CLASS 10 SCIENCE CHAPTER 1 — CHEMICAL REACTIONS AND EQUATIONS

Question Bank

Total practice set: 150+ questions

SECTION A — MCQs

Choose the correct option.

1.

Which observation is not by itself listed in the chapter as an indication of a chemical reaction?

A. Change in colour
B. Evolution of a gas
C. Change in temperature
D. Change in mass

Answer: D

2.

In the equation

Mg + O₂ → MgO

the substances on the left side are called:

A. Products
B. Reactants
C. Catalysts
D. Precipitates

Answer: B

3.

The main purpose of balancing a chemical equation is to ensure that:

A. The number of molecules is always equal on both sides
B. The number of atoms of each element is equal on both sides
C. The products are always solids
D. The reaction occurs faster

Answer: B

4.

Which of the following is a correctly balanced equation?

A. Mg + O₂ → MgO
B. 2Mg + O₂ → 2MgO
C. Mg + 2O₂ → MgO
D. 2Mg + 2O₂ → MgO

Answer: B

5.

Which symbol represents an aqueous solution?

A. (s)
B. (l)
C. (g)
D. (aq)

Answer: D

6.

Which equation represents a combination reaction?

A. CaCO₃ → CaO + CO₂
B. Fe + CuSO₄ → FeSO₄ + Cu
C. CaO + H₂O → Ca(OH)₂
D. Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl

Answer: C

7.

A reaction in which two or more substances combine to form one product is called:

A. Decomposition
B. Combination
C. Displacement
D. Double displacement

Answer: B

8.

Which of the following is a decomposition reaction?

A. C + O₂ → CO₂
B. 2H₂ + O₂ → 2H₂O
C. CaCO₃ → CaO + CO₂
D. Fe + CuSO₄ → FeSO₄ + Cu

Answer: C

9.

The decomposition of calcium carbonate by heating is an example of:

A. Photochemical decomposition
B. Thermal decomposition
C. Electrolytic decomposition
D. Displacement

Answer: B

10.

Which form of energy causes the decomposition of silver chloride?

A. Heat
B. Electricity
C. Light
D. Sound

Answer: C

11.

The grey colour obtained when silver chloride is exposed to sunlight is due to formation of:

A. Chlorine
B. Silver
C. Silver oxide
D. Hydrogen chloride

Answer: B

12.

Which reaction is an example of displacement?

A. Fe + CuSO₄ → FeSO₄ + Cu
B. Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl
C. CaCO₃ → CaO + CO₂
D. CaO + H₂O → Ca(OH)₂

Answer: A

13.

Iron can displace copper from copper sulphate because:

A. Copper is more reactive than iron
B. Iron is more reactive than copper
C. Both have equal reactivity
D. Copper sulphate contains no copper

Answer: B

14.

Which substance is the precipitate in:

Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl?

A. Na₂SO₄
B. BaCl₂
C. BaSO₄
D. NaCl

Answer: C

15.

A reaction involving exchange of ions between two compounds is:

A. Combination
B. Decomposition
C. Double displacement
D. Oxidation

Answer: C

16.

Which process involves gain of oxygen?

A. Reduction
B. Oxidation
C. Precipitation
D. Neutralisation

Answer: B

17.

Loss of oxygen from a substance is called:

A. Oxidation
B. Reduction
C. Corrosion
D. Rancidity

Answer: B

18.

CuO + H₂ → Cu + H₂O

CuO undergoes:

A. Oxidation
B. Reduction
C. Combination
D. Displacement only

Answer: B

19.

In the same reaction, hydrogen undergoes:

A. Oxidation
B. Reduction
C. Decomposition
D. Precipitation

Answer: A

20.

A reaction in which oxidation and reduction occur simultaneously is called:

A. Combination reaction
B. Redox reaction
C. Decomposition reaction
D. Precipitation reaction

Answer: B

21.

Which of the following is an exothermic reaction?

A. A reaction in which energy is absorbed
B. A reaction in which heat is released
C. A reaction requiring sunlight only
D. A reaction producing a precipitate

Answer: B

22.

Respiration is considered exothermic because:

A. Glucose is produced
B. Oxygen is absorbed
C. Energy is released
D. Carbon dioxide is absorbed

Answer: C

23.

