NCERT Class 10 Science Chapter 1 — Chemical Reactions and Equations | Full Notes, Solutions & Tricks (Convex Classes, Jaipur)

If you ask any topper in Jaipur which Class 10 Science chapter decides whether your Chemistry foundation is strong or shaky, most will say Chapter 1 — Chemical Reactions and Equations. It’s short, it’s scoring, and it’s also the chapter where careless students lose the easiest marks — usually because they never actually learned why an equation balances, they just memorised the answer.

At Convex Classes, Jaipur, we’ve taught this chapter to hundreds of Class 10 students, and this guide is built the way we teach it in our own batches: concept first, trick second, NCERT answer last — so you’re never just copying a solution, you actually understand it.

Why This Chapter Matters More Than Students Think

Chemical Reactions and Equations isn’t just “chapter 1 marks.” It’s the entry point to everything you’ll study in Chemistry for the next 3–4 years:

• Class 11–12 Physical & Inorganic Chemistry (redox reactions, electrochemistry) builds directly on the oxidation-reduction concept you learn here.
• Balancing equations here is the same logical skill you’ll use in stoichiometry later.
• Board exams almost always pick 4–6 marks directly from this chapter — combination, decomposition, displacement, double displacement, and corrosion/rancidity questions repeat with only the compounds changed.

So the goal isn’t to “finish” this chapter. It’s to master the pattern of questions, because PYQs show the same pattern tested every single year with different chemicals.

Concept Breakdown (In Convex Classes’ Own Words)

1. What Actually Is a Chemical Reaction?

A chemical reaction is a process where the bonds between atoms break and new bonds form, producing a substance with properties completely different from what you started with. This is the one-line difference examiners look for: a physical change (like ice melting) changes the form; a chemical change changes the identity of the substance.

Quick identification tips students forget:

• Change in colour, smell, or state
• Formation of a precipitate (insoluble solid)
• Gas evolution (bubbles)
• Temperature change (reaction feels hot or cold)

If you spot two or more of these in an activity-based question, it’s almost certainly asking you to identify a chemical reaction.

2. Chemical Equations — Why “Balancing” Isn’t Optional

A chemical equation is a shorthand for a chemical reaction using formulae instead of names. But an unbalanced equation is scientifically wrong, not just incomplete — because it violates the Law of Conservation of Mass (matter can neither be created nor destroyed in a chemical reaction). If your equation has 4 hydrogen atoms on the left and 2 on the right, you’ve claimed atoms vanished — which is impossible.

Convex Classes’ balancing method (works for every reaction in this chapter):

1. List every element and count atoms on both sides.
2. Start balancing with the element that appears in the most compounds last, and the one appearing in the fewest compounds first.
3. Use whole-number coefficients only — never fractions in the final answer.
4. Recheck every element after each change (a common student mistake is balancing one element and forgetting you’ve disturbed another).

3. The Four (Really Five) Reaction Types — A Comparison Table

Reaction Type	What Happens	General Form	Example
Combination	Two or more reactants form one product	A + B → AB	CaO + H₂O → Ca(OH)₂
Decomposition	One reactant breaks into two or more products (needs heat/light/electricity)	AB → A + B	CaCO₃ →(heat) CaO + CO₂
Displacement	A more reactive element pushes out a less reactive one from its compound	A + BC → AC + B	Fe + CuSO₄ → FeSO₄ + Cu
Double Displacement	Two compounds exchange ions, often forming a precipitate	AB + CD → AD + CB	Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl
Oxidation-Reduction (Redox)	One substance gains oxygen/loses electrons (oxidised), another loses oxygen/gains electrons (reduced) — happens simultaneously	—	CuO + H₂ → Cu + H₂O

Memory trick we use in class: Decomposition is the opposite of Combination — if Combination is “getting together,” Decomposition is “breaking up,” almost always needing outside energy (heat, light, or electricity) to happen, exactly like breaking up something stable usually needs some push.

