Case Based Questions: Chemical Reactions and Equations | Science Class 10 PDF Download

Shruti performed an experiment in the laboratory where she burned a magnesium ribbon in the presence of oxygen. She noticed that the ribbon burned with a bright white flame and a white powder was formed as a product.

(a) Identify the chemical reaction taking place and write the balanced chemical equation. (1 mark)
(b) Explain why magnesium needs to be cleaned with sandpaper before burning. (2 marks)
(c) Identify and explain the type of reaction occurring in this experiment. (1 mark)
OR
(c) How does this reaction follow the law of conservation of mass? (1 mark)

Ans:
(a) The reaction is a combination reaction where magnesium reacts with oxygen to form magnesium oxide.
Equation: Mg + O₂ → MgO
(b) Magnesium must be cleaned with sandpaper to:

• Remove the oxide layer on its surface.
• Ensure a better reaction with oxygen.

(c) The reaction is a combination reaction because two reactants (magnesium and oxygen) combine to form a single product, magnesium oxide. 
OR
(c) The mass of reactants (magnesium + oxygen) equals the mass of the product (magnesium oxide), proving the law of conservation of mass.

Q2: Read the source below and answer the questions that follow:

Aryan took a few zinc granules in a test tube and added dilute sulphuric acid to it. He noticed the formation of bubbles and felt the test tube becoming warm. He then passed the gas through a soap solution and found that bubbles were formed, which burst with a ‘pop’ sound.

(a) Write the balanced chemical equation for the reaction taking place. (1 mark)
(b) What is the identity of the gas formed, and how can it be tested? (2 marks)
(c) What type of reaction is this? Justify your answer. (1 mark)
OR
(c) Why does the test tube become warm? Explain with reference to energy changes in the reaction. (1 mark)

Ans:
(a) Balanced equation: Zn + H₂SO₄ → ZnSO₄ + H₂
(b) The gas formed is hydrogen (H₂). It can be tested by:

• Bringing a burning matchstick near the bubbles.
• A ‘pop’ sound confirms the presence of hydrogen gas.

(c) The reaction is a displacement reaction because zinc (Zn) replaces hydrogen (H) from sulphuric acid to form zinc sulfate and hydrogen gas. 
OR
(c) The test tube becomes warm because the reaction is exothermic, meaning it releases heat.

Q3: Read the source below and answer the questions that follow:

A chemistry teacher demonstrated an experiment where she heated ferrous sulfate crystals in a test tube. The green crystals changed color, and a brownish solid was formed along with the release of two gases that had a pungent smell.

(a) Write the balanced chemical equation for the reaction. (1 mark)
(b) Name the type of reaction taking place and explain why it is categorized as such. (2 marks)
(c) What are the names of the gases released? (1 mark)
OR
(c) Give one industrial application of decomposition reactions. (1 mark)

Ans:
(a) Balanced equation:
2FeSO₄ → Fe₂O₃ + SO₂ + SO₃
(b) The reaction is a thermal decomposition reaction because ferrous sulfate decomposes upon heating into iron(III) oxide, sulfur dioxide, and sulfur trioxide.
(c) The gases released are sulfur dioxide (SO₂) and sulfur trioxide (SO₃). 
OR
(c) An industrial application of decomposition reactions is the decomposition of limestone in cement manufacturing.

Q4: Read the source below and answer the questions that follow:

Raj noticed that an old iron gate in his backyard had turned reddish-brown after being exposed to the air for a long time. He asked his teacher about this phenomenon, and she explained that it was a chemical reaction called rusting.

(a) What is the chemical formula of rust? (1 mark)
(b) Write the balanced chemical equation for the rusting of iron. (2 marks)
(c) Suggest two ways to prevent rusting. (1 mark)
OR
(c) How does rusting affect the strength of iron objects? (1 mark)

Ans:
(a) The chemical formula of rust is Fe₂O₃·xH₂O (hydrated iron(III) oxide).
(b) Balanced equation:
4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃
2Fe(OH)₃ → Fe₂O₃·xH₂O (rust)
(c) Rusting can be prevented by:

• Applying paint or oil on iron surfaces to prevent contact with oxygen and moisture.
• Galvanization, i.e., coating iron with a layer of zinc to prevent oxidation.

OR
(c) Rusting weakens iron objects, making them brittle and reducing their durability.

Q5: Read the source below and answer the questions that follow:

In a chemistry practical, Mansi mixed solutions of barium chloride (BaCl₂) and sodium sulfate (Na₂SO₄) in a test tube. She observed the formation of a white precipitate in the solution.

(a) Write the balanced chemical equation for the reaction. (1 mark)
(b) What is the white precipitate formed? (2 marks)
(c) What type of reaction is this? Justify your answer. (1 mark)
OR
(c) Define a precipitation reaction and give another example. (1 mark)

Ans:
(a) Balanced equation:
BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl
(b) The white precipitate formed is barium sulfate (BaSO₄).
(c) This is a double displacement reaction because there is an exchange of ions between the two reactants, forming an insoluble precipitate.
OR
(c) A precipitation reaction is one where an insoluble solid (precipitate) is formed. 
Example: AgNO₃ + NaCl → AgCl (precipitate) + NaNO₃.)

