Chemical Reactions and Equations — Class 10 Science
What CBSE examines here (2026-27). Writing and balancing chemical equations, the five reaction types (combination, decomposition, displacement, double displacement, redox), and corrosion/rancidity are all still fully in the chapter and unchanged. One thing has quietly dropped out: the electron-transfer ("modern") definition of oxidation and reduction — the OIL RIG mnemonic and the terms "oxidising agent" / "reducing agent" appear nowhere in the current NCERT text. Oxidation and reduction are now defined and examined only in terms of gain or loss of oxygen or hydrogen (Exercise Q16 asks for exactly this). The electron-based definition returns in Class 11 Chemistry.
"Chemistry is the music of matter — every reaction is a story."
1. About the Chapter
This is the opening chapter of Class 10 Science. Introduces:
- Chemical reactions and their characteristics
- Writing and balancing chemical equations
- Types of reactions (5 main types)
- Oxidation and reduction (redox)
- Corrosion and rancidity (real-world applications)
Why Important
- Foundation for all chemistry
- Used in industry, medicine, daily life
- Topic appears across science studies
2. What is a Chemical Reaction?
Definition
A chemical reaction is a process in which one or more substances (reactants) are transformed into new substances (products) with different properties.
Characteristics (Signs of Chemical Reaction)
- Change in colour (e.g., iron rusting, leaves turning yellow)
- Change in state (e.g., wax melting then re-solidifying differently)
- Evolution of gas (e.g., bubbling soda)
- Formation of precipitate (solid in liquid)
- Change in temperature (heating or cooling)
- Emission of light (e.g., burning magnesium)
Reactants and Products
Reactants → Products
Example: Hydrogen + Oxygen → Water
- Reactants: H₂, O₂
- Product: H₂O
3. Chemical Equations
Word Equation
Magnesium + Oxygen → Magnesium oxide
Symbol Equation
Mg + O₂ → MgO
Balancing Equations
A balanced equation has equal atoms of each element on both sides (Law of Conservation of Mass).
Unbalanced: Mg + O₂ → MgO
- Mg: 1 on each side ✓
- O: 2 on left, 1 on right ✗
Balanced: 2Mg + O₂ → 2MgO
- Mg: 2 on each side ✓
- O: 2 on each side ✓
Why Balance?
Mass cannot be created or destroyed (Lavoisier's Law of Conservation of Mass, 1789).
Symbols Used
- (s): solid
- (l): liquid
- (g): gas
- (aq): aqueous (dissolved in water)
- ↑: gas evolves
- ↓: precipitate forms
- Δ: heated
4. Types of Chemical Reactions
Type 1: Combination Reaction
Two or more substances combine to form a SINGLE product.
A + B → AB
Examples:
- 2Mg + O₂ → 2MgO (magnesium burns)
- CaO + H₂O → Ca(OH)₂ (calcium oxide → slaked lime; exothermic)
- 2H₂ + O₂ → 2H₂O
Type 2: Decomposition Reaction
A SINGLE substance breaks into two or more products.
AB → A + B
Types:
- Thermal decomposition (by heat): CaCO₃ → CaO + CO₂ (limestone heated)
- Electrolytic (by electricity): 2H₂O → 2H₂ + O₂
- Photochemical (by light): 2AgCl → 2Ag + Cl₂ (silver chloride in photographic film)
Type 3: Displacement Reaction
A more reactive element displaces a less reactive one.
A + BC → AC + B
Example: Iron + Copper sulphate → Iron sulphate + Copper Fe + CuSO₄ → FeSO₄ + Cu
A more reactive metal displaces a less reactive one from its compound — zinc and lead, for instance, are both more reactive than copper and displace it from copper salts. The full reactivity series (K, Na, Ca, Mg, Al, Zn, Fe, Pb, [H], Cu, Hg, Ag, Au) is built up properly in Chapter 3, Metals and Non-metals — this chapter only needs "which of these two is more reactive," not the whole order.
Type 4: Double Displacement Reaction
Two compounds exchange ions.
AB + CD → AD + CB
Example: Sodium sulphate + Barium chloride → Barium sulphate + Sodium chloride Na₂SO₄ + BaCl₂ → BaSO₄ ↓ + 2NaCl
(BaSO₄ is precipitate.)
This is also called precipitation reaction.
Type 5: Redox Reaction (Oxidation-Reduction)
Oxidation = gain of oxygen OR loss of hydrogen. Reduction = loss of oxygen OR gain of hydrogen.
These occur TOGETHER in the same reaction — whenever one substance is oxidised, another is reduced. A reaction showing both is called an oxidation-reduction reaction, or redox reaction.
Example: CuO + H₂ → Cu + H₂O
- CuO loses O → reduced to Cu
- H₂ gains O → oxidised to H₂O
Not every reaction that adds oxygen to something has a matching "reduced" partner you can name this way. In 4Na + O₂ → 2Na₂O, sodium clearly gains oxygen (oxidised) — but there is no compound here losing oxygen, so nothing is classified as "reduced" by this definition. Pairing every oxidation with a reduction needs the electron-transfer view, which this chapter does not use (see the appendix).
5. Everyday Language for Oxidation and Reduction
CBSE now sticks to one working definition for this chapter, and it is worth stating plainly because most guidebooks still teach the older one:
| Oxidation | Reduction | |
|---|---|---|
| In terms of oxygen | gains oxygen | loses oxygen |
| In terms of hydrogen | loses hydrogen | gains hydrogen |
That is the entire definition this chapter examines. There is no need for oxidation numbers, electron counting, or the terms "oxidising agent" / "reducing agent" here — those come back, more rigorously, in Class 11 Chemistry.
