TEMP
01
📘 Chapter 1 · NCERT Science IX

MATTER IN OUR
SURROUNDINGS

Explore the nature of matter — its states, properties, and the fascinating phase transitions governed by temperature and pressure.

3States
6Changes
4 hrsStudy Time
★★★★★Exam Weight
📍 Chapter Snapshot
📖Chapter 01
⏱️Est. Study Time: 4 hrs
🎯Difficulty: Moderate
📝Marks Weightage: High
🔬Type: Chemistry
🧊
Solid
Definite shape & volume. Tightly packed particles.
💧
Liquid
No fixed shape. Definite volume. Fluid particles.
💨
Gas
No fixed shape or volume. Highly compressible.
🎯 Why This Chapter Matters for Exams
📌 State-change diagrams with latent heat are a recurring 3-mark question in SA-II.
📌 Evaporation vs boiling distinctions appear almost every year in objective rounds.
📌 Define Interconversion of states with neat labelled diagrams — 5-mark questions love this.
📌 Latent heat MCQs appear in competitive exams (NTSE, Olympiad) — don't skip numerical values.
💡 Key Concept Highlights
Interparticle forcesLatent Heat of FusionLatent Heat of VaporisationSublimationEvaporationBoiling PointMelting PointPlasma StateBose-Einstein CondensateDiffusionCompressibilityRigidity
⚗️ Important Formula Capsules
Latent HeatQ = mLJ or kJ
Densityρ = m/Vkg/m³
Temp (K)K = °C + 273Kelvin
📚 What You Will Learn
  • 1Characteristics of matter and how particles are arranged in each state
  • 2How temperature and pressure drive changes of state
  • 3The concept of latent heat and why it matters during phase changes
  • 4Factors affecting evaporation and its cooling effect
  • 5Plasma and Bose-Einstein Condensate — the two extra states
🔗 Navigate to Chapter Resources
📄
Full Notes
🧩
MCQs Practice
✅
True / False
📝
NCERT Exercises
🏆 Exam Strategy & Preparation Tips
01
Draw Diagrams
Always draw a state-change diagram with arrows and labels for full marks.
02
Memorise Values
0°C, 100°C, 273K — these numbers appear in every exam.
03
Use Examples
Cite camphor (sublimation), dry ice, iodine crystals as real-life examples.
04
Practice MCQs
State-change questions are almost entirely MCQ in board pattern — do 30+ per session.
Chapter 1 · CBSE · Class IX
🧪

Physical Nature of Matter

NCERT Class 9 Science Matter States of Matter Solid Liquid Gas Kinetic Theory Evaporation Latent Heat Sublimation Physical Nature Particle Theory
📋
Table of Contents
10 sections
📖 Introduction
📘 Definition
📌 Note
Matter is Made Up of Particles
🎨 Particle Model of Matter
Solid Liquid Gas

The diagram shows how particles are arranged in solids, liquids and gases. Particles are most closely packed in solids and farthest apart in gases.

🔷 Characteristics
Characteristics of Particles of Matter
🔷 Characteristics of Particles of Matter

The behaviour of matter can be explained by understanding the properties of its particles.

1. Particles of Matter Have Space Between Them

There exists empty space between the particles of matter. The amount of space varies from one state of matter to another.

Experiment

Take 100 mL water in a measuring cylinder and dissolve a spoonful of sugar in it. The water level remains almost unchanged.

Explanation

Sugar particles occupy the spaces present between water particles. Therefore, the total volume does not increase significantly.

Key Concept

Presence of intermolecular space allows substances to dissolve in one another.

2. Particles of Matter are Continuously Moving

Matter particles are never at rest. They continuously move and possess kinetic energy.

Definition of Kinetic Energy

The energy possessed by a body due to its motion is called kinetic energy.

\[ KE=\frac{1}{2}mv^2 \]

Example

The fragrance of perfume spreads throughout a room even when sprayed at one corner.

Reason

Perfume particles move continuously and mix with air particles through diffusion.

3. Particles Move Faster on Heating

When temperature increases, particles gain kinetic energy and start moving faster.

Observation
  • Hot milk spreads smell faster than cold milk.
  • Ink diffuses more rapidly in hot water than in cold water.
  • Sugar dissolves faster in warm water.
Scientific Explanation

Heating increases the kinetic energy of particles, making them move more rapidly and mix quickly.

4. Particles of Matter Attract Each Other

Particles exert attractive forces on one another. This force is known as intermolecular force of attraction.

Examples
  • A chalk piece remains intact.
  • A rubber band can be stretched but does not break easily.
  • Water forms droplets.
Strength of Attraction
State Force of Attraction
Solid Maximum
Liquid Moderate
Gas Minimum
🗒️ Important
Particle
The smallest constituent unit of matter.
Intermolecular Space
Space present between particles of matter.
Intermolecular Force
Force of attraction between particles.
Diffusion
Intermixing of particles due to their random motion.
Kinetic Energy
Energy possessed by a moving object.
✏️ Example
Solved Examples
Why does the smell of incense sticks spread throughout a room?
Diffusion and motion of particles.
Incense particles continuously move and mix with air particles. Hence the fragrance spreads throughout the room.
Why can sugar dissolve in water without significantly increasing the volume?
Space between particles.
Sugar particles occupy the spaces present between water particles.
📋 Case Study

A student places a drop of blue ink into a beaker containing hot water. The colour spreads quickly throughout the water without stirring.

Question 1: Which property of matter is demonstrated?

Answer: Particles of matter are continuously moving.

Question 2: Why is spreading faster in hot water?

Answer: Higher temperature increases kinetic energy and particle motion.

❌ Common Mistakes
  • Writing that particles stop moving in solids.
  • Confusing intermolecular space with vacuum.
  • Assuming diffusion occurs only in gases.
  • Ignoring the role of temperature in diffusion.
  • Writing that matter is continuous rather than particulate.
⚡ Exam Tip
⚡ Quick Revision
One-Mark Quick Revision
  • Matter has mass and occupies space.
  • Matter is made of tiny particles.
  • Particles have spaces between them.
  • Particles are continuously moving.
  • Higher temperature increases kinetic energy.
  • Particles attract one another.
  • Diffusion occurs due to particle motion.
🧪

States of Matter

📋
Table of Contents
10 sections
📘 Definition
🗒️ Basis Of Classification Of States Of Matter
Property Solid Liquid Gas
Particle Arrangement Very closely packed Less closely packed Far apart
Intermolecular Space Minimum Moderate Maximum
Force of Attraction Maximum Intermediate Very weak
Kinetic Energy Least Moderate Maximum
🎨 Particle Arrangement in Different States
SOLID LIQUID GAS STATES OF MATTER: KINETIC STABILITY MODE

The spacing between particles increases from solid to liquid to gas.

📘 Definition

Solid State

🗺️ Overview

Solids are substances whose particles are tightly packed and held together by strong intermolecular forces.

