Class 10 Chemistry • CBSE / NCERT • Exam Revision
Chemical Reactions and Equations — Solved Numericals & Common Mistakes
Balancing equations is one of those Class 10 Chemistry skills that looks easy until a question adds brackets, polyatomic ions, heat, light, a precipitate, or a word-problem calculation. This guide turns the chapter into a practical revision sheet: concepts, reaction types, solved numerical-style questions, balancing steps, oxidation and reduction, common traps, exam-ready shortcuts, and AI-search-friendly answers.
- What is a chemical reaction?
- Chemical equations and symbols
- How to balance equations step by step
- Types of chemical reactions
- Solved numericals and calculation-based questions
- Oxidation, reduction, corrosion and rancidity
- Common mistakes and how to avoid them
- Exam checklist and rapid revision
- Frequently asked questions
What Is a Chemical Reaction?
A chemical reaction is a process in which one or more substances are transformed into new substances with different chemical properties. The substances present at the start are called reactants, while the substances formed are called products.
| Term | Meaning | Example |
|---|---|---|
| Reactant | Starting substance that takes part in a reaction | Mg and O₂ |
| Product | New substance formed | MgO |
| Chemical equation | Symbolic representation of the reaction | 2Mg + O₂ → 2MgO |
Common observations that indicate a chemical reaction include a colour change, temperature change, gas formation, formation of a precipitate, change of state, or release/absorption of energy. One observation alone is not always sufficient; interpret it with the substances involved.
2. Chemical Equations: Word, Formula and Balanced Forms
A word equation gives the reaction in words. A chemical equation uses symbols and formulae. A balanced chemical equation uses coefficients so that the number of atoms of each element is equal on both sides.
What do the symbols mean?
| Symbol | Meaning |
|---|---|
| → | Forms / yields |
| + | Reacts with / together with |
| (s) | Solid |
| (l) | Liquid |
| (g) | Gas |
| (aq) | Aqueous solution |
| Δ | Heat supplied |
| ↑ | Gas evolved (often used in school-level notation) |
| ↓ | Precipitate formed |
How to Balance a Chemical Equation
The law of conservation of mass is the reason equations must be balanced. Atoms are rearranged during a chemical reaction; they are not created or destroyed.
Reliable 5-step method
- Write the correct chemical formulae for all reactants and products.
- Count the atoms of each element on both sides.
- Start with an element appearing in one formula on each side; leave H and O until later when practical.
- Change coefficients in front of formulae, never subscripts inside formulae.
- Recount every element and reduce coefficients to the smallest whole-number ratio.
Worked Example 1: H₂ + O₂ → H₂O
Step 1: Oxygen is 2 on the left and 1 on the right.
Step 2: Hydrogen is now 2 on the left and 4 on the right.
Final answer: 2H₂ + O₂ → 2H₂O
Worked Example 2: Fe + H₂O → Fe₃O₄ + H₂
Fe₃O₄ contains 3 Fe atoms and 4 O atoms. Put 3 before Fe and 4 before H₂O. That gives 8 H atoms, so put 4 before H₂.
Atom check: Fe = 3, O = 4, H = 8 on both sides.
Types of Chemical Reactions
| Type | General pattern | Example | Recognition clue |
|---|---|---|---|
| Combination | A + B → AB | 2Mg + O₂ → 2MgO | Two or more reactants give one main product |
| Decomposition | AB → A + B | CaCO₃ → CaO + CO₂ | One compound breaks into simpler products |
| Displacement | A + BC → AC + B | Zn + CuSO₄ → ZnSO₄ + Cu | More reactive element replaces another |
| Double displacement | AB + CD → AD + CB | Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl | Ions exchange partners |
| Redox | Oxidation + reduction | CuO + H₂ → Cu + H₂O | Oxidation and reduction occur together |
Combination reaction
Two reactants combine to produce a single product. The number of products is the quickest first clue.
Decomposition reaction
Decomposition may be caused by heat, electricity or light. For example, electrolysis of water can produce hydrogen and oxygen, while silver chloride decomposes in light.
Displacement reaction
Zinc is more reactive than copper, so zinc can displace copper from copper sulphate solution.
Double displacement and precipitation
Barium sulphate is insoluble and forms a precipitate. The reaction is classified as double displacement and, because a solid precipitate forms, it is also a precipitation reaction.
Solved Numericals and Calculation-Based Questions
Although this chapter is largely conceptual, examiners can turn balancing, atom counting, formula interpretation and stoichiometric ratios into numerical-style questions. The safest approach is to treat the balanced equation as a fixed ratio.
