Chemical Reactions and Equations: Worksheet PDF and its Answers | Toppers Hub

Class 10 Science Chapter 1: Chemical Reactions and Equations – Complete Notes, Questions & Answers

Introduction

The first chapter of Class 10 CBSE Science, Chemical Reactions and Equations, lays the foundation for understanding chemistry in real life. From rusting of iron to photosynthesis in plants, all processes involve chemical reactions. In this blog, we will cover:

• Summary of the chapter
• Important definitions and formulas
• Long Answer Questions (with NCERT-style answers)
• Very Short Answer Questions
• Multiple Choice Questions (MCQs) with solutions
• Assertion and Reasoning type questions
• Downloadable worksheet PDF for extra practice (at bottom of this article)

This will help students revise effectively for their board exams.

Long Answer Type Questions

Q1. Explain the type of reactions represented by the following equations
(i) $CaO + CO_2 \rightarrow CaCO_3$
(ii) $Mg + CuSO_4 \rightarrow MgSO_4 + Cu$
(iii) $CuSO_4 + 2NaOH \rightarrow Cu(OH)_2 + Na_2SO_4$
(iv) $2Na + 2H_2O \rightarrow 2NaOH + H_2$
(v) $NH_4NO_2 \rightarrow N_2 + 2H_2O$

Answer:
(i) Combination – two reactants form a single product.
(ii) Displacement – Mg displaces Cu.
(iii) Double displacement – ions exchange between compounds.
(iv) Displacement – Na displaces H₂.
(v) Decomposition – one compound breaks into simpler products.
Thus, reactions can be classified into major types like combination, decomposition, displacement and double displacement depending on how products are formed.

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Q2. What is the difference between displacement and double displacement reactions? Write equations for their reactions.

Answer:
In displacement reactions, a more reactive element replaces a less reactive one. Example:
$Zn + CuSO_4 \rightarrow ZnSO_4 + Cu$.
In double displacement reactions, two compounds exchange ions. Example:
$AgNO_3 + NaCl \rightarrow AgCl + NaNO_3$.
The main difference is that in displacement only one element is replaced, while in double displacement both reactants exchange parts. Both are important in understanding reactivity series and precipitation reactions.

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Q3. (a) Write a balanced chemical equation for photosynthesis and state conditions.
(b) Classify the following as exothermic or endothermic:
(i) Electrolysis of water
(ii) Burning of natural gas
(iii) Decomposition of calcium carbonate
(iv) Burning of magnesium ribbon

Answer:
(a) Photosynthesis:

$$6CO_2(g) + 6H_2O(l) \xrightarrow{sunlight, chlorophyll} C_6H_{12}O_6(aq) + 6O_2(g)$$

Conditions: Sunlight and chlorophyll are essential.

(b) Classification:

• Electrolysis of water → Endothermic
• Burning of natural gas → Exothermic
• Decomposition of calcium carbonate → Endothermic
• Burning of Mg ribbon → Exothermic

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Q4. Write balanced chemical equations for the following reactions.
a) Calcium Hydroxide + Carbon Dioxide → Calcium Carbonate + Water
b) Barium Chloride + Aluminium Sulphate → Barium Sulphate + Aluminium Chloride

Answer:
a) $Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O$
b) $3BaCl_2 + Al_2(SO_4)_3 \rightarrow 3BaSO_4 + 2AlCl_3$

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Q5. Explain oxidation and reduction with two examples in each case.

Answer:

• Oxidation: Addition of oxygen or removal of hydrogen.

Examples:
(i) $2Mg + O_2 \rightarrow 2MgO$
(ii) $2Cu + O_2 \rightarrow 2CuO$

• Reduction: Removal of oxygen or addition of hydrogen.

Examples:
(i) $CuO + H_2 \rightarrow Cu + H_2O$
(ii) $ZnO + C \rightarrow Zn + CO$

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Q6. (a) Define corrosion.
(b) What is corrosion of iron called?
(c) How will you recognise corrosion of iron?
(d) Why is it a serious problem?
(e) How can we prevent corrosion?

Answer:
(a) Corrosion is gradual destruction of metals by chemical/environmental action.
(b) In iron, it is called rusting.
(c) Reddish-brown flaky substance on iron articles.
(d) It weakens structures, damages machinery and causes financial losses.
(e) Preventive methods: painting, galvanising, alloying, oiling, cathodic protection.

