Chemical Reactions and Equations: Class 10 Science Slides

Slide-by-slide content

1. 1. Chemical Reactions and Equations

   CBSE Class 10 Science · Writing, balancing and classifying reactions

   Speaker notes

   Open with a quick question: what changes when milk turns into curd, or when an iron gate rusts? Collect answers and point out that in both cases a new substance forms. Tell the class that today they will learn to describe such changes in the short, exact language of chemistry.

2. 2. By the end of this lesson you can

   • Spot the signs of a chemical reaction
   • Write and balance a chemical equation
   • Name the four main types of reactions
   • Tell exothermic from endothermic reactions
   • Explain oxidation, reduction, corrosion and rancidity
   Speaker notes

   Read the objectives aloud and ask students which one sounds hardest. Promise that balancing, usually the scariest, will be done step by step with a familiar example.

3. 3. What is a chemical reaction?

   • Starting substances (reactants) change into new substances (products)
   • Products have different properties from the reactants
   • Atoms are rearranged; no atom is created or destroyed
   • Examples: curd setting, food cooking, iron rusting
   Speaker notes

   Contrast with a physical change such as ice melting, where no new substance forms. Ask: is dissolving sugar in water a chemical reaction? It is not, because the sugar can be recovered by evaporating the water.

4. 4. Signs that a reaction has happened

   • A change of colour
   • A gas is given off (bubbles or fizzing)
   • The mixture gets hotter or colder
   • A change of state, or a solid (precipitate) appears
   Speaker notes

   Burn a small magnesium ribbon only as a teacher demonstration with tongs and eye protection: it gives a dazzling white flame and leaves white magnesium oxide. Warn students never to look straight at the flame. A sign such as a colour change hints at a reaction but does not prove one on its own.

5. 5. From words to a chemical equation

   • Word equation: magnesium + oxygen → magnesium oxide
   • Replace names with formulas: Mg + O₂ → MgO
   • Reactants go on the left, products on the right
   • The arrow means 'forms' or 'changes into'
   Speaker notes

   Show that the formula version is shorter and tells us exactly which atoms are involved. Ask a student to count the oxygen atoms on each side of Mg + O₂ → MgO; they will find two on the left and one on the right, which sets up the next slide.

6. 6. Why equations must be balanced

   • Law of conservation of mass: mass is neither created nor destroyed
   • Each element needs the same number of atoms on both sides
   • Balance with coefficients (numbers in front of formulas)
   • Never change the subscripts inside a formula
   Speaker notes

   Balanced form: 2Mg + O₂ → 2MgO. Explain that changing O₂ to O or MgO to MgO₂ would describe different substances, which is why only coefficients may change. A common mistake is to write the coefficient as a subscript.

7. 7. Balancing the burning of CNG (methane)

   • Skeleton equation: CH₄ + O₂ → CO₂ + H₂O
   • Carbon: one on each side, already balanced
   • Hydrogen: 4 on the left, so write 2H₂O
   • Oxygen: now 4 on the right, so write 2O₂
   • Balanced: CH₄ + 2O₂ → CO₂ + 2H₂O
   Speaker notes

   CNG autos and buses in many Indian cities run mainly on methane, so this reaction happens inside their engines every second. Work through each step on the board and recount every element at the end. Ask students to balance H₂ + O₂ → H₂O on their own before moving on.

8. 8. Making equations more informative

   • State symbols: (s) solid, (l) liquid, (g) gas
   • (aq) means dissolved in water, an aqueous solution
   • Conditions go above or below the arrow: heat, light, catalyst
   • Example: Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)
   Speaker notes

   State symbols are optional in the textbook unless a question asks for them, but they make an equation much clearer. Ask: in the zinc example, which product would you see as bubbles? The answer is hydrogen gas.

9. 9. Combination and decomposition

   Combination

   • Two or more reactants form one product
   • CaO + H₂O → Ca(OH)₂ (slaked lime)
   • Gives out a lot of heat
   • Slaked lime is used to whitewash walls

   Decomposition

   • One reactant breaks into simpler products
   • CaCO₃ → CaO + CO₂ on heating
   • Needs energy: heat, light or electricity
   • Used to make quicklime from limestone
   Speaker notes

   Point out that one reaction is roughly the reverse pattern of the other: many things combine into one, or one thing splits into many. Adding quicklime to water must be done carefully because the mixture gets very hot. Ask students to classify 2Mg + O₂ → 2MgO; it is a combination reaction.

