Most Probable
SOLUTIONS
Questions for Practice
Chemistry
CHAPTER 1 Chemical Reactions and Equations (3) Oxidation numbers:
1.The given equation represents a redox reaction, Element/compound Oxidation state
specifically the thermite reaction, in which of key atom
aluminium (Al) is oxidised and iron(III) oxide Zn in ZnO +2
(Fe2O3) is reduced.
Zn in Zn (metal) 0
Oxidation: Aluminium is oxidised.
Half-reaction: 2Al → 2Al3+ + 6e– C in C (element) 0
(In the equation, this appears as Al → Al2O3.) C in CO +2
(Fe³+ in Fe2O3 is reduced to Fe) Change observed:
l Redox Reaction: This reaction is a redox reaction, l Zn: +2 → 0 (reduction)
involving the transfer of electrons. Aluminium l C: 0 → +2 (oxidation)
loses electrons (oxidation), and iron (III) gains (4) Half-reactions:
electrons (reduction). + gains electrons):
l Reduction (Zn²
l Oxidation Half-Reaction: Aluminium (Al) loses
Zn² + 2e– → Zn
+
three electrons to form Al³+ ions. The half-
• Oxidation (C loses electrons overall):
reaction shown is correct, but it needs to be
balanced with the oxygen from the oxide. In the C → C²+ + 2e–,
overall reaction, aluminium reacts with oxygen which combines with O²– to form CO.
to form aluminium oxide (Al2O3). (5) Identifying the oxidising/reducing agents:
+ • Carbon is the reducing agent (it reduces ZnO
l Reduction Half-Reaction: Fe³ ions in Fe2O3
gain electrons to form iron atoms (Fe). This is the by taking away its oxygen and is oxidised to
reduction process. CO).
Note: • Zinc oxide (ZnO) acts as the oxidising agent
Thermite Reaction: The reaction of aluminium with (it provides oxygen/accepts electrons and is
iron (III) oxide is known as the thermite reaction, reduced).
producing molten iron and aluminium oxide. This 3. (1) Iron nail in copper sulphate (CuSO4) solution
reaction is highly exothermic (releases a lot of
l Type of reaction: This is a single
heat) and is used in welding and other industrial
displacement reaction because iron, being
applications.
more reactive than copper, displaces copper
2. Reaction: ZnO + C → Zn + CO from the copper sulphate solution.
(1) Type of reaction: l Observations:
l Redox reaction (both oxidation and n The blue colour of the copper sulphate
reduction occur). solution fades and turns pale green due
l More specifically, reduction of a metal to the formation of iron (II) sulphate
oxide by carbon (also called carbothermic (FeSO4).
reduction in metallurgy). n A reddish-brown layer of copper metal
(2) What is reduced? What is oxidised? deposits on the iron nail.
l Zinc is reduced: In ZnO, zinc is +2. In Zn l Balanced Chemical Equation:
(metal), zinc is 0. Fe(s) + CuSO4(aq) → FeSO4(aq) +
Going from +2 → 0 means gain of electrons Cu(s)
⇒ reduction. (2) Iron nail in silver nitrate (AgNO3) solution
l Carbon is oxidised: In C (elemental carbon), l Type of reaction: This is also a single
carbon is 0. In CO, carbon is +2. displacement reaction, as iron is more
Going from 0 → +2 means loss of electrons reactive than silver and displaces it from the
⇒ oxidation. silver nitrate solution.
,2 | OSWAAL CBSE Sample Question Papers, SCIENCE, Class-10
Observations:
l (4) Mechanism of Conduction in Metals: The
n The iron nail is coated with a shiny, reason metals are good conductors is that
dendrite-like layer of silver metal. they have free electrons within their atomic
n The initially colourless silver nitrate structure. When one end is heated, these free
solution may gradually turn light green electrons gain kinetic energy and move around
due to the formation of iron (II) nitrate more rapidly. They collide with other atoms
(Fe(NO3)2) or iron (III) nitrate (Fe(NO3)3) and electrons, transferring their energy and
depending on the conditions. causing the heat to spread throughout the
l Balanced chemical equation (assuming
metal.
the formation of iron (II) nitrate): (5) Thermal Conductivity: The term thermal
Fe(s) + 2AgNO3(aq) conductivity refers to how efficiently a material
conducts or transfers heat. Materials with high
→ Fe (NO3) 2(aq)
thermal conductivity, like aluminium, transfer
+ 2Ag(s)
heat quickly, while materials with low thermal
l Alternatively, if iron (III) nitrate is formed:
conductivity (insulators) transfer heat very
2Fe + 3AgNO3 → 3Ag + 2Fe (NO3)3 slowly or not at all.
