Assertion (A): Burning of magnesium ribbon produces magnesium oxide and is a chemical change. Reason (R): A chemical change results in the formation of new substances.

Both A and R are true and R is the correct explanation of A.

Both A and R are true but R is not the correct explanation of A.

Why does a magnesium ribbon when ignited in air leave a white ash? Write the balanced chemical equation and explain whether it is a combination or decomposition reaction.

The white ash is magnesium oxide (MgO) formed by the combination of magnesium with oxygen. Equation: 2Mg + O₂ → 2MgO. It is a combination reaction because two reactants combine to form a single product.

Define a chemical reaction and give an example with a balanced chemical equation.

A chemical reaction is a process in which one or more substances (reactants) are converted into new substances (products) with different chemical properties. Example: 2H₂ + O₂ → 2H₂O.

Explain the term 'redox reaction' with an example. In the reaction ZnO + C → Zn + CO, identify the substance oxidised and the substance reduced. Give reason for your choice.

A reaction in which both oxidation and reduction take place simultaneously is called a redox reaction. Example: CuO + H₂ → Cu + H₂O. In ZnO + C → Zn + CO: Zinc oxide (ZnO) is reduced to zinc because it loses oxygen. Carbon (C) is oxidised to carbon monoxide (CO) because it gains oxygen. Thus, ZnO is the oxidising agent, and C is the reducing agent.

Balance the following chemical equation: MnO₂ + HCl → MnCl₂ + Cl₂ + H₂O After balancing, explain why this is a redox reaction by identifying which substance is oxidised and which is reduced.

Balanced equation: MnO₂ + 4HCl → MnCl₂ + Cl₂ + 2H₂O In this reaction: - MnO₂ (manganese dioxide) is reduced to MnCl₂ (manganese in +4 oxidation state reduces to +2). It acts as an oxidising agent. - HCl (hydrochloric acid) is oxidised to Cl₂ (chlorine from -1 oxidation state to 0). It acts as a reducing agent. Since both oxidation and reduction occur, it is a redox reaction.

Balance the following chemical equations and state the type of chemical reaction in each case: (a) H2 + O2 → H2O, (b) KClO3 → KCl + O2, (c) Zn + HCl → ZnCl2 + H2. Explain the basis for your classification.

(a) 2H2 + O2 → 2H2O; Combination reaction (two reactants combine to form a single product). (b) 2KClO3 → 2KCl + 3O2; Decomposition reaction (single reactant breaks down into two or more products). (c) Zn + 2HCl → ZnCl2 + H2; Displacement reaction (zinc displaces hydrogen from hydrochloric acid because it is more reactive).

A student strongly heated a white crystalline solid in a dry test tube. He observed brown fumes evolving and a yellow residue left in the tube. Identify the solid. Write the balanced chemical equation for the reaction. What type of reaction is this? Suggest a test to confirm the gas evolved.

The white solid is lead nitrate (Pb(NO3)2). The balanced equation is: 2Pb(NO3)2 → 2PbO + 4NO2 + O2. This is a thermal decomposition reaction. To confirm the gas (NO2): the brown fumes turn moist blue litmus paper red due to acidic nature, or they have a pungent irritating smell. Alternatively, pass the gas through potassium iodide solution; it will turn brown due to liberation of iodine.

The Contact process is used for the industrial manufacture of sulfuric acid. It involves three main steps: Step 1: Sulfur is burned in air to form sulfur dioxide gas. Step 2: Sulfur dioxide is oxidized to sulfur trioxide using a vanadium(V) oxide catalyst at high temperature. Step 3: Sulfur trioxide is dissolved in concentrated sulfuric acid to give oleum, which is later diluted to obtain sulfuric acid. (1) Write balanced chemical equations for all three steps. (2) In step 2, which substance acts as the reducing agent? Justify your answer. (3) Identify the type of reaction in step 3.

Step 1: S + O2 → SO2. Step 2: 2SO2 + O2 ⇌ 2SO3 (catalyst). Step 3: SO3 + H2SO4 → H2S2O7. SO2 is the reducing agent in step 2 (oxidation state of S changes from +4 to +6). Step 3 is a combination reaction.