📘 Definition
A solid is a state of matter having definite shape, definite volume, and rigid structure.
🔷 Characteristics
  • Definite shape and fixed volume.
  • Particles are closely packed.
  • Strong intermolecular attraction.
  • Negligible compressibility.
  • Particles vibrate about fixed positions.
  • Do not flow like liquids.
  • Highest density among the three states in most cases.
1
Example
  • Stone
  • Iron
  • Wood
  • Ice
  • Glass
🤔 Did You Know?
Why Are Solids Rigid?
The particles in solids cannot move freely because of strong intermolecular forces. They can only vibrate about their mean positions.
📘 Definition

Liquid State

📖 Introduction
Liquids have intermediate properties between solids and gases.
📘 Definition
A liquid is a state of matter having fixed volume but no fixed shape.
🔷 Characteristics
  • Definite volume.
  • No definite shape.
  • Take the shape of their container.
  • Can flow easily.
  • Particles can slide past one another.
  • Compressibility is very small.
  • Less force of attraction than solids.
2
Example
  • Water
  • Milk
  • Petrol
  • Kerosene
  • Alcohol
🤔 Did You Know?

Why Are Liquids Called Fluids?

Liquids can flow and change shape continuously. Therefore, liquids are classified as fluids.

📘 Definition

Diffusion in Liquids

Diffusion is the spontaneous mixing of particles of two substances.

Solids, liquids and gases can diffuse into liquids.

3
Example
Examples
  • Ink spreads in water.
  • Sugar dissolves in tea.
  • Oxygen dissolves in water for aquatic animals.
  • Carbon dioxide dissolves in water for aquatic plants.

Dissolved oxygen is essential for respiration of fishes and other aquatic organisms.

🗒️ Defintion

Gas State

🗺️ Overview
Gases possess maximum kinetic energy and minimum force of attraction.
📘 Definition
A gas is a state of matter having neither definite shape nor definite volume.
🔷 Characteristics
  • No fixed shape.
  • No fixed volume.
  • Highly compressible.
  • Very large intermolecular spaces.
  • Lowest density among ordinary states.
  • Diffuse rapidly.
  • Fill the entire container.
4
Example
  • Oxygen
  • Nitrogen
  • Hydrogen
  • Carbon dioxide
  • Water vapour
🛠️ Applications

Practical Applications of Gas Compressibility

  • LPG cylinders
  • CNG fuel tanks
  • Oxygen cylinders in hospitals
  • Scuba diving cylinders
📘 Definition

Pressure Exerted by Gases

Gas particles move randomly in all directions and continuously strike the walls of the container.

The force exerted per unit area by gas particles on the walls of a container is called gas pressure.

\[ Pressure=\frac{Force}{Area} \]

SI Unit of Pressure

Pascal (Pa)

📘 Definition

Plasma: The Fourth State of Matter

📘 Definition

Plasma is an ionized gas containing free electrons and positive ions.

It is formed when very high energy is supplied to a gas.

🔷 Characteristics

Characteristics of Plasma

  • Contains charged particles.
  • Conducts electricity.
  • Responds to magnetic fields.
  • Exists at extremely high temperatures.
5
Example
  • Sun
  • Stars
  • Lightning
  • Neon sign boards
  • Fluorescent lamps
📊 Comparison Table
Comparison of Solids, Liquids and Gases
Property Solid Liquid Gas
Shape Fixed No fixed shape No fixed shape
Volume Fixed Fixed Not fixed
Compressibility Negligible Very small Very high
Diffusion Slowest Moderate Fastest
Force of Attraction Strongest Moderate Weakest
⚡ Exam Tip
❌ Common Mistakes
  • Writing that liquids have no volume.
  • Assuming solids do not diffuse at all.
  • Confusing gas pressure with atmospheric pressure.
  • Writing plasma as a liquid.
  • Ignoring particle theory in descriptive answers.
🧪

Interconversion of States of Matter

📋
Table of Contents
13 sections
🗺️ Overview

Matter can change from one state to another by changing temperature or pressure. Such transformations are called interconversion of states of matter.

When heat is supplied, particles gain kinetic energy and move farther apart. When heat is removed, particles lose kinetic energy and come closer together. This causes a substance to change from solid to liquid, liquid to gas, or vice versa.

📌 How Does a Change of State Occur?
🗒️ Flow Diagram of Interconversion of States
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 42 43 44 45 46 47 Solid 48 49 50 51 Liquid 52 53 54 55 Gas 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 Melting (Fusion) 73 Freezing 74 75 76 Vaporisation 77 Condensation 78 79

The diagram illustrates the conversion between different physical states of matter.

📘 Definition

Melting Point

📘 Definition

Fusion

✏️ Example
Melting of Ice

Ice is the solid form of water. When heat is supplied to ice, its particles absorb energy.

At 0°C and atmospheric pressure, ice begins to melt and changes into liquid water.

Therefore, the melting point of ice is:

\[ \boxed{0^\circ C} \]

🤔 Did You Know?
What Happens to Particles During Melting?
  1. Particles absorb heat energy.
  2. Their kinetic energy increases.
  3. Intermolecular forces weaken.
  4. Particles move away from fixed positions.
  5. The rigid structure breaks down.
  6. The substance becomes a liquid.
🎨 SVG Diagram
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 Solid 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 Heat Energy 73 74 75 76 77 78 Liquid 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95
🤔 Did You Know?
Why Does Temperature Remain Constant During Melting?

A surprising observation is that the temperature of ice remains constant at 0°C while it is melting, even though heat is being supplied.

The supplied heat is not used to increase temperature. Instead, it is used to overcome the intermolecular forces of attraction.

This hidden heat energy is called latent heat.

The term "latent" means hidden.

📘 Definition

Latent Heat of Fusion

✏️ Example
Solved Example
Why does ice at 0°C feel colder than water at 0°C?
Latent heat of fusion.
Ice at 0°C absorbs additional latent heat from the surroundings in order to melt. Water at 0°C does not require this extra heat. Therefore, ice removes more heat from our body and feels colder
🛠️ Applications
  • Ice cream melts when left outside a freezer.
  • Wax melts while making candles.
  • Metals are melted during casting and moulding processes.
  • Glaciers melt due to rising temperatures.
  • Chocolate melts in warm weather.
📋 Case Study
CBSE Case Study Based Question

Riya places an ice cube in a glass. The temperature of the ice remains at 0°C for several minutes although heat is continuously absorbed from the surroundings.

Question 1: What process is taking place?

Answer: Melting (Fusion).

Question 2: Why does the temperature remain constant?

Answer: The absorbed heat is used to overcome intermolecular forces rather than increase temperature.

Question 3: What is this hidden heat called?

Answer: Latent heat of fusion.

❌ Common Mistakes
  • Confusing melting point with boiling point.
  • Writing that temperature increases continuously during melting.
  • Ignoring atmospheric pressure in the definition of melting point.
  • Confusing latent heat with temperature.
  • Using fusion and vaporisation interchangeably.
⚡ Exam Tip
🧪

Latent Heat of Fusion

📋
Table of Contents
12 sections
📘 Definition
🤔 Did You Know?
Why is it Called "Latent" Heat?

