Page 106 - ICSE Chemistry 8

A chemical equa on consists of reactants present on the le -hand side (LHS) and products present on the
right-hand side (RHS). A plus sign ‘+’ is used between the reactants. In case of products, a plus sign is used
if more than one product is formed. An arrow is placed between the reactants and the products, which
points towards the products.
A word equa on represents the chemical reac on using only the names of the reactants involved and
products formed.

For example, sodium reacts with chlorine to form sodium chloride. This reac on can be represented in a
word equa on as shown below.

Sodium + Chlorine  Sodium chloride

This equa on can be wri en again using symbols and formulae to represent a chemical equa on.
Na + Cl  NaCl

Steps to Write a Chemical Equation

• The reactants are always wri en on the le -hand side in a chemical equa on with a plus sign (+)
between them.

• The products are always wri en on the right-hand side in a chemical equa on with a plus sign (+)
between them.
• An arrow ( ) is placed between the reactants and the products to indicate the direc on in
which the reac on is taking place.
• In certain cases, chemical reac ons have reversible arrow ( ) between the reactants and the

products. A reversible arrow shows that the reac on can proceed in either direc on.
• In a chemical equa on, the physical state of reactants and products are indicated as follows.

(i) Solid state is indicated by the symbol (s).
(ii) Liquid state is indicated by the symbol (l). Endothermic Reac ons
(iii) Aqueous solu on, i.e., the solu on made in water is The reac ons in which heat is
absorbed are called endothermic
indicated by the symbol (aq).
reac ons.
(iv) Gaseous state is indicated by the symbol (g).
Exothermic Reac ons
• Any gas evolved during a reac on is represented by an arrow
The reac ons in which heat is
poin ng upwards () proceeding the chemical formula evolved are called exothermic
and any precipitate formed is shown by an arrow poin ng reac ons.
downwards () proceeding its chemical formula.
Catalyst
• An exothermic reac on is indicated by wri ng ‘+ Heat’ in A substance that changes the rate
the product side, i.e., the right-hand side of the chemical of a chemical reac on without
equa on. An endothermic reac on is indicated by wri ng undergoing any chemical change
‘+ Heat’ in the reactant side, i.e., the le -hand side of the in itself is called a catalyst.
chemical equa on.
• Some mes, the pressure, the temperature and the catalyst used in the reac on are also indicated
in the chemical equa on by wri ng them above or below the arrow sign.

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