Here are the answers to your Class 10 Science NCERT – Chapter 1 (Chemical Reactions and Equations) questions, clearly and completely for your CBSE Board exam preparation:
⸻
✍️ Answers
⸻
🔷 Q1.
Which of the statements about the reaction below are incorrect?
2PbO(s) + C(s) → 2Pb(s) + CO_2(g)
Statements:
(a) Lead is getting reduced. ✅
(b) Carbon dioxide is getting oxidised. ❌
(c) Carbon is getting oxidised. ✅
(d) Lead oxide is getting reduced. ✅
Correct answer:
❌ Statement (b) is incorrect because CO₂ is not oxidised further.
Hence, option: (i) (a) and (b) is incorrect — but note here actually only (b) is incorrect.
So most correct answer:
👉 (i) (a) and (b)
⸻
🔷 Q2.
Fe_2O_3 + 2Al → Al_2O_3 + 2Fe
This is a displacement reaction, also called thermite reaction.
✅ Answer: (d) displacement reaction
⸻
🔷 Q3.
When dilute HCl is added to iron fillings:
Fe + 2HCl → FeCl_2 + H_2 ↑
✅ Answer: (a) Hydrogen gas and iron chloride are produced.
⸻
🔷 Q4.
What is a balanced chemical equation? Why should equations be balanced?
✅ A chemical equation with equal number of atoms of each element on both sides is called a balanced chemical equation.
✅ It is balanced to obey the Law of Conservation of Mass — matter is neither created nor destroyed.
⸻
🔷 Q5. Translate and balance:
(a) Hydrogen + Nitrogen → Ammonia
N_2 + 3H_2 → 2NH_3
(b) Hydrogen sulphide + Oxygen → Water + Sulphur dioxide
2H_2S + 3O_2 → 2H_2O + 2SO_2
(c) Barium chloride + Aluminium sulphate → Aluminium chloride + Barium sulphate
3BaCl_2 + Al_2(SO_4)_3 → 2AlCl_3 + 3BaSO_4↓
(d) Potassium + Water → Potassium hydroxide + Hydrogen
2K + 2H_2O → 2KOH + H_2 ↑
⸻
🔷 Q6. Balance:
(a)
2HNO_3 + Ca(OH)_2 → Ca(NO_3)_2 + 2H_2O
(b)
2NaOH + H_2SO_4 → Na_2SO_4 + 2H_2O
(c)
NaCl + AgNO_3 → AgCl↓ + NaNO_3
(d)
BaCl_2 + H_2SO_4 → BaSO_4↓ + 2HCl
⸻
🔷 Q7. Balanced equations:
(a)
Ca(OH)_2 + CO_2 → CaCO_3↓ + H_2O
(b)
Zn + 2AgNO_3 → Zn(NO_3)_2 + 2Ag
(c)
2Al + 3CuCl_2 → 2AlCl_3 + 3Cu
(d)
BaCl_2 + K_2SO_4 → BaSO_4↓ + 2KCl
⸻
🔷 Q8. Equations & type:
(a)
2KBr(aq) + BaI_2(aq) → 2KI(aq) + BaBr_2(s)
Double displacement / precipitation
(b)
ZnCO_3(s) → ZnO(s) + CO_2(g)
Decomposition
(c)
H_2(g) + Cl_2(g) → 2HCl(g)
Combination
(d)
Mg(s) + 2HCl(aq) → MgCl_2(aq) + H_2(g)
Displacement
⸻
🔷 Q9.
✅ Exothermic reactions: release heat.
e.g., Combustion:
C + O_2 → CO_2 + heat
✅ Endothermic reactions: absorb heat.
e.g., Photosynthesis:
6CO_2 + 6H_2O + sunlight → C_6H_{12}O_6 + O_2
⸻
🔷 Q10.
Why is respiration exothermic?
Glucose is broken down with oxygen to release energy:
C_6H_{12}O_6 + 6O_2 → 6CO_2 + 6H_2O + energy
⸻
🔷 Q11.
Why are decomposition & combination opposite?
• Combination: 2+ reactants → 1 product
• Decomposition: 1 reactant → 2+ products
Examples:
2H_2 + O_2 → 2H_2O (Combination)
2H_2O → 2H_2 + O_2 (Decomposition)
⸻
🔷 Q12.
Decomposition with:
• Heat:
CaCO_3 → CaO + CO_2
• Light:
2AgCl → 2Ag + Cl_2
• Electricity:
2H_2O → 2H_2 + O_2
⸻
🔷 Q13.
✅ Displacement: More reactive element displaces less reactive one.
✅ Double displacement: Two compounds exchange ions, often forming a precipitate.
⸻
🔷 Q14.
Recovery of silver:
Cu + 2AgNO_3 → Cu(NO_3)_2 + 2Ag
⸻
🔷 Q15.
✅ Precipitation reaction: Insoluble solid forms in a solution.
e.g.,
BaCl_2 + Na_2SO_4 → BaSO_4↓ + 2NaCl
⸻
🔷 Q16.