During melting, heat is continuously supplied to the solid. However, the temperature of the substance does not increase.

The supplied heat becomes hidden inside the substance and is used to overcome the intermolecular forces of attraction between particles. This hidden heat is called latent heat.

Since the heat is not reflected as a rise in temperature, it is known as latent (hidden) heat.

📌 Particle-Level Explanation
🎨 Melting at the Particle Level
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 SOLID 25 LIQUID 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 Heat Energy 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88

Heat supplied to the solid is used to overcome intermolecular forces, allowing particles to move more freely and form a liquid.

🤔 Did You Know?
Why Does Temperature Remain Constant During Melting?

This is one of the most important conceptual questions in CBSE examinations.

When a solid reaches its melting point, any additional heat supplied is used to break the attractive forces between particles.

Since the supplied energy is not used to increase particle speed, the temperature remains constant until the entire solid melts.

Thus, during melting:

  • Heat is absorbed continuously.
  • Temperature remains unchanged.
  • Potential energy increases.
  • Intermolecular attraction decreases.
🔢 Formula
✏️ Example
Melting of Ice

Ice starts melting at 0°C under normal atmospheric pressure.

Even though heat is continuously supplied, the temperature remains at 0°C until all the ice converts into water.

This absorbed heat is the latent heat of fusion of ice.

The latent heat of fusion of ice is approximately:

\[ 3.34 \times 10^5 \, J\,kg^{-1} \]

🗒️ Importance of Latent Heat of Fusion
  • Explains why ice cools drinks effectively.
  • Used in refrigeration systems.
  • Helps regulate Earth's climate through melting glaciers.
  • Important in metal casting and industrial manufacturing.
  • Forms the basis of many cooling technologies.
✏️ Example
Higher Order Thinking Skill (HOTS)
Why does ice at 0°C cool a drink more effectively than water at 0°C?
  1. 1
    Compare ice and water at the same temperature.
  2. 2
    Think about latent heat.
  3. 3
    Determine which absorbs more heat.

Ice at 0°C must absorb latent heat of fusion to melt. Therefore it absorbs additional heat from the drink, causing greater cooling than water at 0°C.

📋 Case Study

A student places ice cubes in a beaker and starts heating them. The temperature remains at 0°C for several minutes although heat is continuously supplied.

Question 1: Which process is taking place?

Answer: Fusion (melting).

Question 2: Why does temperature remain constant?

Answer: The supplied heat is used to overcome intermolecular forces.

Question 3: What is this absorbed heat called?

Answer: Latent heat of fusion.

❌ Common Mistakes
  • Writing that temperature increases continuously during melting.
  • Confusing latent heat with melting point.
  • Writing latent heat as a temperature.
  • Ignoring the phrase "without change in temperature" in the definition.
  • Using boiling and melting interchangeably.
⚡ Exam Tip
⚡ Quick Revision
  • Fusion means melting.
  • Latent means hidden.
  • Temperature remains constant during melting.
  • Heat is used to overcome intermolecular forces.
  • \(Q = mL_f\)
  • SI unit: \(J\,kg^{-1}\)
  • Ice at 0°C is a better coolant than water at 0°C.
🧪

Boiling Point

📋
Table of Contents
15 sections
🗺️ Overview
When a liquid is heated continuously, its particles gain kinetic energy and move faster. At a certain temperature, the particles acquire sufficient energy to overcome the intermolecular forces of attraction and escape into the gaseous state. This temperature is called the boiling point.
📘 Definition
🤔 Did You Know?
What Happens During Boiling?

As heat is supplied to a liquid:

  1. Particles absorb heat energy.
  2. Their kinetic energy increases.
  3. Particles move more rapidly.
  4. Intermolecular attraction becomes weaker.
  5. Vapour bubbles form throughout the liquid.
  6. The liquid changes into gas.

Unlike evaporation, boiling occurs throughout the liquid and not merely at its surface.

❓ Question
Why is Boiling Called a Bulk Phenomenon?

A phenomenon occurring throughout the entire mass of a substance is called a bulk phenomenon.

During boiling, particles present in every part of the liquid gain sufficient energy to convert into vapour.

Therefore, boiling takes place throughout the liquid and not only at the surface.

Examples
  • Boiling water in a kettle.
  • Milk boiling in a vessel.
  • Steam generation in industrial boilers.
🎨 Particle-Level View of Boiling
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 LIQUID (Stable) 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 Heat Energy Added 59 60 61 62 63 BOILING (Excited) 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 Vaporization occurring at surface & bottom 106 107

During boiling, particles absorb energy and overcome intermolecular forces, converting the liquid into vapour.

📌 Boiling Point of Water
🤔 Did You Know?
Why Does Temperature Remain Constant During Boiling?

Even though heat is continuously supplied to a boiling liquid, its temperature remains constant until the entire liquid converts into vapour.

The supplied heat is used to overcome intermolecular forces rather than increase temperature.

This energy is known as latent heat of vaporisation.

📘 Definition

Latent Heat of Vaporisation

🔢 Formula
📌 Particle Explanation of Latent Heat of Vaporisation
📊 Difference Between Boiling and Evaporation
Boiling Evaporation
Occurs at a fixed temperature. Occurs at all temperatures.
Bulk phenomenon. Surface phenomenon.
Rapid process. Slow process.
Bubbles are formed. No bubbles are formed.
Requires continuous heating. Can occur without heating.
🛠️ Application
Applications of Boiling
  • Purification of drinking water.
  • Cooking food.
  • Steam generation in power plants.
  • Sterilisation of medical instruments.
  • Industrial chemical processing.
🗒️ Solved Conceptual Example
Solved Conceptual Example
Why does water remain at 100°C while it is boiling?
  1. 1
    Think about latent heat.
  2. 2
    Consider what happens to supplied heat.
  3. 3
    Identify the role of intermolecular forces.
The supplied heat is used to overcome intermolecular forces and convert water into steam. Therefore, temperature remains constant during boiling.
📋 Case Study
CBSE Case Study Question

A student heats water in a beaker. The thermometer shows 100°C and remains constant for several minutes although heating continues.

Question 1: Which process is occurring?

Answer: Boiling or vaporisation.

Question 2: Why does the temperature remain unchanged?

Answer: Heat is used as latent heat of vaporisation.

Question 3: Is boiling a surface or bulk phenomenon?

Answer: Bulk phenomenon.