Note: The reactivity series of metals determines (6) Melting Point: It’s important to note that even
the possibility of displacement. In this case, iron is though the aluminium wire conducts heat and
more reactive than both copper and silver, hence melts the wax, the wire itself does not melt.
the displacement occurs in both cases. This is because aluminium has a relatively high
4. The experiment melting point (around 660°C). The temperature
(1) Set up: An aluminium wire is clamped firmly produced by the candle or spirit lamp is not
at one end to a stand. high enough to melt the aluminium wire.
(2) Attach the pin: A small pin is attached to the Summary: The experiment with the aluminium
free end of the wire using a small amount of wire, pin and wax demonstrates the concept of
wax, like a candle wax. thermal conductivity. It shows that aluminium is
a good conductor of heat, allowing heat to travel
(3) Heat the wire: The wire is then heated near the
along its length, melt the wax and cause the pin
clamped end using a candle or a spirit lamp
to fall. This ability to transfer heat efficiently is
flame.
due to the presence of free electrons in the metal’s
(4) Observe: Carefully observe what happens to structure.
the wax and the pin.
Setup and Observation: 5. (1) Definition of Oxidation
l Oxidation is a process in which:
Aluminum wire Wax Pin n Oxygen is added to a substance, or
n Hydrogen is removed from a substance,
or
n There is loss of electrons (an increase in
Pins fall off oxidation number).
Example from daily life:
l Rusting of iron: Iron reacts with oxygen and
Spirit lamp moisture to form hydrated iron oxide (rust).
4Fe + 3O2 + 6H2O → 4Fe(OH)3
Observations (Here, iron is oxidised to form rust.)
l After a short time, the wax holding the pin at the
(2) Definition of Reduction
free end of the aluminium wire begins to melt.
l Reduction is a process in which:
l As the wax completely melts, the pin detaches
n Oxygen is removed from a substance, or
and falls off the wire.
n Hydrogen is added to a substance, or
Explanation: The concept of thermal conductivity
n There is gain of electrons (a decrease in
(1) Heat Transfer: Heating the wire at the clamped
oxidation number).
end causes heat energy to travel along the
aluminium wire, even though the flame is not Example from daily life:
l Reduction of copper oxide with hydrogen:
directly touching the wax or pin.
(2) Conduction: This transfer of heat energy from CuO + H2 → Cu + H2O
the hotter end of the wire to the cooler end is (Here, copper oxide loses oxygen and is
known as conduction. reduced to copper metal.)
(3) Metals as Conductors: This experiment clearly (3) Both together: Redox reaction
demonstrates that metals, like aluminium, l Oxidation and reduction take place
are good conductors of heat. They allow heat simultaneously.
energy to flow through them easily. l If one substance is oxidised, the other must
be reduced.
, Most Probable Questions | 3
l Example: In the above reaction 2.Step 1: Reaction with small amount of CO2
CuO + H2 → Cu + H2O (milkiness appears).
n H2 is oxidised to H2O. When CO2 is passed into limewater (Ca(OH)2
n CuO is reduced to Cu. solution), it reacts to form insoluble calcium
(4) Following are the daily life examples: carbonate (CaCO3), which makes the solution to
l Respiration:
become milky.
Glucose is oxidised in our body to release Ca(OH)2 + CO2 → CaCO3↓ + H2O
energy: Step 2: Reaction with excess CO2 (milkiness
C6H12O6 + 6O2 → 6CO2 + 6H2O disappears)
+ Energy When excess CO2 is passed, the formed CaCO3
(Oxidation of glucose provides energy for reacts with extra CO2 and water to produce soluble
life processes.) calcium bicarbonate (Ca(HCO3)2). Since it dissolves
in water, the milkiness disappears.
l Corrosion / Rusting: Iron articles (gates,
bridges, ships) are oxidised to rust in CaCO3 + CO2 + H2O → Ca(HCO3)2 (soluble)
presence of air and moisture. Final Answer:
l Cooking of food (Reduction): Metal oxides l Milkiness appears due to insoluble CaCO3.
