Chapter 1 – Chemical Reactions and Equations PYQ with solution

Chapter 1 – Chemical Reactions and Equations

जो भरा नहीं है भावों से
जिसमें बहती रसधार नहीं,
वह हृदय नहीं पत्थर है,
जिसमें मातृभूमि से प्यार नहीं !

और एक विद्यार्थी का सबसे बड़ा देशभक्ति यही है कि वो मन लगाकर पढ़ाई करे

2026

Question 1

Why should a magnesium ribbon be cleaned before burning in air?

Answer:

Magnesium ribbon should be cleaned before burning because a thin layer of magnesium oxide (MgO) forms on its surface when it reacts with oxygen in air. This layer prevents magnesium from burning properly.

Cleaning removes the oxide layer and helps the magnesium ribbon burn easily with a bright white flame.

Topic Explanation

Magnesium is a very reactive metal. When it remains exposed to air, it reacts slowly with oxygen.

The reaction is:

2Mg + O2 → 2MgO

Here:

Mg = Magnesium
O₂ = Oxygen
MgO = Magnesium oxide

The magnesium oxide forms a dull white coating on the ribbon.

Important Concept

This reaction is a:

➤ Combination Reaction

Because two substances combine to form one product.

Magnesium + Oxygen → Magnesium Oxide

Observation During Burning

When magnesium ribbon burns:

It produces a bright white flame
White ash of magnesium oxide is formed
Heat is released

So, it is also an:

➤ Exothermic Reaction

Because heat energy is released.

Exam Writing Tip

Write keywords like:

“oxide layer”
“magnesium oxide coating”
“prevents proper burning”

These keywords help in scoring full marks.

Short Board Answer (2 Marks)

Magnesium ribbon is cleaned before burning because it develops a layer of magnesium oxide on its surface due to reaction with oxygen in air. This layer prevents proper burning. Cleaning removes the oxide layer so that magnesium burns easily.

Question 2

Write the balanced chemical equation for the reaction between iron and steam.

Answer

When iron reacts with steam, it forms iron oxide and hydrogen gas.

3Fe + 4H2O → Fe3O4 + 4H2

Topic Explanation

Here:

Fe = Iron
H₂O = Steam
Fe₃O₄ = Iron oxide
H₂ = Hydrogen gas

Iron cannot react with cold water easily, but it reacts with steam at high temperature.

Type of Reaction

➤ Displacement Reaction

Iron displaces hydrogen from water.

Iron + Steam → Iron Oxide + Hydrogen

Observation

Black coloured iron oxide forms
Hydrogen gas is released

Important Concept

The equation is balanced because the number of atoms on both sides is equal.

Element	Left Side	Right Side
Fe	3	3
H	8	8
O	4	4

Short Board Answer (2 Marks)

The balanced chemical equation is:

3Fe + 4H2O → Fe3O4 + 4H2

Iron reacts with steam to form iron oxide and hydrogen gas.

Question 3

State two observations when lead nitrate solution reacts with potassium iodide solution.

Answer

When lead nitrate reacts with potassium iodide:

A yellow precipitate is formed.
The solution becomes cloudy.

Chemical Equation

Pb(NO3)2 + 2KI → PbI2 + 2KNO3

Topic Explanation

Here:

Pb(NO₃)₂ = Lead nitrate
KI = Potassium iodide
PbI₂ = Lead iodide (yellow precipitate)

The yellow solid formed is called a precipitate because it is insoluble in water.

Type of Reaction

➤ Double Displacement Reaction

The ions exchange their places.

Lead nitrate + Potassium iodide →Lead iodide + Potassium nitrate

Important Concept

What is a Precipitation Reaction?

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

Short Board Answer (2 Marks)

Two observations are:

Yellow precipitate of lead iodide is formed.
The solution becomes cloudy.

Chemical equation:

Pb(NO3)2 + 2KI → PbI2 + 2KNO3

Question 4

What is a displacement reaction? Give one example.

Answer

A reaction in which a more reactive element displaces a less reactive element from its compound is called a displacement reaction.

Example

Zn + CuSO4 → ZnSO4 + Cu

Topic Explanation

Here:

Zn = Zinc
CuSO₄ = Copper sulphate
ZnSO₄ = Zinc sulphate
Cu = Copper

Zinc is more reactive than copper, so zinc displaces copper from copper sulphate solution.

Observation

Blue colour of copper sulphate fades
Brown copper metal deposits

Important Concept

Reactivity Series

A more reactive metal can replace a less reactive metal from its compound.

Short Board Answer (2 Marks)

A displacement reaction is a reaction in which a more reactive element displaces a less reactive element from its compound.

Example:

Zn + CuSO4 → ZnSO4 + Cu

Question 5

Why are decomposition reactions called the opposite of combination reactions?

Answer

Decomposition reactions are called the opposite of combination reactions because:

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

Combination Reaction Example

CaO + H2O → Ca(OH)2

Two substances combine to form one product.

Decomposition Reaction Example

CaCO3 → CaO + CO2

One substance breaks into simpler substances.

Easy Understanding

Combination Reaction	Decomposition Reaction
Many → One	One → Many
Substances combine	Substance breaks
Opposite process	Opposite process

Important Concept

Decomposition reactions usually require:

Heat
Light
Electricity

to break compounds.

Short Board Answer (2 Marks)

Decomposition reactions are opposite to combination reactions because in combination reactions two or more substances combine to form one product, whereas in decomposition reactions one substance breaks into two or more simpler substances.

2025

Question 1

Define oxidation and reduction with one example each.

Answer

Oxidation

The process in which:

Oxygen is added
OR
Hydrogen is removed

is called oxidation.

Example of Oxidation

2Cu + O₂ → 2CuO

Here, copper combines with oxygen to form copper oxide.
So, copper is oxidised.

Topic Explanation

Cu = Copper
O₂ = Oxygen
CuO = Copper oxide

Copper gains oxygen.
Therefore, oxidation takes place.

Reduction

The process in which:

Oxygen is removed
OR
Hydrogen is added

is called reduction.