Numerical 1: Find the total number of oxygen atoms
Question: How many oxygen atoms are represented by 3 molecules of CO₂?
One CO₂ molecule contains 2 oxygen atoms.
So, 3 × 2 = 6 oxygen atoms.
Answer: 6 oxygen atoms.
Numerical 2: Atom count in a coefficient-containing equation
Question: In 2Al₂(SO₄)₃, find the number of Al, S and O atoms.
One Al₂(SO₄)₃ contains Al = 2, S = 3 and O = 12.
Multiply each by 2:
- Al = 2 × 2 = 4
- S = 2 × 3 = 6
- O = 2 × 12 = 24
Answer: Al = 4, S = 6, O = 24 atoms.
Numerical 3: Coefficient ratio from a balanced equation
Question: In 2H₂ + O₂ → 2H₂O, how many molecules of water are formed when 5 molecules of oxygen react completely?
The balanced equation gives the ratio O₂ : H₂O = 1 : 2.
For 5 O₂ molecules: 5 × 2 = 10 H₂O molecules.
Answer: 10 water molecules, provided sufficient hydrogen is available.
Numerical 4: Conservation of mass
Question: A reaction uses 12 g carbon and 32 g oxygen to form carbon dioxide. What mass of CO₂ is expected if all reactants react completely?
By conservation of mass, total mass of reactants = total mass of products.
12 g + 32 g = 44 g.
Answer: 44 g CO₂.
Numerical 5: Identify the limiting amount using the equation ratio
Question: For 2H₂ + O₂ → 2H₂O, suppose 4 mol H₂ and 1 mol O₂ are available. Which reactant limits the amount of water?
The equation requires 2 mol H₂ for every 1 mol O₂. One mol O₂ can therefore use 2 mol H₂ and produce 2 mol H₂O. Since 4 mol H₂ is available but only 1 mol O₂ is available, oxygen is used up first.
Answer: O₂ is the limiting reactant; 2 mol H₂O can form if the reaction goes to completion.
Numerical 6: Percentage composition-style check
Question: What percentage by mass of oxygen is present in H₂O? Use H = 1 and O = 16.
Molar mass of H₂O = 2(1) + 16 = 18.
Mass due to oxygen = 16.
Percentage of oxygen = (16 ÷ 18) × 100 ≈ 88.89%.
Answer: approximately 88.89% oxygen by mass.
Oxidation, Reduction, Corrosion and Rancidity
At Class 10 level, oxidation is commonly described as addition of oxygen or removal of hydrogen, while reduction can be described as removal of oxygen or addition of hydrogen. In modern chemistry, oxidation is also associated with loss of electrons and an increase in oxidation state; reduction involves electron gain or the reverse change. IUPAC gives the more general electron-based definitions.
| Concept | Easy Class 10 description | Example |
|---|---|---|
| Oxidation | Gain of oxygen / loss of hydrogen | 2Cu + O₂ → 2CuO |
| Reduction | Loss of oxygen / gain of hydrogen | CuO + H₂ → Cu + H₂O |
| Redox | Oxidation and reduction occur together | CuO + H₂ → Cu + H₂O |
| Corrosion | Gradual deterioration of a metal by environmental reactions | Rusting of iron |
| Rancidity | Oxidation-related spoilage of fats/oils causing unpleasant smell or taste | Stale oily food |
Example: CuO + H₂ → Cu + H₂O
CuO loses oxygen, so CuO is reduced. Hydrogen gains oxygen and becomes water, so hydrogen is oxidised. Because both changes happen in the same reaction, it is a redox reaction.
Common Mistakes Students Make
- Changing subscripts while balancing: Use coefficients only.
- Writing an incorrect formula first: Balancing cannot repair a wrong chemical formula.
- Stopping after balancing one element: Recount every element.
- Forgetting brackets: In Al₂(SO₄)₃, sulphate occurs three times, so O = 3 × 4 = 12.
- Calling every two-compound reaction double displacement: Check whether ions actually exchange partners.
- Confusing displacement with combination: Zn + CuSO₄ has two products, so it is not combination.
- Confusing oxidation with oxygen only: The broader concept also involves electron transfer/oxidation state.
- Ignoring reaction conditions: Heat, light or electricity can be essential to the reaction.
- Forgetting state symbols when asked: Use (s), (l), (g) and (aq) where relevant.
- Not checking the final coefficient ratio: If all coefficients have a common factor, simplify them.
