Page 19 - NCERT Science Class 10 English Medium

What you have learnt

n A complete chemical equation represents the reactants, products and their physical
states symbolically.

n A chemical equation is balanced so that the numbers of atoms of each type involved
in a chemical reaction are the same on the reactant and product sides of the
equation. Equations must always be balanced.

n In a combination reaction two or more substances combine to form a new single
substance.
n Decomposition reactions are opposite to combination reactions. In a decomposition
reaction, a single substance decomposes to give two or more substances.
n Reactions in which heat is given out along with the products are called exothermic
reactions.
n Reactions in which energy is absorbed are known as endothermic reactions.
n When an element displaces another element from its compound, a displacement
reaction occurs.

n Two different atoms or groups of atoms (ions) are exchanged in double displacement
reactions.
n Precipitation reactions produce insoluble salts.
n Reactions also involve the gain or loss of oxygen or hydrogen by substances.
Oxidation is the gain of oxygen or loss of hydrogen. Reduction is the loss of oxygen
or gain of hydrogen.

E X E R C I S E S

1. Which of the statements about the reaction below are incorrect?
2PbO(s) + C(s) → 2Pb(s) + CO (g)
2
(a) Lead is getting reduced.
(b) Carbon dioxide is getting oxidised.
(c) Carbon is getting oxidised.
(d) Lead oxide is getting reduced.
(i) (a) and (b)
(ii) (a) and (c)
(iii) (a), (b) and (c)
(iv) all
2. Fe O + 2Al → Al O + 2Fe
2 3 2 3
The above reaction is an example of a
(a) combination reaction.
(b) double displacement reaction.

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2024-25