(a) Oxidation (gain of O / loss of H):
C + O_2 → CO_2
2Mg + O_2 → 2MgO
(b) Reduction (loss of O / gain of H):
CuO + H_2 → Cu + H_2O
Fe_2O_3 + 3CO → 2Fe + 3CO_2
⸻
🔷 Q17.
Brown element ‘X’: Copper (Cu)
Black compound: Copper oxide (CuO)
2Cu + O_2 → 2CuO
⸻
🔷 Q18.
✅ Paint prevents iron from rusting by stopping oxygen & moisture contact.
⸻
🔷 Q19.
✅ Nitrogen prevents rancidity by displacing oxygen & slowing oxidation of oils/fats.
⸻
🔷 Q20.
(a) Corrosion:
Slow damage of metals by oxidation.
e.g., Rusting:
4Fe + 3O_2 + 6H_2O → 4Fe(OH)_3
(b) Rancidity:
Oxidation of fats/oils → bad taste & smell.
e.g., spoiled chips.
⸻
If you’d like, I can also prepare:
📄 Printable PDF of this solution
📋 Neat HTML notes page
📊 A quiz for practice
Let me know! 🌟
📘 NCERT Class 10 Science – Chapter 1
✨ Chemical Reactions and Equations
⸻
🔷 Topics covered:
✅ Chemical reactions
✅ Writing & balancing chemical equations
✅ Types of reactions:
• Combination
• Decomposition
• Displacement
• Double displacement
• Oxidation & reduction
⸻
📋 NCERT Questions & Solutions
⸻
🔷 In-text Questions (Page 6)
Q1. Why should a magnesium ribbon be cleaned before burning in air?
✅ Answer: Magnesium ribbon is cleaned by rubbing with sandpaper to remove the oxide layer (MgO) formed on its surface. This ensures that magnesium metal burns readily to form magnesium oxide.
⸻
🔷 In-text Questions (Page 10)
Q2. Why is respiration considered an exothermic reaction?
✅ Answer: During respiration, glucose combines with oxygen to produce CO₂, water and releases energy.
Reaction:
C_6H_{12}O_6 + 6O_2 → 6CO_2 + 6H_2O + energy
Since energy is released, it is exothermic.
⸻
Q3. Why are decomposition reactions called the opposite of combination reactions?
✅ Answer:
• In decomposition reactions, a single compound breaks down into two or more simpler substances.
• In combination reactions, two or more substances combine to form a single product.
Thus, they are opposite in nature.
⸻
🔷 In-text Questions (Page 13)
Q4. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
✅ Answer:
Iron displaces copper from copper sulphate solution because it is more reactive.
Reaction:
Fe + CuSO_4 → FeSO_4 + Cu
The solution turns green due to the formation of iron sulphate.
⸻
Q5. Why is double displacement reaction called precipitation reaction?
✅ Answer:
In some double displacement reactions, an insoluble solid (precipitate) is formed.
Example:
BaCl_2 + Na_2SO_4 → BaSO_4↓ + 2NaCl
Here, white precipitate of BaSO_4 is formed.
⸻
🔷 Exercise Questions (Page 14–15)
Q1. A solution of substance ‘X’ is used for whitewashing.
What is ‘X’? Write its reaction with water.
✅ Answer:
‘X’ is calcium oxide (CaO) or quicklime.
CaO + H_2O → Ca(OH)_2
⸻
Q2. Why does the brown colour of copper sulphate disappear when iron is added?
✅ Answer:
Iron displaces copper from copper sulphate (blue) to form iron sulphate (green) and copper metal (brownish-red).
Reaction:
Fe + CuSO_4 → FeSO_4 + Cu
⸻
Q3. Why do we apply paint on iron articles?
✅ Answer:
To prevent rusting by stopping oxygen and moisture from coming in contact with iron.
⸻
Q4. Oil and fat-containing food is flushed with nitrogen. Why?
✅ Answer:
To prevent rancidity. Nitrogen displaces oxygen and prevents oxidation of oils and fats.
⸻
Q5. Explain the following with an example:
• Corrosion:
Slow oxidation of metals when exposed to air and moisture.
Example: Rusting of iron.
4Fe + 3O_2 + 6H_2O → 4Fe(OH)_3
• Rancidity:
Oxidation of fats and oils in food resulting in bad taste and smell.
⸻
📝 Summary Table of Reactions:
Reaction Type Example
Combination CaO + H_2O → Ca(OH)_2
Decomposition 2Pb(NO_3)_2 → 2PbO + 4NO_2 + O_2
Displacement Zn + CuSO_4 → ZnSO_4 + Cu
Double Displacement Na_2SO_4 + BaCl_2 → BaSO_4 + 2NaCl
Exothermic Respiration
Endothermic Photosynthesis
⸻
If you want, I can also:
📄 Provide these solutions in a PDF or printable format
📋 Design them as a nice HTML notes page
📊 Or create a quiz from this chapter
Let me know what you’d like next! 🌟