CBSE Class 10 Science Chapter 1 Notes — Chemical Reactions and Equations

What is a Chemical Reaction?

A chemical reaction is a process in which substances called reactants are converted into new substances called products, which have different physical and chemical properties.

Signs That a Chemical Reaction Has Occurred

Look for any of these when asked to identify whether a chemical change has taken place:

Tip: Change of state alone (melting, boiling) is a physical change, not a chemical reaction.

Writing and Balancing Chemical Equations

Word Equation → Chemical Equation

Step 1 — Word equation: Magnesium + Oxygen → Magnesium oxide

Step 2 — Unbalanced chemical equation: Mg + O₂ → MgO

Step 3 — Balanced equation: 2Mg + O₂ → 2MgO

Rules for Balancing

1. Count atoms of each element on both sides 2. Add coefficients (numbers in front of formulas) to make them equal 3. Never change subscripts inside a formula (changing H₂O to H₂O₂ changes the substance entirely) 4. Check: total atoms in = total atoms out

State Symbols Used in Equations

Types of Chemical Reactions

1. Combination Reaction

Two or more substances combine to form a single product.

General form: A + B → AB

Examples:

• 2Mg(s) + O₂(g) → 2MgO(s) — magnesium burns in air
• C(s) + O₂(g) → CO₂(g) — carbon burns in air
• CaO(s) + H₂O(l) → Ca(OH)₂(aq) — quicklime + water (exothermic)

2. Decomposition Reaction

A single compound breaks down into two or more simpler products.

General form: AB → A + B

Three types of decomposition:

Thermal decomposition (heat): 2Pb(NO₃)₂(s) →(Δ) 2PbO(s) + 4NO₂(g) + O₂(g)

Electrolytic decomposition (electricity): 2H₂O(l) →(electricity) 2H₂(g) + O₂(g)

Photolytic decomposition (light): 2AgCl(s) →(sunlight) 2Ag(s) + Cl₂(g) ← used in black-and-white photography

3. Displacement Reaction

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

General form: A + BC → AC + B (where A is more reactive than B)

Examples:

• Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s) — iron displaces copper
• Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s) — zinc displaces copper
• Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂↑ — zinc displaces hydrogen

Observation: The blue colour of CuSO₄ solution fades as colourless FeSO₄ forms, and a reddish-brown copper deposit appears on the iron rod.

4. Double Displacement Reaction

Two ionic compounds exchange their ions. Usually forms a precipitate (insoluble product).

General form: AB + CD → AD + CB

Examples:

• Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s)↓ + 2NaCl(aq) — white precipitate of barium sulphate
• NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l) — neutralisation (also double displacement)

5. Oxidation–Reduction (Redox) Reactions

These always occur simultaneously:

Oxidising agent = the substance that oxidises others (itself gets reduced) Reducing agent = the substance that reduces others (itself gets oxidised)

Example: CuO + H₂ → Cu + H₂O

• CuO loses oxygen → CuO is reduced (CuO is the oxidising agent)
• H₂ gains oxygen → H₂ is oxidised (H₂ is the reducing agent)

Exothermic and Endothermic Reactions

Respiration is an exothermic reaction: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy

Corrosion and Rancidity

Corrosion

The slow eating away of a metal surface due to oxidation by air, moisture, or chemicals.

• Rusting: 4Fe + 3O₂ + xH₂O → 2Fe₂O₃·xH₂O (hydrated iron oxide)
• Prevention: painting, galvanising (zinc coating), electroplating, alloying, oil/grease coating

Rancidity

The oxidation of fats and oils in food, producing foul smell and bad taste.

Prevention methods: 1. Storing in airtight containers (no oxygen) 2. Adding antioxidants (e.g., BHA — butylated hydroxyanisole) 3. Refrigerating food (slows oxidation) 4. Nitrogen gas flushing in food packets (inert atmosphere)

Important Equations to Memorise

NCERT Exercise — Key Questions

Q1. Why should magnesium ribbon be cleaned before burning? Magnesium ribbon has a layer of magnesium oxide (MgO) on its surface due to prior reaction with air. This oxide layer prevents proper combustion. Cleaning removes this layer, allowing clean magnesium to burn brightly.