Which substance is used in whitewashing after reacting with water?

A. Calcium oxide
B. Calcium carbonate
C. Copper oxide
D. Sodium chloride

Answer: A

24.

Calcium oxide reacts with water to produce:

A. Calcium carbonate
B. Calcium hydroxide
C. Calcium chloride
D. Calcium sulphate

Answer: B

25.

Which of the following is an example of an endothermic process according to the chapter’s description?

A. Respiration
B. Burning natural gas
C. A decomposition reaction requiring absorbed energy
D. Calcium oxide reacting with water

Answer: C

26.

The reddish-brown coating on iron is associated with:

A. Rancidity
B. Corrosion
C. Precipitation
D. Reduction

Answer: B

27.

The black coating formed on silver is an example of:

A. Corrosion
B. Combination
C. Decomposition
D. Rancidity

Answer: A

28.

The green coating commonly observed on copper is related to:

A. Corrosion
B. Reduction
C. Rancidity
D. Thermal decomposition

Answer: A

29.

Rancidity is mainly associated with oxidation of:

A. Water
B. Metals
C. Fats and oils
D. Salts

Answer: C

30.

Which gas is commonly used in food packets to reduce oxidation of oily foods?

A. Oxygen
B. Nitrogen
C. Hydrogen
D. Carbon dioxide

Answer: B

31.

Which equation represents oxidation of copper?

A. CuO + H₂ → Cu + H₂O
B. 2Cu + O₂ → 2CuO
C. Cu + CuSO₄ → Cu₂SO₄
D. CuO → Cu + O₂

Answer: B

32.

Which reaction involves a single reactant?

A. Combination
B. Decomposition
C. Displacement
D. Double displacement

Answer: B

33.

Which of the following has the form AB + CD → AD + CB?

A. Combination
B. Decomposition
C. Displacement
D. Double displacement

Answer: D

34.

Which of these is not a physical-state symbol?

A. (aq)
B. (g)
C. (p)
D. (s)

Answer: C

35.

In a chemical equation, the arrow generally indicates:

A. Equal quantities
B. Direction of reaction
C. Temperature
D. Physical state

Answer: B

36.

Which statement is correct?

A. Chemical formulae can be altered during balancing
B. Subscripts can be freely changed while balancing
C. Coefficients are changed to balance equations
D. Balanced equations need not conserve atoms

Answer: C

37.

Which equation represents a reaction that produces a gas?

A. Fe + CuSO₄ → FeSO₄ + Cu
B. CaCO₃ → CaO + CO₂
C. Na₂SO₄ + BaCl₂ → BaSO₄ + NaCl
D. CaO + H₂O → Ca(OH)₂

Answer: B

38.

The reaction

2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂

is:

A. Combination
B. Thermal decomposition
C. Displacement
D. Double displacement

Answer: B

39.

Which substance is reduced in:

ZnO + C → Zn + CO?

A. ZnO
B. C
C. Zn
D. CO

Answer: A

40.

Which substance is oxidised in the same reaction?

A. ZnO
B. Zn
C. Carbon
D. CO

Answer: C

SECTION B — ASSERTION AND REASON

Choose:

A. Both Assertion and Reason are true, and Reason correctly explains Assertion.
B. Both are true, but Reason does not correctly explain Assertion.
C. Assertion is true, Reason is false.
D. Assertion is false, Reason is true.

41.

Assertion: Chemical equations should be balanced.

Reason: Atoms are neither created nor destroyed in a chemical reaction.

Answer: A

42.

Assertion: Mg + O₂ → MgO is a balanced equation.

Reason: There are two oxygen atoms on the reactant side and one on the product side.

Answer: D

43.

Assertion: CaO + H₂O → Ca(OH)₂ is a combination reaction.

Reason: Two reactants form a single product.

Answer: A

44.

Assertion: Respiration is an exothermic process.

Reason: Energy is released during respiration.

Answer: A

45.

Assertion: Fe can displace Cu from CuSO₄ solution.

Reason: Iron is more reactive than copper.

Answer: A

46.

Assertion: BaSO₄ formed in the reaction between Na₂SO₄ and BaCl₂ is called a precipitate.

Reason: BaSO₄ is insoluble in water.

Answer: A

47.

