In this chapter, we learn how substances react with each other and form new substances. Chemical reactions are happening all around us—from rusting of iron to digestion of food. This chapter is very important for the SSC board exam because it includes definitions, equations, and reasoning-based questions. If students understand how to write and balance chemical equations properly, this chapter becomes very easy and scoring.
Table of Contents
Important Concepts of the Chapter
Chemical Reaction
A chemical reaction is a process in which one or more substances react to form new substances with different properties.
The substances that take part in a reaction are called reactants, and the substances formed are called products.
In simple words, when old substances change into new substances, a chemical reaction has taken place.
For example, when magnesium burns in air, it forms magnesium oxide.
Chemical Equation
A chemical equation is a symbolic representation of a chemical reaction using chemical formulae.
For example,
Magnesium + Oxygen → Magnesium oxide
This shows what substances react and what products are formed during a reaction.
Balanced and Unbalanced Chemical Equations
If the number of atoms of each element is not equal on both sides of an equation, it is called an unbalanced equation.
When the number of atoms of each element is equal on both sides, it is called a balanced chemical equation.
Balancing an equation follows the law of conservation of mass, which states that mass can neither be created nor destroyed during a chemical reaction.
Steps for Balancing a Chemical Equation
Balancing is done step by step:
- Write the unbalanced equation
- List atoms of each element on both sides
- Balance one element at a time
- Use coefficients, not subscripts
- Write the final balanced equation
This process is frequently asked in exams.
Types of Chemical Reactions
Chemical reactions are classified into different types based on how reactants form products.
- Combination Reaction
- Decomposition Reaction
- Displacement Reaction
- Double Displacement Reaction
Combination Reaction
In a combination reaction, two or more substances combine to form a single product.
In simple words, many reactants form one product.
For example, calcium oxide reacts with water to form calcium hydroxide.
These reactions are generally exothermic.
Decomposition Reaction
In a decomposition reaction, a single compound breaks down into two or more simpler substances.
This reaction usually requires heat, light, or electricity.
For example, heating calcium carbonate produces calcium oxide and carbon dioxide.
Displacement Reaction
In a displacement reaction, a more reactive element displaces a less reactive element from its compound.
For example, zinc displaces copper from copper sulphate solution.
These reactions depend on the reactivity series of metals.
Double Displacement Reaction
In a double displacement reaction, two compounds exchange ions to form two new compounds.
These reactions often result in the formation of a precipitate, gas, or water.
For example, reaction between sodium sulphate and barium chloride forms barium sulphate precipitate.
Oxidation and Reduction
Oxidation is the addition of oxygen or removal of hydrogen.
Reduction is the removal of oxygen or addition of hydrogen.
Both processes occur together in a chemical reaction and are called redox reactions.
For example, when copper oxide reacts with hydrogen, copper oxide is reduced and hydrogen is oxidized.
Effects of Oxidation in Everyday Life
Oxidation causes several effects in daily life.
Corrosion
Corrosion is the gradual destruction of metals by chemical reactions with air, moisture, or chemicals.
Rusting of iron is a common example.
Rancidity
Rancidity is the oxidation of fats and oils in food, causing bad smell and taste.
This is why food items are stored in airtight containers.
Important Laws / Formulae
Law of Conservation of Mass
Statement:
Mass can neither be created nor destroyed in a chemical reaction.
This law is the basis of balancing chemical equations and is frequently asked in theory questions.
Important Definitions
- Chemical Reaction – Process in which new substances are formed from reactants.
- Reactants – Substances that take part in a chemical reaction.
- Products – New substances formed during a reaction.
- Balanced Equation – Equation with equal atoms on both sides.
- Oxidation – Addition of oxygen or removal of hydrogen.
- Reduction – Removal of oxygen or addition of hydrogen.
- Corrosion – Slow destruction of metals due to oxidation.
- Rancidity – Spoiling of food due to oxidation.
Numerical / Process Explanation
This chapter mainly focuses on processes rather than numericals.
Students should clearly understand:
- How to balance chemical equations step by step
- How to identify the type of reaction
- How oxidation and reduction occur simultaneously
- Logical reasoning behind corrosion and rancidity
Process-based questions are commonly asked in SSC exams.
Important Points for SSC Board Exam
- Balancing chemical equations is very important
- Definitions carry 1–2 marks
- Types of reactions are frequently asked
- Oxidation–reduction explanation is common
- Corrosion and rancidity are important theory topics
- Proper chemical symbols and formulae must be used
- Writing states of substances improves answers
Quick Revision Summary
- Chemical reactions form new substances
- Chemical equations represent reactions symbolically
- Balanced equations follow conservation of mass
- Combination reactions form one product
- Decomposition reactions break compounds
- Displacement reactions depend on reactivity
- Oxidation and reduction occur together
- Corrosion damages metals
- Rancidity spoils food items
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