(like copper oxide on utensils) are reduced l It disappears in excess CO2 because CaCO3
to pure metals when cleaned with reducing converts into soluble Ca(HCO3)2.
agents. 3. (1) pH is a measure of how acidic or basic a
l
l Photosynthesis: CO2 is reduced to glucose,
solution is.
and water is oxidised to oxygen. l It is the negative logarithm of hydrogen ion
Final Answer concentration:
l Oxidation: Addition of oxygen / removal of
pH = −log[H+]
hydrogen / loss of electrons. l Scale ranges from 0 to 14:
Example: rusting of iron, burning of fuels. n pH < 7 → Acidic solution
l Reduction: Removal of oxygen / addition of
n pH = 7 → Neutral solution
hydrogen / gain of electrons.
n pH > 7 → Basic solution
Example: reduction of copper oxide by hydrogen.
(2) Importance of pH in daily life:
l Redox reactions: Oxidation and reduction occur
(a) pH in our digestive system
together.
l Our stomach produces HCl
Example: respiration, photosynthesis, corrosion,
(hydrochloric acid) with a pH of 1.5–3.5
combustion.
to help digest food.
CHAPTER 2 Acids, Bases and Salts l Sometimes excess acid causes indigestion
or acidity. To relieve it, antacids (like
1. Methyl orange in acidic solution → Red
l
milk of magnesia, Mg(OH)2) neutralise
Methyl orange in basic solution → Yellow
l
the excess acid.
Comment: (b) pH in tooth decay
Methyl orange is an acid-base indicator. It shows l Tooth enamel (made of calcium
red colour in acidic medium (pH < 3.1) and yellow phosphate) starts to dissolve when the
colour in basic medium (pH > 4.4). In between (pH pH in the mouth falls below 5.5.
3.1–4.4), it appears orange. l This happens because bacteria break
down food particles and produce acids.
l Hence, we use toothpaste (slightly basic)
Colour of methyl orange in acidic
and basic solutions to neutralise acids and protect teeth.
(c) pH of soil for plants
l Plants grow best only in a certain pH
range.
Example: Rice needs slightly acidic soil,
while wheat grows better in neutral to
slightly basic soil.
l Farmers test soil pH and add lime (CaO)
to reduce acidity or manures/fertilisers
Acidic Basic
to adjust pH for healthy crop growth.
Red Yellow
Final Answer:
l pH is a scale that measures the acidity or basicity
Note: Methyl orange is a synthetic indicator
commonly used in acid-base titrations. It has a of a solution.
distinct colour change depending on the pH of the l Daily life importance:
solution. 1. Stomach acid and antacids (digestion).
SOLUTIONS
Questions for Practice
Chemistry
CHAPTER 1 Chemical Reactions and Equations (3) Oxidation numbers:
1.The given equation represents a redox reaction, Element/compound Oxidation state
specifically the thermite reaction, in which of key atom
aluminium (Al) is oxidised and iron(III) oxide Zn in ZnO +2
(Fe2O3) is reduced.
Zn in Zn (metal) 0
Oxidation: Aluminium is oxidised.
Half-reaction: 2Al → 2Al3+ + 6e– C in C (element) 0
(In the equation, this appears as Al → Al2O3.) C in CO +2
(Fe³+ in Fe2O3 is reduced to Fe) Change observed:
l Redox Reaction: This reaction is a redox reaction, l Zn: +2 → 0 (reduction)
involving the transfer of electrons. Aluminium l C: 0 → +2 (oxidation)
loses electrons (oxidation), and iron (III) gains (4) Half-reactions:
electrons (reduction). + gains electrons):
l Reduction (Zn²
l Oxidation Half-Reaction: Aluminium (Al) loses
Zn² + 2e– → Zn
+
three electrons to form Al³+ ions. The half-
• Oxidation (C loses electrons overall):
reaction shown is correct, but it needs to be
balanced with the oxygen from the oxide. In the C → C²+ + 2e–,
overall reaction, aluminium reacts with oxygen which combines with O²– to form CO.
to form aluminium oxide (Al2O3). (5) Identifying the oxidising/reducing agents:
+ • Carbon is the reducing agent (it reduces ZnO
l Reduction Half-Reaction: Fe³ ions in Fe2O3
gain electrons to form iron atoms (Fe). This is the by taking away its oxygen and is oxidised to
reduction process. CO).
