Chemical Equations and Chemical Reactions: How Chemistry Shows Change

Chemistry is not about memorising formulas.
It is about understanding change.
Whenever a substance changes into a new substance with different properties, a chemical reaction has taken place. These changes are described using chemical equations.

If you understand chemical reactions and equations properly, many later chapters become much easier.

What Is a Chemical Reaction?

A chemical reaction is a process in which one or more substances react to form new substances with different chemical properties.
The substances that take part in a reaction are called reactants.
The substances formed after the reaction are called products.

In a chemical reaction:

• Bonds break
• New bonds form
• Atoms rearrange

Atoms are not created or destroyed.

How Do We Know a Chemical Reaction Has Occurred?

A chemical reaction is usually indicated by one or more of the following changes:

• Change in colour
• Evolution of gas
• Formation of a precipitate
• Change in temperature
• Change in state

Example:

When iron rusts, its colour changes and a new substance is formed.

What Is a Chemical Equation?

A chemical equation is a symbolic way of representing a chemical reaction using chemical symbols and formulae.

It shows:

• Reactants on the left-hand side
• Products on the right-hand side

Example:

2Mg + O₂ → 2MgO

This equation shows magnesium reacting with oxygen to form magnesium oxide.

Skeletal Equation vs Balanced Equation

A skeletal equation shows only the formulas of reactants and products.
Example:

Mg + O₂ → MgO

This equation is not balanced.

A balanced equation has equal number of atoms of each element on both sides.

Why Must Chemical Equations Be Balanced?

Chemical equations must be balanced to obey the law of conservation of mass.
This law states that mass is neither created nor destroyed in a chemical reaction.

Balancing ensures that:

• Number of atoms before reaction = Number of atoms after reaction

Balancing of Chemical Equations

Balancing is done by adjusting coefficients, not subscripts.

Steps involved:

• Write skeletal equation
• Count atoms on both sides
• Balance one element at a time
• Check final atom count

Example:

2H₂ + O₂ → 2H₂O

Common Mistakes While Balancing

Students often:

• Change subscripts instead of coefficients
• Forget to balance oxygen and hydrogen at the end
• Ignore polyatomic ions

Remember:

Formulas are never changed during balancing.

Types of Chemical Reactions

Chemical reactions are classified based on how reactants change into products.

Combination Reaction

In a combination reaction, two or more substances combine to form a single product.

General form:

A + B → AB

Example:

CaO + H₂O → Ca(OH)₂

Decomposition Reaction

In a decomposition reaction, a single compound breaks down into two or more simpler substances.

General form:

AB → A + B

Example:

CaCO₃ → CaO + CO₂

Decomposition reactions usually require heat, electricity, or light.

Displacement Reaction

In a displacement reaction, a more reactive element displaces a less reactive element from its compound.

General form:

A + BC → AC + B

Example:

Zn + CuSO₄ → ZnSO₄ + Cu

Double Displacement Reaction

In a double displacement reaction, ions exchange places between two compounds.

General form:

AB + CD → AD + CB

Example:

Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl

Formation of a precipitate is common in such reactions.

Oxidation and Reduction Reactions

Oxidation means:

• Gain of oxygen
• Loss of hydrogen

Reduction means:

• Loss of oxygen
• Gain of hydrogen

Oxidation and reduction always occur together. These are called redox reactions.

Corrosion: A Special Chemical Reaction

Corrosion is the slow destruction of metals by chemical reactions with the environment.

Rusting of iron is a common example.

Rusting requires:

• Oxygen
• Water

Rancidity: Chemical Reaction in Food

Rancidity is the chemical reaction in which fats and oils react with oxygen, leading to bad smell and taste.
It is an example of oxidation reaction.

How Can Rancidity Be Prevented?

Rancidity can be prevented by:

• Storing food in airtight containers
• Refrigeration
• Adding antioxidants
• Using nitrogen gas in food packets

CHEMICAL EQUATIONS & CHEMICAL REACTIONS – FAQs

Last-Moment Notes (Cheat Sheet): CHEMICAL EQUATIONS & CHEMICAL REACTIONS

• Chemical reaction forms new substances with new properties
• Reactants → substances that react
• Products → substances formed
• Chemical equations represent reactions using symbols and formulae
• Reactants are written on left, products on right
• Balanced equation obeys law of conservation of mass
• Number of atoms of each element must be same on both sides
• Coefficients are changed, formulas are never changed while balancing
• Combination reaction: A + B → AB
• Decomposition reaction: AB → A + B
• Displacement reaction: A + BC → AC + B
• Double displacement reaction: AB + CD → AD + CB
• Oxidation: gain of oxygen or loss of hydrogen
• Reduction: loss of oxygen or gain of hydrogen
• Oxidation and reduction always occur together
• Corrosion is slow destruction of metals
• Rusting requires oxygen and moisture
• Rancidity is oxidation of fats and oils
• Prevent rancidity by airtight packing, refrigeration, antioxidants
• Balancing ensures mass conservation, not equal mass on both sides
• Change of state ≠ chemical reaction