When the CNG fuel
 
burns, it produces carbon dioxide and water.
 
So how do we write down  this reaction?
 
We write down the reaction as CH4 plus O2
 
because it is in the presence of
 
oxygen
 
Ok! And it gives carbon dioxide, water and
 
a lot of heat. So we write plus heat after
 
the products. Because of this heat
 
energy our car works. Okay, there's one
 
more example of exothermic reaction and
 
that is respiration. Why is respiration
 
consider to be exothermic reaction?
 
Let us
 
see. Ok, we know that the food we eat like
 
potatoes, rice, they contain carbohydrates.
 
These carbohydrates on digestion convert
 
to glucose. Now this glucose
 
reacts with the oxygen we respire in our
 
cells and forms carbon dioxide and water
 
and along with this, a lot of energy is
 
given out in the form of heat due to
 
which we say that respiration is an
 
exothermic reaction and because of this
 
heat energy we work daily. There is one more
 
example of exothermic reaction and that
 
is decomposition of vegetable matter
 
into compost. In this reaction to a lot
 
of heat energy is evolved. Ok, now let
 
us come to endothermic reactions.
 
Ok! Endothermic reaction means the
 
reactions in which heat is absorbed, Do you
 
remember decomposition reactions? The
 
reaction in which you have to supply some
 
or the other form of energy for
 
substance to decompose. The energy can be
 
heat, light or electricity. That is the
 
reason why we call all the decomposition
 
reactions as endothermic reaction,
 
because the substance to undergo
 
reaction has to absorb energy either in
 
the form of heat, light or electricity.
 
Ok! Let's cosider the same example that
 
we've just seen. Calcium carbonate to
 
decompose requires heat energy. So we
 
heat it. And how do we write this reaction?
 
We write Calcium carbonate CaCO3
 
plus heat or instead of writing plus
 
here we can also write heat on the arrow.
 
So after providing heat to calcium
 
carbonate, it it forms calcium oxide and
 
carbon dioxide. So this is an example of
 
endothermic reaction.