4. Oxidation and Reduction — The Part Students Fear Most (and Shouldn’t)

Forget memorising definitions in isolation. Learn them as a pair, because they never happen alone:

• Oxidation = gain of oxygen, or loss of hydrogen, or loss of electrons
• Reduction = loss of oxygen, or gain of hydrogen, or gain of electrons

The mnemonic that actually works: “OIL RIG” — Oxidation Is Loss, Reduction Is Gain (of electrons).

In CuO + H₂ → Cu + H₂O, CuO loses oxygen (reduced to Cu), and H₂ gains oxygen (oxidised to H₂O). Both happen in the same equation — that’s why it’s called a redox reaction.

Fully Worked NCERT Solutions — In-Text & Exercise Questions

In-Text Questions (Page 6)

Q1. Why should a magnesium ribbon be cleaned before burning in air?

Magnesium metal reacts slowly with atmospheric oxygen even at room temperature, forming a thin, dull layer of magnesium oxide (MgO) on its surface. This layer is chemically stable and acts like a barrier — it prevents the fresh metal underneath from burning properly when you try to ignite it. Cleaning (usually with sandpaper) removes this oxide coat so the magnesium burns with its characteristic bright white flame.

Q2. Balance the following:

i) H₂ + Cl₂ → 2HCl ii) 3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄ + 2AlCl₃ iii) 2Na + 2H₂O → 2NaOH + H₂

Q3. Balanced equations with state symbols:

i) BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq) ii) NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

In-Text Questions (Page 10)

Q1. Substance ‘X’ used for whitewashing: ‘X’ is quicklime (Calcium Oxide, CaO).

When mixed with water, it forms slaked lime: CaO + H₂O → Ca(OH)₂

Q2. Why is gas collected in one test tube double the other (Activity 1.7)?

This activity is the electrolysis of water. Water splits into hydrogen and oxygen gas in a fixed ratio — 2 volumes of hydrogen for every 1 volume of oxygen — because a water molecule (H₂O) has twice as many hydrogen atoms as oxygen atoms. So the test tube collecting hydrogen fills up twice as fast.

In-Text Questions (Page 13)

Q1. Why does copper sulphate solution change colour with an iron nail?

Iron is more reactive than copper (higher in the reactivity series), so it displaces copper from copper sulphate solution. The blue colour of CuSO₄ fades as it’s converted to light green FeSO₄, and reddish-brown copper deposits on the nail. Fe + CuSO₄ → FeSO₄ + Cu

Q2. A double displacement reaction example (other than in Activity 1.10):

NaCl + AgNO₃ → AgCl↓ + NaNO₃ Here, Na⁺ and Ag⁺ exchange partners, and a white precipitate of silver chloride forms — a classic double displacement.

Q3. Identify oxidised and reduced substances:

i) 4Na + O₂ → 2Na₂O — Sodium is oxidised (gains oxygen) ii) CuO + H₂ → Cu + H₂O — Copper is reduced (loses oxygen); hydrogen is oxidised

Exercise Questions (Full Solutions)

Q1. Which statements about 2PbO + C → 2Pb + CO₂ are incorrect?

Answer: (i) (a) and (b) — Lead is reduced (not oxidised, since it loses oxygen), and carbon dioxide cannot be “oxidised” since it’s already the final oxidised product here — it’s carbon that gets oxidised.

Q2. Fe₂O₃ + 2Al → Al₂O₃ + 2Fe is an example of:

Answer: Displacement reaction. Aluminium is more reactive than iron, so it takes oxygen away from Fe₂O₃, freeing iron metal and forming aluminium oxide. (This is actually the thermite reaction, used in welding railway tracks — a great real-world example to mention in your answer for extra marks.)

Q3. Dilute HCl added to iron filings produces:

Answer: Hydrogen gas and iron chloride. Fe + 2HCl → FeCl₂ + H₂

Q4. What is a balanced chemical equation and why balance it?

