CBSE Class 10 Science Chemical Reactions and Equations Notes

Key Topics Covered in This Chapter

•         What is a Chemical Reaction? Signs and Characteristics

•         Chemical Equations — Word and Symbolic Form

•         Balancing Chemical Equations (Law of Conservation of Mass)

•         Types of Chemical Reactions: Combination, Decomposition, Displacement, Double Displacement, Redox

•         Oxidation, Reduction, and Oxidising/Reducing Agents

•         Effects of Oxidation: Corrosion and Rancidity

•         Key Equations to Memorise for Board Exams

1.  Chemical Reactions — Signs and Characteristics

A chemical reaction involves the breaking and forming of chemical bonds, converting reactants into products. The substances that react are called reactants (or reagents), and the new substances formed are called products.

In everyday life, we observe chemical reactions happening all around us. Recognising the signs of a chemical reaction is an important skill.

Signs That a Chemical Reaction Has Occurred

Writing Chemical Equations

A chemical equation is a symbolic representation of a chemical reaction. It uses chemical formulae of reactants and products, separated by an arrow (→). Conditions like heat, light, or catalyst are written above or below the arrow.

We write equations in two stages:

•         Step 1 — Word Equation: Reactants and products written as names, separated by →

•         Step 2 — Chemical/Skeleton Equation: Replace names with chemical formulae

•         Step 3 — Balanced Equation: Add coefficients to satisfy the Law of Conservation of Mass

Example:  Magnesium burns in oxygen.

Word equation:       Magnesium + Oxygen → Magnesium oxide

Skeleton equation:   Mg + O₂ → MgO

Balanced equation:   2Mg + O₂ → 2MgO  ✓

2.  Balancing Chemical Equations

Balancing a chemical equation means making the number of atoms of each element equal on both sides. This is based on the Law of Conservation of Mass (Lavoisier, 1789): matter is neither created nor destroyed in a chemical reaction. We use the hit-and-trial method (balancing by inspection).

Law of Conservation of Mass: Mass of reactants = Mass of products in every chemical reaction.

Steps to Balance a Chemical Equation

1.       Write the skeleton (unbalanced) equation using correct chemical formulae.

2.       Count the atoms of each element on both sides.

3.       Balance elements one by one — start with the element that appears in the fewest compounds.

4.       Add coefficients (whole numbers) in front of formulae to balance atom counts. Never change subscripts.

5.       Verify: recount all atoms on both sides. The equation is balanced when all counts match.

Example:  Balance  Fe + H₂O → Fe₃O₄ + H₂

Skeleton: Fe + H₂O → Fe₃O₄ + H₂

Fe: 1 vs 3  → put 3Fe on left:  3Fe + H₂O → Fe₃O₄ + H₂

O:  1 vs 4  → put 4H₂O on left: 3Fe + 4H₂O → Fe₃O₄ + H₂

H:  8 vs 2  → put 4H₂ on right: 3Fe + 4H₂O → Fe₃O₄ + 4H₂  ✓

Never balance an equation by changing subscripts in a chemical formula. Only coefficients (numbers before formulae) can be changed.

3.  Types of Chemical Reactions

Chemical reactions are classified into five main types based on how atoms are rearranged. Every reaction in the CBSE board exam belongs to one or more of these categories.

Type 1: Combination (Synthesis) Reaction

Two or more substances combine to form a single new substance.  General form:  A + B → AB

A + B  →  AB

•         These reactions are generally exothermic (release heat).

•         Elements or compounds can combine.

Examples:

C + O₂  →  CO₂  (carbon burns in oxygen)

2H₂ + O₂  →  2H₂O  (hydrogen burns in oxygen)

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

Type 2: Decomposition Reaction

A single compound breaks down into two or more simpler substances.  General form:  AB → A + B

AB  →  A + B

Decomposition can be triggered by different energy sources:

•         Thermal decomposition: heat applied (e.g., 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂)

•         Electrolytic decomposition: electric current (e.g., 2H₂O → 2H₂ + O₂)

•         Photolytic decomposition: light energy (e.g., 2AgCl → 2Ag + Cl₂ in sunlight)

2KClO₃  →(MnO₂/heat)→  2KCl + 3O₂  (thermal; used in lab preparation of O₂)

CaCO₃  →(heat)→  CaO + CO₂  (decomposition of limestone; industrial use)

Type 3: Displacement (Single Displacement) Reaction

A more reactive element displaces a less reactive element from its compound.  General form:  A + BC → AC + B

A + BC  →  AC + B   (A is more reactive than B)

•         The activity series (reactivity series) determines which element displaces which.

•         More reactive metals displace less reactive metals from their salt solutions.

