Class 10 Science: Chapter 1 – Chemical Reactions & Equations

Class 10 Science — Chapter 1: Chemical Reactions and Equations

A student-friendly, exam-focused explanation with examples, activities & practice — prepared for Jaymashar Institute.

Contents: Introduction Writing Chemical Equations Balancing Equations Types of Reactions Effects of Oxidation (Corrosion / Rancidity) Activities Practice & Answers Exam Tips / Summary

1. What is a chemical reaction?

A chemical reaction is a process in which one or more substances (called reactants) change into one or more different substances (called products). In a reaction new chemical bonds form and/or old bonds break, producing substances with different properties than the reactants.

Indicators that a chemical reaction has occurred include: change of color, evolution of a gas, formation of a precipitate, and change of temperature.

2. Writing chemical equations

A chemical equation is a short-hand representation of a chemical reaction using chemical formulas and an arrow (→) to separate reactants and products. Example (word form): Hydrogen gas + Oxygen gas → Water. In formula form: H2 + O2 → H2O (this equation must be balanced — see next section).

Word equation example: Magnesium + Oxygen → Magnesium oxide.
Chemical equation (unbalanced): Mg + O2 → MgO.

3. Law of conservation of mass & balancing equations

According to the law of conservation of mass, matter is neither created nor destroyed in a chemical reaction. Therefore, the number of atoms of each element must be the same on both sides of a chemical equation.

How to balance equations (step-by-step) — a reliable method:

1. Write the correct formulas for reactants and products.
2. Count the number of atoms of each element on both sides.
3. Start by balancing elements that appear in only one reactant and one product.
4. Use coefficients (whole numbers) before formulas to balance atoms. Never change subscripts.
5. Check that all atoms balance and coefficients are in the simplest ratio.

Worked examples

Example 1 — Burning magnesium
Unbalanced: Mg + O2 → MgO
Balance steps: oxygen appears as O₂ on left (2 atoms) and as single O in MgO → put coefficient 2 before MgO:

Mg + O2 → 2 MgO  ➜ Now Mg: left 1, right 2 → put 2 before Mg on left

Balanced:

2 Mg + O2 → 2 MgO

Example 2 — Electrolysis of water
Word eqn: water → hydrogen + oxygen (on electrolysis)
Formula (balanced):

2 H2O (l) → 2 H2 (g) + O2 (g)

Example 3 — Combustion of methane
Unbalanced: CH4 + O2 → CO2 + H2O
Balanced:

CH4 + 2 O2 → CO2 + 2 H2O

4. Types of chemical reactions (with simple examples)

4.1 Combination reaction

Two or more reactants combine to form one product.

CaO + H2O → Ca(OH)2 (quicklime + water → slaked lime)

4.2 Decomposition reaction

A single compound breaks into two or more simpler substances (often by heating, electrolysis or light).

2 HgO (s) → 2 Hg (l) + O2 (g) (decomposition on heating)

4.3 Displacement reaction (single)

A more reactive element displaces a less reactive element from its compound.

Zn + CuSO4 → ZnSO4 + Cu

4.4 Double displacement reaction

Exchange of ions between two compounds, often producing a precipitate, gas or water.

AgNO3 + NaCl → AgCl (s) + NaNO3 (AgCl is a white precipitate)

4.5 Oxidation and reduction (basic idea)

Oxidation: gain of oxygen or loss of hydrogen/electrons. Reduction: loss of oxygen or gain of hydrogen/electrons. Many reactions are redox reactions (both happen together).

Example: 2 Fe + O2 → 2 FeO (iron oxidises)

5. Everyday effects of oxidation: Corrosion & Rancidity

Corrosion (rusting of iron)

Corrosion is the gradual deterioration of metals by chemical reactions with substances in their environment (usually oxygen and moisture). Rust is hydrated iron(III) oxide formed when iron reacts with oxygen and water.

Simplified rusting idea: 4 Fe + 3 O2 + x H2O → 2 Fe2O3·xH2O

Rancidity

Fats and oils react with oxygen in air (oxidation) and produce unpleasant-smelling products — this is called rancidity. Storing food in airtight containers, refrigeration, and adding antioxidants prevent rancidity.

6. Useful classroom activities (with observations & equations)

Activity A — Burning magnesium ribbon

1. Hold a cleaned magnesium ribbon with tongs and burn it in a small inner-dish under safety supervision (wear goggles).
2. Observation: bright white flame and a white powder (magnesium oxide) are formed.
3. Equation:

   2 Mg (s) + O2 (g) → 2 MgO (s)

4. Safety: do this only under teacher supervision; use tongs and goggles.

Activity B — Quicklime and water (exothermic)

1. Take a little calcium oxide (quicklime) in a beaker; slowly add water and feel the beaker.
2. Observation: the beaker becomes warm (heat is produced) as slaked lime forms.
3. Equation:

   CaO (s) + H2O (l) → Ca(OH)2 (s)

Activity C — Electrolysis of water

1. Set up electrolysis of water (with small amount of dilute acid or salt to increase conductivity).
2. Observation: gas bubbles collected — hydrogen at cathode (twice the volume) and oxygen at anode.
3. Equation:

   2 H2O (l) → 2 H2 (g) + O2 (g)

7. Practice questions (click to reveal answers)

Short & long answer practice

1. Define a chemical reaction and give two indicators.
2. Balance: Fe + O2 → Fe2O3
3. Give one example of a double displacement reaction with balanced equation.
4. Explain why magnesium ribbon should be cleaned before burning.
5. What is rancidity and how can it be prevented?

Answers / Hints

1. A chemical reaction is... (explain and list indicators such as change of color, gas evolution).
2. Balanced:

   4 Fe + 3 O2 → 2 Fe2O3

3. Example:

   AgNO3 + NaCl → AgCl (s) + NaNO3

4. Because magnesium surface oxidises quickly, forming MgO layer; cleaning removes oxide so fresh metal reacts visibly.
5. Rancidity is oxidation of fats/oils — prevent by airtight containers, refrigeration, antioxidants.

8. Exam tips & quick summary

• Always remember the law of conservation of mass — balance equations accordingly.
• Use coefficients (not subscripts) when balancing.
• Learn typical examples for each reaction type (combination, decomposition, displacement, double displacement).
• For corrosion — moisture and oxygen are necessary; mention prevention methods (painting, galvanising).
• Practice balancing many equations — that develops intuition for coefficients.

Prepared for Jay Ma Sharde Institute — content adapted and explained from NCERT Class 10 Science (Chapter 1).