❌ Common Mistakes
  • Confusing boiling with evaporation.
  • Writing that boiling occurs only at the surface.
  • Ignoring atmospheric pressure in the definition.
  • Assuming temperature rises continuously during boiling.
  • Confusing latent heat of fusion with latent heat of vaporisation.
⚡ Exam Tip
⚡ Quick Revision
  • Boiling point of water = 100°C.
  • Boiling is a bulk phenomenon.
  • Boiling occurs throughout the liquid.
  • Temperature remains constant during boiling.
  • Latent heat of vaporisation is absorbed during boiling.
  • \(Q=mL_v\)
  • Boiling and evaporation are different processes.
🧪

Latent Heat of Vaporization

📋
Table of Contents
14 sections
🗺️ Overview
When a liquid reaches its boiling point, further heating does not immediately increase its temperature. Instead, the supplied heat is used to convert the liquid into vapour. This hidden heat energy is known as the latent heat of vaporization.
📘 Definition
📌 Meaning of the Term
🤔 Did You Know?
Why Does Temperature Remain Constant During Vaporization?

At the boiling point, particles already possess sufficient kinetic energy. The additional heat supplied is not used to increase temperature.

Instead, this energy is used to overcome the intermolecular forces of attraction between liquid particles.

As a result:

  • Heat is continuously absorbed.
  • Temperature remains constant.
  • Liquid gradually converts into vapour.
📌 Particle-Level Explanation
🎨 SVG Diagram
Conversion of Liquid into Vapour
2 Phase Change: Liquid to Vapour 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 47 48 49 50 51 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 LIQUID 90 VAPOUR 91 92 93 94 95 96

During vaporization, liquid particles absorb energy and move far apart, forming vapour.

🤔 Did You Know?
Why Does Steam Cause More Severe Burns Than Boiling Water?

This is one of the most frequently asked conceptual questions in CBSE examinations.

Water vapour (steam) at 100°C contains not only the heat corresponding to its temperature but also the latent heat of vaporization absorbed during boiling.

When steam comes in contact with skin, it condenses into water and releases this latent heat.

Therefore, steam transfers more heat energy than boiling water at the same temperature and causes more severe burns.

Important Conclusion

Steam at 100°C contains more energy than water at 100°C.

🔢 Formula
✏️ Example
Boiling Water

When water is heated to 100°C, it starts boiling.

Further heating does not increase the temperature immediately. Instead, the heat supplied converts liquid water into steam.

This absorbed heat is the latent heat of vaporization of water.

🛠️ Application
Applications of Latent Heat of Vaporization
  • Steam cooking.
  • Steam sterilization in hospitals.
  • Power generation in thermal power plants.
  • Industrial boilers.
  • Steam irons.
  • Distillation processes.
✏️ Example
Solved Conceptual Example
Which contains more energy: water at 100°C or steam at 100°C?
Latent heat of vaporization.
Steam at 100°C contains additional latent heat of vaporization. Therefore, steam possesses more energy than water at the same temperature.
📋 Case Study
CBSE Case Study Question

A laboratory worker accidentally touches boiling water and steam, both at 100°C. The burn caused by steam is more severe.

Question 1: Why is the burn caused by steam more severe?

Answer: Steam contains latent heat of vaporization and releases additional heat during condensation.

Question 2: Which contains more energy?

Answer: Steam at 100°C.

Question 3: What form of energy makes the difference?

Answer: Latent heat of vaporization.

❌ Common Mistakes
  • Confusing latent heat of fusion with latent heat of vaporization.
  • Assuming steam and water at 100°C contain equal energy.
  • Writing that temperature increases during vaporization.
  • Ignoring the phrase "without change in temperature" in the definition.
  • Confusing boiling with evaporation.
⚡ Exam Tip
⚡ Quick Revision
  • Latent heat means hidden heat.
  • Vaporization is the conversion of liquid into vapour.
  • Temperature remains constant during boiling.
  • Heat is used to overcome intermolecular forces.
  • Steam at 100°C contains more energy than water at 100°C.
  • \(Q=mL_v\)
  • Steam causes more severe burns than boiling water.
🧪

Sublimation and Deposition

📋
Table of Contents
16 sections
🗺️ Overview

Most substances change from solid to liquid and then from liquid to gas upon heating. However, certain substances directly change from solid state to gaseous state without passing through the liquid state. This special process is called sublimation.

The reverse process, in which a gas directly changes into a solid without becoming a liquid, is called deposition.

📘 Definition

Sublimation

📘 Definition
Deposition
🎨 SVG Diagram
Interconversion Between Solid and Gas
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 30 31 32 33 34 35 36 37 38 39 41 Solid 43 44 45 46 47 49 Gas 51 52 53 54 56 Sublimation 58 59 60 62 Deposition 64

Sublimation converts a solid directly into a gas, whereas deposition converts a gas directly into a solid.

📌 Particle-Level Explanation of Sublimation
🎨 Particle View of Sublimation
2 Sublimation Particle View - High Compatibility 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 SOLID LATTICE (Vibrating) 111 SUBLIMATION (Direct to Gas) 112

The particles directly move from an ordered arrangement in the solid state to a widely separated gaseous state.

📌 Substances That Undergo Sublimation
✏️ Example
Dry Ice: An Important Example

Solid carbon dioxide (\(CO_2\)) is known as dry ice.

Unlike ordinary ice, dry ice does not melt into a liquid under normal atmospheric conditions.

It directly changes into carbon dioxide gas through sublimation.

Because no liquid is formed, it is called "dry" ice.

🛠️ Application
Applications of Sublimation
  • Purification of substances such as iodine and camphor.
  • Preservation and transportation using dry ice.
  • Air fresheners and deodorizing products.
  • Moth repellents made from naphthalene balls.
  • Freeze-drying of food and medicines.
📊 Difference Between Sublimation and Melting
Sublimation Melting
Solid changes directly into gas. Solid changes into liquid.
No liquid phase is formed. Liquid phase is formed.
Occurs in specific substances. Occurs in most solids.
Example: Camphor. Example: Ice.
📊 Difference Between Sublimation and Deposition
Sublimation Deposition
Solid → Gas Gas → Solid
Heat is absorbed. Heat is released.
Particles move farther apart. Particles come closer together.
Endothermic process. Exothermic process.
✏️ Example
Examples of Deposition in Daily Life
  • Formation of frost on cold surfaces during winter.
  • Snowflake formation in clouds.
  • Deposition of water vapour on freezer walls.
  • Ice crystal formation in extremely cold regions.
📌 Note
NCERT Laboratory Activity
✏️ Example
Higher Order Thinking Skill (HOTS)<
Why do naphthalene balls gradually become smaller when left in a cupboard?
  1. 1
    Identify the state of naphthalene.
  2. 2
    Recall the process of sublimation.
  3. 3
    Determine what happens to the particles.
Naphthalene undergoes sublimation. Its particles directly escape into the air as vapour, causing the size of the balls to decrease gradually.
📋 Case Study
CBSE Case Study Question

A student notices that camphor kept in an open container gradually disappears without forming a liquid.

Question 1: Which process is taking place?

Answer: Sublimation.

Question 2: Does camphor pass through the liquid state?

Answer: No, it changes directly from solid to gas.

Question 3: Name another substance showing similar behaviour.

Answer: Ammonium chloride, iodine, naphthalene or dry ice.

