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Class 10 Chemical Reactions and Equations notes: NCERT Science Chapter 1 revision

CMDLearn · 4 November 2026

These are short revision notes for Chapter 1 of the NCERT Class 10 Science textbook, "Chemical Reactions and Equations" (Reprint 2026-27), pages 1 to 16. Almost every board question on this chapter asks you to balance an equation, name a reaction type or say what is oxidised, so these notes put the reaction types in one table, then go through the chapter section by section, and end with the key words, the set-up to draw, the common mistakes and 8 quick questions of our own with worked answers.

Keep the textbook open next to these notes. The wording here is our own; the textbook is the authority.

How much it counts

CBSE's Class 10 Science curriculum for 2026-27 puts this chapter in Unit I, Chemical Substances: Nature and Behaviour, worth 25 of the 80 theory marks, shared with Acids, Bases and Salts, Metals and Non-metals, and Carbon and its Compounds (curriculum, PDF page 4). For this chapter the unit lists chemical reactions and equations, balanced equations, and the reaction types: combination and decomposition, displacement and double displacement, precipitation, exothermic and endothermic reactions, and oxidation and reduction (PDF page 4). More on the units: CBSE Class 10 Science syllabus 2026-27.

The reaction types on one page

1. How to tell a reaction has happened (pages 1-2)

A chemical change has taken place if you see any of these:

• a change in state;
• a change in colour;
• a gas given off;
• a change in temperature.

The chapter's examples are a magnesium ribbon burning with a dazzling white flame to a white powder, zinc fizzing in dilute acid, and lead nitrate and potassium iodide solutions forming a solid.

2. Writing and balancing equations (section 1.1, pages 2-5)

• A word equation names the reactants on the left and the products on the right, with an arrow pointing to the products (page 2).
• A chemical equation uses formulae instead. If the atoms of each element are not equal on both sides it is a skeletal equation (page 3).
• Equations must be balanced because of the law of conservation of mass: mass is not made or lost, so every atom on the left must appear on the right (page 3).
• Hit-and-trial method (pages 4-5): 1. put a box round each formula and never change what is inside it: put a number in front (4H₂O), never change the formula (not H₂O₄); 2. list the atoms of each element on both sides; 3. start with the compound that has the most atoms, and the element with most atoms in it; 4. balance the remaining elements one at a time; 5. count every element again at the end. The book's example: 3Fe + 4H₂O → Fe₃O₄ + 4H₂.
• State symbols: (s) solid, (l) liquid, (g) gas, (aq) in water. Conditions such as temperature, pressure or a catalyst can be written above or below the arrow (page 5).

3. Types of reactions (section 1.2, pages 6-13)

• Combination (pages 6-7): - quick lime and water give slaked lime and a lot of heat; - burning coal, C + O₂ → CO₂; - 2H₂ + O₂ → 2H₂O. - Slaked lime used for whitewashing slowly takes up carbon dioxide from the air; within two to three days a thin, shiny coat of calcium carbonate covers the wall (page 7).
• Exothermic reactions give out heat (page 7): burning natural gas (CH₄ + 2O₂ → CO₂ + 2H₂O), respiration (glucose + oxygen → carbon dioxide + water + energy), and vegetable matter turning into compost.
• Decomposition (pages 8-10): - by heat (thermal): green ferrous sulphate crystals lose water and break into Fe₂O₃, SO₂ and SO₃; limestone gives quick lime and CO₂, which is used for cement; lead nitrate gives brown NO₂ fumes, PbO and O₂; - by electricity: water splits into hydrogen and oxygen; - by light: white silver chloride turns grey (2AgCl → 2Ag + Cl₂), and silver bromide does the same, which is used in black and white photography.
• Endothermic reactions absorb energy. Decomposition needs heat, light or electricity, so it is endothermic (page 10).
• Displacement (pages 10-11): an iron nail in copper sulphate solution turns brownish and the blue colour fades, Fe + CuSO₄ → FeSO₄ + Cu. Zinc and lead also displace copper, because they are more reactive.
• Double displacement (pages 11-12): sodium sulphate and barium chloride solutions form a white precipitate of BaSO₄, because ions are exchanged between the reactants.
• Oxidation and reduction (pages 12-13): - heating copper powder gives black CuO (oxidation, oxygen gained); passing hydrogen over hot CuO gives brown copper back (CuO is reduced, hydrogen is oxidised); - when one reactant is oxidised and another reduced, it is a redox reaction, for example ZnO + C → Zn + CO and MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂; - Oxidation: a substance takes in oxygen or gives up hydrogen. Reduction: a substance gives up oxygen or takes in hydrogen.

4. Oxidation in everyday life (section 1.3, page 13)

• Corrosion: a metal is attacked by moisture, acids and other substances around it. Examples are iron rusting to a reddish-brown powder, a black coating on silver and a green coating on copper. It damages car bodies, bridges, railings and ships, and costs a great deal every year.
• Rancidity: fats and oils oxidise, and their smell and taste change. It is slowed by antioxidants, airtight containers, and filling chip packets with nitrogen.