Note: • Zinc oxide (ZnO) acts as the oxidising agent
Thermite Reaction: The reaction of aluminium with (it provides oxygen/accepts electrons and is
iron (III) oxide is known as the thermite reaction, reduced).
producing molten iron and aluminium oxide. This 3. (1) Iron nail in copper sulphate (CuSO4) solution
reaction is highly exothermic (releases a lot of
l Type of reaction: This is a single
heat) and is used in welding and other industrial
displacement reaction because iron, being
applications.
more reactive than copper, displaces copper
2. Reaction: ZnO + C → Zn + CO from the copper sulphate solution.
(1) Type of reaction: l Observations:
l Redox reaction (both oxidation and n The blue colour of the copper sulphate
reduction occur). solution fades and turns pale green due
l More specifically, reduction of a metal to the formation of iron (II) sulphate
oxide by carbon (also called carbothermic (FeSO4).
reduction in metallurgy). n A reddish-brown layer of copper metal
(2) What is reduced? What is oxidised? deposits on the iron nail.
l Zinc is reduced: In ZnO, zinc is +2. In Zn l Balanced Chemical Equation:
(metal), zinc is 0. Fe(s) + CuSO4(aq) → FeSO4(aq) +
Going from +2 → 0 means gain of electrons Cu(s)
⇒ reduction. (2) Iron nail in silver nitrate (AgNO3) solution
l Carbon is oxidised: In C (elemental carbon), l Type of reaction: This is also a single
carbon is 0. In CO, carbon is +2. displacement reaction, as iron is more
Going from 0 → +2 means loss of electrons reactive than silver and displaces it from the
⇒ oxidation. silver nitrate solution.
,2 | OSWAAL CBSE Sample Question Papers, SCIENCE, Class-10
Observations:
l (4) Mechanism of Conduction in Metals: The
n The iron nail is coated with a shiny, reason metals are good conductors is that
dendrite-like layer of silver metal. they have free electrons within their atomic
n The initially colourless silver nitrate structure. When one end is heated, these free
solution may gradually turn light green electrons gain kinetic energy and move around
due to the formation of iron (II) nitrate more rapidly. They collide with other atoms
(Fe(NO3)2) or iron (III) nitrate (Fe(NO3)3) and electrons, transferring their energy and
depending on the conditions. causing the heat to spread throughout the
l Balanced chemical equation (assuming
metal.
the formation of iron (II) nitrate): (5) Thermal Conductivity: The term thermal
Fe(s) + 2AgNO3(aq) conductivity refers to how efficiently a material
conducts or transfers heat. Materials with high
→ Fe (NO3) 2(aq)
thermal conductivity, like aluminium, transfer
+ 2Ag(s)
heat quickly, while materials with low thermal
l Alternatively, if iron (III) nitrate is formed:
conductivity (insulators) transfer heat very
2Fe + 3AgNO3 → 3Ag + 2Fe (NO3)3 slowly or not at all.
Note: The reactivity series of metals determines (6) Melting Point: It’s important to note that even
the possibility of displacement. In this case, iron is though the aluminium wire conducts heat and
more reactive than both copper and silver, hence melts the wax, the wire itself does not melt.
the displacement occurs in both cases. This is because aluminium has a relatively high
4. The experiment melting point (around 660°C). The temperature
(1) Set up: An aluminium wire is clamped firmly produced by the candle or spirit lamp is not
at one end to a stand. high enough to melt the aluminium wire.
(2) Attach the pin: A small pin is attached to the Summary: The experiment with the aluminium
free end of the wire using a small amount of wire, pin and wax demonstrates the concept of
wax, like a candle wax. thermal conductivity. It shows that aluminium is
a good conductor of heat, allowing heat to travel
(3) Heat the wire: The wire is then heated near the
along its length, melt the wax and cause the pin
clamped end using a candle or a spirit lamp
to fall. This ability to transfer heat efficiently is
flame.
due to the presence of free electrons in the metal’s
(4) Observe: Carefully observe what happens to structure.
the wax and the pin.
Setup and Observation: 5. (1) Definition of Oxidation
l Oxidation is a process in which:
Aluminum wire Wax Pin n Oxygen is added to a substance, or
n Hydrogen is removed from a substance,
or
n There is loss of electrons (an increase in
Pins fall off oxidation number).
Example from daily life:
l Rusting of iron: Iron reacts with oxygen and
Spirit lamp moisture to form hydrated iron oxide (rust).
4Fe + 3O2 + 6H2O → 4Fe(OH)3
Observations (Here, iron is oxidised to form rust.)
l After a short time, the wax holding the pin at the
(2) Definition of Reduction
free end of the aluminium wire begins to melt.
l Reduction is a process in which:
l As the wax completely melts, the pin detaches
n Oxygen is removed from a substance, or
and falls off the wire.
n Hydrogen is added to a substance, or
Explanation: The concept of thermal conductivity
n There is gain of electrons (a decrease in
(1) Heat Transfer: Heating the wire at the clamped
oxidation number).
end causes heat energy to travel along the
aluminium wire, even though the flame is not Example from daily life:
l Reduction of copper oxide with hydrogen:
directly touching the wax or pin.
