📚 UNIQUE STUDY POINT | By Sumeet Sahu | uniquestudyonline.com
✅ Complete In-Text + Exercise Solutions | CBSE 2025-26 Syllabus
📋 Topics Covered in This Chapter:
1.1 Chemical Equations → 1.2 Types of Chemical Reactions (Combination, Decomposition, Displacement, Double Displacement) → 1.3 Oxidation & Reduction → 1.4 Corrosion → 1.5 Rancidity
❓ Q1. Why should a magnesium ribbon be cleaned before burning in air?
ANS Magnesium ribbon should be cleaned before burning in air because magnesium metal reacts with atmospheric oxygen and moisture to form a layer of magnesium oxide (MgO) on its surface. This white layer of MgO is quite stable and prevents the burning of magnesium ribbon. Therefore, the ribbon needs to be cleaned by sandpaper to remove this oxide layer so that it can burn properly in air.
❓ Q2. Write the balanced equation for the following chemical reactions:
(i) Hydrogen + Chlorine → Hydrogen chloride
H₂(g) + Cl₂(g) → 2HCl(g)
(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
3BaCl₂(aq) + Al₂(SO₄)₃(aq) → 3BaSO₄(s)↓ + 2AlCl₃(aq)
(iii) Sodium + Water → Sodium hydroxide + Hydrogen
2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)↑
❓ Q3. Write a balanced chemical equation with state symbols for the following reactions:
(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s)↓ + 2NaCl(aq)
(ii) Sodium hydroxide solution reacts with hydrochloric acid solution to produce sodium chloride solution and water.
NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
❓ Q1. A solution of a substance \'X\' is used for whitewashing.
(i) Name the substance \'X\' and write its formula.
(ii) Write the reaction of the substance \'X\' with water.
ANS (i) The substance \'X\' is Calcium Oxide (Quick Lime). Its chemical formula is CaO.
(ii) Reaction with water:
CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat
This is an exothermic reaction as a large amount of heat is released. Ca(OH)₂ is called Slaked Lime.
❓ Q2. Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas.
ANS During electrolysis of water, water decomposes into hydrogen and oxygen gases:
2H₂O(l) →[Electricity] 2H₂(g) + O₂(g)
From this equation, 2 volumes of hydrogen are produced for every 1 volume of oxygen. So hydrogen gas collected is double the amount of oxygen. The gas collected in double amount is Hydrogen (H₂).
❓ Q1. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
ANS When an iron nail is dipped in copper sulphate solution, iron displaces copper because iron is more reactive than copper.
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
The blue colour of CuSO₄ solution fades and turns green (FeSO₄). A brown deposit of copper appears on the iron nail. This is a displacement reaction.
❓ Q2. Give an example of a double displacement reaction other than the one given in Activity 1.10.
ANS
Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s)↓ + 2NaCl(aq)
In this reaction, sodium sulphate and barium chloride exchange their ions to form barium sulphate (white precipitate) and sodium chloride. Hence, it is a double displacement reaction.
❓ Q3. Identify the substances that are oxidised and reduced in the following reactions:
(i) 4Na(s) + O₂(g) → 2Na₂O(s)
OXIDISED Sodium (Na) — gains oxygen
REDUCED Oxygen (O₂) — gained by sodium
(ii) CuO(s) + H₂(g) → Cu(s) + H₂O(l)
REDUCED CuO → Cu (loss of oxygen)
OXIDISED H₂ → H₂O (gain of oxygen)
❓ Q1. Which of the statements about the reaction below are 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
✅ Answer: (i) (a) and (b)
PbO is losing oxygen → lead oxide is getting reduced (not lead).
Carbon is gaining oxygen to form CO₂ → carbon is getting oxidised (not CO₂).
So statements (a) and (b) are incorrect.
❓ Q2. Fe₂O₃ + 2Al → Al₂O₃ + 2Fe — The above reaction is an example of:
✅ Answer: (d) Displacement reaction
Aluminium (more reactive) displaces iron from iron oxide. This is also called a thermite reaction, used to join railway tracks.
❓ Q3. What happens when dilute hydrochloric acid is added to iron fillings?
