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Chemical Reactions and Equations Extra Questions and Answers for Class 10 provide additional practice beyond the textbook questions. These questions cover important concepts such as types of chemical reactions, balancing equations, oxidation and reduction, corrosion, and rancidity.
Chemical Reactions and Equations Extra Questions and Answers
Short-Answer
Q. Why should a magnesium ribbon be cleaned before burning in air?
Answer: Magnesium ribbon is cleaned to remove the thin layer of magnesium oxide formed on its surface. This helps magnesium burn easily with a bright flame.
Q. Why should chemical equations be balanced?
Answer: Chemical equations are balanced to follow the law of conservation of mass. The number of atoms of each element must be equal on both sides of the equation.
Q. Define a combination reaction with one example from the Textbook activities.
Answer: When two or more substances combine to form a single product, it is called a combination reaction. Example: CaO + H₂O → Ca(OH)₂ (formation of slaked lime).
Q. Why does the blue colour of copper sulphate solution fade when an iron nail is dipped in it?
Answer: Iron is more reactive than copper. It displaces copper from copper sulphate solution, forming iron sulphate. The solution becomes light in colour and copper gets deposited on the nail.
Q. Define exothermic and endothermic reactions with one example each.
Answer:
- Exothermic reaction: A reaction that gives out heat. Example: CH₄ + 2O₂ → CO₂ + 2H₂O (burning of natural gas).
- Endothermic reaction: A reaction that absorbs heat. Example: CaCO₃ → CaO + CO₂ (thermal decomposition of limestone).
Q. Give two examples of chemical reactions from daily life and explain why they are chemical changes.
Answer:
- Rusting of iron: Iron reacts with oxygen and moisture to form rust. A new substance is formed, so it is a chemical change.
- Digestion of food: Complex food molecules break down into simpler substances. Since new substances are formed, it is a chemical change.
Reason-Based
Q. Why is respiration considered an exothermic reaction?
Answer: Respiration releases energy when glucose combines with oxygen to form carbon dioxide and water. Since heat/energy is given out, it is an exothermic reaction.
Q. Why are decomposition reactions called the opposite of combination reactions?
Answer: In a combination reaction, two or more substances join to form one product. In a decomposition reaction, one substance breaks down into two or more products. That is why they are opposite.
Q. Give reasons:
a) The blue colour of copper sulphate solution fades when an iron nail is dipped in it.
Answer: Iron is more reactive than copper. It displaces copper from copper sulphate solution, forming iron sulphate which is light in colour.
b) Silver articles turn black after some time.
Answer: Silver reacts with sulphur compounds present in air to form silver sulphide, which is black in colour.
Q. Give reasons:
a) The equation Mg + O₂ → MgO is called a skeletal equation.
Answer: Because it is not balanced. The number of atoms on both sides is not equal.
b) We cannot change the formula of a compound while balancing an equation.
Answer: Changing the formula will change the substance itself. We can only adjust the number of molecules (coefficients).
Q. Give reasons:
a) Quick lime reacts vigorously with water.
Answer: Because the reaction is highly exothermic. It releases a lot of heat and forms slaked lime.
b) Silver chloride turns grey when kept in sunlight.
Answer: Silver chloride decomposes in sunlight to form silver metal and chlorine gas. The silver formed is grey.
Q. Give reasons:
a) A white precipitate is formed when sodium sulphate solution is mixed with barium chloride solution.
Answer: Because insoluble barium sulphate (BaSO₄) is formed in the reaction.
b) Silver articles turn black after some time.
Answer: Silver reacts with sulphur compounds in air to form silver sulphide, which is black in colour.
Application-Based
Q. Whitewashing walls gives a shiny finish after 2–3 days. Explain the chemical reactions involved.
Answer: When quick lime (CaO) reacts with water, it forms slaked lime [Ca(OH)₂]. Slaked lime slowly reacts with carbon dioxide in air to form calcium carbonate (CaCO₃). This thin layer of CaCO₃ gives the walls a shiny finish.