❌ Common Mistakes
  • Confusing sublimation with melting.
  • Assuming all solids undergo sublimation.
  • Writing that deposition passes through the liquid state.
  • Confusing evaporation with sublimation.
  • Forgetting that deposition is the reverse of sublimation.
⚡ Exam Tip
⚡ Quick Revision
  • Sublimation: Solid → Gas.
  • Deposition: Gas → Solid.
  • Dry ice is solid carbon dioxide.
  • Camphor and naphthalene undergo sublimation.
  • Deposition is the reverse of sublimation.
  • Frost formation is an example of deposition.
🧪

Evaporation

📋
Table of Contents
15 sections
🗺️ Overview

We often observe that wet clothes dry on a clothesline, perfume spreads in a room, and water stored in an open vessel gradually decreases. These phenomena occur due to evaporation.

📘 Definition
📌 Particle-Level Explanation
🎨 SVG Diagram
How Evaporation Occurs
MOLECULAR DYNAMICS: EVAPORATION & DIFFUSION 1. Thermal Energy (Heat) Increases 2. Intermolecular Bonds Break 3. Escape & Gas Diffusion
🗒️ Characteristics of Evaporation
  • Occurs at all temperatures.
  • Takes place only at the surface of a liquid.
  • Slow process compared to boiling.
  • No bubbles are formed.
  • Causes cooling of the surroundings.
  • Does not require the liquid to reach its boiling point.
📊 Difference Between Evaporation and Boiling
Evaporation Boiling
Occurs at all temperatures. Occurs only at a fixed temperature.
Surface phenomenon. Bulk phenomenon.
Slow process. Rapid process.
No bubbles are formed. Bubbles are formed.
Causes cooling. Requires continuous heating.
🔎 Factors Affecting Evaporation
👁️ Evaporation Causes Cooling
🤔 Did You Know?
Why Does Evaporation Cause Cooling?
  1. Fast-moving particles escape first.
  2. These particles carry away energy.
  3. The average kinetic energy of the remaining particles decreases.
  4. The liquid becomes cooler.
  5. Energy is absorbed from the surroundings.
  6. The surroundings also become cooler.
🎨 SVG Diagram
Cooling Effect of Evaporation
EVAPORATIVE COOLING EFFECT High-energy particles escape, leaving lower-energy (cooler) particles behind. TEMP DROPS
🛠️ Daily Life Applications of Evaporative Cooling
  • Sweating: Sweat evaporates from the skin and removes heat from the body.
  • Earthen Pot (Matka): Water slowly seeps through tiny pores and evaporates, cooling the water.
  • Desert Cooler: Water evaporates from cooling pads and lowers air temperature.
  • Acetone or Perfume: Evaporates rapidly and produces a cooling sensation on the skin.
  • Wet Clothes: Dry because water continuously evaporates.
💡 Concept
Important NCERT Concepts
✏️ Higher Order Thinking Skill (HOTS)

Question: Why are we advised to wear cotton clothes during summer?

Roadmap:

  • Consider sweating.
  • Think about evaporation.
  • Relate evaporation to cooling.

Answer:

Cotton absorbs sweat and exposes it to air. The sweat evaporates rapidly, taking heat from the body and producing a cooling effect.

📋 Case Study
CBSE Case Study Question

Rahul notices that wet clothes dry much faster on a sunny and windy day than on a humid day.

Question 1: Which process is responsible for drying clothes?

Answer: Evaporation.

Question 2: Which two factors increase evaporation in this situation?

Answer: High temperature and high wind speed.

Question 3: Why does humidity reduce evaporation?

Answer: Humid air already contains a large amount of water vapour.

❌ Common Mistakes
  • Confusing evaporation with boiling.
  • Writing that evaporation occurs only at the boiling point.
  • Ignoring the role of humidity.
  • Assuming evaporation heats the surroundings.
  • Writing that evaporation is a bulk phenomenon.
⚡ Exam Tip
⚡ Quick Revision
  • Evaporation occurs at all temperatures.
  • It is a surface phenomenon.
  • Higher temperature increases evaporation.
  • Larger surface area increases evaporation.
  • Higher humidity decreases evaporation.
  • Higher wind speed increases evaporation.
  • Evaporation causes cooling.
🧪

Important Points

📝 Summary
Important Points to remember
· Updated
NCERT Science · Class IX · Chapter 1

Matter in Our Surroundings

A complete AI-powered concept engine — covering all properties, state changes, definitions, formulas, and interactive exercises for this chapter.

Chemistry 5 Core Concepts Tricks & Tips Common Mistakes

Core Concepts

Five foundational ideas that build your complete understanding of Chapter 1. Click any concept to expand.

01 What is Matter? — Particle Nature

Matter is anything that has mass and occupies space (volume). It is made up of tiny particles called atoms or molecules.

Scientists established the particle nature of matter through careful experiments. The key observations were:

  • Particles of matter are extremely small — too small to be seen with the naked eye.
  • Particles of matter have spaces between them (inter-particle spaces).
  • Particles of matter are in constant random motion (kinetic energy).
  • Particles of matter attract each other through intermolecular forces of attraction.
  • Diffusion and osmosis are direct evidence of particle motion.

Memory Aid: Remember "SSMA" — Particles are Small, have Spaces, are in Motion, and have Attraction between them.

Classic experiment — diffusion of potassium permanganate (KMnO₄) in water:

KMnO₄ crystalpurple solid
Placed in waterparticles move
Colour spreadsdiffusion
Uniform purpleuniform mixture
02 States of Matter — Solid, Liquid, Gas

Matter exists in three physical states. The difference lies in the arrangement, distance, and energy of particles.

PropertySolidLiquidGas
ShapeDefiniteNo definite shapeNo definite shape
VolumeDefiniteDefiniteNo definite volume
Particle arrangementVery orderedLess orderedCompletely random
Inter-particle spaceMinimumMore than solidVery large
Inter-particle forceMaximumLess than solidNegligible
Kinetic energyMinimumMore than solidMaximum
CompressibilityNegligibleVery lowHighly compressible
FluidityCannot flowCan flowCan flow in all directions
DensityHighLower than solidVery low

Special Note: Gases are highly compressible — this is why LPG (cooking gas) is stored in cylinders under high pressure. Liquids are almost incompressible, which is why they are used in hydraulic brakes.

03 Change of State — Interconversions

States of matter are interconvertible by changing temperature or pressure. Each process has a specific name.

Solidordered
Melting ↑T
Liquidflowing
Freezing ↓T
Liquidflowing
Vaporisation ↑T
Gasfree
Condensation ↓T
Soliddry ice, camphor
Sublimation ↔ Deposition
Gasdirectly

Latent Heat of Fusion (Lf): The heat energy absorbed by 1 kg of a solid substance to change into liquid at its melting point, without any rise in temperature. For ice: 3.34 × 10⁵ J/kg

Latent Heat of Vaporisation (Lv): The heat energy absorbed by 1 kg of a liquid to change into gas at its boiling point, without any rise in temperature. For water: 22.6 × 10⁵ J/kg

Why does steam cause worse burns than boiling water? Steam at 100°C has extra latent heat of vaporisation (22.6 × 10⁵ J/kg) stored in it. When it condenses on skin, it releases this extra heat, causing more severe burns.