Class 10 Science Chapter 1: Chemical Reactions and Equations — Important Questions & Sample Paper

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Chapter 1 of CBSE Class 10 Science, 'Chemical Reactions and Equations', lays the groundwork for understanding how substances interact and transform. Students learn to represent these changes through chemical equations, which must be balanced to obey the law of conservation of mass. The chapter explores the observable signs of a reaction—change in colour, temperature, state, or evolution of a gas—and introduces the systematic writing of skeleton equations followed by balancing. A major focus is classifying reactions into types: combination (two or more substances unite), decomposition (a compound breaks down), displacement (a more reactive element displaces a less reactive one), double displacement (ions are exchanged), and redox (oxidation and reduction occur simultaneously). Students also encounter exothermic and endothermic reactions, and concepts like oxidation and reduction as addition or removal of oxygen/hydrogen. Exam questions frequently test the ability to write and balance equations for given reactions, identify the type from an equation or description, predict products of a reaction, and determine whether a reaction is redox. For example, recognising that the cold feeling when barium hydroxide mixes with ammonium chloride indicates an endothermic decomposition, or that zinc replacing copper in copper sulphate is a displacement and redox reaction. Mastering this chapter is essential, as it forms the basis for many subsequent chemistry topics.

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Science — Chemical Reactions and Equations

Class 10Time: 3 hrsMax Marks: 80

SECTION A

1.
The reaction in which two compounds exchange their ions to form two new compounds is called:
(a) displacement reaction(b) double displacement reaction(c) decomposition reaction(d) combination reaction
1
2.
The reaction BaCl2 + H2SO4 → BaSO4 + 2HCl is an example of:
(a) combination(b) decomposition(c) displacement(d) double displacement
1
3.
Which of the following will not undergo a displacement reaction?
(a) Zn + CuSO4(b) Fe + ZnSO4(c) Mg + FeSO4(d) Cu + AgNO3
1

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In a CBSE exam, this chapter typically contributes questions across the following types. The last column shows how many original questions of each type we have ready in our bank for this chapter:

Question type	Marks each	In our bank
Multiple Choice (MCQ)	1 mark	13
Assertion–Reason	1 mark	6
Short Answer	2 marks	8
Short Answer	3 marks	6
Long Answer	5 marks	5
Case Study	4 marks	6

44 original, exam-style questions in our bank for this chapter — with answers.

Important & sample questions (with answers)