✅ Answer: (a) Hydrogen gas and iron chloride are produced
Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g)↑
❓ Q4. What is a balanced chemical equation? Why should chemical equations be balanced?
ANS A balanced chemical equation is one in which the number of atoms of each element on the reactant side is equal to the number of atoms on the product side.
Chemical equations should be balanced to satisfy the Law of Conservation of Mass — matter can neither be created nor destroyed in a chemical reaction.
Example:
Unbalanced: H₂ + O₂ → H₂O ❌
Balanced: 2H₂ + O₂ → 2H₂O ✅
❓ Q5. Translate the following statements into chemical equations and balance them:
(a) Hydrogen gas combines with nitrogen to form ammonia.
3H₂(g) + N₂(g) → 2NH₃(g)
(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
2H₂S(g) + 3O₂(g) → 2H₂O(l) + 2SO₂(g)
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
3BaCl₂(aq) + Al₂(SO₄)₃(aq) → 2AlCl₃(aq) + 3BaSO₄(s)↓
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g)↑
❓ Q6. Balance the following chemical equations:
(a) HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O
2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O
(b) NaOH + H₂SO₄ → Na₂SO₄ + H₂O
2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O
(c) NaCl + AgNO₃ → AgCl + NaNO₃
NaCl(aq) + AgNO₃(aq) → AgCl(s)↓ + NaNO₃(aq) (Already balanced)
(d) BaCl₂ + H₂SO₄ → BaSO₄ + HCl
BaCl₂(aq) + H₂SO₄(aq) → BaSO₄(s)↓ + 2HCl(aq)
❓ Q7. Write the balanced chemical equations for the following reactions:
(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s)↓ + H₂O(l)
(b) Zinc + Silver nitrate → Zinc nitrate + Silver
Zn(s) + 2AgNO₃(aq) → Zn(NO₃)₂(aq) + 2Ag(s)
(c) Aluminium + Copper chloride → Aluminium chloride + Copper
2Al(s) + 3CuCl₂(aq) → 2AlCl₃(aq) + 3Cu(s)
(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride
BaCl₂(aq) + K₂SO₄(aq) → BaSO₄(s)↓ + 2KCl(aq)
❓ Q8. Write the balanced chemical equation and identify the type of reaction:
(a) Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(aq)
2KBr(aq) + BaI₂(aq) → 2KI(aq) + BaBr₂(aq)
🔄 Double Displacement Reaction
(b) Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)
ZnCO₃(s) → ZnO(s) + CO₂(g)
💥 Decomposition Reaction
(c) Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)
H₂(g) + Cl₂(g) → 2HCl(g)
🔗 Combination Reaction
(d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)
Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)↑
⚡ Displacement Reaction
❓ Q9. What does one mean by exothermic and endothermic reactions? Give examples.
🔥 Exothermic Reactions (Heat Released)
Reactions in which heat energy is released along with products.
Example 1: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) + Heat
Example 2: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (Respiration)
❄️ Endothermic Reactions (Heat Absorbed)
Reactions in which heat energy is absorbed from surroundings.
Example 1: CaCO₃(s) →[Heat] CaO(s) + CO₂(g)
Example 2: 2FeSO₄(s) →[Heat] Fe₂O₃(s) + SO₂(g) + SO₃(g)
❓ Q10. Why is respiration considered an exothermic reaction? Explain.
ANS Respiration is an exothermic reaction because during respiration, glucose combines with oxygen in our body cells to produce carbon dioxide, water and a large amount of energy.
C₆H₁₂O₆(aq) + 6O₂(g) → 6CO₂(g) + 6H₂O(l) + Energy
Since energy is released, respiration is exothermic. This energy keeps our body warm and helps perform various activities.
❓ Q11. Why are decomposition reactions called the opposite of combination reactions? Write equations.
ANS In combination, two or more substances combine to form one product. In decomposition, one substance breaks into two or more. They are reverse of each other.
🔗 COMBINATION
CaO + H₂O → Ca(OH)₂
Two → One product
💥 DECOMPOSITION
Ca(OH)₂ →[Heat] CaO + H₂O
One → Two products
❓ Q12. Write one equation each for decomposition reactions where energy is supplied as heat, light or electricity.