Example of Reduction

CuO + H₂ → Cu + H₂O

Here, oxygen is removed from copper oxide.
So, copper oxide is reduced.

Topic Explanation

CuO = Copper oxide
H₂ = Hydrogen
Cu = Copper
H₂O = Water

Copper oxide loses oxygen.
Therefore, reduction takes place.

Important Concept

Oxidation and reduction occur together.
Such reactions are called:

➤ Redox Reactions

Short Board Answer (3 Marks)

Oxidation is the addition of oxygen or removal of hydrogen.

Example:

2Cu + O₂ → 2CuO

Reduction is the removal of oxygen or addition of hydrogen.

Example:

CuO + H₂ → Cu + H₂O

Question 2

Balance the following equation:

Fe + H₂O → Fe₃O₄ + H₂

Step-by-Step Balancing

Step 1: Write the unbalanced equation

Fe + H₂O → Fe₃O₄ + H₂

Step 2: Balance iron atoms

There are 3 iron atoms in Fe₃O₄.

So put 3 before Fe.

3Fe + H₂O → Fe₃O₄ + H₂

Step 3: Balance oxygen atoms

There are 4 oxygen atoms in Fe₃O₄.

So put 4 before H₂O.

3Fe + 4H₂O → Fe₃O₄ + H₂

Step 4: Balance hydrogen atoms

Left side hydrogen:

4H₂O = 8 hydrogen atoms

So put 4 before H₂.

3Fe + 4H₂O → Fe₃O₄ + 4H₂

Balanced Equation

3Fe + 4H₂O → Fe₃O₄ + 4H₂

Important Concept

A chemical equation must be balanced because of the:

➤ Law of Conservation of Mass

Atoms cannot be created or destroyed in a chemical reaction.

Short Board Answer (2 Marks)

Balanced chemical equation:

3Fe + 4H₂O → Fe₃O₄ + 4H₂

Question 3

Identify the type of reaction:

CaO + H₂O → Ca(OH)₂

Answer

This is a:

➤ Combination Reaction

because two substances combine to form a single product.

Topic Explanation

CaO = Calcium oxide
H₂O = Water
Ca(OH)₂ = Calcium hydroxide

Calcium oxide reacts with water to form calcium hydroxide.

Observation

Heat is released
The reaction becomes hot

So, it is also an:

➤ Exothermic Reaction

Important Concept

Combination reaction means:

Many reactants → One product

Short Board Answer (2 Marks)

CaO + H₂O → Ca(OH)₂

This is a combination reaction because two substances combine to form one product.

It is also an exothermic reaction because heat is released.

Question 4

What happens when silver chloride is exposed to sunlight? Write the equation.

Answer

Silver chloride decomposes in sunlight and forms silver metal and chlorine gas.

Chemical Equation

2AgCl → 2Ag + Cl₂

Topic Explanation

AgCl = Silver chloride
Ag = Silver
Cl₂ = Chlorine gas

Silver chloride is white in colour.
When exposed to sunlight, it turns grey because silver metal is formed.

Type of Reaction

➤ Photochemical Decomposition Reaction

because decomposition occurs in presence of sunlight.

Important Concept

Decomposition reactions can occur due to:

Heat
Electricity
Sunlight

Short Board Answer (2 Marks)

When silver chloride is exposed to sunlight, it decomposes into silver and chlorine gas.

Equation:

2AgCl → 2Ag + Cl₂

This is a photochemical decomposition reaction.

Question 5

Explain rancidity and mention two methods to prevent it.

Answer

Rancidity is the process in which oils and fats get oxidised and develop unpleasant smell and taste.

Topic Explanation

Food containing oil or fat reacts with oxygen present in air.

This causes:

Bad smell
Bad taste
Spoilage of food

Examples

Chips smelling bad after long time
Butter becoming stale

Methods to Prevent Rancidity

1. Store food in airtight containers

This prevents contact with oxygen.

2. Add antioxidants

Substances like BHA and BHT prevent oxidation.

Other Methods

Refrigeration
Nitrogen gas flushing

Important Concept

Chips packets are filled with nitrogen gas because nitrogen prevents oxidation.

Short Board Answer (3 Marks)

Rancidity is the oxidation of oils and fats causing unpleasant smell and taste.

Methods to prevent rancidity:

Store food in airtight containers
Add antioxidants like BHA and BHT

2024

Question 1

What is a balanced chemical equation? Why is it necessary?

Answer

A chemical equation in which the number of atoms of each element is equal on both sides of the equation is called a balanced chemical equation.

Example of Balanced Equation

2H₂ + O₂ → 2H₂O

Topic Explanation

Left Side

Hydrogen atoms = 4
Oxygen atoms = 2

Right Side

Hydrogen atoms = 4
Oxygen atoms = 2

Both sides have equal number of atoms.
Therefore, the equation is balanced.

Why is it Necessary?

A chemical equation must be balanced to obey the:

➤ Law of Conservation of Mass

Mass can neither be created nor destroyed during a chemical reaction.

So, atoms must remain equal on both sides.

Important Concept

Unbalanced Equation:

H₂ + O₂ → H₂O

Balanced Equation:

2H₂ + O₂ → 2H₂O

Short Board Answer (3 Marks)

A balanced chemical equation is an equation in which the number of atoms of each element is equal on both sides.

Example:

2H₂ + O₂ → 2H₂O

It is necessary because it follows the law of conservation of mass.

Question 2

State the law of conservation of mass.

Answer

The law of conservation of mass states that:

“Mass can neither be created nor destroyed in a chemical reaction.”

The total mass of reactants is always equal to the total mass of products.

Topic Explanation

During a chemical reaction:

Atoms rearrange themselves
No atom disappears
No new atom is created

Therefore, total mass remains constant.

Example

2H₂ + O₂ → 2H₂O

Number of atoms remains same on both sides.

Important Concept

This law is the main reason why chemical equations are balanced.

Short Board Answer (2 Marks)

The law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction. The total mass of reactants is equal to the total mass of products.