Assertion: CuO is oxidised when it reacts with hydrogen.

Reason: CuO loses oxygen in the reaction.

Answer: D

48.

Assertion: Hydrogen is oxidised in CuO + H₂ → Cu + H₂O.

Reason: Hydrogen gains oxygen.

Answer: A

49.

Assertion: Rancidity can be slowed by keeping food in airtight containers.

Reason: Reduced exposure to air slows oxidation of fats and oils.

Answer: A

50.

Assertion: Decomposition reactions are opposite to combination reactions.

Reason: Decomposition produces two or more substances from a single substance.

Answer: A

SECTION C — FILL IN THE BLANKS

51.

The substances that undergo chemical change are called __________.

Answer: reactants

52.

The new substances formed during a chemical reaction are called __________.

Answer: products

53.

A chemical equation in which the number of atoms is not equal on both sides is called a __________ equation.

Answer: skeletal

54.

The law stating that mass can neither be created nor destroyed is the law of __________.

Answer: conservation of mass

55.

The symbol (aq) represents a substance dissolved in __________.

Answer: water

56.

A reaction in which two or more substances form one product is called a __________ reaction.

Answer: combination

57.

A reaction in which one substance breaks into two or more substances is called a __________ reaction.

Answer: decomposition

58.

Decomposition caused by heat is called __________ decomposition.

Answer: thermal

59.

Decomposition caused by light is called __________ decomposition.

Answer: photochemical

60.

A reaction in which a more reactive element replaces a less reactive element is a __________ reaction.

Answer: displacement

61.

The insoluble solid formed in some double displacement reactions is called a __________.

Answer: precipitate

62.

A reaction involving exchange of ions between reactants is called a __________ displacement reaction.

Answer: double

63.

Gain of oxygen is called __________.

Answer: oxidation

64.

Loss of oxygen is called __________.

Answer: reduction

65.

A reaction in which oxidation and reduction occur together is called a __________ reaction.

Answer: redox

66.

A reaction in which heat is released is called an __________ reaction.

Answer: exothermic

67.

A reaction in which energy is absorbed is called an __________ reaction.

Answer: endothermic

68.

The reddish-brown coating formed on iron is associated with __________.

Answer: rusting/corrosion

69.

Oxidation of fats and oils leading to unpleasant smell and taste is called __________.

Answer: rancidity

70.

__________ is used in packets of chips to slow oxidation.

Answer: nitrogen

SECTION D — TRUE OR FALSE

71.

A chemical equation must always conserve the number of atoms of each element.

Answer: True

72.

The formula of a compound may be changed while balancing an equation.

Answer: False

73.

A combination reaction produces a single product.

Answer: True

74.

A decomposition reaction always has two or more reactants.

Answer: False

75.

BaSO₄ formed in the reaction of BaCl₂ and Na₂SO₄ is a precipitate.

Answer: True

76.

Reduction means gain of oxygen.

Answer: False

77.

Oxidation and reduction can occur simultaneously.

Answer: True

78.

Respiration is an exothermic process.

Answer: True

79.

Rancidity is associated with oxidation of fats and oils.

Answer: True

80.

Nitrogen speeds up oxidation of fats and oils.

Answer: False

SECTION E — MATCH THE FOLLOWING

81.

Column A	Column B
1. Combination	A. Fe + CuSO₄ → FeSO₄ + Cu
2. Decomposition	B. CaO + H₂O → Ca(OH)₂
3. Displacement	C. CaCO₃ → CaO + CO₂
4. Double displacement	D. Na₂SO₄ + BaCl₂ → BaSO₄ + NaCl

Answer: 1-B, 2-C, 3-A, 4-D

82.

Column A	Column B
1. Oxidation	A. Loss of oxygen
2. Reduction	B. Gain of oxygen
3. Exothermic	C. Energy absorbed
4. Endothermic	D. Heat released

Answer: 1-B, 2-A, 3-D, 4-C

83.

Column A	Column B
1. (s)	A. Gas
2. (l)	B. Aqueous
3. (g)	C. Solid
4. (aq)	D. Liquid

Answer: 1-C, 2-D, 3-A, 4-B

84.

