Chemical Reactions and Equations Class 10 Notes
Free Class 10 chemistry notes on balancing equations, reaction types, oxidation and reduction, corrosion and rancidity, with a downloadable PDF.
A chemical reaction changes reactants into products with different chemical identities. A chemical equation records that change using formulas. In a balanced equation, each element has the same number of atoms on both sides; atoms are rearranged rather than created or destroyed.
How to balance chemical equations
Balance Al + O₂ → Al₂O₃
- 1Start with correct formulas. Oxygen occurs in groups of 2 on the left and 3 on the right.
- 2Use six oxygen atoms: put 3 before O₂ and 2 before Al₂O₃.
- 3There are now four aluminium atoms on the right, so put 4 before Al.
- 4Balanced equation: 4Al + 3O₂ → 2Al₂O₃. Check Al: 4 each side; O: 6 each side.
- 5Use the smallest whole-number coefficients. Add physical states and required conditions when known. Never change the subscripts in a formula.
Types of chemical reactions with examples
| Type | Pattern | Example and condition |
|---|---|---|
| Combination | Several reactants form one product | 2Mg + O₂ → 2MgO when magnesium burns. |
| Decomposition | One reactant forms several products | CaCO₃ → CaO + CO₂ on heating. |
| Displacement | An element replaces another in a compound | Zn + CuSO₄ → ZnSO₄ + Cu in solution. |
| Double displacement | Ions exchange partners | AgNO₃ + NaCl → AgCl + NaNO₃ in aqueous solution; AgCl precipitates. |
| Redox | Oxidation and reduction occur together | CuO + H₂ → Cu + H₂O on heating. |
These categories can overlap. Burning magnesium is both a combination reaction and an oxidation process. Exothermic and endothermic describe energy transfer, rather than a separate pattern of how many reactants become how many products.
Oxidation and reduction: the difference
In the oxygen-based model used in this chapter, oxidation is gain of oxygen and reduction is loss of oxygen. In CuO + H₂ → Cu + H₂O, hydrogen gains oxygen and is oxidised, while copper(II) oxide loses oxygen and is reduced. The changes occur together. More general definitions use electron transfer.
Corrosion and rancidity: the difference
| Feature | Corrosion | Rancidity |
|---|---|---|
| What changes | A metal reacts with its surroundings. | Fats and oils oxidise and deteriorate. |
| Example | Rusting of iron in moist air. | Oil developing an unpleasant smell. |
| Ways to slow it | Protective paint or suitable coatings. | Reduce oxygen exposure; appropriate cool storage can help. |
Revision questions
- 1. Balance H₂ + O₂ → H₂O. Answer: 2H₂ + O₂ → 2H₂O.
- 2. Which is reduced in ZnO + C → Zn + CO? Answer: ZnO loses oxygen.
- 3. Can you change MgO to MgO₂ to balance an equation? No; that changes the substance.
- 4. Why can one reaction have several labels? The labels describe different features, such as product pattern, oxygen transfer and energy change.
Important questions and interactive MCQs
NCERT exercise answer companion
Topic reference: NCERT Science, Chemical Reactions and Equations