Question 3

Write one example each of:

(a) Combination reaction

(b) Double displacement reaction

(a) Combination Reaction

Answer

CaO + H₂O → Ca(OH)₂

Topic Explanation

CaO = Calcium oxide
H₂O = Water
Ca(OH)₂ = Calcium hydroxide

Two substances combine to form one product.

Therefore, it is a combination reaction.

Important Concept

Many reactants → One product

(b) Double Displacement Reaction

Answer

BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl

Topic Explanation

BaCl₂ = Barium chloride
Na₂SO₄ = Sodium sulphate
BaSO₄ = Barium sulphate
NaCl = Sodium chloride

The ions exchange places.

Therefore, it is a double displacement reaction.

Observation

White precipitate of barium sulphate is formed.

Important Concept

Exchange of ions takes place.

Short Board Answer (3 Marks)

(a) Combination reaction:

CaO + H₂O → Ca(OH)₂

(b) Double displacement reaction:

BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl

Question 4

Why do respiration and digestion involve exothermic reactions?

Answer

Respiration and digestion are exothermic reactions because they release energy in the form of heat.

Topic Explanation

During Respiration

Glucose reacts with oxygen to produce energy.

Equation:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy

Energy is released.
Therefore, respiration is exothermic.

During Digestion

Food breaks down into simpler substances and energy is released for body activities.

Important Concept

Exothermic Reaction

A reaction that releases heat energy is called an exothermic reaction.

Examples:

Respiration
Burning of fuel
Combustion

Short Board Answer (2 Marks)

Respiration and digestion are exothermic reactions because they release energy in the form of heat during breakdown of food substances.

Question 5

Differentiate between exothermic and endothermic reactions.

Answer

Exothermic Reaction	Endothermic Reaction
Heat is released	Heat is absorbed
Surroundings become hot	Surroundings become cool
Energy is given out	Energy is taken in

Example of Exothermic Reaction

CH₄ + 2O₂ → CO₂ + 2H₂O + Heat

Burning of methane releases heat.

Example of Endothermic Reaction

6CO₂ + 6H₂O + Sunlight → C₆H₁₂O₆ + 6O₂

Photosynthesis absorbs sunlight energy.

Topic Explanation

Exothermic Reaction

Releases heat
Temperature increases

Examples:

Respiration
Burning

Endothermic Reaction

Absorbs heat
Temperature decreases

Examples:

Photosynthesis
Melting of ice

Short Board Answer (3 Marks)

Exothermic Reaction	Endothermic Reaction
Heat is released	Heat is absorbed
Energy is given out	Energy is taken in

Examples:

Exothermic:
CH₄ + 2O₂ → CO₂ + 2H₂O + Heat

Endothermic:
6CO₂ + 6H₂O + Sunlight → C₆H₁₂O₆ + 6O₂

2023

Question 1

Balance the equation:

Al + CuCl₂ → AlCl₃ + Cu

Step-by-Step Balancing

Step 1: Write the unbalanced equation

Al + CuCl₂ → AlCl₃ + Cu

Step 2: Balance chlorine atoms

Left side chlorine = 2
Right side chlorine = 3

LCM of 2 and 3 = 6

Put:

3 before CuCl₂
2 before AlCl₃

Al + 3CuCl₂ → 2AlCl₃ + Cu

Step 3: Balance aluminium atoms

Right side has 2 aluminium atoms.

So put 2 before Al.

2Al + 3CuCl₂ → 2AlCl₃ + Cu

Step 4: Balance copper atoms

Left side has 3 copper atoms.

So put 3 before Cu.

2Al + 3CuCl₂ → 2AlCl₃ + 3Cu

Balanced Equation

2Al + 3CuCl₂ → 2AlCl₃ + 3Cu

Topic Explanation

Al = Aluminium
CuCl₂ = Copper chloride
AlCl₃ = Aluminium chloride
Cu = Copper

Aluminium is more reactive than copper, so it displaces copper from copper chloride solution.

Type of Reaction

➤ Displacement Reaction

because a more reactive metal displaces a less reactive metal.

Short Board Answer (2 Marks)

Balanced chemical equation:

2Al + 3CuCl₂ → 2AlCl₃ + 3Cu

Question 2

Why is respiration considered an exothermic reaction?

Answer

Respiration is considered an exothermic reaction because energy is released during the breakdown of glucose.

Chemical Equation

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy

Topic Explanation

During respiration:

Glucose combines with oxygen
Food breaks down
Energy is released

This released energy helps in:

Body movement
Growth
Breathing
Other life processes

Important Concept

Exothermic Reaction

A reaction that releases heat energy is called an exothermic reaction.

Examples:

Respiration
Burning of fuel
Combustion

Short Board Answer (2 Marks)

Respiration is an exothermic reaction because energy is released during breakdown of glucose.

Equation:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy

Question 3

What do you observe when zinc reacts with dilute hydrochloric acid?

Answer

The following observations are seen:

Bubbles of hydrogen gas are formed.
Zinc gradually dissolves.
Heat is produced.

Chemical Equation

Zn + 2HCl → ZnCl₂ + H₂

Topic Explanation

Zn = Zinc
HCl = Hydrochloric acid
ZnCl₂ = Zinc chloride
H₂ = Hydrogen gas

Zinc reacts with hydrochloric acid and displaces hydrogen.

Type of Reaction

➤ Displacement Reaction

because zinc displaces hydrogen from acid.

Important Concept

The reaction also releases heat, so it is an:

➤ Exothermic Reaction

Short Board Answer (2 Marks)

When zinc reacts with dilute hydrochloric acid:

Hydrogen gas bubbles are produced
Zinc dissolves gradually

Equation:

Zn + 2HCl → ZnCl₂ + H₂

Question 4

Explain thermal decomposition reaction with an example.

Answer

A decomposition reaction in which a compound breaks down into simpler substances by heating is called a thermal decomposition reaction.