Equation:
- CaO + H₂O → Ca(OH)₂
- Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
Q. Write the balanced chemical equation for the reaction of sodium with water. Also mention the physical states of reactants and products.
Answer: 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g), Here, sodium (solid) reacts with water (liquid) to form sodium hydroxide (aqueous solution) and hydrogen gas.
Q. Whitewashing walls gives a shiny finish after 2–3 days. Explain the chemical reactions involved and name the compound responsible for the shine.
Answer: Quick lime reacts with water to form slaked lime. Slaked lime reacts with carbon dioxide in air to form calcium carbonate. Calcium carbonate (CaCO₃) is the compound responsible for the shiny finish.
Q. Explain two methods used to prevent corrosion of metals.
Answer:
- Painting or oiling: A protective layer prevents contact of metal with air and moisture.
- Galvanisation: Iron is coated with a thin layer of zinc. Zinc prevents rusting by forming a protective oxide layer.
Q. How can rancidity of food be prevented? Give examples.
Answer:
- By adding antioxidants: Substances like BHA are added to food items containing oil.
- By airtight packing: Food is kept in sealed containers to reduce contact with oxygen.
- By flushing with nitrogen gas: Chips packets are filled with nitrogen to prevent oxidation of oils.
Case-Based
Q. A student heats ferrous sulphate crystals in a boiling tube.
- What change in colour is observed?
- Which gases are released?
- Write the balanced chemical equation.
- Identify the type of reaction.
Answer:
- The crystals change from green to brown.
- Gases released are sulphur dioxide (SO₂) and sulphur trioxide (SO₃).
- Equation: 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃
- Type: Decomposition reaction.
Q. A student tries to balance the equation: Fe + H₂O → Fe₃O₄ + H₂
- Which element should be balanced first?
- Write the final balanced equation.
- Which method of balancing is used here?
Answer:
- Oxygen should be balanced first.
- Final equation: 3Fe + 4H₂O → Fe₃O₄ + 4H₂
- Method: Hit‑and‑trial method.
Q. A student dips iron nails in copper sulphate solution. After 20 minutes, the nails turn brownish and the solution’s blue colour fades.
- Which type of reaction is this?
- Write the balanced chemical equation.
- Which new compound is formed in solution?
- Why does the iron nail turn brownish?
Answer:
- Type: Displacement reaction.
- Equation: Fe + CuSO₄ → FeSO₄ + Cu
- New compound: Iron sulphate (FeSO₄).
- Nails turn brownish due to deposition of copper metal.
Q. A student heats lead nitrate in a boiling tube.
- What change is observed?
- Name the gas responsible for brown fumes.
- Write the balanced equation.
- Identify the type of reaction.
Answer:
- Observation: Brown fumes are seen.
- Gas: Nitrogen dioxide (NO₂).
- Equation: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂
- Type: Thermal decomposition reaction.
Experiment-Based
Q. In Activity, water is electrolysed using carbon electrodes.
- Why is dilute sulphuric acid added?
- Why is the volume of hydrogen collected double that of oxygen?
- Write the equation for the reaction.
Answer: Dilute sulphuric acid is added to make water a good conductor of electricity. Hydrogen collected is double because each water molecule has 2 hydrogen atoms and 1 oxygen atom. Equation: 2H₂O → 2H₂ + O₂
Q. In Activity, zinc granules are added to dilute sulphuric acid.
- What observations are made?
- Write the balanced equation with state symbols.
- Is the reaction exothermic or endothermic?
Answer:
- Observation: Bubbles of hydrogen gas are seen, and the test tube becomes warm.
- Equation: Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)
- Reaction: Exothermic (heat is released).
Q. A student heats ferrous sulphate crystals in a boiling tube.
- What colour change is observed?
- Which gases are released?
- Write the balanced chemical equation.
- Identify the type of reaction.
Answer:
- Colour changes from green to brown.
- Gases released: SO₂ and SO₃.
- Equation: 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃
- Type: Decomposition reaction.
Q. In Activity, water is electrolysed using carbon electrodes.
- Why is dilute sulphuric acid added?
