Chemical Equation


When the chemical reaction is represented using symbols of substances, it is called chemical equation.

Example: A + B ⇨ C + D

In this equation, A and B are called reactants and C and D are called the products. An arrow shows the direction of the chemical reaction. A condition, if any, is written generally above the arrow.

Reactant: Substances which take part in a chemical reaction are called reactants.

Product: New substance formed after a chemical reaction is called product.

When hydrogen reacts with oxygen, it gives water. This reaction can be represented by following chemical equation.

Hydrogen + Oxygen ⇨ Water

H2 + O2 ⇨ H2O

In the first equation, words are used and in second symbols of substances are used to write the chemical equation. For convenience, a symbol of substance is used to represent chemical equations.

Chemical Equation is a way to represent the chemical reaction in a concise and informative way.

Chemical equation can be divided into two types – Balanced Chemical Equation and Unbalanced Chemical Equation.

Balanced Chemical Equation: A balanced chemical equation has number atoms of each element equal on both sides.

Example: Zn + H2SO4 ⇨ ZnSO4 + H2

In this equation, numbers of zinc, hydrogen and sulphate are equal on both sides, so it is a balanced chemical equation.

According to Law of Conservation of Mass, mass can neither be created nor destroyed in a chemical reaction. To obey this law, the total mass of elements present in reactants must be equal to the total mass of elements present in products.

Unbalanced Chemical Equation: When the number of atoms of each element in reactants is not equal to the number of atoms of each element present in the product, the chemical equation is called unbalanced chemical equation.

Example: Fe + H2O ⇨ Fe3O4 + H2

In the above equation, the number of atoms of elements are not equal on two sides of the reaction. For example, on the left hand side only one iron atom is present, while three iron atoms are present on the right hand side. Therefore, it is an unbalanced chemical equation.

Writing the symbols of the Physical States of substances in Chemical equation

By writing the physical states of substances a chemical equation becomes more informative.

Gaseous state is represented by symbol ‘g’

Liquid state is represented by symbol ‘l’

Solid state is written by symbol ‘s’

Aqueous solution is written by symbol ‘aq’

Writing the condition in which reaction takes place: The condition is generally written above and/or below the arrow of a chemical equation.

By writing the symbols of physical state of substances and condition under which reaction takes place, a chemical equation can be made more informative.

Chemical Reaction

The transformation of one chemical substance into another chemical substance is known as Chemical Reaction. For example: rusting of iron, setting of milk into curd, digestion of food, respiration, etc.

In chemical reaction new substance is formed which is completely different in properties from the original substance, so in a chemical reaction chemical change takes place.

Signs of Chemical Reaction

Change of state of the substance

Change of colour of the substance

Evolution of heat

Absorption of heat

Evolution of gas

Evolution of light

Evolution of sound, etc.

Types of Reaction

Double Displacement Reaction

Reactions in which ions are exchanged between two reactants forming new compounds are called double displacement reactions.

AB + CD ⇨ AC + BD

Precipitation reaction

Double displacement reaction, in which precipitate is formed, is also known as precipitation reaction.

A double replacement reaction is specifically classified as a precipitation reaction when the chemical equation in question occurs in aqueous solution and one of the of the products formed is insoluble. 

Neutralization Reaction

Neutralization reactions are also examples of double displacement reaction.

Neutralization is a type of chemical reaction in which a strong acid and strong base react with each other to form water and salt. Have you ever been unlucky enough to be stung by a wasp or a bee? Bee stings are acidic in nature, which is why a household remedy for a bee sting is baking soda or sodium bicarbonate, which is a basic substance. A wasp sting, on the other hand, is mildly basic, so a household remedy for this will be vinegar, also known as acetic acid. These simple treatments ease these painful stings by a process called neutralization.

Have you seen people popping antacid tablets when they get a Heartburn or acidity, both complaints can be relieved by taking an antacid. The antacid is alkaline/basic and helps neutralize the stomach's acidic environment.

