How to write chemical equations

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What is a chemical reaction and equation?

Chemical reaction:

When there's a change in the chemical form of a substance we say a chemical reaction has taken place. Changes can be in state, temperature, colour or there could be evolution of gas.

Chemical equation:

The symbolic representation of a reaction is called a chemical equation. The best and consise way to write chemical equations is using chemical formulae.

Rules to write chemical formulae

(i) the valencies or charges on the ion must balance.

(ii) when a compound consists of a metal and a non-metal, the name or symbol of the metal is written first. For example: calcium oxide (CaO), sodium chloride (NaCl), etc., where oxygen and chlorine are non-metals and are written on the right, whereas calcium and sodium are metals, and are written on the left.

(iii) in compounds formed with polyatomic ions, the ion is enclosed in a bracket before writing the number to indicate the ratio.

How to write chemical formulae

Step 1: Write the symbols and the valencies of the combining atoms

Step 2: Criss-cross the valencies and write as subscripts. The so obtain is the chemical formula.

Definitions

(i) Reactants:

Substances undergoing a chemical change are called reactants.

(ii) Products:

New substances formed during a reaction are the products 

How to chemical reaction  

Step 1: Write the symbols and valencies of the elements involved in the reaction.

Step 2: Obtain the chemical formulae. Simplify the formulae if needed.

Step 3: Write the whole things as equation with plus signs b/w the reactants on the left side and for the products on the right side. and you obtained skeletal chemical equation.

Questions

(i)  Which of the following chemical processes involves chemical reaction?

(a) Keeping petrol in a china dish in open.

(b) Liquefaction of air.

(c) Storing oxygen gas under high pressure in a gas cylinder.

(d) Heating copper wire in the presence of air at high temperature.

(ii) Which of the following is not a physical change?

(a) Boiling of water to give water vapour.

(b) Melting of ice to give water.

(c) Dissolution of salt in water.

(d) Combustion of Liquefied Petroleum Gas (LPG). 

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