Chemical equation is a symbolic representation of actual chemical change in terms of symbols and formulae of the reactants and products. The substances which react among themselves are called reactants. These are written on left hand side of the equation. The substances formed during the chemical change are called products. These are written on the right hand side of the equation. For example, reaction between zinc and hydrochloric acid to form zinc chloride and hydrogen can be represented by a chemical equation.     

            .

Essentials of a chemical equation. A chemical equation must satisfy the following conditions:

(i) It should represent a true chemical change i.e. the change represented by the chemical equation should be practically possible.
For example, the equation



is not a true chemical equation since copper does not react with sulphuric acid to produce copper sulphate and hydrogen.

(ii) It should be arithmetically balanced. 
Atoms of each element on both the sides of the arrow  must be equal. For example,

   

is an arithmetically balanced equation.

(iii) It should be molecular. If any elementary gas takes part in a chemical equation it must be expressed in the molecular form e.g.

The drawbacks of a chemical equation can be partially removed or we can make a chemical equation more informative as follows:

1. By specifying the physical states of the reactants and products in a chemical equation by putting the letter (s) for solid, (l) for liquid and (g) for gaseous state after the formula of the substance. The word (aq), which means aqueous solution, is used when the substance is in the form of solution, For example, reaction between sodium and water to form sodium hydroxide solution and hydrogen gas may be written as:



2. The concentrations of the reactants used in the form of solution can be shown by the word dil. for dilute and conc. for concentrated e.g.



3. By using a down arrow  for the compound precipitated and an upward arrow  for a gas evolved. For example,

(i) Evolution of hydrogen gas.
 
(ii) Precipitation of AgCl.
  

4. By mentioning the condition of reaction such as heat (), pressure, catalyst on the arrow head.
 

5. In case of exothermic reactions, the sign "+ heat" is written along with the products.
    
In case of enodthermic reactions, the sign "-heat" is written along with products. 

A chemical equation gives a lot of information viz.,
1. It tells the names of the reactants and products.

2. It tells the relative number of molecules of reactants and products.

3. It tells the relative number of moles of reactants and the products.

4. It tells the relative weights of reactants and the products.

5. It tells the relative volumes of reacting gases and those of products formed in case of gaseous reactants. For example, the chemical equation.

conveys the following information:
1. Zinc reacts with hydrochloric acid to produce zinc chloride and hydrogen.

2. One atom of zinc reacts with two molecules of hydrogen chloride to produce one molecule of zinc chloride and one molecule of hydrogen.

3. One mole of zinc reacts with two moles of hydrogen chloride to produce one mole of zinc chloride and one mole of hydrogen.

4. 65 gram of zinc reacts with 73 gram of hydrochloric acid to give 136 gram of zinc chloride and 2 gram of hydrogen.

5. 22·4 litres of hydrogen is produced at S.T.P. when one mole of zinc reacts with 2 moles of hydrogen chloride.

A chemical equation does not give any information regarding:
1. The physical states of the reactants and the products, whether these are solids, liquids, gases or in the form of solution.

2. In case of solution, their concentration i.e. whether concentrated or dilute.

3. The time taken for the completion of the reaction.

4. Whether any heat is absorbed or given out during the chemical change.

5. The speed of a chemical reaction, whether it is slow or fast.

6. The conditions of temperature, pressure or catalyst etc. at which the reaction takes place.