Key words at a glance

• Reactant / product: what you start with / what forms.
• Skeletal equation: an unbalanced chemical equation.
• Balanced equation: equal atoms of every element on both sides.
• Combination / decomposition: many into one / one into many.
• Displacement / double displacement: an element replaces another / two compounds exchange ions.
• Precipitate: an insoluble solid formed in a reaction.
• Exothermic / endothermic: gives out heat / takes in energy.
• Oxidation / reduction: gain of oxygen or loss of hydrogen / loss of oxygen or gain of hydrogen.
• Corrosion / rancidity: metals attacked by their surroundings / fats and oils oxidised.

The set-up to draw: electrolysis of water

Board papers often ask for this diagram (Fig. 1.6, page 9). Practise it:

1. A plastic mug with two carbon electrodes fixed through rubber stoppers in its base.
2. Water with a few drops of dilute sulphuric acid, covering the electrodes.
3. Two test tubes filled with water, turned upside down over the electrodes.
4. Wires from the electrodes to a 6 V battery with a switch.
5. Bubbles at both electrodes collecting in the tubes. Label which tube holds hydrogen and which oxygen, and write the reaction as decomposition by electricity.

Mistakes that cost marks

• Changing a formula to balance. Write 2H₂O, never H₂O₂: that is a different substance.
• Stopping before the final count. Check every element at the end, oxygen last.
• Calling every reaction with oxygen "combination". Burning natural gas gives two products; it is exothermic, but not combination.
• Mixing up displacement and double displacement. One element taking another's place versus two compounds swapping partners.
• Naming the wrong substance as oxidised. The substance that gains oxygen is oxidised; the one that gives up oxygen is reduced.
• Forgetting state symbols when the question asks for them. (aq) for solutions, (s) for a precipitate, (g) for gases.

Quick check: 8 questions of our own

These are our own questions, written for these notes and checked against the textbook pages named; no practising teacher has reviewed them yet. Every equation and number was re-checked by a script.

Question 1. Balance each equation by the hit-and-trial method, using the smallest whole numbers: (a) Al + HCl → AlCl₃ + H₂; (b) C₂H₆ + O₂ → CO₂ + H₂O; (c) Fe₂O₃ + CO → Fe + CO₂.

Answer:

• (a) 2Al + 6HCl → 2AlCl₃ + 3H₂. Start with AlCl₃, the compound with the most atoms: 3 Cl on the right needs 3HCl, which leaves 3 H against 2 in H₂. Double both: 6HCl and 2AlCl₃ give 6 H, balanced by 3H₂. Then 2Al. Check: Al 2 = 2, H 6 = 6, Cl 6 = 6.
• (b) 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O. One C₂H₆ gives 2CO₂ and 3H₂O, which needs 7 O atoms, that is 3½ O₂. To avoid a half, double everything. Check: C 4 = 4, H 12 = 12, O 14 = 14.
• (c) Fe₂O₃ + 3CO → 2Fe + 3CO₂. 2 Fe on the left needs 2Fe. Each CO takes one O from Fe₂O₃ to become CO₂, so its 3 O atoms need 3CO. Check: Fe 2 = 2, C 3 = 3, O 6 = 6.

(Balancing steps, pages 3-5.)

Question 2. Name the type of each reaction: (a) 2KClO₃ → 2KCl + 3O₂, on heating; (b) Mg + CuSO₄ → MgSO₄ + Cu; (c) CaCl₂ + Na₂CO₃ → CaCO₃ + 2NaCl, where CaCO₃ is an insoluble white solid; (d) 2SO₂ + O₂ → 2SO₃.

Answer: (a) thermal decomposition; (b) displacement; (c) double displacement, and also precipitation; (d) combination. (a) One reactant breaks into two products when heated (page 8). (b) Magnesium takes copper's place in its compound (pages 10-11). (c) The two compounds exchange ions, and the insoluble CaCO₃ is a precipitate (pages 11-12). (d) Two reactants form a single product (pages 6-7). Each equation balances: (a) K 2 = 2, Cl 2 = 2, O 6 = 6; (c) Ca, C, Na 2 = 2, Cl 2 = 2, O 3 = 3; (d) S 2 = 2, O 6 = 6.

Question 3. Write a balanced equation with state symbols: marble chips (solid calcium carbonate) are dropped into dilute hydrochloric acid, giving a solution of calcium chloride, water and carbon dioxide gas.

Answer: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g). The 2 Cl in CaCl₂ need 2HCl, which also supplies the 2 H in water. Check: Ca 1 = 1, C 1 = 1, O 3 = 3, H 2 = 2, Cl 2 = 2. The state symbols follow page 5: (s) solid, (aq) in water, (l) liquid, (g) gas.

Question 4. Sort these into exothermic and endothermic, and give the chapter's reason for each: (a) vegetable waste turning into compost in a pit; (b) silver bromide on photographic film darkening in light; (c) respiration in our cells; (d) lead nitrate heated in a boiling tube; (e) natural gas burning on a stove.