Column A	Column B
1. Corrosion	A. Fats and oils
2. Rancidity	B. Metal deterioration
3. Precipitate	C. Insoluble solid
4. Reactants	D. Substances undergoing change

Answer: 1-B, 2-A, 3-C, 4-D

SECTION F — ONE-WORD / VERY SHORT ANSWER

85.

What is the name given to substances present before a chemical reaction?

Answer: Reactants

86.

What is the term for a new substance formed in a chemical reaction?

Answer: Product

87.

What type of reaction is represented by A + B → AB?

Answer: Combination

88.

What type of reaction is represented by AB → A + B?

Answer: Decomposition

89.

What is the name of the insoluble solid formed during a precipitation reaction?

Answer: Precipitate

90.

Which process causes iron to develop a reddish-brown coating?

Answer: Corrosion/rusting

91.

What is the oxidation-related spoilage of fats and oils called?

Answer: Rancidity

92.

Which gas is used to protect chips from oxidation?

Answer: Nitrogen

93.

What type of reaction occurs when iron displaces copper from CuSO₄?

Answer: Displacement

94.

What type of reaction occurs when Na₂SO₄ reacts with BaCl₂?

Answer: Double displacement / precipitation

SECTION G — IDENTIFY THE TYPE OF REACTION

95.

2Mg + O₂ → 2MgO

Answer: Combination; also oxidation

96.

CaCO₃ → CaO + CO₂

Answer: Decomposition; specifically thermal decomposition when heated

97.

Fe + CuSO₄ → FeSO₄ + Cu

Answer: Displacement

98.

Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl

Answer: Double displacement; precipitation

99.

2AgCl → 2Ag + Cl₂ in sunlight

Answer: Photochemical decomposition

100.

CaO + H₂O → Ca(OH)₂ + Heat

Answer: Combination; exothermic

101.

CuO + H₂ → Cu + H₂O

Answer: Redox reaction

102.

2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂ on heating

Answer: Thermal decomposition

103.

C + O₂ → CO₂

Answer: Combination

104.

2H₂ + O₂ → 2H₂O

Answer: Combination

SECTION H — BALANCE THE EQUATIONS

105.

Mg + O₂ → MgO

Answer: 2Mg + O₂ → 2MgO

106.

H₂ + Cl₂ → HCl

Answer: H₂ + Cl₂ → 2HCl

107.

Fe + H₂O → Fe₃O₄ + H₂

Answer: 3Fe + 4H₂O → Fe₃O₄ + 4H₂

108.

Na + H₂O → NaOH + H₂

Answer: 2Na + 2H₂O → 2NaOH + H₂

109.

BaCl₂ + Na₂SO₄ → BaSO₄ + NaCl

Answer: BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl

110.

Pb(NO₃)₂ → PbO + NO₂ + O₂

Answer: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂

111.

CaCO₃ → CaO + CO₂

Answer: Already balanced

112.

Fe + CuSO₄ → FeSO₄ + Cu

Answer: Already balanced

113.

H₂ + O₂ → H₂O

Answer: 2H₂ + O₂ → 2H₂O

114.

Al + CuCl₂ → AlCl₃ + Cu

Answer: 2Al + 3CuCl₂ → 2AlCl₃ + 3Cu

SECTION I — WRITE THE CHEMICAL EQUATION

115.

Magnesium burns in oxygen to form magnesium oxide.

Answer: 2Mg + O₂ → 2MgO

116.

Calcium oxide reacts with water to form calcium hydroxide.

Answer: CaO + H₂O → Ca(OH)₂

117.

Iron reacts with copper sulphate to form iron sulphate and copper.

Answer: Fe + CuSO₄ → FeSO₄ + Cu

118.

Sodium sulphate reacts with barium chloride to produce barium sulphate and sodium chloride.

Answer: Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl

119.

Calcium carbonate decomposes on heating.

Answer: CaCO₃ —Heat→ CaO + CO₂

120.

Silver chloride decomposes in sunlight.

Answer: 2AgCl —Sunlight→ 2Ag + Cl₂

121.

Copper reacts with oxygen on heating.

Answer: 2Cu + O₂ —Heat→ 2CuO

122.

Copper oxide reacts with hydrogen on heating.

Answer: CuO + H₂ —Heat→ Cu + H₂O

SECTION J — VERY SHORT ANSWER QUESTIONS

123.

Why is Mg + O₂ → MgO called a skeletal equation?