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Q7. What is a balanced chemical equation? Why should the chemical equation be balanced?

Answer:
A balanced chemical equation has equal number of atoms of each element on both sides. For example,
$2H_2 + O_2 \rightarrow 2H_2O$.
It is balanced to obey the Law of Conservation of Mass. If not balanced, the equation would be incorrect and misleading for stoichiometric calculations. Balanced equations ensure accuracy in both theoretical and practical chemistry.

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Q8. What is rancidity? How can it be prevented?

Answer:
Rancidity is oxidation of oils and fats leading to unpleasant smell and taste in food. Examples include spoiling of chips and nuts.
Prevention:

• Storing in airtight containers
• Refrigeration
• Nitrogen flushing in packets
• Adding antioxidants like BHA and BHT.

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Q9. Explain redox reaction with examples.

Answer:
A reaction in which oxidation and reduction occur simultaneously is called a redox reaction. Example:
$Zn + CuSO_4 \rightarrow ZnSO_4 + Cu$.
Here Zn is oxidised to $Zn^{2+}$ and Cu²⁺ is reduced to Cu. Another example is
$CuO + H_2 \rightarrow Cu + H_2O$.
In this reaction, hydrogen is oxidised and copper oxide is reduced. Redox reactions are common in respiration, combustion and electrochemical processes.

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Q10. Balance the following chemical equations.
a) $Al_2(SO_4)_3 + NaOH \rightarrow Al(OH)_3 + Na_2SO_4$
b) $Mg(OH)_2 + HCl \rightarrow MgCl_2 + H_2O$
c) $Al(OH)_3 \rightarrow Al_2O_3 + H_2O$
d) $CaCO_3 + HCl \rightarrow CaCl_2 + H_2O + CO_2$
e) $BaCl_2 + H_2SO_4 \rightarrow BaSO_4 + HCl$

Answer:
a) $Al_2(SO_4)_3 + 6NaOH \rightarrow 2Al(OH)_3 + 3Na_2SO_4$
b) $Mg(OH)_2 + 2HCl \rightarrow MgCl_2 + 2H_2O$
c) $2Al(OH)_3 \rightarrow Al_2O_3 + 3H_2O$
d) $CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2$
e) $BaCl_2 + H_2SO_4 \rightarrow BaSO_4 + 2HCl$

Very Short Answer Type Questions

Q1. What can be seen when a strip of copper is placed in silver nitrate?
Ans: Silver deposits on copper, solution turns blue.

Q2. State one industrial application of reduction process.
Ans: Extraction of metals such as iron from ores.

Q3. Why does colour of copper sulphate change when iron nail is dipped?
Ans: Iron displaces copper, solution turns green.

Q4. What happens when water is added to quick lime? Is it endothermic or exothermic?
Ans: Slaked lime forms; reaction is exothermic.

Q5. On what basis is a chemical equation balanced? Name oxidising and reducing agent in $CuO + H_2 \rightarrow Cu + H_2O$.
Ans: On conservation of mass; Oxidising agent: CuO, Reducing agent: H₂.

Q6. Why should magnesium ribbon be cleaned before burning?
Ans: To remove oxide layer for proper burning.

Q7. Which type of reaction produces insoluble salts?
Ans: Double displacement (precipitation) reaction.

Q8. How does food become rancid?
Ans: Due to oxidation of oils and fats.

Q9. Which gas is filled in chips packets to prevent rancidity? Why?
Ans: Nitrogen; prevents oxidation.

Q10. Which is a chemical change – rusting or melting of iron?
Ans: Rusting is chemical change.

