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NCERT Class 10 Science Chapter 1, Chemical Reactions and Equations, explained simply

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Chemical Reactions and Equations opens the NCERT Class 10 Science textbook (Reprint 2026-27). It teaches three skills that the rest of the chemistry chapters keep using: writing a reaction as a balanced equation, naming the type of a reaction, and saying which substance is oxidised and which is reduced. This guide explains each one in plain words, shows how the CBSE sample paper asks them, and ends with a self-check and a way to test an AI's answer.

Keep the textbook open next to it. Our explanations and examples are our own; the textbook is the authority.

How much it counts in the board exam

CBSE's Class 10 Science curriculum for 2026-27 places this chapter in Unit I, Chemical Substances: Nature and Behaviour, worth 25 of the 80 theory marks. The unit also holds Acids, Bases and Salts; Metals and Non-metals; and Carbon and its Compounds, and CBSE does not split the 25 marks by chapter. For this chapter the curriculum lists: chemical equations, balanced equations, and the reaction types (combination, decomposition, displacement, double displacement, precipitation, exothermic and endothermic, oxidation and reduction).

The 2026-27 sample paper (Science 086) has three sections, one each for Biology, Chemistry and Physics. In the Chemistry section the chapter appears here:

QuestionMarksWhat it asks
Q171Is a chameleon's change of colour a chemical change or a physical one?
Q221A biscuit label lists two antioxidants: which other substance goes into food packets to do the same job?
Q29, option A5Who is right about rusting and the making of ammonia being oxidation-reduction reactions; then find and correct the errors in two written equations
Q29, option B5Who has understood decomposition correctly; then judge two lab observations (a metal added to a salt solution, a metal added to an acid)

The idea of oxidation and reduction also turns up in the Biology section: Q1 asks what happens to carbon dioxide and water in photosynthesis.

The curriculum also adds a note: information printed in the textbook's boxes (such as the "Do You Know?" box on whitewashing) is not assessed in the year-end exam. It is still worth reading.

1. Chemical change, and how you can tell (start of the chapter)

In a chemical change a new substance forms. The chapter's opening activities (burning magnesium ribbon, zinc in dilute acid, lead nitrate with potassium iodide) give four signs to look for:

A sign is a clue, not a proof. The sample paper's Q17 is built on exactly this: a chameleon changes colour, but no new substance forms (only the spacing of crystals in its skin changes), so it is a physical change.

2. Writing and balancing an equation (section 1.1)

A word equation names the reactants on the left, an arrow, and the products on the right: magnesium + oxygen → magnesium oxide. Writing formulae instead gives a skeletal equation: Mg + O₂ → MgO.

Count the atoms. There are 2 oxygen atoms on the left and 1 on the right, so this equation is not balanced. Mass cannot be created or destroyed in a reaction (the law of conservation of mass from Class 9), so each element needs the same number of atoms on both sides.

The rule that matters most: change only the numbers in front of formulae, never the small numbers inside them. Writing MgO₂ to fix the oxygen would describe a different substance.

The book calls its method "hit-and-trial", and works it on iron and steam. In short:

  1. Write the skeletal equation and list the atoms of each element on each side.
  2. Start with the formula that has the most atoms, and with its largest number.
  3. Put a number in front of a formula, recount, and move to the next element that is still unequal.
  4. Finish by counting every element again on both sides.

Worked: aluminium burning in oxygen. Al + O₂ → Al₂O₃

Worked: burning propane (cooking gas). C₃H₈ + O₂ → CO₂ + H₂O

State symbols make an equation say more: (s) solid, (l) liquid, (g) gas and (aq) for a solution in water. The book also writes conditions such as heat, sunlight or a catalyst above or below the arrow.

3. The types of reaction (section 1.2)

TypeWhat happensExample
CombinationTwo or more substances join into one productCaO(s) + H₂O(l) → Ca(OH)₂(aq): quicklime and water give slaked lime, and a lot of heat
DecompositionOne substance breaks into two or more simpler onesCaCO₃ → CaO + CO₂ on heating (limestone gives quicklime)
DisplacementA more reactive element pushes a less reactive one out of its compoundFe + CuSO₄ → FeSO₄ + Cu: the blue solution fades and the iron nail turns brown with copper
Double displacementTwo compounds swap partners (they exchange ions)Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl
PrecipitationAny reaction that produces an insoluble solid (a precipitate). In this chapter's examples it is a double displacement.The white BaSO₄ above; or Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃, a yellow solid

Decomposition needs energy, in one of three forms

The sample paper's Q29 option B tests a common confusion here. The reactant in a decomposition must be a single compound; the products can be elements (hydrogen and oxygen from water) or compounds (magnesium oxide and carbon dioxide from magnesium carbonate). Both are decomposition.

