Chemical Reactions – Types of Reactions
Chemical Reactions are one of the most important topics in Class 10 Science. Understanding different types of chemical reactions makes it much easier to identify reactions in board exams and write their correct names with suitable examples.
In this chapter, we will learn about Combination, Decomposition, Displacement, Double Displacement, Precipitation, Oxidation, Reduction, Redox, Exothermic and Endothermic reactions in a simple and exam-friendly way.
1. What is a Chemical Reaction?
A chemical reaction is a process in which one or more substances change into new substances having different properties.
The substances that take part in a chemical reaction are called reactants, while the new substances formed are called products.
Reactants → Products
Example
2Mg + O₂ → 2MgO
- Reactants: Mg + O₂
- Product: MgO
Here, magnesium reacts with oxygen and forms magnesium oxide.
Signs of a Chemical Reaction
A chemical reaction may be identified by one or more observable changes:
- Change in colour
- Change in temperature
- Evolution of gas
- Formation of a precipitate
- Change in state
These signs help us understand that a chemical change may have taken place.
2. Combination Reaction
A combination reaction is a reaction in which two or more substances combine to form a single product.
General Form
A + B → AB
The important point is that multiple reactants combine and produce only one product.
Example 1
CaO + H₂O → Ca(OH)₂
Calcium oxide combines with water to form calcium hydroxide.
Example 2
2Mg + O₂ → 2MgO
Magnesium combines with oxygen to form magnesium oxide.
Memory Trick
Many → One = Combination
3. Decomposition Reaction
A decomposition reaction is a reaction in which one compound breaks down into two or more simpler substances.
General Form
AB → A + B
This is exactly opposite to a combination reaction.
Example
CaCO₃ → CaO + CO₂
Calcium carbonate breaks down into calcium oxide and carbon dioxide.
Types of Decomposition Reactions
Depending on the energy supplied, decomposition reactions can occur in different ways:
- Thermal Decomposition → Heat
- Photochemical Decomposition → Light
- Electrolytic Decomposition → Electricity
Photochemical Decomposition
2AgCl → 2Ag + Cl₂
This reaction takes place in the presence of sunlight.
Electrolytic Decomposition
2H₂O → 2H₂ + O₂
This decomposition takes place using electricity.
Memory Trick
One → Many = Decomposition
4. Displacement Reaction
A displacement reaction occurs when a more reactive element displaces a less reactive element from its compound.
General Form
A + BC → AC + B
Here, element A is more reactive than element B, so A replaces B from the compound BC.
Example 1
Zn + CuSO₄ → ZnSO₄ + Cu
Here, zinc is more reactive than copper. Therefore, zinc displaces copper from copper sulphate.
Example 2
Fe + CuSO₄ → FeSO₄ + Cu
Iron is more reactive than copper, so iron displaces copper.
Key Concept
More Reactive → Displaces Less Reactive
Reactivity Series
The given reactivity series is:
K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au
The position of elements in this series helps determine whether one element can displace another from its compound.
5. Double Displacement Reaction
In a double displacement reaction, two compounds exchange their ions to form two new compounds.
General Form
AB + CD → AD + CB
In this reaction, the ions of the two compounds exchange partners.
Example
Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl
Here, the ions exchange to form barium sulphate and sodium chloride.
BaSO₄ is formed as a white precipitate.
Memory Trick
Exchange → Double Displacement
6. Precipitation Reaction
A precipitation reaction is a reaction in which an insoluble solid, called a precipitate, is formed.
Example
Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl
In this reaction:
BaSO₄ → White precipitate
The downward arrow ↓ indicates the formation of a precipitate.
What is a Precipitate?
A precipitate is an insoluble solid formed during a reaction.
Precipitation reactions are generally a type of double displacement reaction.
7. Oxidation
Oxidation can be understood as the addition of oxygen or removal of hydrogen.
Example
2Cu + O₂ → 2CuO
Here, copper gains oxygen and forms copper oxide.
Therefore:
Gain of Oxygen → Oxidation
Easy Trick
Oxidation → Oxygen Added
8. Reduction
Reduction can be understood as the removal of oxygen or addition of hydrogen.
Example
CuO + H₂ → Cu + H₂O
Here, CuO loses oxygen and changes into copper.
Therefore:
Loss of Oxygen → Reduction
Easy Trick
Reduction → Oxygen Removed
9. Redox Reaction
When oxidation and reduction occur simultaneously in the same reaction, the reaction is called a Redox Reaction.
The word Redox is formed from:
Reduction + Oxidation = Redox
Example
CuO + H₂ → Cu + H₂O
In this reaction:
- CuO → Cu = Reduction because oxygen is removed.
- H₂ → H₂O = Oxidation because oxygen is added to hydrogen.
Since both processes occur together:
Both together = REDOX
10. Exothermic Reaction
An exothermic reaction is a reaction that releases heat.
General Representation
Reactants → Products + Heat
Example
CH₄ + 2O₂ → CO₂ + 2H₂O + Heat
Heat is released during the reaction.
Other Examples
- Burning of fuels
- Respiration
Memory Trick
Exo → Heat OUT
11. Endothermic Reaction
An endothermic reaction is a reaction that absorbs energy or heat.
Example
CaCO₃ → CaO + CO₂
This decomposition takes place on heating, so energy is absorbed for the reaction to occur.
Memory Trick
Endo → Heat IN
12. Difference Between Major Types of Reactions
| Reaction Pattern / Feature | Type of Reaction |
|---|---|
| A + B → AB | Combination |
| AB → A + B | Decomposition |
| A + BC → AC + B | Displacement |
| AB + CD → AD + CB | Double Displacement |
| Insoluble solid forms | Precipitation |
| Oxidation + Reduction together | Redox |
| Heat is released | Exothermic |
| Heat is absorbed | Endothermic |
13. How to Identify the Reaction Type?
When a reaction is given in a board examination, do not immediately try to memorize its name. First observe its pattern.