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Q1. The reaction in which two compounds exchange their ions to form two new compounds is called:
1 mark
Multiple Choice (MCQ)
(A) displacement reaction(B) double displacement reaction(C) decomposition reaction(D) combination reaction
▸ Answer▾ Answer
double displacement reaction
Q2. The reaction BaCl2 + H2SO4 → BaSO4 + 2HCl is an example of:
1 mark
Multiple Choice (MCQ)
(A) combination(B) decomposition(C) displacement(D) double displacement
▸ Answer▾ Answer
double displacement
Q3. Which of the following will not undergo a displacement reaction?
1 mark
Multiple Choice (MCQ)
(A) Zn + CuSO4(B) Fe + ZnSO4(C) Mg + FeSO4(D) Cu + AgNO3
▸ Answer▾ Answer
Fe + ZnSO4
Q4. Rusting of iron is an example of:
1 mark
Multiple Choice (MCQ)
(A) slow combustion(B) corrosion(C) decomposition(D) reduction
▸ Answer▾ Answer
corrosion
Q5. Assertion (A): Burning of magnesium ribbon produces magnesium oxide and is a chemical change. Reason (R): A chemical change results in the formation of new substances.
1 mark
Assertion–Reason
(A) Both A and R are true and R is the correct explanation of A.(B) Both A and R are true but R is not the correct explanation of A.(C) A is true but R is false.(D) A is false but R is true.
▸ Answer▾ Answer
Both A and R are true and R is the correct explanation of A.
Q6. Why does a magnesium ribbon when ignited in air leave a white ash? Write the balanced chemical equation and explain whether it is a combination or decomposition reaction.
2 marks
Short Answer
▸ Answer▾ Answer
The white ash is magnesium oxide (MgO) formed by the combination of magnesium with oxygen. Equation: 2Mg + O₂ → 2MgO. It is a combination reaction because two reactants combine to form a single product.
Q7. Define a chemical reaction and give an example with a balanced chemical equation.
2 marks
Short Answer
▸ Answer▾ Answer
A chemical reaction is a process in which one or more substances (reactants) are converted into new substances (products) with different chemical properties. Example: 2H₂ + O₂ → 2H₂O.
Q8. Explain the term 'redox reaction' with an example. In the reaction ZnO + C → Zn + CO, identify the substance oxidised and the substance reduced. Give reason for your choice.
3 marks
Short Answer
▸ Answer▾ Answer
A reaction in which both oxidation and reduction take place simultaneously is called a redox reaction. Example: CuO + H₂ → Cu + H₂O. In ZnO + C → Zn + CO: Zinc oxide (ZnO) is reduced to zinc because it loses oxygen. Carbon (C) is oxidised to carbon monoxide (CO) because it gains oxygen. Thus, ZnO is the oxidising agent, and C is the reducing agent.
Q9. Balance the following chemical equation: MnO₂ + HCl → MnCl₂ + Cl₂ + H₂O After balancing, explain why this is a redox reaction by identifying which substance is oxidised and which is reduced.
3 marks
Short Answer
▸ Answer▾ Answer
Balanced equation: MnO₂ + 4HCl → MnCl₂ + Cl₂ + 2H₂O In this reaction: - MnO₂ (manganese dioxide) is reduced to MnCl₂ (manganese in +4 oxidation state reduces to +2). It acts as an oxidising agent. - HCl (hydrochloric acid) is oxidised to Cl₂ (chlorine from -1 oxidation state to 0). It acts as a reducing agent. Since both oxidation and reduction occur, it is a redox reaction.
Q10. Balance the following chemical equations and state the type of chemical reaction in each case: (a) H2 + O2 → H2O, (b) KClO3 → KCl + O2, (c) Zn + HCl → ZnCl2 + H2. Explain the basis for your classification.
5 marks
Long Answer
▸ Answer▾ Answer
(a) 2H2 + O2 → 2H2O; Combination reaction (two reactants combine to form a single product). (b) 2KClO3 → 2KCl + 3O2; Decomposition reaction (single reactant breaks down into two or more products). (c) Zn + 2HCl → ZnCl2 + H2; Displacement reaction (zinc displaces hydrogen from hydrochloric acid because it is more reactive).
Q11. A student strongly heated a white crystalline solid in a dry test tube. He observed brown fumes evolving and a yellow residue left in the tube. Identify the solid. Write the balanced chemical equation for the reaction. What type of reaction is this? Suggest a test to confirm the gas evolved.
5 marks
Long Answer
▸ Answer▾ Answer
The white solid is lead nitrate (Pb(NO3)2). The balanced equation is: 2Pb(NO3)2 → 2PbO + 4NO2 + O2. This is a thermal decomposition reaction. To confirm the gas (NO2): the brown fumes turn moist blue litmus paper red due to acidic nature, or they have a pungent irritating smell. Alternatively, pass the gas through potassium iodide solution; it will turn brown due to liberation of iodine.
Q12. The Contact process is used for the industrial manufacture of sulfuric acid. It involves three main steps: Step 1: Sulfur is burned in air to form sulfur dioxide gas. Step 2: Sulfur dioxide is oxidized to sulfur trioxide using a vanadium(V) oxide catalyst at high temperature. Step 3: Sulfur trioxide is dissolved in concentrated sulfuric acid to give oleum, which is later diluted to obtain sulfuric acid.
4 marks
Case Study
1. (i) Write balanced chemical equations for all three steps.2 marks
2. (ii) In step 2, which substance acts as the reducing agent? Justify your answer.1 mark
3. (iii) Identify the type of reaction in step 3.1 mark
▸ Answer▾ Answer
Step 1: S + O2 → SO2. Step 2: 2SO2 + O2 ⇌ 2SO3 (catalyst). Step 3: SO3 + H2SO4 → H2S2O7. SO2 is the reducing agent in step 2 (oxidation state of S changes from +4 to +6). Step 3 is a combination reaction.

Frequently asked questions

How do I balance a chemical equation in this chapter?

Begin with the correct skeleton equation. Count atoms of each element on reactant and product sides. Adjust only the coefficients (numbers in front of formulas) to equalize atom counts, typically leaving hydrogen and oxygen for last. Practice with simple reactions like H2 + O2 → H2O to build confidence.

What is the difference between displacement and double displacement reactions?

In a displacement reaction, a more reactive element pushes out a less reactive element from its compound (e.g., Zn + CuSO4 → ZnSO4 + Cu). In double displacement, two ionic compounds exchange their ions to form two new compounds, often with a precipitate (e.g., BaCl2 + Na2SO4 → BaSO4 + 2NaCl).

How can I identify if a reaction is a redox reaction?

Check for change in oxidation numbers: if any element’s oxidation number increases (oxidation) and another’s decreases (reduction), it’s redox. Alternatively, look for addition/removal of oxygen or hydrogen. Example: 2Na + 2H2O → 2NaOH + H2 is redox because Na undergoes oxidation and H undergoes reduction.

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