Multiple Choice Questions (MCQs) 

Q1. Oxidation is a process which involves:
a) Addition of Oxygen
b) Addition of Hydrogen
c) Addition of Nitrogen
d) None of the above

Right Answer: a) Addition of Oxygen

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Q2. A substance added to food containing fats and oils is called:
a) Oxidant
b) Rancid
c) Coolant
d) Antioxidant

Right Answer: d) Antioxidant

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Q3. The correct formula of rust is:
a) Fe₂O₃
b) Fe₃O₄
c) Fe₂O₃·xH₂O
d) Fe₂O₄·xH₂O

Right Answer: c) Fe₂O₃·xH₂O

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Q4. Pb + CuCl₂ → PbCl₂ + Cu. The above reaction is an example of:
a) Combination
b) Decomposition
c) Displacement
d) Double Displacement

Right Answer: c) Displacement

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Q5. The reaction of H₂ gas with oxygen gas to form water is an example of:
a) Combination reaction
b) Redox reaction
c) Endothermic reaction
d) All of these

Right Answer: d) All of these

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Q6. MnO₂ + 4HCl → MnCl₂ + H₂O + Cl₂. The above reaction is an example of:
a) Oxidation
b) Reduction
c) Combination
d) Displacement

Right Answer: d) Displacement

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Q7. Burning of coal is:
a) Exothermic Reaction
b) Endothermic Reaction
c) Oxidation Reaction
d) None of these

Right Answer: a) Exothermic Reaction

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Q8. Rancidity occurs when oily foods are:
a) Oxidised
b) Reduction
c) Decomposed
d) All of these

Right Answer: a) Oxidised

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Q9. It is necessary to balance a chemical equation in order to satisfy the law of:
a) Conservation of motion
b) Conservation of mass
c) Conservation of momentum
d) Conservation of energy

Right Answer: b) Conservation of mass

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Q10. When a gas is passed through lime water it becomes milky, the gas may be:
a) NO₂
b) NH₃
c) CO₂
d) None of these

Right Answer: c) CO₂

Assertion–Reason Questions

Directions :- Two statements (Assertion-A and Reason- R) are given. Select the correct answer to these questions from codes a,b,c and d as given below. 

 (a) Both A and R are true, and R is the correct explanation of assertion. 

 (b) Both A and R are true, but R is not the correct explanation of the assertion. 

 (c) A is true but R is false. 

 (d) A is false but R is true. 

 Q1. Assertion: Stoichiometric co-efficients are the numbers which are put before compounds or elements to balance the chemical equation. 

 Reason:- Chemical Equation is balanced to justify the law of conservation of mass. 

 Q2. Assertion: When Zinc rod is dipped into aqueous solution of copper sulphate, the colour of the solution changes. 

 Reason: Zinc being more reactive displaces less reactive copper from its aqueous solution resulting in the formation of Zinc sulphate which is colourless. 

 Q3. Assertion: Heat is required for the decomposition of lead nitrate. 

 Reason: Decomposition reactions are endothermic. 

 Q4. Assertion: During displacement reactions precipitates are usually formed. 

 Reason: Precipitation reaction produce insoluable salt.   

Q1. Both A and R are true; R explains A → (a)
Q2. Both A and R are true; R explains A → (a)
Q3. Both A and R are true; R explains A → (a)
Q4. A true, R true but not correct explanation → (d)

Case-Based Questions

Q. Read the following and answer any four questions:

 Electrolysis is a process of decomposition of an electrolyte by the passage of electricity through the aqueous solution or molten (fused) state. During the electrolysis of water as shown in the diagram given below:- 

 (a) Identify the gases evolved at anode and cathode. 

 (b) Why are the amounts of gases collected in the two test tubes are of not the same volume? 

 (c) What type of reaction is this? 

 (d) Why should we use salt water? 

 (e) Write the reaction involved.       

(Answers) 

(a) Anode gas → Oxygen; Cathode gas → Hydrogen
(b) Unequal volumes due to 2:1 ratio of H₂:O₂
(c) Reaction type → Decomposition
(d) Salt water used to improve conductivity

(e) Reaction: $2H_2O \xrightarrow{electricity} 2H_2 + O_2$

Q. Read the following and answer any four questions: 

Oxidation is the gain of oxygen or loss of hydrogen. Reduction is the loss of oxygen or gain of hydrogen in the reaction. 

 MnO2 + 4HCl MnCl + 2H2O + (Cl2) 2 

 (a) Name the compound oxidised. 

 (b) Name the compound reduced.

 (c) Name the oxidising agent. 

 (d) Name the reducing agent. 

 (e) Define oxidation and reduction on its basis.

(Answers) 

(a) Compound oxidised → HCl
(b) Compound reduced → MnO₂
(c) Oxidising agent → MnO₂
(d) Reducing agent → HCl
(e) Oxidation = gain of O/loss of H; Reduction = loss of O/gain of H.

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