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Schooling · Class 10 · Science · Chemical Reactions and Equations · Chapter on a page

CBSE Class 10 · Science · NCERT Science

Chemical Reactions and Equations: chapter on a page

A cut of Shishya's notes for this chapter: the first paragraph of the introduction and the key ideas, formulas and facts, common mistakes and quick revision lists, word for word. Nothing is added or reworded; the worked examples are left out (they are on the chapter page).

About the notes: Written by Shishya's AI on 26 Sept 2026 from the chapter's title and class level, in Shishya's own words — not a copy or summary of the textbook. Read the official chapter for the book's own text, activities and exercises.

This note's current text has no applied fact-check on record.

Mind map

Drawn from the same notes by a fixed rule (no AI): each label is the item's own words — its bold term, its label before a colon, a short formula as written, or its first words.

Mind map of Chemical Reactions and Equations: Key ideas, Formulas and facts, Watch out forKey ideasA chemical reaction involves…A chemical equation uses symbols and…Equations must be balancedPhysical states are shown as (s) for…Combination reactionDecomposition reactionDisplacement reactionFormulas and factsLaw of conservation of massCombination reaction exampleDecomposition by heat (thermal…Decomposition by electricity…Displacement reaction exampleDouble displacement reactionOxidation-reduction (redox)Watch out forWriting unbalanced equations and…Changing subscripts in formulas to…Confusing physical change with…Thinking all reactions that release…Forgetting to write physical statesChemical Reactionsand Equations
The mind map as a list
  • Key ideas
    • A chemical reaction involves…
    • A chemical equation uses symbols and…
    • Equations must be balanced
    • Physical states are shown as (s) for…
    • Combination reaction
    • Decomposition reaction
    • Displacement reaction
  • Formulas and facts
    • Law of conservation of mass
    • Combination reaction example
    • Decomposition by heat (thermal…
    • Decomposition by electricity…
    • Displacement reaction example
    • Double displacement reaction
    • Oxidation-reduction (redox)
  • Watch out for
    • Writing unbalanced equations and…
    • Changing subscripts in formulas to…
    • Confusing physical change with…
    • Thinking all reactions that release…
    • Forgetting to write physical states

What this chapter is about

This chapter introduces the language chemists use to describe chemical changes. When substances interact and form new substances with different properties, a chemical reaction has occurred. You learn to represent these changes using chemical equations, where symbols and formulas replace long descriptions.

Key ideas

  • A chemical reaction involves breaking of bonds in reactants and forming new bonds to produce products with different properties.
  • A chemical equation uses symbols and formulas to show reactants (left side) and products (right side), separated by an arrow (→).
  • Equations must be balanced: the number of atoms of each element must be equal on both sides, following the law of conservation of mass.
  • Physical states are shown as (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous (dissolved in water).
  • Combination reaction: two or more substances combine to form a single product. Example: A + B → AB.
  • Decomposition reaction: a single compound breaks into two or more simpler substances. Example: AB → A + B.
  • Displacement reaction: a more reactive element replaces a less reactive one from its compound. Example: A + BC → AC + B.
  • Oxidation is gain of oxygen or loss of hydrogen; reduction is loss of oxygen or gain of hydrogen. Reactions involving both are called redox reactions.

Formulas and facts to remember

  • Law of conservation of mass: In a chemical reaction, total mass of reactants equals total mass of products.
  • Combination reaction example: 2Mg + O₂ → 2MgO (magnesium burns in air to form magnesium oxide).
  • Decomposition by heat (thermal decomposition): CaCO₃ → CaO + CO₂ (limestone breaks down on heating).
  • Decomposition by electricity (electrolysis): 2H₂O → 2H₂ + O₂ (water splits into hydrogen and oxygen).
  • Displacement reaction example: Zn + CuSO₄ → ZnSO₄ + Cu (zinc displaces copper from copper sulphate solution).
  • Double displacement reaction: exchange of ions between two compounds. Example: Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl.
  • Oxidation-reduction (redox): In CuO + H₂ → Cu + H₂O, CuO is reduced (loses oxygen) and H₂ is oxidised (gains oxygen).
  • Signs of a chemical reaction: change in colour, evolution of gas, formation of precipitate, change in temperature, change in smell.

Common mistakes

  • Writing unbalanced equations and assuming they are correct → always count atoms of each element on both sides before finishing.
  • Changing subscripts in formulas to balance an equation → only change coefficients (numbers before formulas), never subscripts.
  • Confusing physical change with chemical change → physical changes (melting ice) do not produce new substances; chemical changes do.
  • Thinking all reactions that release gas are decomposition → gas can also form in displacement or double displacement reactions.
  • Forgetting to write physical states → states (s), (l), (g), (aq) give important information about the reaction conditions.

Quick revision

  • Chemical equation: reactants → products, using symbols and formulas.
  • Balance equations by adjusting coefficients, not subscripts.
  • Combination: many → one; Decomposition: one → many.
  • Displacement: reactive element kicks out less reactive one.
  • Oxidation = gain of oxygen or loss of hydrogen; Reduction = opposite.
  • Redox reactions have both oxidation and reduction happening together.