SHORT ANSWER QUESTIONS (2–3 Marks)
✅ Q1. Write a balanced chemical equation for the reaction of zinc with dilute sulphuric acid. Name the type of reaction.
📅 CBSE 2010
Ans:
Zn + H₂SO₄ → ZnSO₄ + H₂ ↑
Type: Displacement reaction
✅ Q2. Why does the colour of copper sulphate solution change when an iron nail is dipped in it? Write the reaction.
📅 CBSE 2011
Ans:
Iron displaces copper from copper sulphate because it is more reactive.
Reaction: Fe + CuSO₄ → FeSO₄ + Cu
The solution turns green due to formation of FeSO₄.
✅ Q3. Define oxidation and reduction with one example each.
📅 CBSE 2012
Ans:
- Oxidation: Gain of oxygen or loss of hydrogen.
E.g.: C + O₂ → CO₂ - Reduction: Loss of oxygen or gain of hydrogen.
E.g.: CuO + H₂ → Cu + H₂O
✅ Q4. Write the chemical equation for thermal decomposition of lead nitrate. Mention the change observed.
📅 CBSE 2014
Ans:
2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂
Observation: Brown fumes of NO₂ are released.
✅ Q5. Identify the type of reaction and give one example each:
(i) Thermal decomposition
(ii) Photochemical decomposition
📅 CBSE 2016
Ans:
(i) CaCO₃ → CaO + CO₂ (Thermal)
(ii) 2AgCl → 2Ag + Cl₂ (Photochemical, in sunlight)
✅ Q6. Why is respiration considered an exothermic reaction?
📅 CBSE 2017
Ans:
In respiration, glucose is broken down to release energy:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy
Since energy is released, it is an exothermic reaction.
✅ Q7. What is a redox reaction? Give one example.
📅 CBSE 2015
Ans:
A redox reaction involves both oxidation and reduction.
Example: Zn + CuO → ZnO + Cu
(Zn is oxidised, CuO is reduced)
✅ Q8. Explain with an example how displacement reaction is different from double displacement reaction.
📅 CBSE 2018
Ans:
- Displacement: One element replaces another.
E.g.: Zn + CuSO₄ → ZnSO₄ + Cu - Double Displacement: Exchange of ions between compounds.
E.g.: Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl
✅ Q9. Why is it important to balance a chemical equation?
📅 CBSE 2011
Ans:
To satisfy the law of conservation of mass, where the total mass of reactants = total mass of products.
✅ **Q10. Write a balanced chemical equation for the reaction of:
(i) Calcium oxide with water
(ii) Barium chloride with sulphuric acid**
📅 CBSE 2020
Ans:
(i) CaO + H₂O → Ca(OH)₂
(ii) BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl
📗 LONG ANSWER QUESTIONS (5 Marks)
✅ **Q11. Define the following with one example each:
(a) Combination reaction
(b) Decomposition reaction
(c) Displacement reaction
(d) Double displacement reaction
(e) Redox reaction**
📅 CBSE 2014
Ans:
(a) Combination: Two substances combine to form one.
→ H₂ + Cl₂ → 2HCl
(b) Decomposition: One substance breaks down.
→ CaCO₃ → CaO + CO₂
(c) Displacement: One element replaces another.
→ Zn + CuSO₄ → ZnSO₄ + Cu
(d) Double Displacement: Exchange of ions.
→ Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl
(e) Redox: Oxidation and reduction occur.
→ CuO + H₂ → Cu + H₂O
✅ **Q12. Write the balanced chemical equations for the following and identify the type of reaction:
(i) Rusting of iron
(ii) Reaction of zinc with hydrochloric acid
(iii) Decomposition of lead nitrate**
📅 CBSE 2015
Ans:
(i) 4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃ (Oxidation)
(ii) Zn + 2HCl → ZnCl₂ + H₂ (Displacement)
(iii) 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂ (Decomposition)
✅ **Q13. Define corrosion. What happens when:
(a) Silver is exposed to air
(b) Copper vessels are exposed to moist air
(c) Iron nails are left in water**
📅 CBSE 2017
Ans:
Corrosion is the deterioration of metal due to atmospheric gases.