Example

CaCO₃ → CaO + CO₂

Topic Explanation

CaCO₃ = Calcium carbonate
CaO = Calcium oxide
CO₂ = Carbon dioxide

When calcium carbonate is heated, it decomposes into calcium oxide and carbon dioxide.

Important Concept

Decomposition Reaction

One substance breaks into two or more simpler substances.

Thermal means:

“By heat”

Daily Life Use

This reaction is used in manufacture of cement and lime.

Short Board Answer (2 Marks)

A decomposition reaction caused by heat is called thermal decomposition reaction.

Example:

CaCO₃ → CaO + CO₂

Question 5

What is corrosion? How can it be prevented?

Answer

Corrosion is the slow eating away of metals due to reaction with air, moisture or chemicals.

Example

Rusting of iron.

Topic Explanation

Iron reacts with oxygen and water present in air to form rust.

Rusting Equation

4Fe + 3O₂ + xH₂O → 2Fe₂O₃·xH₂O

Observation

Brown coating forms on iron
Metal becomes weak

Methods to Prevent Corrosion

1. Painting

Paint prevents contact with air and moisture.

2. Oiling or Greasing

Oil layer protects metal surface.

3. Galvanisation

Iron is coated with zinc.

4. Alloy Formation

Example: Stainless steel.

Important Concept

Corrosion damages:

Bridges
Vehicles
Iron pipes
Machines

Short Board Answer (3 Marks)

Corrosion is the slow destruction of metals due to reaction with air and moisture.

Example: Rusting of iron.

Methods to prevent corrosion:

Painting
Oiling or greasing
Galvanisation
Alloy formation

2022

Question 1

Give reasons:

(a) Chips packets are flushed with nitrogen gas.

(b) Iron articles are painted.

(a) Chips packets are flushed with nitrogen gas

Answer

Chips packets are filled with nitrogen gas to prevent rancidity.

Topic Explanation

Chips contain oil and fats.
These oils react with oxygen present in air and become spoiled.

This process is called:

➤ Rancidity

Rancidity causes:

Bad smell
Bad taste
Spoilage of food

Nitrogen gas does not react easily with oil and fats, so it prevents oxidation.

Important Concept

Oxidation causes rancidity.

Nitrogen gas prevents contact between oxygen and food.

Short Board Answer

Chips packets are flushed with nitrogen gas to prevent rancidity caused by oxidation of oils and fats.

(b) Iron articles are painted

Answer

Iron articles are painted to prevent rusting.

Topic Explanation

Iron reacts with:

Oxygen
Moisture

and forms rust.

Rusting Equation

4Fe + 3O₂ + xH₂O → 2Fe₂O₃·xH₂O

How Paint Helps

Paint forms a protective layer on iron.

This prevents contact with:

Air
Water

Therefore, rusting does not occur easily.

Important Concept

Rusting is a type of corrosion.

Short Board Answer

Iron articles are painted to prevent contact with air and moisture, which prevents rusting.

Question 2

Write the balanced equation for the reaction of barium chloride with sodium sulphate.

Answer

BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl

Topic Explanation

BaCl₂ = Barium chloride
Na₂SO₄ = Sodium sulphate
BaSO₄ = Barium sulphate
NaCl = Sodium chloride

The ions exchange places during the reaction.

Type of Reaction

➤ Double Displacement Reaction

because ions exchange their positions.

Observation

A white insoluble precipitate of barium sulphate is formed.

Important Concept

This is also a:

➤ Precipitation Reaction

because an insoluble solid is formed.

Short Board Answer (2 Marks)

Balanced equation:

BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl

It is a double displacement reaction.

Question 3

Name the type of reaction in which energy is absorbed. Give one example.

Answer

The reaction in which energy is absorbed is called an:

➤ Endothermic Reaction

Example

6CO₂ + 6H₂O + Sunlight → C₆H₁₂O₆ + 6O₂

Topic Explanation

This equation represents:

➤ Photosynthesis

Plants absorb sunlight energy to prepare food.

Important Concept

Endothermic Reaction

Heat or energy is absorbed
Surroundings become cool

Other Examples

Melting of ice
Evaporation

Short Board Answer (2 Marks)

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

Example:

6CO₂ + 6H₂O + Sunlight → C₆H₁₂O₆ + 6O₂

Question 4

What is a precipitation reaction?

Answer

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

The insoluble solid formed is called a precipitate.

Example

Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃

Topic Explanation

Pb(NO₃)₂ = Lead nitrate
KI = Potassium iodide
PbI₂ = Lead iodide

Lead iodide forms a yellow insoluble solid.

Observation

Yellow precipitate appears in the solution.

Important Concept

Most precipitation reactions are:

➤ Double Displacement Reactions

Short Board Answer (2 Marks)

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

Example:

Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃

Question 5

State two conditions necessary for rusting of iron.

Answer

The two conditions necessary for rusting are:

Presence of oxygen (air)
Presence of water or moisture

Topic Explanation

Iron reacts with oxygen and water to form rust.

Rusting Equation

4Fe + 3O₂ + xH₂O → 2Fe₂O₃·xH₂O

Important Concept

If either:

air
OR
moisture

is absent, rusting does not occur.

Experiment Understanding

Iron does not rust:

In dry air
In boiled water covered with oil

because one condition is missing.

Short Board Answer (2 Marks)

The two conditions necessary for rusting are:

Presence of oxygen
Presence of water or moisture

2021

Question 1

Write any two observations in a chemical reaction.

Answer

Two common observations in a chemical reaction are:

Change in colour
Evolution of gas

Topic Explanation

When a chemical reaction occurs, some visible changes are observed.

These changes help us identify that a chemical reaction has taken place.

Common Observations in Chemical Reactions

1. Change in Colour

Example:

Fe + CuSO₄ → FeSO₄ + Cu

Blue copper sulphate solution becomes green.

2. Evolution of Gas

Example:

Zn + 2HCl → ZnCl₂ + H₂

Hydrogen gas bubbles are produced.

Other Possible Observations

Change in temperature
Formation of precipitate
Change in state

Important Concept

These observations indicate formation of new substances.