04 Evaporation — Factors and Cooling Effect

Evaporation is the process by which a liquid changes to vapour at any temperature below its boiling point. It is a surface phenomenon — only surface particles escape.

Factors that increase the rate of evaporation:

  • Increase in temperature: More particles gain enough kinetic energy to escape.
  • Increase in surface area: More particles exposed at the surface. (A wet shirt spread out dries faster.)
  • Decrease in humidity: Drier air absorbs more moisture. (Clothes dry faster on a dry day.)
  • Increase in wind speed: Vapour above the surface is carried away, maintaining a low vapour concentration.

Cooling Effect of Evaporation: During evaporation, particles with the highest kinetic energy escape, reducing the average kinetic energy of the remaining liquid, which lowers its temperature. This is why:
• We feel cool after sweating
• Earthen pots keep water cool (seepage + evaporation)
• Acetone on hand feels cold

Evaporation vs Boiling: Evaporation is a surface phenomenon occurring at any temperature. Boiling is a bulk phenomenon occurring at a fixed temperature (boiling point).

05 Effect of Pressure & Temperature on States

Both temperature and pressure are used to change the state of matter:

Effect of Temperature
  • Increasing temperature increases particle kinetic energy
  • Particles overcome intermolecular forces
  • Solid → Liquid at Melting Point
  • Liquid → Gas at Boiling Point
  • Boiling point of water = 100°C = 373 K
  • Melting point of ice = 0°C = 273 K
Effect of Pressure
  • Increasing pressure brings particles closer
  • Gas → Liquid (liquefaction) under high pressure
  • Liquid → Solid under high pressure
  • LPG: gas liquefied by high pressure in cylinders
  • Compressed Natural Gas (CNG) used as fuel
  • Dry ice (solid CO₂): CO₂ solidified at high pressure

Kelvin Scale: Temperature in Kelvin = Temperature in Celsius + 273
0°C = 273 K  |  100°C = 373 K  |  −273°C = 0 K (Absolute Zero)

Why –273°C is special: At absolute zero (0 K), particles theoretically have no kinetic energy and all motion stops. This is the lowest possible temperature in the universe.

Formula Capsules

All key expressions, conversions, and values for Chapter 1 — ready for quick revision.

Temperature Conversion
KELVIN ↔ CELSIUS
K = °C + 273
Celsius to Kelvin: add 273. Kelvin to Celsius: subtract 273.
FAHRENHEIT ↔ CELSIUS
°F = (9/5 × °C) + 32
Not in NCERT exam but useful for context. Not required for Class 9.
Temperature PointCelsius (°C)Kelvin (K)
Absolute Zero−2730 K
Melting Point of Ice0273 K
Boiling Point of Water100373 K
Normal Body Temperature37310 K
Latent Heat
HEAT ENERGY
Q = mL
Q = heat energy (J), m = mass (kg), L = latent heat (J/kg)
FUSION (ICE)
Lf(ice) = 3.34 × 10⁵ J/kg
Heat needed to melt 1 kg of ice at 0°C (no temperature change).
VAPORISATION
Lv(water) = 22.6 × 10⁵ J/kg
Heat needed to vaporise 1 kg of water at 100°C (no temperature change).

Exam Tip: In problems, if temperature is not changing during a state change, use Q = mL. If temperature is changing, use Q = mcΔT (specific heat capacity, usually given). They are always separate calculations.

Density
DENSITY
ρ = m / V
ρ = density (kg/m³), m = mass (kg), V = volume (m³). Solids have highest density; gases have lowest.
Key Values to Memorise
QuantityValueSignificance
Melting point of ice0°C / 273 KIce becomes water at standard pressure
Boiling point of water100°C / 373 KWater becomes steam at 1 atm
Melting point of wax~60–65°CExample of a low-melting solid
Boiling point of alcohol~78°CLower than water — example of volatile liquid
Sublimation point of dry ice−78°CCO₂ directly converts to gas
Latent heat of fusion (ice)3.34 × 10⁵ J/kgEnergy absorbed melting ice
Latent heat of vaporisation (water)22.6 × 10⁵ J/kgEnergy absorbed vaporising water

Step-by-Step AI Solver

Seven worked problems — each broken into clear logical steps with reasoned explanations.

Problem 01 · Temperature Conversion
Convert the following temperatures to Kelvin: (a) 25°C   (b) −40°C   (c) 150°C
1

Recall the formula: K = °C + 273. This formula comes from the fact that the Kelvin scale starts 273 units below 0°C (absolute zero).

2

Apply to (a) 25°C:
K = 25 + 273

K = 298 K
3

Apply to (b) −40°C:
K = −40 + 273

K = 233 K
4

Apply to (c) 150°C:
K = 150 + 273

K = 423 K
✓

Verify: All Kelvin values are positive (since −273°C is the minimum). 233 K, 298 K, 423 K — all correct.

Problem 02 · Latent Heat Calculation
How much heat energy is required to melt 5 kg of ice at 0°C? (Lf of ice = 3.34 × 10⁵ J/kg)
1

Identify given values:
Mass m = 5 kg  |  Latent heat of fusion Lf = 3.34 × 10⁵ J/kg  |  Temperature is not changing (state change at 0°C)

2

Select the correct formula: Since the temperature is NOT changing (it stays at 0°C during melting), we use the latent heat formula: Q = m × L

3

Substitute values:
Q = 5 × 3.34 × 10⁵

4

Calculate:
Q = 16.7 × 10⁵ = 1.67 × 10⁶ J

Q = 1.67 × 10⁶ J = 1670 kJ
✓

Interpretation: 1.67 million joules of heat must be supplied to 5 kg of ice at 0°C to convert it completely to water at 0°C — with NO rise in temperature during the process.

Problem 03 · Boiling Point Under Pressure
A pressure cooker is used to cook food faster. Explain why food cooks faster in a pressure cooker, with reference to boiling point.
1

Recall the principle: The boiling point of a liquid depends on the pressure acting on its surface. Higher external pressure → higher boiling point.

2

Inside a pressure cooker: The lid is sealed. As water heats, steam builds up inside but cannot escape. This increases the pressure inside the cooker above atmospheric pressure (1 atm).

3

Effect on boiling point: The increased pressure raises the boiling point of water — from 100°C to approximately 120–130°C. Water boils at a higher temperature inside.

4

Effect on cooking: Food is cooked in hotter water/steam (120–130°C instead of 100°C), so chemical reactions in cooking happen faster.

✓

Conclusion: Increased pressure → increased boiling point → food cooked at higher temperature → faster cooking. This is an application of the effect of pressure on the boiling point of liquids.

Problem 04 · Evaporation & Cooling
Why does sprinkling water on a hot floor on a summer day cool the surroundings? Explain using the concept of evaporation.
1

Identify the process: When water is sprinkled on a hot floor, evaporation takes place rapidly because the floor is hot (high temperature increases rate of evaporation).