(a) Silver → turns black due to Ag₂S
(b) Copper → forms green coating of CuCO₃·Cu(OH)₂
(c) Iron → rusts forming Fe₂O₃·xH₂O
✅ Q14. What are exothermic and endothermic reactions? Give one example each.
📅 CBSE 2012
Ans:
- Exothermic: Releases heat.
Example: C + O₂ → CO₂ + heat - Endothermic: Absorbs heat.
Example: CaCO₃ → CaO + CO₂
✅ Q15. State the differences between physical and chemical changes with examples.
📅 CBSE 2013
Ans:
| Physical Change | Chemical Change |
|---|---|
| No new substance formed | New substance formed |
| Reversible | Usually irreversible |
| Example: Melting ice | Example: Rusting of iron |
✅ Q16. Write one balanced equation for each of the following reactions and name them:
(a) Thermal decomposition
(b) Photochemical decomposition
(c) Displacement reaction
(d) Double displacement reaction
(e) Combination reaction
📅 CBSE 2022
Ans:
(a) CaCO₃ → CaO + CO₂
(b) 2AgCl → 2Ag + Cl₂
(c) Fe + CuSO₄ → FeSO₄ + Cu
(d) Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl
(e) C + O₂ → CO₂
✅ Q17. Explain oxidation and reduction with examples of redox reactions. Identify oxidised and reduced substances.
📅 CBSE 2016
Ans:
Oxidation: Gain of oxygen
Reduction: Loss of oxygen
Example: CuO + H₂ → Cu + H₂O
- CuO is reduced to Cu
- H₂ is oxidised to H₂O
✅ Q18. A solution of a substance ‘X’ is used for white-washing.
(a) Name the substance X.
(b) Write its formula.
(c) Write the chemical reaction involved in its preparation.
📅 CBSE 2014
Ans:
(a) Calcium oxide
(b) CaO
(c) CaO + H₂O → Ca(OH)₂
✅ Q19. What happens when ferrous sulphate crystals are heated? Write the balanced chemical equation.
📅 CBSE 2019
Ans:
FeSO₄·7H₂O → Fe₂O₃ + SO₂ + SO₃ + H₂O
- Green crystals turn brown
- Gas with rotten egg smell is released
✅ **Q20. Give reasons:
(a) Silver chloride turns grey in sunlight.
(b) Iron articles are painted.
(c) Vinegar and lemon juice conduct electricity.**
📅 CBSE 2023
Ans:
(a) Due to photochemical decomposition of AgCl into Ag.
(b) To prevent rusting.
(c) They contain acids that release ions in water.
CBSE Class 10 Chemistry: Chapter 1 – Chemical Reactions and Equations Short & Long Answer Questions with Solutions (Reflecting patterns from past CBSE Board Exams)
Part A: Short Answer Questions (2-3 Marks)
- Question: Why is it necessary to balance a chemical equation? State the Law of Conservation of Mass.
Answer: It is necessary to balance a chemical equation to satisfy the Law of Conservation of Mass. This law states that mass can neither be created nor destroyed in a chemical reaction. Therefore, the total mass of the elements present in the reactants must be equal to the total mass of the elements present in the products. This implies that the number of atoms of each element must be the same on both sides of a chemical equation. - Question: Write balanced chemical equations for the following reactions: (a) Hydrogen gas combines with nitrogen to form ammonia. (b) Hydrogen sulphide gas reacts with sulphur dioxide to form sulphur and water.
Answer: (a) 3H2(g)+N2(g)→2NH3(g) (b) 2H2S(g)+SO2(g)→3S(s)+2H2O(l) - Question: What happens chemically when quicklime is added to water? Write the balanced chemical equation for the reaction.
Answer: When quicklime (calcium oxide, CaO) is added to water, a vigorous and highly exothermic reaction occurs to form slaked lime (calcium hydroxide, Ca(OH)2).