- Why is the volume of hydrogen collected double that of oxygen?
- Write the equation for the reaction.
Answer:
- Dilute sulphuric acid makes water conduct electricity.
- Hydrogen collected is double because water has two hydrogen atoms for every oxygen atom.
- Equation: 2H₂O → 2H₂ + O₂
Q. In Activity, copper powder is heated in a china dish.
- What change is observed on the surface of copper?
- Write the balanced equation.
- What happens when hydrogen gas is passed over the heated black substance?
Answer:
- Copper surface turns black due to formation of copper oxide (CuO).
- Equation: 2Cu + O₂ → 2CuO
- When hydrogen is passed, black CuO is reduced back to brown copper metal.
Q. Describe the experiment of adding water to quicklime.
- What observation is made?
- Write the balanced equation.
- Name the type of reaction.
Answer:
- Observation: Quicklime reacts vigorously with water, releasing heat.
- Equation: CaO + H₂O → Ca(OH)₂
- Type: Combination reaction (also exothermic).
Differentiate
Q. Differentiate between corrosion and rancidity (with examples).
Answer:
- Corrosion: It is the slow damage of metals due to reaction with air, moisture, or acids. Example: Rusting of iron, green coating on copper.
- Rancidity: It is the spoilage of food containing oils/fats due to oxidation. Example: Oily food becoming foul‑smelling when kept for long.
Q. Differentiate between:
a) Combination and Decomposition reactions (with examples).
Answer:
- Combination: Two or more substances combine to form one product. Example: CaO + H₂O → Ca(OH)₂.
- Decomposition: One substance breaks down into two or more products. Example: CaCO₃ → CaO + CO₂.
b) Displacement and Double Displacement reactions (with examples).
Answer:
- Displacement: One element replaces another in a compound. Example: Fe + CuSO₄ → FeSO₄ + Cu.
- Double Displacement: Exchange of ions between two compounds. Example: Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl.
Q. Differentiate between:
a) Skeletal equation and balanced equation.
Answer:
- Skeletal equation: Shows reactants and products but atoms are not balanced. Example: Mg + O₂ → MgO.
- Balanced equation: Atoms of each element are equal on both sides. Example: 2Mg + O₂ → 2MgO.
b) Word equation and chemical equation.
Answer:
- Word equation: Written using names of substances. Example: Magnesium + Oxygen → Magnesium oxide.
- Chemical equation: Written using chemical symbols and formulae. Example: 2Mg + O₂ → 2MgO.
Q. Differentiate between:
a) Thermal decomposition and photolytic decomposition (with examples).
Answer:
- Thermal decomposition: A compound breaks down by heating. Example: CaCO₃ → CaO + CO₂.
- Photolytic decomposition: A compound breaks down by light. Example: 2AgCl → 2Ag + Cl₂ (in sunlight).
b) Exothermic and endothermic reactions (with examples).
Answer:
- Exothermic: Heat is released. Example: CH₄ + 2O₂ → CO₂ + 2H₂O.
- Endothermic: Heat is absorbed. Example: CaCO₃ → CaO + CO₂.
Q. Differentiate between:
a) Displacement and Double Displacement reactions (with examples).
Answer:
- Displacement: One element replaces another. Example: Zn + CuSO₄ → ZnSO₄ + Cu.
- Double Displacement: Exchange of ions between compounds. Example: NaOH + HCl → NaCl + H₂O.
b) Oxidation and Reduction (with examples).
Answer:
- Oxidation: Gain of oxygen or loss of hydrogen. Example: 2Cu + O₂ → 2CuO (copper oxidised).
- Reduction: Loss of oxygen or gain of hydrogen. Example: CuO + H₂ → Cu + H₂O (CuO reduced).
Competency-Based
Q. Identify the type of reaction in each case:
- a) CH₄ + 2O₂ → CO₂ + 2H₂O
- b) 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂
- c) NaOH + HCl → NaCl + H₂O
Answer:
- a) Combustion (Exothermic reaction).