6. Effects of Oxidation in Daily Life
Corrosion
Slow oxidation of metals.
Rusting of iron: 4Fe + 3O₂ + xH₂O → 2Fe₂O₃.xH₂O (hydrated iron oxide)
- Requires both oxygen and water
- Forms reddish-brown rust
- Major economic loss globally
Prevention:
- Painting (barrier)
- Greasing/oiling
- Galvanisation (zinc coating)
- Electroplating (chromium)
- Alloying (stainless steel: iron + chromium + nickel)
Rancidity
Oxidation of oils/fats in food.
- Develops bad smell and taste
- Common in fried foods, butter
Prevention:
- Sealed packaging (no oxygen)
- Refrigeration (slows oxidation)
- Antioxidants (BHA, BHT)
- Vacuum packing
- Nitrogen flushing (replace oxygen with N₂)
7. Worked Examples
Example 1: Balance the equation
Balance: Al + HCl → AlCl₃ + H₂
- Al: 1 → ?
- H: 1 → 2
- Cl: 1 → 3
Try: 2Al + 6HCl → 2AlCl₃ + 3H₂
- Al: 2 = 2 ✓
- H: 6 = 6 ✓
- Cl: 6 = 6 ✓
Example 2: Identify Type
Identify: 2KClO₃ →(heat) 2KCl + 3O₂
- One substance breaks into more → DECOMPOSITION
Example 3: Identify Type
Identify: Zn + 2HCl → ZnCl₂ + H₂
- Zn displaces H from HCl → DISPLACEMENT
Example 4: Identify Type
Identify: AgNO₃ + NaCl → AgCl↓ + NaNO₃
- Ion exchange between two compounds → DOUBLE DISPLACEMENT (also precipitation)
Example 5: Identify Oxidised and Reduced Substances
In CuO + H₂ → Cu + H₂O:
- CuO loses oxygen → CuO is REDUCED (to Cu)
- H₂ gains oxygen → H₂ is OXIDISED (to H₂O)
8. Common Mistakes
-
Unbalanced equations
- Always balance by checking atoms of each element.
-
Wrong subscripts
- You can CHANGE coefficients (in front), NOT subscripts (in formula). H₂O is always H₂O, not H₃O.
-
Confusing types
- Decomposition: 1 → many. Combination: many → 1.
-
Reaching for electrons or "oxidising/reducing agent" in this chapter
- This chapter's definition of oxidation and reduction runs on oxygen and hydrogen only. Save OIL RIG and oxidising/reducing agents for Class 11 — using them here answers a question this chapter isn't asking.
-
Assuming every oxidation needs a visible "reduction" partner
- True in general, but not always nameable with just the oxygen/hydrogen definition — see the note after Type 5 above (4Na + O₂ → 2Na₂O has an oxidised substance but no compound losing oxygen to point to as "reduced").
9. Indian Heritage
Ancient Indian Chemistry
- Rasaśastra (alchemy) — medieval Indian chemistry texts
- Charaka (~600 BCE) — described various chemical processes
- Indian metallurgy: zinc extraction discovered in Zawar (Rajasthan) — earliest in world
Modern Indian Chemistry
- C.V. Raman — Nobel 1930 (Raman Effect in chemistry)
- Har Gobind Khorana — Nobel 1968 (biochemistry)
- C.N.R. Rao — Bharat Ratna 2014 (materials chemistry)
10. Conclusion
Chemical Reactions and Equations is THE foundation of Class 10 Chemistry:
- Master writing and balancing equations
- Memorise 5 types of reactions
- Understand oxidation, reduction, corrosion, rancidity
- Connect to real-world applications
Practice:
- 15+ balancing problems
- Identify each reaction type
- Daily life examples (rusting, food spoilage, fireworks)
This chapter prepares you for Chapters 2-4 (Acids/Bases, Metals/Non-metals, Carbon).
Every reaction tells a story — learn to read it.
Appendix — beyond the current syllabus
Not examinable in CBSE 2026-27. The current NCERT chapter defines oxidation and reduction purely in terms of gaining or losing oxygen or hydrogen — the electron-transfer definition below, and the terms "oxidising agent" / "reducing agent," do not appear anywhere in the current text. They are kept here because older guidebooks, previous editions, and most solution websites still teach this chapter with electrons front and centre.
The electron-transfer (modern) definition
Oxidation is the loss of electrons; reduction is the gain of electrons. Memory aid: OIL RIG — Oxidation Is Loss, Reduction Is Gain (of electrons).
This is a more powerful definition because it also classifies reactions the oxygen/hydrogen rule cannot — for example, Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s) involves no oxygen or hydrogen changing hands at all, yet iron is oxidised (Fe → Fe²⁺, losing 2 electrons) and copper is reduced (Cu²⁺ → Cu, gaining 2 electrons).
Oxidising agent and reducing agent
- Oxidising agent: the substance that causes oxidation in the other reactant, by accepting its electrons — and is itself reduced in the process.
- Reducing agent: the substance that causes reduction in the other reactant, by donating electrons — and is itself oxidised in the process.
Example. In 2Mg + O₂ → 2MgO: magnesium loses electrons (oxidised) and is therefore the reducing agent; oxygen gains electrons (reduced) and is therefore the oxidising agent. This resolves the gap noted earlier in the main chapter — under the oxygen-only definition, 4Na + O₂ → 2Na₂O had an oxidised substance (Na) with no compound to call "reduced"; under the electron-transfer definition, O₂ is reduced (0 → −2), completing the pair.