High-Value Reaction Equations to Revise
| Reaction | Balanced equation | Type / idea |
|---|---|---|
| Magnesium burns in oxygen | 2Mg + O₂ → 2MgO | Combination |
| Calcium oxide with water | CaO + H₂O → Ca(OH)₂ | Combination |
| Calcium carbonate on heating | CaCO₃ → CaO + CO₂ | Decomposition |
| Zinc and copper sulphate | Zn + CuSO₄ → ZnSO₄ + Cu | Displacement |
| Barium chloride and sodium sulphate | BaCl₂ + Na₂SO₄ → BaSO₄↓ + 2NaCl | Double displacement / precipitation |
| Hydrogen reduces copper oxide | CuO + H₂ → Cu + H₂O | Redox |
| Iron with water | 3Fe + 4H₂O → Fe₃O₄ + 4H₂ | Redox / displacement-type chemistry |
Exam Strategy: A 60-Second Equation Check
- Formula check: Are all reactants and products written correctly?
- Atom check: Count each element on both sides.
- Coefficient check: Did you change only coefficients?
- Classification check: Identify the reaction from what actually changes.
- Condition check: Add heat/light/electricity if given or required.
- Answer check: If it is numerical, include unit and a clear final statement.
For a broader chapter-by-chapter overview, continue with the Class 10 Chemistry Complete Guide. For focused practice on this exact chapter, use the Chemical Reactions and Equations Class 10 Notes, Types & Balancing Tricks.
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Trusted External References
- NCERT official textbooks — use the prescribed textbook as the primary reference for chapter content.
- CBSE Academic Science curriculum PDF — verify syllabus and examination requirements against the latest official release.
- IUPAC Gold Book — oxidation — useful for the modern scientific definition of oxidation.
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Frequently Asked Questions
What is a chemical reaction?
A chemical reaction is a process in which reactants are transformed into new substances called products.
Why must chemical equations be balanced?
They are balanced to represent conservation of atoms and therefore conservation of mass in the reaction.
What is the easiest way to balance a chemical equation?
Write correct formulae, count atoms, adjust coefficients only, recount every element, and reduce to the smallest whole-number ratio.
Can I change subscripts while balancing an equation?
No. Subscripts are part of a compound's chemical formula. Changing them changes the substance. Use coefficients instead.
What are the main types of chemical reactions in Class 10?
Combination, decomposition, displacement and double displacement are the main structural types. Redox describes simultaneous oxidation and reduction; precipitation is identified when an insoluble solid forms.
What is a combination reaction?
It is a reaction in which two or more substances combine to form a single product, such as 2Mg + O₂ → 2MgO.
What is a decomposition reaction?
It is a reaction in which one compound breaks into two or more simpler substances, such as CaCO₃ → CaO + CO₂ on heating.
What is a displacement reaction?
A more reactive element replaces a less reactive element from its compound, for example Zn + CuSO₄ → ZnSO₄ + Cu.
What is oxidation?
At Class 10 level, oxidation can be described as gain of oxygen or loss of hydrogen. In modern chemistry it is also associated with electron loss or an increase in oxidation state.
What is reduction?
At Class 10 level, reduction can be described as loss of oxygen or gain of hydrogen. In modern chemistry it is also associated with electron gain or a decrease in oxidation state.
What is corrosion?
Corrosion is the gradual deterioration of a metal because of reactions with substances in its environment; rusting of iron is a common example.
What is rancidity?
Rancidity is the undesirable change in smell or taste of fats and oils, commonly associated with oxidation.
Are there numericals in Chemical Reactions and Equations?
Yes. Calculation-style questions can involve atom counting, conservation of mass, coefficient ratios, formula interpretation and simple stoichiometric relationships based on balanced equations.
How can I avoid losing marks in equation questions?
Check formulae first, change coefficients rather than subscripts, count every element, include conditions where required, identify the reaction type carefully, and show working for numerical questions.
Final Revision Sheet
Remember the workflow: Correct formula → Balance → Classify → Explain → Calculate → Check units.
- Combination: many reactants → one product.
- Decomposition: one compound → simpler products.
- Displacement: more reactive element replaces less reactive element.
- Double displacement: ions exchange partners.
- Oxidation and reduction happen together in a redox reaction.
- Never change subscripts during balancing.
- For numerical questions, use the balanced-equation ratio and show the calculation.
Final takeaway: Do not memorise balancing as a guessing game. Treat every equation as an atom-counting problem. Once formulae are correct and coefficients are checked systematically, most Class 10 Chemical Reactions and Equations questions become much more predictable.