Q2. Write the balanced equation when hydrogen gas is passed over heated copper(II) oxide. CuO(s) + H₂(g) → Cu(s) + H₂O(l) This is a redox reaction — CuO is reduced to Cu, and H₂ is oxidised to H₂O.

Q3. Why does the colour of copper sulphate solution change when an iron nail is dipped in it? Iron (Fe) is more reactive than copper (Cu). Iron displaces copper from CuSO₄ solution: Fe + CuSO₄ → FeSO₄ + Cu The blue colour of CuSO₄ fades (as Cu²⁺ ions are replaced by Fe²⁺ ions which form pale green FeSO₄). Reddish-brown copper deposits on the iron nail.

Q4. Give an example each of: (a) exothermic reaction, (b) endothermic reaction. (a) Exothermic: Burning of natural gas — CH₄ + 2O₂ → CO₂ + 2H₂O + Heat (b) Endothermic: Decomposition of calcium carbonate — CaCO₃ →(Δ) CaO + CO₂

Board Exam Tips

• 5–6 marks typically come from this chapter in CBSE boards
• Questions include: identify the type of reaction, balance the given equation, define a term, or explain an observation
• Always identify the type of reaction when writing about any reaction
• Write balanced equations — unbalanced equations may lose marks even if the substances are correct
• For displacement reactions, always mention which element is more reactive
• Learn to spot redox reactions by identifying what gains/loses oxygen or hydrogen

📚 Lakhmir Singh Science Class 10 — Best Reference Book

📝 Oswaal CBSE Question Bank Science Class 10 — Previous Year Questions

NCERT Exercise Solutions

Intext Questions (Remaining)

Page 6, Q2: Write the balanced equation for the following chemical reactions: (a) Hydrogen + Chlorine → Hydrogen chloride (b) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride (c) Sodium + Water → Sodium hydroxide + Hydrogen

Page 10, Q1: A solution of a substance X is used for whitewashing. Name X and write its formula. What happens when water is added to X?

Page 13, Q1: Why do we apply paint on iron articles?

Chapter-End Exercise

Q1. Which of the statements about the reaction below is incorrect? 2PbO(s) + C(s) → 2Pb(s) + CO₂(g) (a) Lead is getting reduced (b) Carbon dioxide is getting oxidised (c) Carbon is getting oxidised (d) Lead oxide is getting reduced

Q3. What does one observe when zinc granules are treated with dilute sulphuric acid? Write a balanced equation.

• Bubbles of hydrogen gas are evolved (effervescence is observed)
• The zinc granules slowly dissolve
• The solution becomes colourless zinc sulphate

Equation: Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂↑

This is a displacement reaction — zinc displaces hydrogen from dilute sulphuric acid.

Q4. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

This is a displacement reaction.

• The blue colour of CuSO₄ fades because Cu²⁺ ions are replaced by Fe²⁺ ions (which form pale green FeSO₄)
• A reddish-brown deposit of copper metal appears on the iron nail

Q5. Give an example of a double displacement reaction.

Q6. Identify the substances that are oxidised and reduced in the following reaction: 4Na(s) + O₂(g) → 2Na₂O(s)

In this combination reaction, sodium is the reducing agent and oxygen is the oxidising agent.

Q7. What is a precipitation reaction? Give an example.

Example: NaOH(aq) + FeCl₃(aq) → Fe(OH)₃(s)↓ + 3NaCl(aq) The reddish-brown precipitate of iron(III) hydroxide is formed.

Another example: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s)↓ + 2NaCl(aq)

Q8. Explain the following in terms of gain or loss of oxygen with one example each: (a) Oxidation (b) Reduction

(b) Reduction — loss of oxygen: When a substance loses oxygen, it is reduced. Example: CuO + H₂ → Cu + H₂O — Copper oxide loses oxygen and is reduced to copper.

Note: In the above example, CuO is the oxidising agent (gets reduced) and H₂ is the reducing agent (gets oxidised).