A large amount of heat is released.
CaO(s)+H2O(l)→Ca(OH)2(aq)+Heat - Question: Explain rancidity. List two ways to prevent or slow down rancidity.
Answer: Rancidity is the process of oxidation of fats and oils in food items, causing them to become stale, produce an unpleasant smell, and change taste. This happens when they are exposed to air (oxygen) for a long time. Two ways to prevent or slow down rancidity are: (i) Adding antioxidants to food items containing fats and oils. (ii) Flushing food packages with inert gases like nitrogen (e.g., in potato chip bags). - Question: What is a precipitation reaction? Give an example with a balanced chemical equation.
Answer: A precipitation reaction is a chemical reaction in which two soluble ionic compounds in an aqueous solution combine to form an insoluble solid product called a precipitate. The precipitate separates out from the solution. Example: When barium chloride solution reacts with sodium sulphate solution, a white precipitate of barium sulphate is formed.
BaCl2(aq)+Na2SO4(aq)→BaSO4(s)↓+2NaCl(aq) - Question: A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed. Write the chemical equation for the reaction.
Answer: The shiny brown coloured element ‘X’ is Copper (Cu). When heated in air (oxygen), it forms black coloured Copper(II) Oxide (CuO). 2Cu(s)+O2(g)Heat2CuO(s) - Question: Differentiate between exothermic and endothermic reactions with one example each. Answer:
- Exothermic Reaction: A chemical reaction that releases energy, usually in the form of heat, to the surroundings. The temperature of the surroundings increases. Example: Burning of natural gas: CH4(g)+2O2(g)→CO2(g)+2H2O(l)+Heat
- Endothermic Reaction: A chemical reaction that absorbs energy from the surroundings, usually in the form of heat. The temperature of the surroundings decreases. Example: Decomposition of calcium carbonate: CaCO3(s)Heat
CaO(s)+CO2(g)
- Question: Why do we apply paint on iron articles?
Answer: We apply paint on iron articles to prevent them from rusting (corrosion). Paint forms a protective layer on the surface of the iron, preventing its direct contact with moisture (water vapour) and oxygen present in the air, which are essential for rusting to occur. - Question: What is a redox reaction? Identify the substance oxidized and reduced in the following reaction: ZnO(s)+C(s)→Zn(s)+CO(g)
Answer: A redox (reduction-oxidation) reaction is a chemical reaction in which both oxidation and reduction occur simultaneously. Oxidation involves the gain of oxygen or loss of hydrogen/electrons, while reduction involves the loss of oxygen or gain of hydrogen/electrons.
In the given reaction: ZnO(s)+C(s)→Zn(s)+CO(g)- ZnO is reduced to Zn (loss of oxygen).
- C is oxidized to CO (gain of oxygen).
- Question: Give an example of a decomposition reaction caused by:
(a) Heat (b) Light (c) Electricity.
Answer: (a) Decomposition by Heat (Thermal decomposition): CaCO3(s)HeatCaO(s)+CO2(g)
(b) Decomposition by Light (Photolytic decomposition): 2AgCl(s)Sunlight2Ag(s)+Cl2(g)
(c) Decomposition by Electricity (Electrolytic decomposition): 2H2O(l)Electricity2H2(g)+O2(g)
Part B: Long Answer Questions (3-5 Marks)
- Question: Classify the following reactions into different types:
(a) AgNO3(aq)+NaCl(aq)→AgCl(s)+NaNO3(aq)
(b) CaO(s)+H2O(l)→Ca(OH)2(aq)
(c) 2KClO3(s)Heat2KCl(s)+3O2(g)
(d) Fe(s)+CuSO4(aq)→FeSO4(aq)+Cu(s) (e) C6H12O6(aq)+6O2(g)→6CO2(g)+6H2O(l)+Energy
Answer: (a) Double Displacement / Precipitation Reaction: Two compounds exchange ions to form two new compounds, one of which is an insoluble precipitate.