10. 10. Three ways to break a compound down

    • Heat (thermal): 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃
    • Electricity (electrolysis): 2H₂O → 2H₂ + O₂
    • Light (photolysis): 2AgCl → 2Ag + Cl₂
    • White silver chloride turns grey in sunlight
    Speaker notes

    In the electrolysis of water, the volume of hydrogen collected is twice that of oxygen, which matches the 2 : 1 ratio in the equation. The action of light on silver salts was the basis of black-and-white photography. Heating ferrous sulphate gives off gases with a sharp smell, so it should be done only under supervision.

11. 11. Displacement and double displacement

    Displacement

    • A more reactive metal pushes out a less reactive one
    • Fe + CuSO₄ → FeSO₄ + Cu
    • Blue solution fades; iron gets a brown copper coat

    Double displacement

    • Two compounds swap their ions
    • Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl
    • White BaSO₄ precipitate forms
    Speaker notes

    Iron is more reactive than copper, so it displaces copper from copper sulphate solution. A reaction that produces an insoluble solid is also called a precipitation reaction. Ask: would copper displace iron from iron sulphate? No, because copper is less reactive.

12. 12. Exothermic and endothermic reactions

    Exothermic

    • Release heat to the surroundings
    • Burning natural gas or LPG
    • Respiration in our cells
    • Quicklime reacting with water

    Endothermic

    • Absorb energy from the surroundings
    • Heating limestone to make quicklime
    • Electrolysis of water
    • Silver chloride splitting in sunlight
    Speaker notes

    Respiration is exothermic: glucose reacts with oxygen and releases the energy our body uses. Most decomposition reactions are endothermic because energy is needed to break the reactant apart. Ask students which type a burning candle is.

13. 13. Oxidation and reduction

    • Oxidation: a substance gains oxygen or loses hydrogen
    • Reduction: a substance loses oxygen or gains hydrogen
    • CuO + H₂ → Cu + H₂O
    • Here CuO is reduced and H₂ is oxidised
    • Both happen together: a redox reaction
    Speaker notes

    Heating copper powder in air forms black copper oxide, 2Cu + O₂ → 2CuO, which is oxidation. Passing hydrogen over hot copper oxide turns it back to brown copper. A common misconception is that oxidation always needs oxygen gas; losing hydrogen also counts.

14. 14. Oxidation at home: corrosion and rancidity

    • An iron gate rusts faster in the humid monsoon
    • Silver anklets and utensils slowly turn black
    • Copper vessels develop a green coating
    • Old namkeen and oil smell and taste stale (rancid)
    • Chips packets are filled with nitrogen to slow rancidity
    Speaker notes

    Corrosion is the slow damage of a metal by air, moisture or chemicals around it; rusting of iron is the most costly example. Rancidity is the oxidation of fats and oils in food. Ask students how their families prevent these: painting and oiling gates, storing food in airtight containers, and adding antioxidants.

15. 15. Quick check

    In CuO + H₂ → Cu + H₂O, which substance is oxidised?

    • CuO
    • H₂
    • Cu
    • H₂O
    Speaker notes

    Answer: B, hydrogen, because it gains oxygen to become water. Copper oxide loses oxygen, so it is reduced. Let students vote before revealing the answer, then ask them to explain their choice.

16. 16. Key takeaways

    • Reactions rearrange atoms into new substances
    • Balanced equations obey conservation of mass
    • Four types: combination, decomposition, displacement, double displacement
    • Reactions either release heat or absorb energy
    • Oxidation and reduction always happen together
    Speaker notes

    Ask each student to write one balanced equation and label its reaction type as an exit ticket. Then move to the five-question quiz. Homework: find two examples of corrosion or rancidity at home and suggest how to prevent each.

Key terms

Chemical equation

A short way of showing a reaction using formulas, with reactants on the left and products on the right of an arrow.

Balanced equation

An equation with the same number of atoms of each element on both sides, as required by the law of conservation of mass.

Combination reaction

A reaction in which two or more reactants form a single product.

Decomposition reaction

A reaction in which one reactant breaks down into two or more simpler products, using heat, light or electricity.

Precipitate

An insoluble solid that forms when two solutions react, as in many double displacement reactions.

Exothermic reaction

A reaction that releases heat to the surroundings, such as respiration or burning fuel.

Oxidation

Gain of oxygen or loss of hydrogen by a substance; reduction is the opposite.

Rancidity

The unpleasant change in smell and taste of fats and oils in food caused by oxidation.

Quick quiz

Frequently asked questions