Chemical reactions are the core of Class 10 Chemistry and one of the highest-scoring areas in CBSE board exams. Questions are frequently asked in the form of equation writing, identifying reaction types, reasoning-based questions, case studies, and even MCQs based on color changes or conditions.
This extended guide is designed not just for memorization but for deep conceptual clarity. If you understand these reactions well, you can confidently attempt nearly 70–80% of Chemistry questions in your board exam.
Why Are Chemical Reactions So Important in CBSE Exams?
Chemical reactions are repeatedly tested because they combine multiple concepts together—chemical equations, balancing, oxidation-reduction, reaction types, and practical applications.
By mastering reactions, you will be able to:
- Identify reaction types instantly in MCQs
- Predict products logically instead of memorizing blindly
- Solve assertion-reason questions with confidence
- Answer case-based and competency-based questions
- Score full marks in equation-based questions
Most importantly, CBSE tends to repeat similar reactions every year with slight variations.
1. Combination Reactions
Combination reactions involve two or more substances combining to form a single product. These reactions are usually straightforward and commonly asked in exams.
Magnesium Burning:
2Mg + O₂ → 2MgO
Explanation:
- Magnesium reacts vigorously with oxygen
- It produces a bright white flame
- The product formed is magnesium oxide, which is white in color
Other Important Examples:
C + O₂ → CO₂
2H₂ + O₂ → 2H₂O
Key Concept:
These reactions are generally exothermic, meaning heat is released. Students should always ensure proper balancing, especially in hydrogen reactions.
2. Decomposition Reactions
Decomposition reactions involve breaking down a compound into simpler substances using heat, electricity, or light.
Thermal Decomposition of Calcium Carbonate:
CaCO₃ → CaO + CO₂
Explanation:
- This reaction occurs on heating limestone
- It is a key reaction in the lime cycle
Decomposition of Lead Nitrate:
2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂
Observations:
- Lead oxide is yellow
- Nitrogen dioxide gas is brown (toxic fumes)
Decomposition of Ferrous Sulphate:
2FeSO₄ → Fe₂O₃ + SO₂ + SO₃
Important Notes:
- FeSO₄·7H₂O first loses water molecules
- Green ferrous sulphate turns into brown ferric oxide
Photochemical Decomposition:
2AgCl → 2Ag + Cl₂ (in sunlight)
Application:
- Used in black and white photography
3. Displacement Reactions
In displacement reactions, a more reactive metal displaces a less reactive metal from its compound.
Example:
Fe + CuSO₄ → FeSO₄ + Cu
Observations:
- Blue CuSO₄ solution turns green
- Reddish-brown copper is deposited
Concept Insight:
These reactions depend on the reactivity series. Always remember that a metal higher in the series displaces a lower one.
4. Double Displacement Reactions
These reactions involve exchange of ions between two compounds and often result in precipitation.
Example 1:
Na₂SO₄ + BaCl₂ → BaSO₄ ↓ + 2NaCl
- White precipitate of BaSO₄ is formed
Example 2:
PbCl₂ + 2KI → PbI₂ ↓ + 2KCl
- Yellow precipitate of PbI₂ is formed
Exam Tip:
Precipitation reactions are frequently asked in lab-based questions and MCQs.
5. Redox Reactions
Redox reactions involve oxidation (loss of electrons) and reduction (gain of electrons) occurring simultaneously.
Example:
Cu + O₂ → CuO
- Copper gets oxidized
- Copper oxide is black
Reduction Reaction:
CuO + H₂ → Cu + H₂O
Explanation:
- CuO loses oxygen → Reduction
- H₂ gains oxygen → Oxidation
Key Idea:
Remember: OIL RIG (Oxidation Is Loss, Reduction Is Gain of electrons)
6. Electrolysis Reaction
Electrolysis involves decomposition of a compound using electricity.
Electrolysis of Water:
2H₂O(l) → 2H₂(g) + O₂(g)
Observations:
- Hydrogen gas is released at cathode
- Oxygen gas is released at anode
- Volume of hydrogen is double that of oxygen
Concept Insight:
This confirms the formula of water (H₂O).
7. Acids, Bases, and Salts Reactions
These reactions form the foundation of many board questions.
Acid + Metal:
Zn + 2HCl → ZnCl₂ + H₂
Acid + Carbonate:
Na₂CO₃ + 2HCl → 2NaCl + CO₂ + H₂O
Acid + Bicarbonate:
NaHCO₃ + HCl → NaCl + CO₂ + H₂O
Neutralization Reaction:
HCl + NaOH → NaCl + H₂O
Exception:
HNO₃ does not usually produce hydrogen gas because it is a strong oxidizing agent.