Step 1: Count the Reactants and Products
If two or more substances combine to form one product:
Combination Reaction
If one compound breaks into two or more substances:
Decomposition Reaction
Step 2: Check for Replacement
If an element replaces another element from a compound:
Displacement Reaction
Step 3: Check for Exchange
If two compounds exchange ions:
Double Displacement Reaction
Step 4: Look for a Solid
If an insoluble solid is formed:
Precipitation Reaction
Step 5: Check Oxygen
Gain of oxygen:
Oxidation
Loss of oxygen:
Reduction
Both occurring together:
Redox Reaction
Step 6: Check Energy
Heat released:
Exothermic Reaction
Heat absorbed:
Endothermic Reaction
14. Quick Memory Trick for Reaction Types
The four basic reaction patterns can be remembered as:
Combine → Break → Replace → Exchange
- Combine → Combination
- Break → Decomposition
- Replace → Displacement
- Exchange → Double Displacement
For oxidation and energy-based reactions, remember:
- Oxygen Gain → Oxidation
- Oxygen Loss → Reduction
- Heat Out → Exothermic
- Heat In → Endothermic
15. Important Chemical Equations for Board Exam
| Reaction Type | Important Equation |
|---|---|
| Combination | CaO + H₂O → Ca(OH)₂ |
| Decomposition | CaCO₃ → CaO + CO₂ |
| Displacement | Zn + CuSO₄ → ZnSO₄ + Cu |
| Double Displacement | Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl |
| Redox | CuO + H₂ → Cu + H₂O |
16. Super Shortcut for Revision
Before the board exam, revise these shortcuts:
2 → 1 = Combination
1 → 2+ = Decomposition
Element + Compound = Displacement
Compound + Compound = Double Displacement
Insoluble Solid = Precipitation
Oxygen Gain = Oxidation
Oxygen Loss = Reduction
Oxidation + Reduction = Redox
Heat Out = Exothermic
Heat In = Endothermic
17. Common Mistakes to Avoid
- ❌ Confusing combination and decomposition reactions.
- ❌ Forgetting that displacement depends on the relative reactivity of elements.
- ❌ Confusing displacement with double displacement.
- ❌ Forgetting that a precipitate is an insoluble solid.
- ❌ Mixing up oxidation and reduction.
- ❌ Forgetting that oxidation and reduction can occur simultaneously in a redox reaction.
- ❌ Confusing heat released with heat absorbed.
Exam Tip: Instead of memorizing only the names, identify the pattern of the reaction first. The pattern will usually tell you the reaction type.
18. Board Exam Quick Revision Table
| Concept | Quick Revision |
|---|---|
| Chemical Reaction | Reactants change into new products |
| Combination | Many → One |
| Decomposition | One → Many |
| Displacement | More reactive element replaces less reactive element |
| Double Displacement | Exchange of ions |
| Precipitation | Insoluble solid is formed |
| Oxidation | Oxygen added |
| Reduction | Oxygen removed |
| Redox | Oxidation + Reduction together |
| Exothermic | Heat released |
| Endothermic | Heat absorbed |
19. Important Questions for Board Exam
Students should practise questions based on these important patterns:
- What is a chemical reaction? Give an example.
- What are the signs of a chemical reaction?
- Define combination reaction with an example.
- Define decomposition reaction and explain its types.
- What is a displacement reaction? Explain with an example.
- What is a double displacement reaction?
- What is a precipitate? Give an example of a precipitation reaction.
- Define oxidation and reduction with examples.
- What is a redox reaction? Explain using CuO + H₂ → Cu + H₂O.
- Differentiate between exothermic and endothermic reactions.
- Identify the type of reaction from a given chemical equation.
20. Frequently Asked Questions
Q1. What is a combination reaction?
A reaction in which two or more substances combine to form a single product is called a combination reaction.
A + B → AB
Q2. What is a decomposition reaction?
A reaction in which one compound breaks down into two or more simpler substances is called a decomposition reaction.
AB → A + B
Q3. What is a displacement reaction?
It is a reaction in which a more reactive element displaces a less reactive element from its compound.
Q4. What is a precipitate?
A precipitate is an insoluble solid formed during a chemical reaction.
Q5. How can oxidation be remembered?
For this chapter, remember:
Oxidation → Oxygen Added
Q6. How can reduction be remembered?
Remember:
Reduction → Oxygen Removed
Q7. What is a redox reaction?
A reaction in which oxidation and reduction occur simultaneously is called a redox reaction.
Q8. What is the easiest way to remember exothermic and endothermic reactions?
Exo → Heat OUT
Endo → Heat IN
21. Download / Study PDF
Use the PDF below for revision and practice:
Final Thoughts
Understanding the types of chemical reactions is essential for Class 10 Science because many questions require students to identify a reaction from its chemical equation.
The easiest approach is to focus on the reaction pattern. If many substances combine to form one product, think Combination. If one compound breaks into several products, think Decomposition. If one element replaces another, think Displacement. If two compounds exchange ions, think Double Displacement.
For oxidation and reduction, remember the simple rule:
Oxygen Gain → Oxidation
Oxygen Loss → Reduction
When both happen together, the reaction is a Redox Reaction.
Finally, remember:
Combine → Break → Replace → Exchange
and
Heat Out → Exothermic | Heat In → Endothermic
Revise the important equations, practise identifying reaction patterns, and use the quick-revision table before your board examination.