2

Mechanism of evaporation cooling: The water particles at the surface gain kinetic energy. The most energetic particles escape first as vapour into the air.

3

Energy source: The escaping particles take energy from the surroundings (floor, air). This energy is the latent heat of vaporisation, absorbed from the environment.

4

Effect on average kinetic energy: After high-energy particles leave, the remaining liquid and the floor have a lower average kinetic energy, which means a lower temperature.

✓

Conclusion: Evaporation absorbs heat from surroundings → floor temperature drops → surroundings feel cooler. This is the cooling effect of evaporation.

Problem 05 · Sublimation Identification
A student places some camphor in an open dish and leaves it for a week. When she returns, the dish is empty. What process occurred? Would the same happen with wax? Explain.
1

Process with camphor: Camphor is a substance that undergoes sublimation — it converts directly from solid to vapour without passing through the liquid state. The camphor vapour escaped into the air, leaving the dish empty.

2

Why no liquid was seen: The camphor never became liquid. Sublimation means solid → gas directly. This is possible when the vapour pressure of a substance exceeds atmospheric pressure at a temperature below its melting point.

3

Would wax do the same? No. Wax does NOT sublimate under normal conditions. Wax melts to a liquid first (melting point ≈ 60–65°C), and only then slowly evaporates. It follows the normal solid → liquid → gas path.

✓

Key difference: Camphor, iodine, ammonium chloride, and dry ice (CO₂) are common examples of sublimation. Wax, ice, and most substances follow melting → evaporation sequence.

Problem 06 · Heat for Vaporisation
Calculate the heat needed to vaporise 2.5 kg of water already at 100°C. (Lv = 22.6 × 10⁵ J/kg)
1

Given: m = 2.5 kg, Lv = 22.6 × 10⁵ J/kg, water is already at boiling point (no heating needed).

2

Formula: Q = m × Lv (only latent heat applies since temperature is constant at 100°C).

3

Calculate:
Q = 2.5 × 22.6 × 10⁵
Q = 56.5 × 10⁵ J

Q = 5.65 × 10⁶ J = 5650 kJ
✓

Note: Compare this with melting 5 kg of ice (1.67 × 10⁶ J). Vaporising 2.5 kg of water requires over 3× the energy — confirming that Lv ≫ Lf.

Problem 07 · Reasoning Question
Ice at 0°C feels colder than water at 0°C. Why? Use the concept of latent heat of fusion to explain.
1

Observation: Both ice and water are at 0°C, so their temperatures are equal. Yet ice feels much colder when touched.

2

What ice does when it touches the hand: Ice at 0°C needs to melt before it can warm up. To melt, it absorbs its latent heat of fusion from the surroundings — including from your hand.

3

Energy comparison: Ice absorbs Lf = 3.34 × 10⁵ J/kg of extra heat from your hand just to melt. Water at 0°C does NOT need this extra energy — it simply absorbs heat to warm up.

4

Net result: Ice removes significantly more heat from your hand (both for melting AND for warming up), making it feel much colder.

✓

Conclusion: Ice at 0°C absorbs extra latent heat of fusion from the hand during melting. Water at 0°C does not. Hence, ice at 0°C feels colder than water at 0°C even though both are at the same temperature.

Concept-Building Questions

Original questions — not from the textbook — organised by concept with full step-by-step solutions.

Group A — Particle Nature of Matter
Easy

A drop of ink spreads throughout a glass of water within minutes even without stirring. Name the process responsible and explain what it tells us about the nature of particles of matter.

FULL SOLUTION
1

Process: The spreading of ink is called diffusion — the spontaneous mixing of particles from a region of higher concentration to lower concentration.

2

What it reveals: (a) Particles of matter are extremely small and invisible to naked eye. (b) Particles of matter are in constant random motion. (c) There are spaces between particles that allow other particles to move into them.

3

Conclusion: Diffusion is evidence that particles are always moving and have spaces between them.

Medium

If you dissolve 10 g of salt in 100 mL of water, the final volume is less than 110 mL. What does this tell you about the particle model of matter?

FULL SOLUTION
1

Observation: Volume after mixing is less than the sum of individual volumes (10 mL from salt + 100 mL water = less than 110 mL). Some volume seems to "disappear".

2

Explanation — Inter-particle spaces: Water particles have spaces between them. When salt dissolves, the small Na⁺ and Cl⁻ ions fit into these spaces. The salt particles occupy the free inter-particle spaces, so the total volume is less than expected.

3

Conclusion: This proves that particles of matter have inter-particle spaces between them and these spaces can be occupied by smaller particles of another substance.

Group B — States of Matter
Medium

A diver uses a compressed air cylinder. The cylinder contains air at very high pressure. Using the particle model, explain why so much air can be stored in a small cylinder.

FULL SOLUTION
1

Gas particle property: Gas particles have very large inter-particle spaces. Under normal conditions, a gas occupies much more volume than a liquid or solid of the same mass.

2

Effect of high pressure: Applying high pressure forces the gas particles closer together, reducing the inter-particle spaces dramatically. This compresses a large volume of air into a small cylinder.

3

Why gases are so compressible: Unlike solids and liquids (where particles are already close), gas particles have huge empty spaces between them. High pressure can reduce these spaces significantly.

✓

Conclusion: Gases are highly compressible because of large inter-particle spaces. High pressure brings particles together, allowing large amounts of gas to be stored in small containers.

Hard

Water at high altitudes (like mountain tops) boils below 100°C. Explain this using the concept of boiling point and pressure. What problem does this cause for mountaineers cooking food?

FULL SOLUTION
1

Why boiling point changes with pressure: Boiling occurs when the vapour pressure of the liquid equals the external (atmospheric) pressure. Liquid boils more easily when external pressure is lower.

2

At high altitude: Atmospheric pressure decreases with altitude (less air column above). At mountain tops, atmospheric pressure may be 0.5–0.7 atm instead of 1 atm.

3

Effect on water: With lower external pressure, water achieves its vapour pressure at a lower temperature. Water boils at ~70–85°C on mountain tops instead of 100°C.

4

Problem for mountaineers: Food cooked in boiling water at 70°C cannot reach the 100°C temperatures needed for proper cooking. Food remains undercooked even after long cooking times. Solution: Use pressure cookers.

✓

Principle: Boiling point is directly related to external pressure. Lower pressure → lower boiling point. Higher pressure → higher boiling point.

Group C — Evaporation & State Changes
Medium

Laundry dries faster on a windy, dry day than on a calm, humid day. Use the concept of evaporation to explain this fully.

FULL SOLUTION
1

Two factors involved: Wind speed and humidity. Both affect the rate of evaporation.

2

Effect of wind (windy day): Wind continuously replaces the humid air above the wet clothes with drier air. This maintains a low vapour concentration above the surface, allowing more water molecules to escape continuously. Higher wind speed → faster evaporation.