Answer: Because the equation has not yet been balanced; the number of oxygen atoms differs on the two sides.

124.

What does an arrow in a chemical equation indicate?

Answer: It indicates the direction of the reaction, from reactants towards products.

125.

Why are coefficients used during balancing?

Answer: To make the number of atoms of each element equal on both sides without changing the chemical formulae.

126.

What is meant by a balanced chemical equation?

Answer: An equation in which the number of atoms of every element is equal on the reactant and product sides.

127.

What is a combination reaction?

Answer: A reaction in which two or more substances combine to form a single product.

128.

What is a decomposition reaction?

Answer: A reaction in which a single substance breaks down into two or more simpler substances.

129.

What is an exothermic reaction?

Answer: A reaction in which heat is released along with the products.

130.

What is an endothermic reaction?

Answer: A reaction in which energy is absorbed.

131.

What is a displacement reaction?

Answer: A reaction in which a more reactive element displaces a less reactive element from its compound.

132.

What is a double displacement reaction?

Answer: A reaction in which ions of two reactants exchange partners to form new compounds.

133.

What is oxidation according to the chapter?

Answer: Gain of oxygen or loss of hydrogen.

134.

What is reduction?

Answer: Loss of oxygen or gain of hydrogen.

135.

What is a redox reaction?

Answer: A reaction in which oxidation and reduction take place simultaneously.

SECTION K — SHORT ANSWER QUESTIONS

136.

Why should chemical equations be balanced?

Answer: Chemical equations are balanced to obey the law of conservation of mass. The number of atoms of each element must remain the same before and after the reaction.

137.

Explain why changing a subscript is not allowed while balancing an equation.

Answer: A subscript is part of the chemical formula and changing it changes the identity of the substance. Only coefficients can be changed to balance an equation.

138.

Differentiate between combination and decomposition reactions.

Answer: In a combination reaction, two or more substances combine to form a single product. In decomposition, a single substance breaks down into two or more products.

139.

Explain thermal decomposition with an equation.

Answer: Decomposition caused by heat is called thermal decomposition.

CaCO₃ —Heat→ CaO + CO₂

140.

Explain photochemical decomposition with an example.

Answer: Decomposition caused by light is called photochemical decomposition.

2AgCl —Sunlight→ 2Ag + Cl₂

141.

What happens when an iron nail is placed in copper sulphate solution?

Answer: Iron displaces copper because iron is more reactive. Copper gets deposited on the nail and iron sulphate is formed.

Fe + CuSO₄ → FeSO₄ + Cu

142.

What is a precipitation reaction? Give an example.

Answer: A reaction in which an insoluble solid is formed is called a precipitation reaction.

Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl

BaSO₄ is the precipitate.

143.

Explain oxidation and reduction using CuO + H₂ → Cu + H₂O.

Answer: CuO loses oxygen and is reduced to Cu. Hydrogen gains oxygen and is oxidised to water. Therefore, both processes occur together and the reaction is a redox reaction.

144.

Why is respiration an exothermic reaction?

Answer: During respiration, glucose reacts with oxygen and energy is released. Therefore, respiration is an exothermic process.

145.

How can rancidity be prevented or slowed?

Answer: It can be slowed by using antioxidants, storing food in airtight containers and reducing oxygen exposure, such as by flushing suitable food packets with nitrogen.

SECTION L — LONG ANSWER QUESTIONS

146.

Explain the different signs that may indicate that a chemical reaction has taken place.

Answer points:

Change in state
Change in colour
Evolution of gas
Change in temperature
Formation of new substances is the underlying chemical change.

147.

Explain the five major types of chemical reactions studied in this chapter with one example each.

Answer points:

Combination — CaO + H₂O → Ca(OH)₂
Decomposition — CaCO₃ → CaO + CO₂
Displacement — Fe + CuSO₄ → FeSO₄ + Cu
Double displacement — Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl
Redox — CuO + H₂ → Cu + H₂O

148.

Describe the process of balancing a chemical equation using the hit-and-trial method.

Answer points:

Write the skeletal equation.
Count atoms on both sides.
Select an appropriate element/compound to balance.
Add coefficients, not subscripts.
Balance remaining elements.
Check every element.
Reduce to the smallest whole-number coefficients.