(2) Conduction: This transfer of heat energy from CuO + H2 → Cu + H2O
the hotter end of the wire to the cooler end is (Here, copper oxide loses oxygen and is
known as conduction. reduced to copper metal.)
(3) Metals as Conductors: This experiment clearly (3) Both together: Redox reaction
demonstrates that metals, like aluminium, l Oxidation and reduction take place
are good conductors of heat. They allow heat simultaneously.
energy to flow through them easily. l If one substance is oxidised, the other must
be reduced.
, Most Probable Questions | 3
l Example: In the above reaction 2.Step 1: Reaction with small amount of CO2
CuO + H2 → Cu + H2O (milkiness appears).
n H2 is oxidised to H2O. When CO2 is passed into limewater (Ca(OH)2
n CuO is reduced to Cu. solution), it reacts to form insoluble calcium
(4) Following are the daily life examples: carbonate (CaCO3), which makes the solution to
l Respiration:
become milky.
Glucose is oxidised in our body to release Ca(OH)2 + CO2 → CaCO3↓ + H2O
energy: Step 2: Reaction with excess CO2 (milkiness
C6H12O6 + 6O2 → 6CO2 + 6H2O disappears)
+ Energy When excess CO2 is passed, the formed CaCO3
(Oxidation of glucose provides energy for reacts with extra CO2 and water to produce soluble
life processes.) calcium bicarbonate (Ca(HCO3)2). Since it dissolves
in water, the milkiness disappears.
l Corrosion / Rusting: Iron articles (gates,
bridges, ships) are oxidised to rust in CaCO3 + CO2 + H2O → Ca(HCO3)2 (soluble)
presence of air and moisture. Final Answer:
l Cooking of food (Reduction): Metal oxides l Milkiness appears due to insoluble CaCO3.
(like copper oxide on utensils) are reduced l It disappears in excess CO2 because CaCO3
to pure metals when cleaned with reducing converts into soluble Ca(HCO3)2.
agents. 3. (1) pH is a measure of how acidic or basic a
l
l Photosynthesis: CO2 is reduced to glucose,
solution is.
and water is oxidised to oxygen. l It is the negative logarithm of hydrogen ion
Final Answer concentration:
l Oxidation: Addition of oxygen / removal of
pH = −log[H+]
hydrogen / loss of electrons. l Scale ranges from 0 to 14:
Example: rusting of iron, burning of fuels. n pH < 7 → Acidic solution
l Reduction: Removal of oxygen / addition of
n pH = 7 → Neutral solution
hydrogen / gain of electrons.
n pH > 7 → Basic solution
Example: reduction of copper oxide by hydrogen.
(2) Importance of pH in daily life:
l Redox reactions: Oxidation and reduction occur
(a) pH in our digestive system
together.
l Our stomach produces HCl
Example: respiration, photosynthesis, corrosion,
(hydrochloric acid) with a pH of 1.5–3.5
combustion.
to help digest food.
CHAPTER 2 Acids, Bases and Salts l Sometimes excess acid causes indigestion
or acidity. To relieve it, antacids (like
1. Methyl orange in acidic solution → Red
l
milk of magnesia, Mg(OH)2) neutralise
Methyl orange in basic solution → Yellow
l
the excess acid.
Comment: (b) pH in tooth decay
Methyl orange is an acid-base indicator. It shows l Tooth enamel (made of calcium
red colour in acidic medium (pH < 3.1) and yellow phosphate) starts to dissolve when the
colour in basic medium (pH > 4.4). In between (pH pH in the mouth falls below 5.5.
3.1–4.4), it appears orange. l This happens because bacteria break
down food particles and produce acids.
l Hence, we use toothpaste (slightly basic)
Colour of methyl orange in acidic
and basic solutions to neutralise acids and protect teeth.
(c) pH of soil for plants
l Plants grow best only in a certain pH
range.
Example: Rice needs slightly acidic soil,
while wheat grows better in neutral to
slightly basic soil.
l Farmers test soil pH and add lime (CaO)
to reduce acidity or manures/fertilisers
Acidic Basic
to adjust pH for healthy crop growth.
Red Yellow
Final Answer:
l pH is a scale that measures the acidity or basicity
Note: Methyl orange is a synthetic indicator
commonly used in acid-base titrations. It has a of a solution.
distinct colour change depending on the pH of the l Daily life importance:
solution. 1. Stomach acid and antacids (digestion).