(b) Combination Reaction: Two or more reactants combine to form a single product. (Also an exothermic reaction).
(c) Decomposition Reaction (Thermal Decomposition): A single compound breaks down into two or more simpler substances upon heating.
(d) Displacement Reaction: A more reactive element displaces a less reactive element from its compound.
(e) Redox Reaction / Exothermic Reaction: Glucose is oxidized by oxygen to produce carbon dioxide, water, and energy. Respiration is an example. - Question: Describe an activity to show that iron is more reactive than copper. Also, write the balanced chemical equation for the reaction. What changes would you observe?
Answer:Activity:- Take a clean iron nail and a test tube containing copper sulphate solution (blue colour).
- Immerse the iron nail in the copper sulphate solution.
- Leave it undisturbed for about 20-30 minutes. Observations:
- The blue colour of the copper sulphate solution starts to fade and eventually changes to light green.
- A reddish-brown coating appears on the surface of the iron nail. Explanation: Iron is more reactive than copper. When the iron nail is dipped in copper sulphate solution, iron displaces copper from the copper sulphate solution, forming ferrous sulphate (light green) and depositing reddish-brown copper metal on the iron nail. Balanced Chemical Equation: Fe(s)+CuSO4(aq)→FeSO4(aq)+Cu(s) (Iron) (Copper Sulphate – blue) (Ferrous Sulphate – light green) (Copper – reddish brown) This experiment demonstrates that iron is more reactive than copper because it can displace copper from its salt solution.
- Question: Explain the process of ‘corrosion’ of metals. Give two examples of corrosion in everyday life and suggest two methods to prevent it.
Answer: Corrosion: Corrosion is the process of slow deterioration of metals due to the attack of atmospheric gases (like oxygen, carbon dioxide, sulphur dioxide, hydrogen sulphide) and moisture on their surface. It is essentially an oxidation reaction where the metal reacts with substances in its environment to form unwanted compounds (e.g., oxides, sulphides, carbonates).
Examples in everyday life:- Rusting of Iron: When iron articles are exposed to air and moisture, a reddish-brown flaky substance called rust (Fe2O3.xH2O) is formed on their surface.
- Blackening of Silver: Silver articles lose their shine and turn black when exposed to air due to the formation of silver sulphide (Ag2S) because of reaction with hydrogen sulphide gas present in the air.
- Green coating on Copper: Copper articles develop a green layer (basic copper carbonate, CuCO3.Cu(OH)2) when exposed to moist air for a long time.
Methods to prevent corrosion: - Painting: Applying paint on metal surfaces provides a barrier, preventing contact with air and moisture (e.g., painting iron grills, bridges).
- Oiling/Greasing: Applying oil or grease forms a protective layer, commonly used for machine parts.
- Galvanization: Coating iron or steel with a thin layer of zinc. Zinc is more reactive than iron and corrodes preferentially, protecting the iron.
- Electroplating: Coating a less reactive metal with a layer of a more resistant metal (e.g., chromium plating on car parts).
- Question: (a) Why is respiration considered an exothermic reaction?
(b) Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Answer: (a) Respiration as an Exothermic Reaction: Respiration is a biochemical process in living organisms where glucose (food) combines with oxygen in the cells to produce carbon dioxide, water, and energy. Since energy is released during this process, it is considered an exothermic reaction. Equation: C6H12O6(aq)+6O2(g)→6CO2(g)+6H2O(l)+Energy
(b) Decomposition vs. Combination Reactions:
* Combination Reaction: Two or more simple substances combine to form a single, more complex product. Energy is usually released (exothermic). Example: C(s)+O2(g)→CO2(g)
* Decomposition Reaction: A single complex compound breaks down into two or more simpler substances. This process typically requires energy (heat, light, or electricity) to break bonds, so they are usually endothermic.