Short Board Answer (2 Marks)

Two observations in a chemical reaction are:

Change in colour
Evolution of gas

Question 2

Why do we apply paint on iron articles?

Answer

Paint is applied on iron articles to prevent rusting.

Topic Explanation

Iron reacts with:

Oxygen
Moisture

present in air and forms rust.

Rusting Equation

4Fe + 3O₂ + xH₂O → 2Fe₂O₃·xH₂O

How Paint Prevents Rusting

Paint forms a protective layer on iron surface.

This layer prevents contact of iron with:

Air
Water

Therefore, rusting is prevented.

Important Concept

Rusting is a type of corrosion.

Short Board Answer (2 Marks)

Paint is applied on iron articles to prevent contact with air and moisture, which prevents rusting.

Question 3

Write the balanced equation for burning of magnesium ribbon.

Answer

2Mg + O₂ → 2MgO

Topic Explanation

Mg = Magnesium
O₂ = Oxygen
MgO = Magnesium oxide

When magnesium ribbon burns in air, it combines with oxygen and forms magnesium oxide.

Observation

Magnesium burns with bright white flame
White ash of magnesium oxide is formed

Type of Reaction

➤ Combination Reaction

because two substances combine to form one product.

Important Concept

This reaction also releases heat.

Therefore, it is an:

➤ Exothermic Reaction

Short Board Answer (2 Marks)

Balanced equation:

2Mg + O₂ → 2MgO

Magnesium burns in oxygen to form magnesium oxide.

Question 4

Explain displacement reaction with an example.

Answer

A reaction in which a more reactive element displaces a less reactive element from its compound is called a displacement reaction.

Example

Zn + CuSO₄ → ZnSO₄ + Cu

Topic Explanation

Zn = Zinc
CuSO₄ = Copper sulphate
ZnSO₄ = Zinc sulphate
Cu = Copper

Zinc is more reactive than copper, so it displaces copper from copper sulphate solution.

Observation

Blue colour of copper sulphate fades
Brown copper metal deposits

Important Concept

A more reactive metal replaces a less reactive metal from its salt solution.

Short Board Answer (2 Marks)

A displacement reaction is a reaction in which a more reactive element displaces a less reactive element from its compound.

Example:

Zn + CuSO₄ → ZnSO₄ + Cu

Question 5

What happens during oxidation?

Answer

During oxidation:

Oxygen is added
OR
Hydrogen is removed

from a substance.

Example

2Cu + O₂ → 2CuO

Copper combines with oxygen to form copper oxide.

So, copper gets oxidised.

Topic Explanation

Cu = Copper
O₂ = Oxygen
CuO = Copper oxide

Addition of oxygen causes oxidation.

Important Concept

Oxidation and reduction occur together in many reactions.

These reactions are called:

➤ Redox Reactions

Another Example

CuO + H₂ → Cu + H₂O

Here:

CuO loses oxygen → Reduction
H₂ gains oxygen → Oxidation

Short Board Answer (2 Marks)

Oxidation is the addition of oxygen or removal of hydrogen from a substance.

Example:

2Cu + O₂ → 2CuO

2020

Question 1

Balance the equation:

H₂ + O₂ → H₂O

Step-by-Step Balancing

Step 1: Write the unbalanced equation

H₂ + O₂ → H₂O

Step 2: Balance oxygen atoms

Left side has:

2 oxygen atoms

Right side has:

1 oxygen atom

So put 2 before H₂O.

H₂ + O₂ → 2H₂O

Step 3: Balance hydrogen atoms

Right side now has:

4 hydrogen atoms

So put 2 before H₂.

2H₂ + O₂ → 2H₂O

Balanced Equation

2H₂ + O₂ → 2H₂O

Topic Explanation

H₂ = Hydrogen
O₂ = Oxygen
H₂O = Water

Hydrogen combines with oxygen to form water.

Type of Reaction

➤ Combination Reaction

because two substances combine to form one product.

Important Concept

A balanced equation follows the:

➤ Law of Conservation of Mass

Atoms are equal on both sides.

Short Board Answer (2 Marks)

Balanced equation:

2H₂ + O₂ → 2H₂O

Question 2

What is the significance of a balanced chemical equation?

Answer

A balanced chemical equation shows that the number of atoms of each element is equal on both sides of the equation.

Topic Explanation

Balanced equations are important because they obey the:

➤ Law of Conservation of Mass

Mass can neither be created nor destroyed during a chemical reaction.

Example

2H₂ + O₂ → 2H₂O

Left Side

Hydrogen atoms = 4
Oxygen atoms = 2

Right Side

Hydrogen atoms = 4
Oxygen atoms = 2

Atoms are equal on both sides.

Important Concept

Balanced equations help to know:

Correct number of atoms
Correct amount of reactants and products

Short Board Answer (2 Marks)

A balanced chemical equation is important because it follows the law of conservation of mass and shows equal number of atoms on both sides.

Question 3

Define rancidity.

Answer

Rancidity is the process in which oils and fats get oxidised and develop unpleasant smell and taste.

Topic Explanation

Food containing oils and fats reacts with oxygen present in air.

This causes:

Bad smell
Bad taste
Spoilage of food

Examples

Stale butter
Bad smelling chips

Prevention of Rancidity

Airtight containers
Refrigeration
Nitrogen gas flushing
Antioxidants

Important Concept

Oxidation is the main cause of rancidity.

Short Board Answer (2 Marks)

Rancidity is the oxidation of oils and fats causing unpleasant smell and taste.

Question 4

Why is photosynthesis an endothermic reaction?

Answer

Photosynthesis is an endothermic reaction because plants absorb sunlight energy during the process.

Chemical Equation

6CO₂ + 6H₂O + Sunlight → C₆H₁₂O₆ + 6O₂

Topic Explanation

CO₂ = Carbon dioxide
H₂O = Water
C₆H₁₂O₆ = Glucose
O₂ = Oxygen

Plants use sunlight energy to prepare food.

Since energy is absorbed, the reaction is endothermic.