An equation is balanced when the number of atoms of each element is equal on both sides. Balancing is compulsory because chemical reactions must obey the Law of Conservation of Mass — atoms are never created or destroyed, only rearranged.

Q5. Translate and balance:

a) N₂ + 3H₂ → 2NH₃ b) 2H₂S + 3O₂ → 2H₂O + 2SO₂ c) 3BaCl₂ + Al₂(SO₄)₃ → 2AlCl₃ + 3BaSO₄ d) 2K + 2H₂O → 2KOH + H₂

Q6. Balance these equations:

a) 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O b) 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O c) NaCl + AgNO₃ → AgCl + NaNO₃ (already balanced) d) BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl

Q7. Write balanced equations:

• 2Ca(OH)₂ + 2CO₂ → 2CaCO₃ + 2H₂O
• Zn + 2AgNO₃ → Zn(NO₃)₂ + 2Ag
• 2Al + 3CuCl₂ → 2AlCl₃ + 3Cu
• BaCl₂ + K₂SO₄ → BaSO₄ + 2KCl

Q8. Balance and identify reaction type:

• 2KBr + BaI₂ → 2KI + BaBr₂ — Double displacement
• ZnCO₃ → ZnO + CO₂ — Decomposition
• H₂ + Cl₂ → 2HCl — Combination
• Mg + 2HCl → MgCl₂ + H₂ — Displacement

Q9. Exothermic vs Endothermic (with examples):

An exothermic reaction releases energy to the surroundings, usually felt as heat (e.g., burning of fuel, respiration). An endothermic reaction absorbs energy from the surroundings (e.g., photosynthesis, decomposition of calcium carbonate on heating).

Q10. Why is respiration exothermic?

During respiration, glucose combines with oxygen inside our cells and breaks down to release carbon dioxide, water, and energy, which the body uses to maintain temperature and carry out life processes — since energy is released, it’s exothermic. C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy

Q11. Why is decomposition called the opposite of combination? Give examples.

Combination joins two or more simple substances into one compound; decomposition breaks one compound into two or more simpler substances — exactly reversed. Decomposition usually needs external energy (heat/light/electricity) since breaking bonds requires energy input, unlike combination which often releases energy. Examples: CaCO₃ →(heat) CaO + CO₂ ; 2HgO →(heat) 2Hg + O₂

Q12. One decomposition example each for heat, light, and electricity:

• Heat: 2KClO₃ →(heat) 2KCl + 3O₂
• Electricity: 2NaCl(molten) →(electrolysis) 2Na + Cl₂
• Light: 2H₂O₂ →(light) 2H₂O + O₂ (this is why hydrogen peroxide is stored in dark/brown bottles!)

Q13. Difference between displacement and double displacement:

In displacement, one element replaces another element in a compound (single exchange). In double displacement, two compounds swap ions with each other (mutual exchange), and one product is often an insoluble precipitate.

• Displacement: Zn + CuSO₄ → ZnSO₄ + Cu
• Double displacement: Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl

Q14. Recovery of silver from silver nitrate using copper:

Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag Copper is more reactive than silver, so it displaces silver from the solution.

Q15. What is a precipitation reaction?

A reaction in which two soluble salts react in solution to form an insoluble solid (precipitate) that settles out. Examples: K₂S + CdSO₄ → CdS↓ + K₂SO₄ MgCl₂ + 2NaOH → Mg(OH)₂↓ + 2NaCl

Q16. Oxidation and Reduction in terms of gain/loss of oxygen (2 examples each):

• Oxidation: 2Mg + O₂ → 2MgO ; C + O₂ → CO₂
• Reduction: CuO + H₂ → Cu + H₂O ; ZnO + C → Zn + CO

Q17. Shiny brown element ‘X’ turns black on heating in air:

‘X’ is copper. On heating, it reacts with atmospheric oxygen to form black copper(II) oxide. 2Cu + O₂ → 2CuO

Q18. Why do we paint iron articles?