Example: CaCO3(s)HeatCaO(s)+CO2(g) Therefore, decomposition reactions are considered the opposite of combination reactions because in combination, substances combine to form one product, while in decomposition, one substance breaks down into multiple products. They also often involve opposite energy changes (release vs. absorption).
- Question: Give reasons for the following observations:
(a) Silver chloride is stored in dark coloured bottles.
(b) Chips manufacturers usually flush bags of chips with gas such as nitrogen.
(c) On heating blue coloured powder of copper (II) nitrate in a boiling tube, copper oxide, oxygen gas and a brown gas ‘X’ is formed.
(i) Write a balanced chemical equation of the reaction.
(ii) Identify the brown gas ‘X’.
(iii) Identify the type of reaction.
Answer: (a) Silver chloride is stored in dark coloured bottles because silver chloride (AgCl) is sensitive to light. When exposed to sunlight, it undergoes photolytic decomposition to form silver metal and chlorine gas. Storing it in dark bottles prevents this decomposition, preserving the compound.
2AgCl(s)Sunlight2Ag(s)+Cl2(g)
(b) Chips manufacturers flush bags of chips with nitrogen gas to prevent rancidity. Chips contain fats and oils which can get oxidized when exposed to oxygen in the air, leading to an unpleasant smell and taste (rancidity). Nitrogen is an inert gas (unreactive) and prevents this oxidation, thus keeping the chips fresh for a longer period.
(c) Reaction: Copper(II) nitrate (Cu(NO3)2) is heated.
(i) Balanced Chemical Equation: 2Cu(NO3)2(s)Heat2CuO(s)+4NO2(g)+O2(g)
(ii) Brown gas ‘X’ is Nitrogen dioxide (NO2).
(iii) Type of reaction is Thermal Decomposition reaction. - Question: Define oxidation and reduction in terms of gain/loss of oxygen and hydrogen. Give one example of a redox reaction with the reactants and products, and identify the oxidizing and reducing agents.
Answer:- Oxidation: It is a process that involves the:
- Gain of oxygen.Loss of hydrogen.Loss of electrons.
- Loss of oxygen.Gain of hydrogen.Gain of electrons.
- CuO is losing oxygen to form Cu, so CuO is reduced.
- H2 is gaining oxygen to form H2O, so H2 is oxidized.
- Oxidizing agent: The substance that causes oxidation (i.e., undergoes reduction itself) is CuO.
- Reducing agent: The substance that causes reduction (i.e., undergoes oxidation itself) is H2.
- Oxidation: It is a process that involves the:
- Question: You are given the following materials: Iron nails, copper sulphate solution, barium chloride solution, copper powder, ferrous sulphate crystals, and quick lime. Identify the type of chemical reaction taking place when:
(a) Barium chloride solution is mixed with copper sulphate solution and a white precipitate is observed.
(b) On heating, copper powder in air in a china dish, the surface of copper powder becomes black. (c) On heating green ferrous sulphate crystals, reddish-brown solid is left and a gas having the smell of burning sulphur is noticed.
(d) Iron nails when left dipped in blue copper sulphate solution become brownish in colour and the blue colour of copper sulphate solution fades away.
Answer: (a) Double Displacement (Precipitation) Reaction: BaCl2(aq)+CuSO4(aq)→BaSO4(s)↓+CuCl2(aq)
(b) Combination (Oxidation) Reaction: 2Cu(s)+O2(g)Heat2CuO(s)
(c) Decomposition (Thermal Decomposition) Reaction: 2FeSO4(s)HeatFe2O3(s)+SO2(g)+SO3(g)
(d) Displacement Reaction: Fe(s)+CuSO4(aq)→FeSO4(aq)+Cu(s) - Question: Give reasons for the following:
(a) Aluminium is a highly reactive metal, yet it is used to make utensils.
(b) The colour of copper sulphate solution changes when an iron nail is dipped in it.
(c) A solution of a substance ‘X’ is used for white-washing. Name the substance ‘X’ and write its formula. Also write the reaction of the substance ‘X’ with water.