Important Concept

Endothermic Reaction

A reaction that absorbs heat or energy is called an endothermic reaction.

Other Examples

Melting of ice
Evaporation

Short Board Answer (2 Marks)

Photosynthesis is an endothermic reaction because sunlight energy is absorbed during the process.

Equation:

6CO₂ + 6H₂O + Sunlight → C₆H₁₂O₆ + 6O₂

Question 5

What is meant by corrosion?

Answer

Corrosion is the slow destruction of metals due to reaction with air, moisture or chemicals.

Topic Explanation

Metals react with substances present in the environment and gradually get damaged.

Example

Rusting of iron.

Rusting Equation

4Fe + 3O₂ + xH₂O → 2Fe₂O₃·xH₂O

Effects of Corrosion

Weakens metals
Damages machines
Damages bridges and vehicles

Prevention Methods

Painting
Oiling
Galvanisation
Alloy formation

Important Concept

Rusting is the most common example of corrosion.

Short Board Answer (2 Marks)

Corrosion is the slow destruction of metals due to reaction with air and moisture.

Example: Rusting of iron.

2019

Question 1

Give one example of a decomposition reaction carried out using electricity.

Answer

Electrolysis of water is a decomposition reaction carried out using electricity.

Chemical Equation

2H₂O → 2H₂ + O₂

Topic Explanation

H₂O = Water
H₂ = Hydrogen gas
O₂ = Oxygen gas

When electric current is passed through water, it breaks down into hydrogen and oxygen gases.

Type of Reaction

➤ Electrolytic Decomposition Reaction

because decomposition occurs due to electricity.

Important Concept

Decomposition Reaction

One substance breaks into two or more simpler substances.

Observation

Bubbles of gases are formed
Hydrogen gas is produced at one electrode
Oxygen gas is produced at another electrode

Short Board Answer (2 Marks)

Electrolysis of water is a decomposition reaction carried out using electricity.

Equation:

2H₂O → 2H₂ + O₂

Question 2

Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

Answer

The blue colour of copper sulphate solution changes to green because iron displaces copper from copper sulphate solution and forms iron sulphate.

Chemical Equation

Fe + CuSO₄ → FeSO₄ + Cu

Topic Explanation

Fe = Iron
CuSO₄ = Copper sulphate
FeSO₄ = Iron sulphate
Cu = Copper

Copper sulphate solution is blue in colour.

Iron sulphate solution is green in colour.

Why Does This Happen?

Iron is more reactive than copper.
Therefore, iron displaces copper from copper sulphate solution.

Observation

Blue solution becomes green
Brown copper deposits appear on iron nail

Type of Reaction

➤ Displacement Reaction

Short Board Answer (2 Marks)

The blue colour of copper sulphate solution changes to green because iron displaces copper from copper sulphate and forms green coloured iron sulphate solution.

Equation:

Fe + CuSO₄ → FeSO₄ + Cu

Question 3

Balance the following equation:

Na + H₂O → NaOH + H₂

Step-by-Step Balancing

Step 1: Write the unbalanced equation

Na + H₂O → NaOH + H₂

Step 2: Balance sodium atoms

Put 2 before NaOH.

Na + H₂O → 2NaOH + H₂

Now sodium atoms on right side = 2

So put 2 before Na.

2Na + H₂O → 2NaOH + H₂

Step 3: Balance oxygen atoms

Right side has 2 oxygen atoms.

So put 2 before H₂O.

2Na + 2H₂O → 2NaOH + H₂

Step 4: Check hydrogen atoms

Left side:

2H₂O = 4 hydrogen atoms

Right side:

2NaOH = 2 hydrogen atoms
H₂ = 2 hydrogen atoms

Total = 4 hydrogen atoms

Equation is balanced.

Balanced Equation

2Na + 2H₂O → 2NaOH + H₂

Topic Explanation

Na = Sodium
H₂O = Water
NaOH = Sodium hydroxide
H₂ = Hydrogen gas

Sodium reacts vigorously with water.

Observation

Hydrogen gas is released
Heat is produced

Important Concept

This is also an:

➤ Exothermic Reaction

because heat is released.

Short Board Answer (2 Marks)

Balanced equation:

2Na + 2H₂O → 2NaOH + H₂

Question 4

State one use of exothermic reactions in everyday life.

Answer

Exothermic reactions are used in respiration to provide energy for body activities.

Topic Explanation

During respiration:

Glucose reacts with oxygen and releases energy.

Chemical Equation

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy

Importance in Daily Life

The released energy helps in:

Walking
Breathing
Digestion
Body movement

Important Concept

Exothermic Reaction

A reaction that releases heat energy is called an exothermic reaction.

Other Uses

Burning fuels for cooking
Heat packs

Short Board Answer (2 Marks)

Exothermic reactions are used in respiration to provide energy for body activities.

Question 5

Explain double displacement reaction with an example.

Answer

A reaction in which two compounds exchange their ions to form new compounds is called a double displacement reaction.

Example

BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl

Topic Explanation

BaCl₂ = Barium chloride
Na₂SO₄ = Sodium sulphate
BaSO₄ = Barium sulphate
NaCl = Sodium chloride

The positive and negative ions exchange places.

Observation

A white precipitate of barium sulphate is formed.

Important Concept

Most double displacement reactions are precipitation reactions.

Short Board Answer (2 Marks)

A reaction in which two compounds exchange ions to form new compounds is called a double displacement reaction.

Example:

BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl

2018

Question 1

What happens when ferrous sulphate crystals are heated? Write the equation.

Answer

When ferrous sulphate crystals are heated, they decompose to form ferric oxide, sulphur dioxide and sulphur trioxide gases.

Chemical Equation

2FeSO₄ → Fe₂O₃ + SO₂ + SO₃

Topic Explanation

FeSO₄ = Ferrous sulphate
Fe₂O₃ = Ferric oxide
SO₂ = Sulphur dioxide
SO₃ = Sulphur trioxide

Ferrous sulphate crystals are green in colour.