Combination reaction

A combination reaction is a general category of chemical reactions. It may be defined as a chemical reaction in which two or more substances combine to form a single substance under suitable conditions.

Combination reactions are also known as synthesis because in these reactions new substances are synthesized.

The general equation that describes a combination reaction is:

Decomposition Reaction

Decomposition is a type of chemical reaction.  It is defined as the reaction in which a single compound splits into two or more simple substances under suitable conditions. It is just the opposite of the combination reaction.

In a combination reaction, a substance is formed as a result of the chemical combination, while in a decomposition reaction, the substance breaks into new substances.

The digestion of food in our body is accompanied by a number of decomposition reactions. The major constituents of our food such as carbohydrates, fats, proteins, etc., decompose to form a number of simpler substances.  These substances further react, releasing large amounts of energy, which keeps our body working.

Precipitation Reaction

Exothermic and Endothermic Reaction

Reactions which produce energy are called exothermic reaction. On the other hand, reactions which absorb energy are called endothermic reaction. Most of the combination reactions are exothermic. The decomposition reactions are endothermic.

Respiration is a combination reaction in which energy is released. When quicklime (calcium oxide) is added to water, it combines to form calcium hydroxide and releases energy. Cooking involves chemical reactions which are endothermic as cooking is possible because of heating.

Oxidation and Reduction Reaction

Oxidation: Addition of oxygen or non-metallic element or removal of hydrogen or metallic element from a compound is known as oxidation.

Elements or compounds in which oxygen or non-metallic element is added or hydrogen or metallic element is removed are called to be oxidized.

Oxidizing agent: Compounds which can add oxygen or a non-metallic compound or remove hydrogen or metallic element are known as oxidizing agents.

Reduction: Addition of hydrogen or metallic element or removal of oxygen or non-metallic element from a compound is called reduction. The compound or element which goes under reduction is called to be reduced.

Reducing agent: Compounds or elements which can cause reduction are called reducing agents.

In a chemical reaction oxidation and reduction both take place simultaneously and such reactions are also known as REDOX REACTIONS. In the word REDOX, ‘Red’ stands for reduction and ‘Ox’ stands for oxidation.

Example: When iron reacts with air, it forms iron oxide (rust)

4Fe + 3O2 ⇨ 2Fe2O3

In this reaction, oxygen is added to iron, thus, iron is oxidized. Here oxygen is an oxidizing agent.

When cupric oxide reacts with hydrogen, it gives copper and water.

CuO + H2 ⇨ Cu + H2O

In this reaction, oxygen is removed from copper and oxygen is added to hydrogen. So, cupric oxide is reduced to copper and hydrogen is oxidized to water. Cupric oxide is oxidizing agent and hydrogen is reducing agent.

When sodium hydroxide reacts with hydrochloric acid, it gives sodium chloride and water.

NaOH + HCl ⇨ NaCl + H2O

In this reaction, sodium hydroxide is reduced to sodium chloride since hydrogen is removed from sodium hydroxide. Hydrochloric acid is oxidized to water since oxygen is added to hydrogen chloride and non-metallic element chloride is removed. Sodium hydroxide is an oxidising agent and hydrochloric acid is reducing agent.

In this reaction oxidation and reduction both takes place simultaneously, thus it is an example of redox reaction.

Significance of Oxidation Reduction in Everyday Life:

Respiration is oxidation reaction in which food is oxidized to produce energy.

Iron gets oxidized to form rust; which leads to corrosion of iron in the long run.

Most of the metals react with atmospheric oxygen and it leads to formation of a layer on the metal article. The metal gets corroded in the long run.

Rusting of iron can be prevented by painting the iron article. This can also be prevented by applying a layer of zinc over the iron article. This process is known as galvanization.

Fried food gets oxidized when exposed to air. This spoils the taste of the food and the food becomes unfit for consumption. The spoiling of fried food because of oxidation is called rancidity. Fried food is often packed in airtight packets to prevent rancidity.

We are able to utilize various types of fuel because of oxidation. Oxidation of fuel helps in producing energy.