Answer: (a) Aluminium is a highly reactive metal, yet it is used to make utensils because when aluminium is exposed to air, it quickly reacts with oxygen to form a thin, strong, and highly protective layer of aluminium oxide (Al2O3) on its surface. This oxide layer is very stable and prevents further corrosion of the underlying metal, making it resistant to further attack by air and water, and thus safe for use in utensils.
(b) The colour of copper sulphate solution changes when an iron nail is dipped in it because iron is more reactive than copper. When the iron nail is dipped in the blue copper sulphate solution, iron displaces copper from the solution, forming light green coloured ferrous sulphate solution and reddish-brown copper metal gets deposited on the nail. Fe(s)+CuSO4(aq)→FeSO4(aq)+Cu(s)
(c) Substance ‘X’ is Calcium oxide (Common name: Quicklime). Its formula is CaO. Reaction with water: CaO(s)+H2O(l)→Ca(OH)2(aq) (Calcium hydroxide, slaked lime) - Question: Explain the conditions for the formation of rust. Write the chemical equation for the formation of rust. List two ways by which rusting of iron can be prevented.
Answer: Conditions for Rusting: Rusting of iron requires the presence of both:- Oxygen (from air)
- Moisture (water or water vapour) In the absence of either oxygen or water, rusting will not occur. Chemical Equation for Rusting: Rust is hydrated iron(III) oxide. The process is complex but can be represented as: 4Fe(s)+3O2(g)+2xH2O(l)→2Fe2O3⋅xH2O(s) (Rust) Two ways to prevent rusting:
- Painting: Applying a coat of paint on iron surfaces acts as a barrier, preventing direct contact with air and moisture.
- Galvanization: Coating the iron object with a thin layer of zinc metal. Zinc is more reactive than iron, so it corrodes preferentially, protecting the iron underneath.
- Oiling/Greasing: Applying a layer of oil or grease reduces contact with air and moisture, useful for machine parts.
- Electroplating: Coating the iron with another metal like chromium or nickel.
- Question: What is a balanced chemical equation? State the importance of balancing chemical equations. How would you balance the following chemical equation step-by-step:
Fe(s)+H2O(g)→Fe3O4(s)+H2(g)
Answer: Balanced Chemical Equation: A balanced chemical equation is one in which the number of atoms of each element on the reactant side (LHS) is equal to the number of atoms of the same element on the product side (RHS).
Importance of Balancing Chemical Equations: Chemical equations are balanced to satisfy the Law of Conservation of Mass, which states that matter can neither be created nor destroyed in a chemical reaction. Therefore, the total mass of the reactants must be equal to the total mass of the products, ensuring that the number of atoms of each element remains constant before and after the reaction.
Step-by-step Balancing of Fe(s)+H2O(g)→Fe3O4(s)+H2(g):
Step 1: Write the unbalanced equation and list atoms. Reactants: Fe=1, H=2, O=1 Products: Fe=3, H=2, O=4
Step 2: Balance the element with the maximum number of atoms (Oxygen).
There are 4 oxygen atoms on the RHS (Fe3O4) and 1 on the LHS (H2O). To balance oxygen, place a coefficient ‘4’ before H2O: Fe(s)+4H2O(g)→Fe3O4(s)+H2(g)
Step 3: Balance the next element (Hydrogen). Now, there are 4×2=8 hydrogen atoms on the LHS (4H2O) and 2 on the RHS (H2). To balance hydrogen, place a coefficient ‘4’ before H2: Fe(s)+4H2O(g)→Fe3O4(s)+4H2(g)
Step 4: Balance the remaining element (Iron). There is 1 iron atom on the LHS (Fe) and 3 on the RHS (Fe3O4). To balance iron, place a coefficient ‘3’ before Fe: 3Fe(s)+4H2O(g)→Fe3O4(s)+4H2(g)
Step 5: Verify the balanced equation. Reactants: Fe=3, H=8, O=4 Products: Fe=3, H=8, O=4 The equation is now balanced.