On heating:

The crystals first lose water
Colour changes from green to brown
Gases with burning sulphur smell are produced

Type of Reaction

➤ Thermal Decomposition Reaction

because decomposition occurs due to heat.

Important Concept

Decomposition Reaction

One substance breaks into simpler substances.

Short Board Answer (3 Marks)

When ferrous sulphate crystals are heated, they decompose into ferric oxide, sulphur dioxide and sulphur trioxide.

Equation:

2FeSO₄ → Fe₂O₃ + SO₂ + SO₃

Question 2

Why are chemical equations balanced?

Answer

Chemical equations are balanced to obey the law of conservation of mass.

Topic Explanation

According to the law of conservation of mass:

“Mass can neither be created nor destroyed.”

Therefore:

Number of atoms of each element must be equal on both sides.

Example

Balanced equation:

2H₂ + O₂ → 2H₂O

Left Side

Hydrogen atoms = 4
Oxygen atoms = 2

Right Side

Hydrogen atoms = 4
Oxygen atoms = 2

Important Concept

Balanced equations show the correct ratio of reactants and products.

Short Board Answer (2 Marks)

Chemical equations are balanced to satisfy the law of conservation of mass and to make the number of atoms equal on both sides.

Question 3

State the observations when a candle burns.

Answer

The following observations are seen when a candle burns:

Heat is produced
Light is produced
Wax melts
Carbon dioxide and water vapour are formed

Topic Explanation

When candle wax burns, it reacts with oxygen present in air.

Chemical Reaction

Wax + Oxygen → Carbon dioxide + Water + Heat + Light

Important Concept

Burning of candle is a:

➤ Combustion Reaction

and also an:

➤ Exothermic Reaction

because heat is released.

Physical and Chemical Changes Together

Physical Change

Wax melts

Chemical Change

Wax burns and new substances form

Short Board Answer (2 Marks)

Observations during burning of candle:

Heat and light are produced
Wax melts
Carbon dioxide and water vapour are formed

Question 4

Explain combination reaction with one example.

Answer

A reaction in which two or more substances combine to form a single product is called a combination reaction.

Example

CaO + H₂O → Ca(OH)₂

Topic Explanation

CaO = Calcium oxide
H₂O = Water
Ca(OH)₂ = Calcium hydroxide

Two reactants combine to form one product.

Observation

Heat is released
Reaction vessel becomes hot

Important Concept

Combination reactions are usually exothermic reactions.

Another Example

2Mg + O₂ → 2MgO

Short Board Answer (2 Marks)

A reaction in which two or more substances combine to form one product is called a combination reaction.

Example:

CaO + H₂O → Ca(OH)₂

Question 5

Differentiate between displacement and double displacement reactions.

Answer

Displacement Reaction	Double Displacement Reaction
One element displaces another element	Two compounds exchange ions
Involves one compound and one element	Involves two compounds
More reactive element replaces less reactive element	Exchange of positive and negative ions occurs

Example of Displacement Reaction

Zn + CuSO₄ → ZnSO₄ + Cu

Zinc displaces copper.

Example of Double Displacement Reaction

BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl

Ions exchange places.

Topic Explanation

Displacement Reaction

A more reactive element replaces a less reactive element.

Double Displacement Reaction

Two compounds exchange ions to form new compounds.

Short Board Answer (3 Marks)

Displacement Reaction	Double Displacement Reaction
One element replaces another	Exchange of ions occurs
One element and one compound react	Two compounds react

Examples:

Displacement:
Zn + CuSO₄ → ZnSO₄ + Cu

Double displacement:
BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl

2017

Question 1

Write the balanced chemical equation for the reaction between hydrogen and chlorine.

Answer

H₂ + Cl₂ → 2HCl

Topic Explanation

H₂ = Hydrogen
Cl₂ = Chlorine
HCl = Hydrogen chloride

Hydrogen combines with chlorine to form hydrogen chloride gas.

Checking Balance

Left Side

Hydrogen atoms = 2
Chlorine atoms = 2

Right Side

Hydrogen atoms = 2
Chlorine atoms = 2

Equation is balanced.

Type of Reaction

➤ Combination Reaction

because two substances combine to form one product.

Important Concept

Hydrogen chloride gas dissolves in water to form hydrochloric acid.

Short Board Answer (2 Marks)

Balanced equation:

H₂ + Cl₂ → 2HCl

Question 2

Why is respiration called a slow combustion reaction?

Answer

Respiration is called a slow combustion reaction because food reacts slowly with oxygen and releases energy gradually.

Chemical Equation

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy

Topic Explanation

In respiration:

Glucose combines with oxygen
Energy is released slowly
No flame is produced

Why “Slow Combustion”?

Combustion means burning in presence of oxygen.

In respiration:

Oxidation of food occurs slowly
Energy is released in small amounts over time

Therefore, it is called slow combustion.

Important Concept

Respiration is also an:

➤ Exothermic Reaction

because heat energy is released.

Short Board Answer (2 Marks)

Respiration is called a slow combustion reaction because glucose reacts slowly with oxygen and releases energy gradually.

Question 3

Explain photolytic decomposition reaction with one example.

Answer

A decomposition reaction in which a compound breaks down in presence of sunlight is called a photolytic decomposition reaction.

Example

2AgCl → 2Ag + Cl₂

Topic Explanation

AgCl = Silver chloride
Ag = Silver
Cl₂ = Chlorine gas

Silver chloride is white in colour.

When exposed to sunlight:

It decomposes
Grey coloured silver is formed

Important Concept

“Photo” means light.

Sunlight causes decomposition.

Other Example

2AgBr → 2Ag + Br₂

Uses

This reaction is used in:

Black and white photography

Short Board Answer (2 Marks)

A decomposition reaction caused by sunlight is called photolytic decomposition reaction.

Example:

2AgCl → 2Ag + Cl₂

Question 4

What is rust? Mention two methods to prevent rusting.

Answer

Rust is a brown flaky substance formed when iron reacts with oxygen and moisture.