🔥 (i) Thermal Decomposition (Heat)
2Pb(NO₃)₂(s) →[Heat] 2PbO(s) + 4NO₂(g) + O₂(g)
☀️ (ii) Photolytic Decomposition (Light)
2AgCl(s) →[Sunlight] 2Ag(s) + Cl₂(g)
(This is why AgCl is stored in dark bottles)
⚡ (iii) Electrolytic Decomposition (Electricity)
2H₂O(l) →[Electricity] 2H₂(g) + O₂(g)
❓ Q13. What is the difference between displacement and double displacement reactions? Write equations.
⚡ Displacement Reaction
A more reactive element displaces a less reactive element from its compound.
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
🔄 Double Displacement Reaction
Two reactants exchange their ions to form two new products.
Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s)↓ + 2NaCl(aq)
Key Difference: In displacement, only one element is displaced. In double displacement, both reactants exchange ions simultaneously.
❓ Q14. In refining of silver, recovery of silver from silver nitrate solution involves displacement by copper metal. Write the reaction.
ANS
Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s)
Copper (more reactive) displaces silver. Solution turns blue due to Cu(NO₃)₂ and silver metal is deposited.
❓ Q15. What do you mean by a precipitation reaction? Explain by giving examples.
ANS A precipitation reaction is one in which two solutions react to form an insoluble substance (precipitate) that settles down.
Ex 1: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s)↓ + 2NaCl(aq)
Ex 2: AgNO₃(aq) + NaCl(aq) → AgCl(s)↓ + NaNO₃(aq)
The insoluble substance formed is called a precipitate (marked with ↓).
❓ Q16. Explain the following in terms of gain or loss of oxygen with two examples each:
(a) Oxidation = Gain of Oxygen 🔥
Ex 1: 2Cu + O₂ → 2CuO (Copper gains oxygen)
Ex 2: 2Mg + O₂ → 2MgO (Magnesium gains oxygen)
(b) Reduction = Loss of Oxygen ❄️
Ex 1: CuO + H₂ → Cu + H₂O (CuO loses oxygen)
Ex 2: Fe₂O₃ + 3CO → 2Fe + 3CO₂ (Fe₂O₃ loses oxygen)
❓ Q17. A shiny brown coloured element \'X\' on heating in air becomes black in colour. Name element \'X\' and the black compound formed.
ANS Element \'X\' is Copper (Cu). The black compound formed is Copper Oxide (CuO).
2Cu(s) + O₂(g) →[Heat] 2CuO(s)
(Shiny brown copper → Black copper oxide) — Oxidation reaction
❓ Q18. Why do we apply paint on iron articles?
ANS We apply paint on iron articles to prevent rusting (corrosion). Iron reacts with oxygen and moisture in air to form reddish-brown rust (Fe₂O₃.xH₂O). Paint provides a protective coating that prevents iron from direct contact with air and moisture, thus preventing rusting.
❓ Q19. Oil and fat containing food items are flushed with nitrogen. Why?
ANS Food items are flushed with nitrogen to prevent rancidity. Fats and oils get oxidised when exposed to air, causing bad smell and taste. Nitrogen is an inert gas — it replaces oxygen in the packet, preventing oxidation and keeping food fresh longer.
❓ Q20. Explain the following terms with one example each:
(a) Corrosion 🔧
The process in which metals are gradually eaten away by action of air, moisture, or chemicals on their surface.
Example: Rusting of iron
4Fe + 3O₂ + xH₂O → 2Fe₂O₃.xH₂O (Rust)
Other examples: Green coating on copper (CuCO₃), Black coating on silver (Ag₂S)
(b) Rancidity 🍔
The process of oxidation of fats and oils in food items leading to unpleasant smell and taste.
Example: Butter/ghee kept open for long time develops bad smell.
Prevention methods:
✅ Adding antioxidants ✅ Airtight containers ✅ Flushing with nitrogen
✅ Refrigeration ✅ Storing away from light
✅ Chapter 1 Complete — Chemical Reactions and Equations
📚 UNIQUE STUDY POINT by Sumeet Sahu
📍 Amitesh Nagar, Indore, MP | uniquestudyonline.com
📱 Download our Android App for more study materials!