Answer: exothermic: (a), (c), (e); endothermic: (b), (d). The chapter names compost formation, respiration and burning natural gas as reactions that give out heat (page 7). Silver bromide decomposes using light energy and lead nitrate decomposes using heat; decomposition reactions take in energy as heat, light or electricity, so they are endothermic (pages 9-10).

Question 5. In a lime kiln, 25 g of limestone (CaCO₃) is heated until it has fully decomposed, and 14 g of quick lime (CaO) is left. (a) Write the equation. (b) What mass of carbon dioxide was given off? (c) Which law did you use? (d) Check your answer with Ca = 40, C = 12 and O = 16.

Answer: (a) CaCO₃ → CaO + CO₂, on heating; (b) 11 g; (c) the law of conservation of mass; (d) it agrees. (a) This is the chapter's thermal decomposition of limestone (page 8). (b) Mass is neither made nor lost, so 25 − 14 = 11 g of CO₂ left the kiln (page 3). (d) CaCO₃ = 40 + 12 + 48 = 100, CaO = 56 and CO₂ = 44, so 100 g gives 56 g + 44 g. For 25 g, a quarter of that: 14 g + 11 g.

Question 6. For each reaction, name the substance oxidised and the substance reduced, and say why: (a) Fe₂O₃ + 3CO → 2Fe + 3CO₂; (b) H₂S + Cl₂ → 2HCl + S.

Answer: (a) CO is oxidised, Fe₂O₃ is reduced; (b) H₂S is oxidised, Cl₂ is reduced. (a) CO gains oxygen and becomes CO₂, while Fe₂O₃ loses oxygen and becomes iron. (b) No oxygen takes part, so use hydrogen: H₂S loses its hydrogen (and becomes sulphur), so it is oxidised; Cl₂ gains hydrogen (and becomes HCl), so it is reduced. The rule is on page 13: gaining oxygen or losing hydrogen is oxidation, and losing oxygen or gaining hydrogen is reduction. Both are redox reactions, because one substance is oxidised while the other is reduced (page 12).

Question 7. (a) A zinc strip is left in blue copper sulphate solution. What two changes would you see, and what is the equation? (b) A copper strip is left in colourless zinc sulphate solution. Will anything happen? (c) What type of reaction is (a)?

Answer: (a) the blue colour fades and a reddish-brown coat of copper forms on the zinc; Zn + CuSO₄ → ZnSO₄ + Cu; (b) no; (c) displacement. (a) Zinc is more reactive than copper and displaces it from its compound (page 11); copper is the reddish-brown metal, and the copper sulphate that gives the blue colour is used up. (b) Copper is less reactive than zinc, so it cannot push zinc out of zinc sulphate. (c) One element replaces another in a compound (page 11).

Question 8. (a) A wall is whitewashed with slaked lime. Why does it look shiny after two or three days? Write the equation. (b) If 7.4 g of calcium hydroxide on the wall reacted fully, what mass of calcium carbonate would form? (Ca = 40, O = 16, H = 1, C = 12.) (c) A family keeps ghee in an airtight steel tin. Which everyday oxidation process does this slow down?

Answer: (a) a thin layer of calcium carbonate forms; Ca(OH)₂ + CO₂ → CaCO₃ + H₂O; (b) 10 g; (c) rancidity. (a) Calcium hydroxide slowly reacts with carbon dioxide in the air to form calcium carbonate, which gives the shine (page 7). Check: Ca 1 = 1, O 4 = 4, H 2 = 2, C 1 = 1. (b) Ca(OH)₂ = 40 + 2 × 17 = 74 and CaCO₃ = 100, so 74 g gives 100 g; 7.4 g gives 10 g. (c) Fats and oils go rancid when they are oxidised, and airtight containers slow this down (page 13).

Revise with questions

Test yourself next: Class 10 Chemical Reactions and Equations, 8 practice questions with answers. For a walk through the chapter with worked examples, see NCERT Class 10 Science Chapter 1 explained. The next chapter's notes: Class 10 Acids, Bases and Salts notes. For timed practice, the free Class 10 revision papers include a Science paper laid out in 1, 2, 3 and 5-mark sections.

Where CMDLearn fits

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Sources

All checked on 4 November 2026. NCERT page numbers are the printed page numbers of the textbook, which match the PDF pages of this chapter.

• NCERT, Science, Textbook for Class X, Chapter 1 "Chemical Reactions and Equations" (Reprint 2026-27), pages 1-16: signs of a reaction (pages 1-2); section 1.1, writing and balancing equations (pages 2-5); section 1.2, types of reactions (pages 6-13); section 1.3, corrosion and rancidity (page 13); "What you have learnt" (page 14). ncert.nic.in/textbook/pdf/jesc101.pdf
• CBSE, Science curriculum, Class X, 2026-27: unit marks and Unit I contents (PDF page 4): cbseacademic.nic.in/web_material/CurriculumMain27/SecPart1/Science_SecP1_2026-27.pdf

The facts, equations, section numbers and figure numbers are from the documents above. The wording of these notes, the diagram steps, the list of mistakes and the 8 questions are our own.

Practise this class: Class 10 worksheets, chapter by chapter · free practice, marked instantly · timed revision papers