Class 10 Science – Chapter 1: Chemical Reactions and Equations (Complete Notes)

🔹 Chapter Summary

Chemical Reaction: A process in which one or more substances (reactants) are converted into new substances (products) with different properties.
Indicators of a Chemical Reaction: Change in state, colour, temperature, evolution of gas, formation of precipitate.
Chemical Equation: A representation of a chemical reaction using symbols and formulas.
Balanced Equation: An equation in which the number of atoms of each element on both sides is equal (Law of Conservation of Mass).
Types of Reactions: Combination, Decomposition, Displacement, Double Displacement, Oxidation–Reduction, Precipitation, Exothermic, Endothermic.

🔹 All Important Chemical Reactions (Word + Formula Form)

1. Combination Reaction

Two or more substances combine to form one compound.

Calcium oxide + Water → Calcium hydroxide
CaO + H₂O → Ca(OH)₂

2. Decomposition Reaction

One compound breaks down into simpler compounds/elements.

Thermal decomposition:
- Calcium carbonate → Calcium oxide + Carbon dioxide
- CaCO₃ → CaO + CO₂
Electrolytic decomposition:
- Water → Hydrogen + Oxygen
- 2H₂O → 2H₂ + O₂
Photolytic decomposition:
- Silver chloride → Silver + Chlorine (in sunlight)
- 2AgCl → 2Ag + Cl₂

3. Displacement Reaction

A more reactive element displaces a less reactive one.

Zinc + Copper sulphate → Zinc sulphate + Copper
Zn + CuSO₄ → ZnSO₄ + Cu

4. Double Displacement Reaction

Exchange of ions between two compounds.

Sodium sulphate + Barium chloride → Barium sulphate (ppt) + Sodium chloride
Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl

5. Oxidation and Reduction (Redox Reactions)

Oxidation: Addition of oxygen / removal of hydrogen.
Reduction: Addition of hydrogen / removal of oxygen.
Example:
- Copper(II) oxide + Hydrogen → Copper + Water
- CuO + H₂ → Cu + H₂O

6. Precipitation Reaction

Formation of insoluble salt (precipitate).

Silver nitrate + Sodium chloride → Silver chloride (ppt) + Sodium nitrate
AgNO₃ + NaCl → AgCl↓ + NaNO₃

7. Exothermic Reaction

Releases heat/energy.

Carbon + Oxygen → Carbon dioxide + Heat
C + O₂ → CO₂ + Heat
Respiration: Glucose + O₂ → CO₂ + H₂O + Energy

8. Endothermic Reaction

Absorbs heat/energy.

Nitrogen + Oxygen → Nitric oxide (heat required)
N₂ + O₂ → 2NO
Photosynthesis:
- 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (in presence of sunlight and chlorophyll)

🔹 Quick Revision Sheet (Tabular Form)

Reaction Type	Example (Word Equation)	Chemical Equation
Combination	CaO + H₂O → Ca(OH)₂	Calcium oxide + Water → Calcium hydroxide
Decomposition	CaCO₃ → CaO + CO₂	Calcium carbonate → Calcium oxide + CO₂
Displacement	Zn + CuSO₄ → ZnSO₄ + Cu	Zinc + Copper sulphate → Zinc sulphate + Copper
Double Displacement	Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl	Sodium sulphate + Barium chloride → Barium sulphate + Sodium chloride
Redox	CuO + H₂ → Cu + H₂O	Copper oxide + Hydrogen → Copper + Water
Precipitation	AgNO₃ + NaCl → AgCl↓ + NaNO₃	Silver nitrate + Sodium chloride → Silver chloride + Sodium nitrate
Exothermic	C + O₂ → CO₂ + Heat	Carbon + Oxygen → Carbon dioxide + Heat
Endothermic	6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂	Photosynthesis reaction

🔹 X-Type (Reasoning Based) Questions

Why does magnesium ribbon need to be cleaned before burning?
Why is respiration considered an exothermic reaction?
Why is photosynthesis considered an endothermic reaction?
Define oxidation and reduction with examples.

🔹 Y-Type (Application/HOTS) Questions

A student mixes zinc with copper sulphate solution. Predict the observation and write the balanced equation.
Write an activity to show decomposition of water using electricity.
Explain with an activity how light can decompose silver chloride.
Which type of chemical reaction occurs in neutralisation of an acid with a base? Give an example.

✅ Final Tip for Students: Always write chemical reactions in both word and balanced symbolic forms. Practicing the equations and types regularly will make it easy to answer one-mark and three-mark questions in CBSE exams.