Page 12 - NCERT Science Class 10 English Medium

Do You Know? A solution of slaked lime produced by the reaction 1.13 is used for whitewashing

walls. Calcium hydroxide reacts slowly with the carbon dioxide in air to form a thin
layer of calcium carbonate on the walls. Calcium carbonate is formed after two to
three days of whitewashing and gives a shiny finish to the walls. It is interesting to note
that the chemical formula for marble is also CaCO .
3
+ H O(l)
Ca(OH) (aq) + CO (g)
(1.14)
CaCO (s)
→
2
3
2
2
(Calcium
(Calcium
carbonate)
hydroxide)
Let us discuss some more examples of combination reactions.
(i) Burning of coal
C(s) + O (g) → CO (g) (1.15)
2 2
(ii) Formation of water from H (g) and O (g)
2 2
2H (g) + O (g) → 2H O(l) (1.16)
2 2 2
In simple language we can say that when two or more substances
(elements or compounds) combine to form a single product, the reactions
are called combination reactions.
In Activity 1.4, we also observed that a large amount of heat is evolved.
This makes the reaction mixture warm. Reactions in which heat is
released along with the formation of products are called exothermic
chemical reactions.
Other examples of exothermic reactions are –
(i) Burning of natural gas
CH (g) + 2O (g) → CO (g) + 2H O (g) (1.17)
4 2 2 2
(ii) Do you know that respiration is an exothermic process?

We all know that we need energy to stay alive. We get this energy
from the food we eat. During digestion, food is broken down into simpler
substances. For example, rice, potatoes and bread contain
carbohydrates. These carbohydrates are broken down to form glucose.
This glucose combines with oxygen in the cells of our body and provides
energy. The special name of this reaction is respiration, the process of
which you will study in Chapter 6.

C H O (aq) + 6O (aq) → 6CO (aq) + 6H O(l) + energy (1.18)
6 12 6 2 2 2
(Glucose)

(iii) The decomposition of vegetable matter into compost is also an
example of an exothermic reaction.
Identify the type of the reaction taking place in Activity 1.1, where
heat is given out along with the formation of a single product.

Chemical Reactions and Equations 7

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