8. Metal and Non-Metal Reactions
Metals react with oxygen to form basic oxides, while non-metals form acidic oxides.
Metal Reaction:
2Mg + O₂ → 2MgO
Non-Metal Reaction:
C + O₂ → CO₂
Concept Tip:
- Metal oxides → Basic
- Non-metal oxides → Acidic
9. Chlor-Alkali Process
2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + H₂(g) + Cl₂(g)
Explanation:
- Electrolysis of brine (salt solution)
- Hydrogen is released at cathode
- Chlorine is released at anode
Applications:
- Production of NaOH, chlorine, and hydrogen
10. Important Industrial Reactions
Bleaching Powder Formation:
Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O
Plaster of Paris Formation:
CaSO₄·2H₂O → CaSO₄·½H₂O + H₂O
Solvay Process:
NaCl + NH₃ + CO₂ + H₂O → NaHCO₃ + NH₄Cl
Importance:
These reactions are frequently asked in 2–3 mark questions.
11. Organic Chemistry Reactions
Esterification:
C₂H₅OH + CH₃COOH → CH₃COOC₂H₅ + H₂O
- Formation of pleasant-smelling esters
Saponification:
CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH
- Soap formation reaction
Addition Reaction:
C₂H₄ + H₂ → C₂H₆ (Ni catalyst)
Substitution Reaction:
CH₄ + Cl₂ → CH₃Cl + HCl (sunlight)
Exam Tip:
Conditions like catalyst and sunlight are frequently tested.
12. Important Biological Reactions
Photosynthesis:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Respiration:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy
Concept Insight:
These are inverse processes and often appear in case-based questions.
Exam Tips for Chemical Reactions
- Always balance equations step-by-step
- Include physical states (s, l, g, aq)
- Memorize color changes for scoring questions
- Practice writing reactions regularly
- Focus on previous year questions (PYQs)
Common Mistakes to Avoid
- Writing incomplete or unbalanced equations
- Ignoring reaction conditions
- Confusing reaction types
- Skipping color observations
FAQs
Q1. How many reactions are enough for boards?
Around 30–40 NCERT-based reactions are generally sufficient for Class 10 CBSE boards. However, the key is not just memorizing them but understanding the concept behind each reaction. If you understand why a reaction occurs, you will be able to write similar reactions even if the question is twisted.
Q2. Are color changes really important?
Yes, color changes are extremely important because CBSE often asks observation-based questions. For example, changes like blue to green (CuSO₄ to FeSO₄) or formation of brown fumes (NO₂) are commonly tested. These are easy marks if you revise them properly.
Q3. Which reactions are most important?
Decomposition, displacement, redox, and acid-base reactions are among the most important. Additionally, reactions from metals and non-metals, carbon compounds, and industrial processes are frequently asked in exams. Focus especially on reactions given in NCERT examples and exercises.
Q4. How can I remember reactions effectively?
Instead of rote learning, try to understand patterns such as reaction types and reactivity series. Practice writing reactions regularly and revise them daily. Group similar reactions together—for example, all decomposition reactions—so your brain remembers them logically.
Q5. Are NCERT reactions enough for scoring full marks?
Yes, NCERT is more than enough for scoring full marks in CBSE exams. Most questions are directly or indirectly based on NCERT examples, in-text questions, and exercises. However, make sure you practice previous year questions along with NCERT.
Q6. Do I need to learn reaction conditions?
Yes, reaction conditions like heat, pressure, catalysts, and sunlight are very important. CBSE often asks questions where missing conditions can make your answer incomplete. Always write conditions along with the equation for full marks.
Q7. How are reactions asked in exams?
Chemical reactions can be asked in multiple formats such as writing balanced equations, identifying reaction types, completing reactions, assertion-reason questions, and case-based questions. Sometimes, reactions are hidden inside word problems.
Q8. What is the best revision strategy?
The best strategy is to revise reactions daily in short sessions. First revise NCERT, then solve previous year questions, and finally test yourself by writing reactions without looking. This helps in long-term retention.
Q9. How to avoid silly mistakes?
To avoid mistakes, always balance equations carefully, check valency, and include states and conditions. Practice regularly so that writing reactions becomes automatic during the exam.
Q10. Can I skip organic reactions?
No, organic reactions are very important and scoring. Topics like esterification, saponification, addition, and substitution reactions are frequently asked. Skipping them can cost you easy marks.
Conclusion
Chemical reactions are one of the easiest and most scoring areas in Class 10 Science if prepared properly. With consistent practice, conceptual clarity, and regular revision, you can easily achieve high marks in Chemistry.
Focus on understanding, not memorization, and practice daily to build confidence before your CBSE Board Exam 2026.
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