149.

Explain corrosion and rancidity and state methods of reducing their effects.

Answer points:

Corrosion: deterioration of metals due to attack by surrounding substances such as moisture and acids.

Examples: rusting of iron, black coating on silver, green coating on copper.

Rancidity: oxidation of fats and oils causing undesirable smell and taste.

Prevention: protective measures against corrosion; antioxidants, airtight storage and nitrogen flushing for rancidity.

SECTION M — CASE-BASED QUESTIONS

Case Study 1

A student observes that a magnesium ribbon burns with a bright white flame and leaves behind a white powder. The teacher explains that magnesium has reacted with oxygen.

150.

What is the white powder?

A. MgO
B. MgCO₃
C. MgSO₄
D. MgCl₂

Answer: A

151.

Write the balanced equation.

Answer: 2Mg + O₂ → 2MgO

152.

What type of reaction is this?

Answer: Combination reaction; magnesium is also oxidised because it gains oxygen.

153.

Why is magnesium ribbon cleaned before burning?

Answer: To remove the surface coating and expose magnesium so that it can react properly.

Case Study 2

An iron nail is placed in blue copper sulphate solution. After some time, the solution changes and a coating forms on the nail.

154.

Which metal is displaced?

A. Iron
B. Copper
C. Sulphur
D. Oxygen

Answer: B

155.

Write the equation.

Answer: Fe + CuSO₄ → FeSO₄ + Cu

156.

What type of reaction occurs?

Answer: Displacement reaction.

157.

Why can iron displace copper?

Answer: Iron is more reactive than copper.

Case Study 3

Two colourless solutions of sodium sulphate and barium chloride are mixed. A white insoluble solid appears.

158.

Name the white solid.

Answer: Barium sulphate (BaSO₄)

159.

Write the balanced equation.

Answer: Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl

160.

What is the white solid called?

Answer: Precipitate

161.

What type of reaction is involved?

Answer: Double displacement reaction and precipitation reaction.

SECTION N — COMPETENCY / APPLICATION QUESTIONS

162.

A student writes:

2H₂ + O₂ → H₂O

Is this equation balanced? If not, correct it.

Answer: No. Correct equation:

2H₂ + O₂ → 2H₂O

163.

A student balances an equation by changing H₂O into H₂O₂. Is this correct? Explain.

Answer: No. Changing the formula changes the substance itself. Balancing must be done by changing coefficients only.

164.

A compound AB breaks into A and B when heated. What type of reaction is this?

Answer: Decomposition reaction; specifically thermal decomposition.

165.

A metal X removes metal Y from its salt solution. What can you infer about the relative reactivity of X and Y?

Answer: X is more reactive than Y.

166.

Two aqueous compounds exchange ions and produce an insoluble solid. Identify the two reaction classifications that apply.

Answer: Double displacement and precipitation.

167.

A substance gains oxygen while another substance loses oxygen in the same reaction. What type of reaction is occurring?

Answer: Redox reaction.

168.

An oily food develops an unpleasant smell after being left exposed to air for a long period. Name the process.

Answer: Rancidity.

169.

Why might airtight packaging help preserve such food?

Answer: It reduces exposure to air/oxygen and slows oxidation of fats and oils.

170.

A reaction makes its surroundings warmer. Which type of reaction may this indicate?

Answer: An exothermic reaction.

SECTION O — HIGH-LEVEL MIXED QUESTIONS

171.

A student is given four equations:

A + B → AB
AB → A + B
A + BC → AC + B
AB + CD → AD + CB

Identify the reaction type represented by each.

Answer:

Combination
Decomposition
Displacement
Double displacement

172.

Consider:

CuO + H₂ → Cu + H₂O

Answer the following:

Which substance loses oxygen?
Which substance gains oxygen?
Which substance is reduced?
Which substance is oxidised?
Why is the reaction called redox?

Answer:

CuO loses oxygen.
H₂ gains oxygen.
CuO is reduced.
H₂ is oxidised.
Oxidation and reduction occur simultaneously.

173.

A reaction has two reactants and two products. Can you immediately conclude that it is double displacement? Explain.

Answer: No. The number of reactants/products alone is not sufficient. We must check whether ions or groups of atoms exchange between the two reactants.

174.

