CBSE Class 10 Science — Chemical Reactions and Equations

Chapter Overview & Weightage

Chemical Reactions and Equations is Chapter 1 in CBSE Class 10 Science (NCERT). It is the opening chapter of the chemistry section and establishes vocabulary and concepts used throughout Class 10 and Class 11 chemistry.

This chapter carries 12–15 marks in the CBSE Class 10 Science board exam. Balancing equations, types of reactions, and oxidation-reduction identification are consistently high-scoring topics. At least one 5-mark question from this chapter appears in every board exam.

What this chapter covers:

Chemical equations and their characteristics
Balancing chemical equations
Types of chemical reactions (combination, decomposition, displacement, double displacement, redox)
Oxidation and reduction
Effects of oxidation in daily life (corrosion, rancidity)

Key Concepts You Must Know

What Makes a Chemical Reaction?

A chemical reaction occurs when:

A new substance with new properties is formed
The process is generally irreversible

Signs of a chemical reaction:

Change in colour
Evolution of gas (effervescence)
Formation of precipitate
Change in temperature
Change in smell

Writing Chemical Equations

A word equation names substances in words. A symbol equation uses chemical formulas.

Skeletal equation (unbalanced): $Mg + O_2 \rightarrow MgO$

Balanced equation: $2Mg + O_2 \rightarrow 2MgO$

Why balance? The Law of Conservation of Mass states matter is neither created nor destroyed. Atoms on both sides must be equal.

Types of Reactions

1. Combination (Synthesis): Two or more substances combine to form one new substance.

A + B \rightarrow AB

Example: $CaO + H_2O \rightarrow Ca(OH)_2$ (quicklime + water → slaked lime, releases heat)

2. Decomposition: One substance breaks down into two or more simpler substances.

AB \rightarrow A + B

Types: thermal (heat), electrolytic (electricity), photolytic (light) Example: $2Pb_3O_4 \xrightarrow{\Delta} 6PbO + O_2$

3. Displacement: A more reactive element displaces a less reactive element from its compound.

A + BC \rightarrow AC + B

Example: $Zn + CuSO_4 \rightarrow ZnSO_4 + Cu$ (Zinc is more reactive than copper)

4. Double Displacement: Exchange of ions between two compounds, often forming a precipitate.

AB + CD \rightarrow AD + CB

Example: $Na_2SO_4 + BaCl_2 \rightarrow BaSO_4\downarrow + 2NaCl$

5. Oxidation-Reduction (Redox): Oxidation and reduction always occur simultaneously.

Oxidation: gain of oxygen / loss of hydrogen / loss of electrons
Reduction: loss of oxygen / gain of hydrogen / gain of electrons

Important Formulas

Write the skeletal equation
Count atoms of each element on both sides
Add coefficients (whole numbers) to balance — start with the most complex molecule
Never change subscripts — only coefficients
Balance H and O last

If a substance:

Gains O or Loses H → it is OXIDISED
Loses O or Gains H → it is REDUCED

The substance that gets oxidised is the reducing agent. The substance that gets reduced is the oxidising agent.

Solved Previous Year Questions

PYQ 1 — Balancing Equations

Q: Balance the following chemical equation: (CBSE Board 2023)

Fe + H_2O \rightarrow Fe_3O_4 + H_2

Solution:

Step 1 — Unbalanced: $Fe + H_2O \rightarrow Fe_3O_4 + H_2$

Step 2 — Balance Fe: 3 on right, put coefficient 3 on left: $3Fe + H_2O \rightarrow Fe_3O_4 + H_2$

Step 3 — Balance O: 4 on right (in $Fe_3O_4$ ), need 4 $H_2O$ on left: $3Fe + 4H_2O \rightarrow Fe_3O_4 + H_2$

Step 4 — Balance H: 8 H on left (4 × $H_2O$ ), need 4 $H_2$ on right:

\boxed{3Fe + 4H_2O \rightarrow Fe_3O_4 + 4H_2}

Verify: Fe: 3=3 ✓, H: 8=8 ✓, O: 4=4 ✓

PYQ 2 — Types of Reactions

Q: Identify the type of reaction and the oxidising and reducing agents in: (CBSE 2022)

CuO + H_2 \rightarrow Cu + H_2O

Solution:

Type: This is a redox reaction (displacement type within redox context).

Oxidation-Reduction analysis:

$H_2 \rightarrow H_2O$ : H₂ gains oxygen → H₂ is oxidised → H₂ is the reducing agent
$CuO \rightarrow Cu$ : CuO loses oxygen → CuO is reduced → CuO is the oxidising agent

PYQ 3 — Corrosion and Rancidity

Q: What is rancidity? State two methods to prevent it. (CBSE Board — 2 marks)

Solution:

Rancidity is the deterioration of fats and oils due to oxidation, producing unpleasant smell and taste.

Prevention methods:

Antioxidants — substances like BHA (Butylated Hydroxyanisole) added to packaged food that get preferentially oxidised, protecting the fats
Inert gas packaging — chips and snacks are packed in nitrogen gas, which displaces oxygen and prevents oxidation
(Also acceptable): Refrigeration (slows oxidation), opaque packaging (prevents light-induced oxidation)

Difficulty Distribution

Difficulty	Topic	Marks
Easy (25%)	Identify type of reaction; word to symbol equation	1–2 marks
Medium (45%)	Balance equations; identify oxidising/reducing agents; state effects	2–3 marks
Hard (30%)	Write balanced equation for described reaction; combined analysis	4–5 marks

Expert Strategy

In board exams, equations with fractions (like $\frac{1}{2}O_2$ ) are not acceptable — always multiply through to clear fractions. For example: $H_2 + \frac{1}{2}O_2 \rightarrow H_2O$ must be written as $2H_2 + O_2 \rightarrow 2H_2O$ .

Memorise these key balanced equations — they appear directly in board exams:

$2Mg + O_2 \rightarrow 2MgO$ (Magnesium burning)
$Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2\uparrow$ (Zinc + dilute acid)
$2HCl + Na_2CO_3 \rightarrow 2NaCl + H_2O + CO_2\uparrow$ (Acid + carbonate)
$3Fe + 4H_2O \rightarrow Fe_3O_4 + 4H_2$ (Rust formation at high temperature)

Common Traps

Trap 1 — Changing subscripts instead of coefficients: If you have $H_2$ on one side and $H_4$ on another, you CANNOT write $H_4$ as a product — there’s no such stable molecule. Only add coefficients in front of the formula, never change the formula itself.

Trap 2 — Confusing oxidising and reducing agents: The reducing agent is oxidised (it causes reduction by giving electrons/getting itself oxidised). Students flip this. Remember: Reducing agent gets oxidised (loses electrons). Oxidising agent gets reduced (gains electrons).

Trap 3 — Not writing state symbols: CBSE board answers should include state symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous. Missing state symbols lose marks in some questions.

Trap 4 — Confusing decomposition with displacement: In decomposition, one substance breaks into simpler parts. In displacement, one element pushes another out of its compound. A reaction like $2KClO_3 \rightarrow 2KCl + 3O_2$ is decomposition (one compound splits). A reaction like $Fe + CuSO_4 \rightarrow FeSO_4 + Cu$ is displacement (Fe replaces Cu).