- b) Thermal decomposition.
- c) Neutralisation (Double displacement).
Q. Identify the substances oxidised and reduced in the following reactions:
- a) CuO + H₂ → Cu + H₂O
- b) 4Na + O₂ → 2Na₂O
Answer:
- a) CuO is reduced to Cu, H₂ is oxidised to H₂O.
- b) Na is oxidised to Na₂O, O₂ is reduced.
Q. Balance the following equations and identify the type of reaction:
- a) H₂ + Cl₂ → HCl
- b) BaCl₂ + Al₂(SO₄)₃ → BaSO₄ + AlCl₃
Answer:
- a) Balanced: H₂ + Cl₂ → 2HCl (Combination reaction).
- b) Balanced: 3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄ + 2AlCl₃ (Double displacement reaction).
Q. Identify the type of reaction in each case:
- a) C + O₂ → CO₂
- b) CaCO₃ → CaO + CO₂
- c) 2AgCl → 2Ag + Cl₂ (in sunlight)
Answer:
- a) Combination reaction.
- b) Thermal decomposition reaction.
- c) Photolytic decomposition reaction.
Q. Identify the substances oxidised and reduced in the following reactions:
- a) CuO + H₂ → Cu + H₂O
- b) ZnO + C → Zn + CO
Answer:
- a) CuO is reduced to Cu, H₂ is oxidised to H₂O.
- b) ZnO is reduced to Zn, C is oxidised to CO.
Q. Write balanced equations and identify the type of reaction in each case:
- a) Pb + CuCl₂ → PbCl₂ + Cu
- b) NaOH + HCl → NaCl + H₂O
- c) 2Cu + O₂ → 2CuO
Answer:
- a) Pb + CuCl₂ → PbCl₂ + Cu (Displacement reaction).
- b) NaOH + HCl → NaCl + H₂O (Neutralisation / Double displacement).
- c) 2Cu + O₂ → 2CuO (Oxidation / Combination reaction).
HOTS
Q. Respiration is considered an exothermic reaction. Justify with the help of a balanced chemical equation. Why is this reaction essential for life?
Answer: Respiration breaks down glucose in the presence of oxygen to release energy.
Equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy
It is exothermic because energy is released. This energy is essential for life as it helps in all body activities like growth, movement, and repair.
Q. Why is the law of conservation of mass considered the basis of balancing chemical equations? Illustrate with one example from the chapter.
Answer: The law states that mass can neither be created nor destroyed in a chemical reaction. So, the number of atoms of each element must be equal on both sides of the equation.
Example: Zn + H₂SO₄ → ZnSO₄ + H₂ (atoms of Zn, H, S, O are equal on both sides).
Q. Respiration is considered an exothermic reaction. Justify with the help of a balanced chemical equation. Why is this reaction essential for survival?
Answer: Respiration releases energy by breaking down glucose with oxygen. Equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy
It is essential for survival because the released energy is used for vital processes like breathing, circulation, and maintaining body temperature.
Q. Corrosion and rancidity are both effects of oxidation. Compare them and explain how they can be prevented in daily life.
Answer:
- Corrosion: Oxidation of metals, e.g., rusting of iron. Prevented by painting, oiling, or galvanisation.
- Rancidity: Oxidation of oils/fats in food, e.g., stale smell of oily food. Prevented by airtight containers, antioxidants, or flushing with nitrogen gas.
Q. Explain how the law of conservation of mass is verified by the reaction between zinc and sulphuric acid.
Answer: In the reaction Zn + H₂SO₄ → ZnSO₄ + H₂, the number of atoms of Zn, H, S, and O are equal on both sides. This shows that mass is conserved, verifying the law of conservation of mass.
Q. Why do decomposition reactions require energy input while combination reactions often release energy?
Answer:
- Decomposition reactions: A single compound breaks into simpler substances. Breaking bonds requires energy, so heat, light, or electricity is absorbed.
- Combination reactions: Two or more substances combine to form one product. New bonds are formed, and energy is released, making them exothermic.
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