CBSE Class 10 Science Chapter 1
Chemical Reactions & Equations
Complete NCERT Notes
Key Definitions
1. Chemical Reaction
A chemical reaction is a process where reactants convert into products with different properties. Bonds break and new bonds form between atoms. Magnesium burns in air producing white MgO powder from shiny metal ribbon. This shows reactants disappear, new substances form. Energy changes occur - heat/light released. Physical/chemical properties change completely. Reaction is irreversible. Examples include rusting, digestion, respiration. Conservation of mass applies - atoms rearrange but total count remains same. This fundamental process explains all chemical changes in nature and industry. (100 words)
2Mg(s) + O₂(g) → 2MgO(s)
2. Chemical Equation
Chemical equation symbolically represents reaction using element symbols and formulae. Reactants shown left, products right, arrow indicates direction. Coefficients show relative amounts. State symbols: (s)solid, (l)liquid, (g)gas, (aq)aqueous. H₂ + O₂ → H₂O shows hydrogen and oxygen forming water. Equations save time, show exact proportions, enable calculations. Universal language for chemists worldwide. Must be balanced to obey conservation laws. Conditions like heat(Δ), electricity shown above arrow. Essential tool for studying reaction stoichiometry and predicting products. (98 words)
3. Balanced Equation
Balanced chemical equation has equal atoms each element both sides. Follows Lavoisier's Law of Conservation of Mass - matter neither created nor destroyed. H₂ + O₂ → H₂O unbalanced (2O left, 1O right). Correct: 2H₂ + O₂ → 2H₂O (4H, 2O both sides). Use coefficients only, never change subscripts (creates new compound). Balance step-wise: metals first, oxygen next, hydrogen last. Verify final count. Essential for accurate stoichiometry calculations and understanding true reaction proportions. (92 words)
2H₂(g) + O₂(g) → 2H₂O(l)
All NCERT Chemical Equations
| S.No |
Equation |
NCERT Reference |
| 1 | 2Mg(s) + O₂(g) → 2MgO(s) | Activity 1.1 |
| 2 | H₂(g) + O₂(g) → H₂O(l) | Example 1.1 |
| 3 | 2H₂(g) + O₂(g) → 2H₂O(l) | Balanced Ex 1.1 |
| 4 | 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g) | Activity 1.3 |
| 5 | CaO(s) + H₂O(l) → Ca(OH)₂(aq) | Example 1.2 |
| 6 | CaCO₃(s) → CaO(s) + CO₂(g) | Thermal Decomposition |
| 7 | 2H₂O(l) → 2H₂(g) + O₂(g) | Electrolysis |
| 8 | 2AgCl(s) → 2Ag(s) + Cl₂(g) | Photolysis |
| 9 | Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s) | Example 1.4 |
| 10 | Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq) | Example 1.5 |
| 11 | CuO(s) + H₂(g) → Cu(s) + H₂O(l) | Example 1.6 |
| 12 | 4Fe + 3O₂ + xH₂O → 2Fe₂O₃.xH₂O | Corrosion |
Types of Chemical Reactions
1. Combination Reaction
Two or more substances combine to form single product. Usually exothermic. Quicklime reacts violently with water forming slaked lime used for whitewashing. Heat evolved so much that water boils. CaO powder becomes Ca(OH)₂ slurry. (52 words)
CaO(s) + H₂O(l) → Ca(OH)₂(aq)
2. Decomposition Reaction
Single compound breaks into two or more simpler substances. Needs energy input. Three types: thermal (heat), electrolytic (electricity), photolytic (light). CaCO₃ heated gives quicklime + CO₂ for cement industry. (42 words)
CaCO₃(s) → CaO(s) + CO₂(g) Δ
3. Displacement Reaction
More reactive element displaces less reactive from compound. Zn > Cu so Zn displaces Cu from CuSO₄. Blue solution becomes colourless, reddish Cu deposits. Follows reactivity series order. (36 words)
Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
4. Double Displacement
Exchange of ions between two compounds. Forms precipitate. Na₂SO₄ + BaCl₂ gives white BaSO₄ precipitate. Acid-base neutralization also double displacement. (28 words)
Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl
Oxidation & Reduction
Oxidation: Gain of oxygen/loss of hydrogen
Reduction: Loss of oxygen/gain of hydrogen
Always occur together (Redox reaction)
CuO(s) + H₂(g) → Cu(s) + H₂O(l)
CuO reduced to Cu, H₂ oxidized to H₂O
Daily Life Effects
Corrosion
4Fe + 3O₂ + xH₂O → 2Fe₂O₃.xH₂O (Rust)
Prevention: Painting, Galvanization, Alloying
Rancidity
Oxidation of food fats causing bad smell/taste
Prevention: Airtight packing, Nitrogen flushing, Antioxidants
Board Exam Tips
- Always balance equations using coefficients
- Write state symbols (s,l,g,aq)
- Show balancing steps in 3-mark questions
- Identify reaction type correctly
- Mention observation/colour change