Rusting Equation

4Fe + 3O₂ + xH₂O → 2Fe₂O₃·xH₂O

Topic Explanation

Fe = Iron
O₂ = Oxygen
H₂O = Water

Rust is hydrated iron oxide.

Effects of Rusting

Iron becomes weak
Surface gets damaged

Methods to Prevent Rusting

1. Painting

Paint prevents contact with air and moisture.

2. Galvanisation

Iron is coated with zinc.

Other Methods

Oiling
Greasing
Alloy formation

Important Concept

Rusting is a type of corrosion.

Short Board Answer (2 Marks)

Rust is a brown flaky substance formed on iron due to reaction with air and moisture.

Methods to prevent rusting:

Painting
Galvanisation

Question 5

Identify the oxidising agent in the following reaction:

CuO + H₂ → Cu + H₂O

Answer

Oxidising Agent = CuO (Copper oxide)

Topic Explanation

CuO = Copper oxide
H₂ = Hydrogen
Cu = Copper
H₂O = Water

In this reaction:

Hydrogen gains oxygen to form water
Copper oxide gives oxygen

Therefore, copper oxide acts as the oxidising agent.

Important Concept

Oxidising Agent

A substance that gives oxygen or removes hydrogen is called an oxidising agent.

Understanding Oxidation and Reduction

Hydrogen:

H₂ → H₂O

Hydrogen gains oxygen.

So, hydrogen is oxidised.

Copper Oxide:

CuO → Cu

Copper oxide loses oxygen.

So, copper oxide is reduced.

Important Concept

Oxidation and reduction occur together.

These reactions are called:

➤ Redox Reactions

Short Board Answer (2 Marks)

CuO is the oxidising agent because it provides oxygen to hydrogen.

2016

Question 1

What is a chemical reaction? State its characteristics.

Answer

A process in which one or more substances change into new substances with different properties is called a chemical reaction.

Topic Explanation

In a chemical reaction:

Old substances are called reactants
New substances formed are called products

Example

2Mg + O₂ → 2MgO

Magnesium reacts with oxygen to form magnesium oxide.

Characteristics of Chemical Reactions

1. Change in Colour

Example:

Fe + CuSO₄ → FeSO₄ + Cu

Blue solution changes to green.

2. Evolution of Gas

Example:

Zn + 2HCl → ZnCl₂ + H₂

Hydrogen gas bubbles are formed.

3. Change in Temperature

Heat may be released or absorbed.

4. Formation of Precipitate

An insoluble solid may form.

Example:

Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃

Yellow precipitate forms.

5. Change in State

Example:

Wax melts on burning.

Important Concept

Chemical reactions always produce new substances.

Short Board Answer (3 Marks)

A chemical reaction is a process in which new substances are formed.

Characteristics:

Change in colour
Evolution of gas
Change in temperature
Formation of precipitate
Change in state

Question 2

Why do silver articles become black after some time?

Answer

Silver articles become black after some time because silver reacts with sulphur compounds present in air and forms silver sulphide.

Chemical Equation

2Ag + H₂S → Ag₂S + H₂

Topic Explanation

Ag = Silver
H₂S = Hydrogen sulphide
Ag₂S = Silver sulphide

Silver sulphide is black in colour.

Therefore, a black coating forms on silver articles.

Important Concept

This process is a type of:

➤ Corrosion

Example

Silver jewellery becoming black
Silver utensils losing shine

Short Board Answer (2 Marks)

Silver articles become black because silver reacts with sulphur compounds in air and forms black coloured silver sulphide.

Question 3

Explain exothermic reaction with one example.

Answer

A reaction in which heat energy is released is called an exothermic reaction.

Example

CH₄ + 2O₂ → CO₂ + 2H₂O + Heat

Topic Explanation

CH₄ = Methane
O₂ = Oxygen
CO₂ = Carbon dioxide
H₂O = Water

When methane burns, heat is released.

Therefore, it is an exothermic reaction.

Important Concept

Exothermic Reactions

Release heat
Surroundings become hot

Other Examples

Respiration
Burning of fuels

Short Board Answer (2 Marks)

An exothermic reaction is a reaction that releases heat energy.

Example:

CH₄ + 2O₂ → CO₂ + 2H₂O + Heat

Question 4

Balance the equation:

Pb(NO₃)₂ → PbO + NO₂ + O₂

Step-by-Step Balancing

Step 1: Write the unbalanced equation

Pb(NO₃)₂ → PbO + NO₂ + O₂

Step 2: Balance lead atoms

Lead atoms are already balanced.

Step 3: Balance nitrogen atoms

Left side has 2 nitrogen atoms.

So put 2 before NO₂.

Pb(NO₃)₂ → PbO + 2NO₂ + O₂

Step 4: Count oxygen atoms

Left Side

Pb(NO₃)₂ has:

6 oxygen atoms

Right Side

PbO = 1 oxygen
2NO₂ = 4 oxygen
O₂ = 2 oxygen

Total = 7 oxygen atoms

Not balanced.

Step 5: Multiply whole equation by 2

2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂

Final Balanced Equation

2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂

Topic Explanation

Lead nitrate decomposes on heating.

Type of Reaction

➤ Thermal Decomposition Reaction

because heat causes decomposition.

Short Board Answer (3 Marks)

Balanced equation:

2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂

Question 5

State one example each of:

(a) Corrosion

(b) Rancidity

(a) Corrosion

Answer

Rusting of iron is an example of corrosion.

Topic Explanation

Iron reacts with oxygen and moisture to form rust.

Rusting Equation

4Fe + 3O₂ + xH₂O → 2Fe₂O₃·xH₂O

Short Board Answer

Rusting of iron is an example of corrosion.

(b) Rancidity

Answer

Spoilage of oily food due to bad smell and taste is an example of rancidity.

Topic Explanation

Oils and fats react with oxygen and become spoiled.

Example

Chips smelling bad
Butter becoming stale

Important Concept

Rancidity occurs due to oxidation.

Short Board Answer

Spoilage of oily food due to oxidation is an example of rancidity.

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