Why can a reaction be both a combination reaction and an exothermic reaction?

Answer: These classifications describe different features. Combination describes the number of reactants/products, while exothermic describes energy change. Therefore, a single reaction can satisfy both.

175.

Why can a decomposition reaction be classified according to the source of energy supplied?

Answer: Different decomposition reactions may use different forms of energy. Heat gives thermal decomposition, light gives photochemical decomposition, and electricity can cause electrolytic decomposition.

176.

A student observes a colour change after placing an iron nail in CuSO₄ solution. Explain the change using reactivity.

Answer: Iron is more reactive than copper, so it displaces copper from CuSO₄. Iron sulphate forms and copper is deposited, producing observable changes.

177.

Why is the reaction

Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl

both a double displacement and precipitation reaction?

Answer: It is double displacement because the ions exchange partners. It is precipitation because insoluble BaSO₄ is formed.

178.

Explain why balancing an equation does not mean making the number of molecules equal on both sides.

Answer: The requirement is equality of the number of atoms of each element, not necessarily equality in the number of molecules.

179.

Why are physical-state symbols useful in chemical equations?

Answer: They provide information about whether each reactant or product is solid, liquid, gas or present as an aqueous solution.

180.

A student says, “Every reaction involving oxygen is necessarily oxidation.” Is this statement supported by the chapter’s definition? Explain carefully.

Answer: The chapter defines oxidation in terms of gain of oxygen or loss of hydrogen. Therefore, the reaction must be examined to determine whether a particular substance gains oxygen or loses hydrogen; merely seeing oxygen somewhere in an equation is not enough to identify which substance is oxidised.

RAPID-FIRE REVISION — 20 QUESTIONS

181.

What is the product of burning magnesium in oxygen?

Answer: Magnesium oxide

182.

What colour is magnesium oxide?

Answer: White

183.

What is the formula of quick lime?

Answer: CaO

184.

What is the formula of slaked lime?

Answer: Ca(OH)₂

185.

What gas is produced when calcium carbonate decomposes?

Answer: CO₂

186.

What colour does silver chloride become in sunlight?

Answer: Grey

187.

Which metal is deposited when iron is placed in CuSO₄?

Answer: Copper

188.

What is the formula of the precipitate in the BaCl₂ + Na₂SO₄ reaction?

Answer: BaSO₄

189.

What is gain of oxygen called?

Answer: Oxidation

190.

What is loss of oxygen called?

Answer: Reduction

191.

What is simultaneous oxidation and reduction called?

Answer: Redox reaction

192.

What type of reaction releases heat?

Answer: Exothermic

193.

What type of reaction absorbs energy?

Answer: Endothermic

194.

What process causes rusting?

Answer: Corrosion

195.

What process causes oily food to develop an unpleasant smell?

Answer: Rancidity

196.

Which gas is used to slow rancidity in chips packets?

Answer: Nitrogen

197.

What does (g) represent?

Answer: Gas

198.

What does (aq) represent?

Answer: Aqueous solution

199.

What method is used in the chapter for balancing equations?

Answer: Hit-and-trial method

200.

What fundamental law is obeyed by a balanced chemical equation?

Answer: Law of conservation of mass

⭐ MUST-PRACTISE QUESTIONS

If you are preparing for a Class 10 examination, make sure you can answer these without looking at notes:

What is a chemical reaction? Give four observations that indicate one.
What is a balanced chemical equation? Why is balancing necessary?
Explain the hit-and-trial method.
Why must chemical formulae not be changed while balancing?
Explain combination and decomposition reactions with equations.
Explain thermal, photochemical and electrolytic decomposition.
Differentiate between exothermic and endothermic reactions.
Explain displacement with the Fe + CuSO₄ reaction.
Explain double displacement using Na₂SO₄ + BaCl₂.
What is a precipitation reaction?
Define oxidation and reduction using gain/loss of oxygen and hydrogen.
Explain redox reaction using CuO + H₂.
What is corrosion? Give examples.
What is rancidity? How can it be prevented?
Balance chemical equations accurately.
Identify the type of reaction from an unfamiliar equation.
Write equations from word statements.
Explain observations in chemical-reaction experiments.
Distinguish displacement from double displacement.
Solve case-based questions involving reaction type, equation and observation.