Chemical Reactions and Equations CBSE Class X Important Notes

1)Chemical reaction— Chemical changes or chemical reactions are the changes in
which one or more new substances are formed.
2)Chemical Equations – Representation of a chemical reaction in terms of symbols and
formulae of the reactants and products is known as chemical equation.
3)Balanced Chemical equation
s – The chemical equation in which the no. of atoms of
different
elements is same on both sides of the arrow is called balanced chemical equation.
4) The chemical reactions can be classified into different types such as—
a) Combination reaction – The reactions in which two or more substances combine to
form a new substance are called combination reaction. For example,
2Mg(s) + O2 (g)  2 MgO (s)
b) Decomposition reaction - The reaction in which a single compound breaks up into
two or more simpler substances are called decomposition reactions. For example,
2Pb (NO3)2 (s)  2PbO (s) + 4NO2 (g) +O2 (g)
The decomposition of a substance by passing electric current through it is known as
electrolysis (Electrical Decomposition).
The decomposition of a substance on heating is known as thermal decomposition.
The decomposition of a substance by absorbing light energy is called photochemical
decomposition.
c) Displacement reactions -The chemical reactions in which a more reactive element
displaces a less reactive element from a compound are known as displacement
reactions. For example,
i) Zn (s) + CuSO4 (aq)  ZnSO4 (aq) + Cu (s).
ii) Cu (s) + 2AgNO3 (aq)  Cu (NO3)2 (aq) +2Ag (s).
d) Double Displacement Reactions - The chemical reactions in which compounds react
to form two different compounds by mutual exchange of ions are called double
displacement reactions.
These reactions take place in solution two common types of this reaction are
precipitation reactions and neutralization reactions
i) Precipitation reaction : In this reactions, aqueous solution of two salts are mixed
whereby
Some salts precipitate due to mutual exchange of ions between the two salts. For example

AgNO3 (aq) + NaCI (aq) -----> AgCl(s) + NaNO3.
ii) Neutralization reaction: In this type of reaction an acid reacts with a base to form
salt and water by exchange of ions.
NaOH (aq) + HCl (aq)  NaCl (aq) + H2O.
e) Redox reaction: Chemical reaction which shows both oxidation and reduction
reaction.
Oxidation: Reaction that involves the gain of oxygen or loss of hydrogen.
Reduction: Reaction that shows the loss of oxygen or gain of hydrogen.
Both oxidation and reduction take place simultaneously and hence called redox reaction.
ZnO + C  Zn +CO
ZnO reduce to Zn ---- reduction
C oxidize to CO ------oxidation
f)Exothermic reaction and endothermic reaction: On the basis of energy changes during
chemical
reaction, they can be classified as
i) Exothermic reaction: A chemical reaction in which heat energy is produced.
C + O2
 CO2 (g) + heat
ii) Endothermic reaction: A chemical reaction in which heat energy is absorbed.
CaCO3 + Heat  CaO + CO2
5 Corrosion – The process of slow conversion of metals into their undesirable compounds due
to their reaction with oxygen, water, acids, gases etc. present in the atmosphere is called
corrosion.
Rusting – Iron when reacts with oxygen and moisture forms red substance called rust. Chemical
composition of rust is Fe2O3.xH2O. Its chemical name is Hydrated Iron (III) oxide.
6 Rancidity – The taste and odour of food materials containing fat and oil changes when they
are left exposed to air for long time. This is called rancidity. It is caused due to oxidation of fat
and oil present in food material.
It can be prevented by using various methods such as by adding antioxidants to the food
materials. Storing food in air tight container and by flushing out air with nitrogen.