Painting creates a physical barrier that stops the iron surface from coming into contact with oxygen and moisture, which are both essential for rusting to occur. This is a preventive measure against corrosion.

Q19. Why are oil/fat-containing foods flushed with nitrogen?

Oxygen in air reacts slowly with fats and oils, making them rancid (bad smell/taste). Nitrogen is an unreactive (inert) gas, so replacing air with nitrogen inside the packet prevents this oxidation, keeping the food fresh longer.

Q20. Explain with one example each:

• Corrosion: Slow eating away of a metal surface due to reaction with air, moisture, or chemicals in the environment — e.g., rusting of iron (forms hydrated iron oxide, Fe₂O₃·xH₂O).
• Rancidity: Slow oxidation of fats/oils in food, causing an unpleasant smell and taste — e.g., stale, old cooking oil or chips smelling “off.”

Common Mistakes Convex Classes Students Make (and How to Avoid Them)

1. Forgetting state symbols when asked specifically — (s), (l), (g), (aq) are worth marks on their own.
2. Balancing polyatomic ions incorrectly — treat groups like SO₄²⁻ or NO₃⁻ as single units while balancing rather than splitting them into S, O, N separately; it’s faster and avoids errors.
3. Mixing up “reactive” with “reduced” — a common trap in reasoning questions (like Q1 and Q2 above). Always double-check who is gaining and who is losing oxygen.
4. Skipping the “why” in exothermic/endothermic answers — examiners want the reasoning (energy released/absorbed), not just the label.
5. Writing decomposition without the energy source — heat, light, or electricity should be explicitly named or shown above the arrow.

Extra Practice Questions (Beyond NCERT) — For Board-Level Practice

1. A silver article turns black over time when exposed to air. Identify the type of reaction and write the balanced equation.
2. Why is the reaction between quicklime and water considered both a combination reaction and an exothermic reaction?
3. Identify the oxidising and reducing agents in: MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂
4. A student adds dilute sulphuric acid to zinc granules in a test tube. State two observations and write the balanced equation.
5. Give one example of a reaction that is both a decomposition reaction and endothermic in nature, and explain why heating is essential for it.

(Convex Classes students: these are discussed in detail with step-by-step boards in our regular batches — ask your subject teacher or message us on WhatsApp for the full solved set.)

Convex Classes’ Quick Revision Checklist Before Your Test

• Can you write and balance all NCERT equations from memory, not just recognise them?
• Can you name the reaction type for any equation given in exam, even unfamiliar compounds?
• Do you know at least 2 real-life examples each for oxidation, reduction, corrosion, and rancidity?
• Can you explain OIL RIG and apply it to a new redox equation?
• Have you practiced writing state symbols alongside every equation?

Need Personal Guidance on This Chapter?

Reading notes online is a good start, but board exam scoring comes from practice, doubt-clearing, and structured revision — which is exactly what we do at Convex Classes, Jaipur in our Class 10 Science batches.

If you’d like a free demo class, chapter-wise test series, or personal doubt support for Class 10 Science, message us directly on WhatsApp: 8290601516.

FAQs

Q1. Is Chapter 1 Chemical Reactions and Equations easy to score in Class 10 board exams?

Yes, it’s one of the highest-scoring chapters if you focus on balancing equations correctly and identifying reaction types — the pattern of questions barely changes year to year.

Q2. How many reaction types are there in this chapter?

Four main types (combination, decomposition, displacement, double displacement), plus the cross-cutting concept of oxidation-reduction, which can occur within any of these.

Q3. What is the easiest way to remember OIL RIG?

Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons) — apply it to any equation by tracking which substance gains or loses oxygen/hydrogen/electrons.

Q4. Where can Jaipur students get in-person doubt support for this chapter?

Convex Classes, Jaipur offers dedicated Class 10 Science batches with chapter tests and doubt sessions — WhatsApp 8290601516 for details.