Q6: Read the source below and answer the questions that follow:

During a construction project, workers added quicklime (CaO) to water to prepare mortar. Rahul, a student observing this, noticed that the mixture released heat and became warm as the reaction proceeded.

(a) Write the balanced chemical equation for the reaction. (1 mark)
(b) What type of chemical reaction is this? Explain. (2 marks)
(c) Why does the reaction release heat? (1 mark)
OR
(c) Give one real-life application of this reaction. (1 mark)

Ans:
(a) Balanced equation:
CaO + H₂O → Ca(OH)₂ + Heat
(b) This is a combination reaction because two reactants (calcium oxide and water) combine to form a single product, calcium hydroxide.
(c) The reaction is exothermic as it releases heat due to the formation of new chemical bonds. 
OR
(c) This reaction is used in whitewashing walls, where Ca(OH)₂ reacts with CO₂ in air to form a protective layer of CaCO₃.)

Q7: Read the source below and answer the questions that follow:

A science teacher demonstrated an experiment where she passed electricity through water containing a few drops of acid. After a few minutes, gases were collected in two test tubes, and the volume of one gas was twice that of the other.

(a) Name the gases collected at the two electrodes and write the balanced equation for the reaction. (1 mark)
(b) Why is the volume of one gas double that of the other? (2 marks)
(c) What type of reaction is this? (1 mark)
OR
(c) Name one industrial use of electrolysis. (1 mark)

Ans:
(a) The gases collected are hydrogen (H₂) at the cathode and oxygen (O₂) at the anode.
Balanced equation:
2H₂O → 2H₂ + O₂
(b) The volume of hydrogen is double that of oxygen because:

• In water (H₂O), two hydrogen atoms combine to form H₂ gas.
• Only one oxygen atom forms O₂ gas.

(c) This is a decomposition reaction because water breaks down into hydrogen and oxygen using electricity. 
OR
(c) Electrolysis is used in electroplating to coat metals like silver or gold onto other surfaces.

Q8: Read the source below and answer the questions that follow:

Anjali noticed that an old iron fence in her neighborhood had developed reddish-brown patches over time. She also observed that some iron objects in her kitchen remained unaffected by rusting.

(a) What is the chemical formula of rust? (1 mark)
(b) What are the conditions necessary for rusting? (2 marks)
(c) How can rusting be prevented? (1 mark)
OR
(c) Why does rusting weaken iron objects over time? (1 mark)

Ans:
(a) The chemical formula of rust is Fe₂O₃·xH₂O (hydrated iron(III) oxide).
(b)The chemical formula of rust is Fe₂O₃·xH₂O (hydrated iron(III) oxide). (b) Rusting occurs under specific conditions:

• Oxygen must be present.
• Moisture (water) is essential.

(c) Rusting can be prevented by:

• Galvanization (coating iron with zinc)
• Painting or oiling iron surfaces 

OR
(c) Rusting weakens iron by slowly corroding the surface, leading to structural damage and reduced durability.

Q9: Read the source below and answer the questions that follow:

Ravi observed a gas stove in his kitchen and wondered about the reaction taking place when the gas burned with a blue flame. He learned that methane (CH₄) is the main component of natural gas, which reacts with oxygen to produce energy.

(a) Write the balanced chemical equation for the burning of methane. (1 mark)
(b) What type of reaction is this? Explain. (2 marks)
(c) Why is the flame blue when methane burns completely? (1 mark)
OR
(c) Why is this reaction considered important for daily life? (1 mark)

Ans:
(a) Balanced equation:
CH₄ + 2O₂ → CO₂ + 2H₂O + Energy
(b) This is a combustion reaction because methane burns in the presence of oxygen to release energy in the form of heat and light.
(c) The blue flame indicates complete combustion, meaning that methane is burning efficiently without producing smoke or soot. 
OR
(c) This reaction is important as it is used in cooking, heating, and generating electricity.

Q10: Read the source below and answer the questions that follow:

Rohan performed an experiment by dipping an iron nail in a beaker containing copper sulfate (CuSO₄) solution. After some time, he observed that the blue color of the solution faded, and a reddish-brown coating appeared on the nail.

(a) Write the balanced chemical equation for this reaction. (1 mark)
(b) Why does the blue color of the solution fade? (2 marks)
(c) What type of reaction is this? (1 mark)
OR
(c) Give another example of a displacement reaction. (1 mark)

Ans:
(a) Balanced equation:
Fe + CuSO₄ → FeSO₄ + Cu
(b) The blue color fades because iron displaces copper from copper sulfate, forming iron sulfate (FeSO₄), which is light green in color.
(c) This is a displacement reaction because a more reactive element (iron) replaces a less reactive element (copper). 
OR
(c) Another displacement reaction is Zn + CuSO₄ → ZnSO₄ + Cu.