Zn + CuSO₄  →  ZnSO₄ + Cu  (Zn more reactive than Cu; blue colour fades)

Fe + CuSO₄  →  FeSO₄ + Cu  (iron displaces copper; nails turn brown)

2Al + 6HCl  →  2AlCl₃ + 3H₂  (Al displaces H from acid)

Type 4: Double Displacement Reaction

Ions of two compounds exchange places to form two new compounds.  General form:  AB + CD → AD + CB

AB + CD  →  AD + CB

•         Usually takes place in aqueous solution.

•         Often results in formation of a precipitate, gas, or water.

•         Reactions forming a precipitate are called precipitation reactions.

Na₂SO₄ + BaCl₂  →  BaSO₄↓ + 2NaCl  (white BaSO₄ precipitate — precipitation reaction)

NaOH + HCl  →  NaCl + H₂O  (neutralisation — also a double displacement)

AgNO₃ + NaCl  →  AgCl↓ + NaNO₃  (white AgCl precipitate)

Type 5: Oxidation–Reduction (Redox) Reactions

Oxidation and reduction always occur simultaneously in a redox reaction. The substance that gets oxidised is called the reducing agent; the substance that causes oxidation (and itself gets reduced) is the oxidising agent.

CuO + H₂  →  Cu + H₂O

  CuO: Cu is REDUCED (loses oxygen)  →  CuO is the oxidising agent

  H₂:  H₂ is OXIDISED (gains oxygen) →  H₂ is the reducing agent  ✓

Tip: OIL RIG — Oxidation Is Loss, Reduction Is Gain (of electrons). Helpful memory aid!

4.  Effects of Oxidation in Daily Life

Oxidation reactions have significant practical consequences in everyday life. Two major effects are corrosion and rancidity.

Corrosion

Corrosion is the process by which metals are eaten away (damaged) due to reaction with moisture, oxygen, acids, or other substances in the environment. The most common example is the rusting of iron.

4Fe + 3O₂ + xH₂O  →  2Fe₂O₃ · xH₂O   (Rust — hydrated iron oxide)

•         Silver tarnishes: silver surface reacts with sulphur compounds in air to form black Ag₂S.

•         Copper turns green: copper reacts with CO₂, H₂O, O₂ to form green patina (basic copper carbonate).

•         Prevention of corrosion: painting, galvanising (zinc coating), alloying (stainless steel), electroplating.

Rancidity

Rancidity is the oxidation of fats and oils in food, producing unpleasant smell and taste. It spoils food and reduces its nutritional value. Rancidity happens when food is exposed to air (oxygen), heat, light, or moisture over time.

•         Prevention of rancidity:

•         Adding antioxidants (e.g., BHA, BHT) to packaged food.

•         Storing food in airtight containers (reducing exposure to oxygen).

•         Flushing packets with nitrogen gas (inert atmosphere — used in chips/snack packets).

•         Refrigeration — low temperature slows oxidation reactions.

Packets of chips are flushed with nitrogen gas to prevent rancidity — a common board exam question!

5.  Important Equations to Memorise

These chemical equations are frequently asked in CBSE Class 10 board examinations. Learn both the equation and the type of reaction for each.

6.  Types of Reactions — Quick Summary

Use this comparison table to quickly review and distinguish all five types of chemical reactions:

7.  Board Exam Practice Questions

These questions cover all question types from CBSE Class 10 Science (Chapter 1) board examinations. Practise thoroughly for each mark category.

1 Mark Questions

1.       Define a chemical reaction. Give one example.

2.       State the Law of Conservation of Mass.

3.       Identify the type of reaction: 2SO₂ + O₂ → 2SO₃

4.       What is a precipitation reaction? Give one example.

5.       What is rancidity? How can it be prevented?

3 Mark Questions

1.       Balance the following equation and identify its type: Fe₂O₃ + CO → Fe + CO₂

2.       Explain with a chemical equation what happens when lead nitrate solution is heated.

3.       Define oxidation and reduction with examples. What is a redox reaction?

4.       Iron nails are dipped in copper sulphate solution. What do you observe? Write the chemical equation and name the type of reaction.

5 Mark Questions

1.       Write the balanced chemical equations for the following reactions and classify each: (a) Magnesium in oxygen, (b) Barium chloride + aluminium sulphate, (c) Sodium + water, (d) Potassium permanganate on heating, (e) Zinc + silver nitrate.

2.       What is corrosion? Explain with the example of rusting of iron. Write the chemical equation for rusting and give four methods to prevent it.

3.       Distinguish between combination and decomposition reactions with two examples each. How are these reactions opposite to each other?