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Bihar Board Class 10 · Science · Matric exam 2027

Chapter 1: Chemical Reactions and Equations — notes

The chapter’s 7 parts in the book’s order — the first idea of each and the points Medha Ma’am writes on the board — and the summary at the end. The full lesson and a check question for each part are in the app.

This chapter’s important questions →

  1. 1. What is a Chemical Reaction?

    Okay, so let's begin by noticing simple things around you.

    • • Chemical reaction: nature/identity of substance changes
    • • Signs: change in state, change in colour, gas evolved, change in temperature
    • • Example: Mg ribbon burns → white Magnesium oxide
    • • Example: Zinc + acid → gas + heat

  2. 2. Writing Chemical Equations

    Okay, so you already know long sentences about reactions can be written shorter.

    • • Word-equation: Magnesium + Oxygen → Magnesium oxide
    • • Reactants (LHS) → Products (RHS)
    • • Chemical equation: Mg + O2 → MgO
    • • This is a skeletal (unbalanced) equation

  3. 3. Balancing a Chemical Equation

    Okay, so — do you remember the law of conservation of mass, from Class IX?

    • • Law of conservation of mass: atoms same before & after
    • • Step 1: Fe + H2O → Fe3O4 + H2 (unbalanced)
    • • Balance O: Fe + 4H2O → Fe3O4 + H2
    • • Balance H: Fe + 4H2O → Fe3O4 + 4H2
    • • Balance Fe: 3Fe + 4H2O → Fe3O4 + 4H2
    • • Final: 3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g)

  4. 4. Combination Reactions and Exothermic Reactions

    Okay, so let's talk about calcium oxide, the quick lime.

    • • Combination reaction: two or more reactants → one product
    • • CaO(s) + H2O(l) → Ca(OH)2(aq)
    • • C(s) + O2(g) → CO2(g)
    • • 2H2(g) + O2(g) → 2H2O(l)
    • • Exothermic: heat is released with products
    • • Respiration: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy

  5. 5. Decomposition Reactions (Heat, Light, Electricity)

    Okay, so let's look at decomposition reactions.

    • • Decomposition: one reactant → two or more products
    • • 2FeSO4(s) --Heat--> Fe2O3(s) + SO2(g) + SO3(g)
    • • CaCO3(s) --Heat--> CaO(s) + CO2(g)
    • • 2Pb(NO3)2(s) --Heat--> 2PbO(s) + 4NO2(g) + O2(g)
    • • 2AgCl(s) --Sunlight--> 2Ag(s) + Cl2(g)
    • • Endothermic: energy is absorbed

  6. 6. Displacement and Double Displacement Reactions

    Okay, so let's talk about what happens when you dip iron nails in copper sulphate solution.

    • • Displacement: Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
    • • Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
    • • Pb(s) + CuCl2(aq) → PbCl2(aq) + Cu(s)
    • • Double displacement: Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq)
    • • Precipitate: insoluble substance formed

  7. 7. Oxidation, Reduction and Everyday Effects

    Okay, so let's talk about oxidation and reduction.

    • • Oxidation: gain of oxygen (or loss of hydrogen)
    • • Reduction: loss of oxygen (or gain of hydrogen)
    • • 2Cu + O2 --Heat--> 2CuO (copper oxidised)
    • • CuO + H2 --Heat--> Cu + H2O (CuO reduced, H2 oxidised)
    • • Corrosion: metal attacked by moisture, acids etc (e.g. rusting)
    • • Rancidity: oxidation of fats/oils spoils smell and taste

Summary

Okay, so let's go back over everything we did in this chapter. A chemical reaction changes the nature and identity of a substance. How do you know one has happened? A change in state, a change in colour, a gas forming, or a change in temperature. We first wrote these as word-equations, then as chemical equations using formulae. And remember — a skeletal equation must be balanced, so the number of atoms of each element stays the same on both sides. That's the law of conservation of mass. Then we looked at the main types of reactions. In a combination reaction, two or more substances join into one, and this often releases heat, so it's exothermic. A decomposition reaction is the opposite — one substance breaks into simpler ones, using heat, light or electricity, and here energy is absorbed, so it's endothermic. We also saw displacement reactions, where a more reactive element pushes out another from its compound. And double displacement reactions, where ions are exchanged, sometimes forming an insoluble precipitate. Finally, oxidation is gain of oxygen, or loss of hydrogen. Reduction is loss of oxygen, or gain of hydrogen. When both happen together, we call it a redox reaction. And in daily life, oxidation shows up as corrosion of metals, and as rancidity in fats and oils. Now, out of all these reaction types, which one do you think happens the most around you, every single day?

AskMedha · Medha Ma’am — Bihar Board Class 10 Science, Chapter 1. Written from the textbook, checked twice.

Questions parents and students ask

How do I get the "Chemical Reactions and Equations" notes as a PDF?
Press "Save as PDF" — the phone’s print sheet opens; choose "Save as PDF" there. The PDF holds the notes only.
Where are these notes written from?
From the chapter’s own textbook text, part by part, in the book’s own terms; a second, independent model then checked every part against the book. The full lesson, a check question for each part and the practice are in the AskMedha app.

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