Exothermic and endothermic

A quick way to name the type

  1. One reactant and several products? Decomposition.
  2. Several reactants and one product? Combination.
  3. An element plus a compound gives a different element plus a compound? Displacement.
  4. Two compounds give two new compounds by swapping parts? Double displacement (and precipitation if one product is an insoluble solid).
  5. Then, separately, ask: did any substance gain or lose oxygen (or hydrogen)? If yes, it is also a redox reaction. A reaction can be two types at once.

4. Oxidation and reduction (section 1.2.5)

In the book's definition:

The two always happen together, so these are called redox reactions. Take hot copper oxide with hydrogen: CuO + H₂ → Cu + H₂O. Copper oxide loses its oxygen, so copper oxide is reduced; hydrogen gains that oxygen, so hydrogen is oxidised.

Name the substance, not just the element. In 2PbO + C → 2Pb + CO₂, it is lead oxide that is reduced (it loses oxygen) and carbon that is oxidised. "Lead is reduced" is the trap in the book's first exercise.

Q29 option A of the sample paper asks about two reactions you can now judge:

The second half of each answer comes from the book's point that one reactant is oxidised while the other is reduced (p. 12), not from the gain/loss rule. The CBSE marking scheme gives exactly this answer.

5. Oxidation in daily life (section 1.3)

6. Mistakes that cost marks

  1. Changing a formula to balance an equation. You may only change the numbers in front.
  2. Writing a wrong formula, then balancing it. In the sample paper's Q29, one equation gives AgCl₂ and MgNO₃; the real formulae are AgCl and Mg(NO₃)₂. Fix the formulae first, then balance.
  3. Mixing up displacement and double displacement. Displacement starts with an element and a compound; double displacement starts with two compounds.
  4. Forgetting reactivity. A metal displaces only a less reactive metal. Iron cannot push magnesium out of magnesium sulphate, so nothing happens there (observation (i) in Q29 option B is wrong; the marking scheme also rejects observation (ii): a metal in dilute acid gives a few hydrogen bubbles, not brisk effervescence).
  5. Calling every colour change a chemical reaction. Ask whether a new substance formed.

7. Self-check: 6 questions with answers

Try them on paper first. Answers are below.

  1. Balance: Fe + O₂ → Fe₂O₃.
  2. On heating, 2KClO₃ → 2KCl + 3O₂. What type of reaction is this? Is the equation balanced?
  3. Will copper displace zinc from zinc sulphate solution? Why or why not?
  4. In 2Mg + CO₂ → 2MgO + C, which substance is oxidised and which is reduced?
  5. Exothermic or endothermic: (a) burning LPG on a stove; (b) silver bromide breaking down in sunlight?
  6. Find the error and correct it: Zn + HCl → ZnCl + H₂.

Answers: 1) 4Fe + 3O₂ → 2Fe₂O₃. Check: Fe 4 = 4, O 6 = 6. 2) Decomposition (thermal decomposition, as it needs heat). Yes, it is balanced: K 2 = 2, Cl 2 = 2, O 6 = 6. 3) No. Copper is less reactive than zinc, so it cannot displace zinc. 4) Magnesium gains oxygen, so magnesium is oxidised. Carbon dioxide loses oxygen, so carbon dioxide is reduced. 5) (a) Exothermic: heat is given out. (b) Endothermic: the light energy is taken in to break it down. 6) Zinc chloride is ZnCl₂, not ZnCl. Correct and balanced: Zn + 2HCl → ZnCl₂ + H₂. Check: Zn 1 = 1, H 2 = 2, Cl 2 = 2.

8. How to check an AI's answer for this chapter

Chatbots write equations fast, and they still get them wrong. These checks take a minute:

More on using AI well: how to use ChatGPT for homework without cheating yourself.

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Sources

All checked on 20 October 2026.

The definitions, section numbers, reactions named from the book and the question types are from the documents above. All explanations, worked examples and self-check questions on this page are our own; every equation was checked by counting atoms.

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