3

Effect of humidity (dry day): Low humidity means the surrounding air already contains very little water vapour. There is a large concentration gradient between the wet clothes and the air. More water molecules escape per second. High humidity → fewer molecules can escape (air is saturated).

✓

Conclusion: On a windy, dry day: both factors (low humidity + high wind) maximise evaporation rate → clothes dry quickly. On a calm, humid day: both factors oppose evaporation → clothes dry slowly or remain damp.

Hard

A nurse applies spirit (ethanol) to a patient's arm before an injection. The patient feels a cold sensation. Immediately after, the nurse blows gently on the area. The cold feeling intensifies. Explain both observations using particle theory.

FULL SOLUTION
1

Observation 1 — Cold feeling with spirit: Spirit (ethanol, boiling point 78°C) is more volatile than water. It evaporates rapidly at skin temperature (37°C). Evaporation absorbs latent heat of vaporisation from the skin, reducing skin temperature → cold sensation.

2

Observation 2 — Blowing intensifies cold: Blowing increases wind speed over the evaporating surface. This removes the vapour already accumulated above the spirit layer, maintaining a low vapour concentration. The concentration gradient between liquid spirit and air increases, driving faster evaporation.

3

Combined effect: Faster evaporation → more heat absorbed per second → greater temperature drop → more intense cold sensation.

✓

Factors applied: Cooling effect of evaporation (Obs 1) + increase in wind speed increases evaporation rate (Obs 2). Both follow directly from the particle nature of matter.

Exam Tips, Tricks & Common Mistakes

High-value exam strategies and the exact errors students make — with corrections.

Smart Tricks & Exam Tips

Trick 1 — Kelvin Conversion: "Kelvin = Celsius + 273." Never subtract 273 when going °C → K. The Kelvin scale was designed so 0 K (absolute zero) = −273°C, so you always ADD 273 going from Celsius to Kelvin.

Trick 2 — Latent Heat Memory: Lv (22.6 × 10⁵) is about 6.8× larger than Lf (3.34 × 10⁵). This means it takes nearly 7× more energy to vaporise water than to melt ice. "Vaporisation = Vastly more energy."

Trick 3 — SSMA for Particle Properties: Small, Spaces, Motion, Attraction — the four properties of particles of matter. Each one explains a real-world phenomenon.

Trick 4 — Evaporation Factors (THWS): Temperature ↑, Humidity ↓, Wind speed ↑, Surface area ↑ — all four increase the rate of evaporation. Learn them as THWS.

Trick 5 — State Change Direction: Adding energy: S→L→G (melting, vaporisation, sublimation). Removing energy: G→L→S (condensation, freezing, deposition). "Energy IN = going up the state ladder."

Trick 6 — Diffusion Speed: Gases diffuse faster than liquids because gas particles have greater kinetic energy and larger inter-particle spaces. NH₃ (lighter) diffuses faster than HCl (heavier) in the classic white ring experiment.

Common Mistakes & Corrections

Mistake 1: Saying "temperature rises while a substance is melting or boiling."
✓ Correction: During a state change (melting/boiling), temperature remains constant. All heat energy goes into breaking intermolecular bonds (latent heat), not raising temperature.

Mistake 2: Confusing "evaporation" with "boiling" and using them interchangeably.
✓ Correction: Evaporation = surface phenomenon, occurs at ANY temperature. Boiling = bulk phenomenon, occurs only at a FIXED boiling point. Boiling is much faster and involves the entire liquid.

Mistake 3: Writing K = °C − 273 (subtracting instead of adding).
✓ Correction: K = °C + 273. Kelvin temperatures are always HIGHER than Celsius. 0°C = 273 K (not −273 K).

Mistake 4: Saying gases cannot be liquefied or "gas always remains a gas."
✓ Correction: Gases can be liquefied by increasing pressure and/or decreasing temperature. LPG is a liquefied gas. CO₂ becomes dry ice when solidified.

Mistake 5: Saying "solids cannot flow" and then incorrectly saying glass or glaciers prove otherwise.
✓ Correction: Glass is actually a supercooled liquid (amorphous solid), not a crystalline solid. Glaciers flow due to immense pressure over very long timescales. For NCERT purposes, solids have definite shape and cannot flow.

Mistake 6: Thinking sublimation only applies to dry ice.
✓ Correction: Several common substances sublime: camphor, iodine, ammonium chloride (NH₄Cl), naphthalene (mothballs), and dry ice (CO₂). Learn all of them — any could appear in exams.

Mistake 7: Saying the cooling effect of evaporation is because "water is cold."
✓ Correction: Cooling effect is NOT because water itself is cold. It is because evaporation absorbs latent heat from the surrounding body/surface. Even warm liquids like ethanol cool the skin when they evaporate.

Interactive Learning Modules

Six different activity types to reinforce understanding through active recall and application.

Module 1 — MCQ Quiz (10 Questions)
Q 1 / 10
Score: 0
CORRECT OUT OF 10
Module 2 — True or False
Diffusion occurs only in gases, not in liquids.
Diffusion occurs in all states — gas, liquid, and even slow diffusion in solids. It is fastest in gases and slowest in solids.
The boiling point of water decreases when external pressure decreases.
Correct. At lower pressure (e.g. on mountains), water boils below 100°C because it needs less energy to match the reduced external pressure.
During melting, the temperature of ice rises continuously.
Temperature remains CONSTANT at 0°C during melting. Heat energy is used to break intermolecular bonds (latent heat), not to raise temperature.
Camphor undergoes sublimation under normal conditions.
Correct. Camphor converts directly from solid to vapour (sublimation) without forming a liquid. Other examples: iodine, naphthalene, NH₄Cl.
Solids are incompressible because their particles have maximum kinetic energy.
Solids are incompressible because particles are very close together with minimum inter-particle space — not because of kinetic energy. Solids actually have the MINIMUM kinetic energy.
Evaporation is a surface phenomenon that can occur at any temperature.
Correct. Evaporation occurs only at the surface and can happen at any temperature below the boiling point — unlike boiling which is a bulk phenomenon at a fixed temperature.
Latent heat of vaporisation is smaller than latent heat of fusion for water.
L_v (22.6 × 10⁵ J/kg) is much larger than L_f (3.34 × 10⁵ J/kg). It takes nearly 7 times more energy to vaporise water than to melt ice.
Increasing surface area increases the rate of evaporation.
Correct. Larger surface area exposes more particles at the surface, increasing the number of particles that can escape per unit time.
Module 3 — Fill in the Blanks

TYPE YOUR ANSWER AND PRESS ENTER OR CLICK CHECK

1. The conversion of a solid directly into gas without passing through the liquid state is called .
2. The temperature at which a solid changes to liquid at standard pressure is called its .
3. On the Kelvin scale, 0°C equals K.
4. The process by which particles of a liquid escape from the surface to form vapour at any temperature is called .
5. The state of matter with the maximum inter-particle space is .
6. Heat absorbed by 1 kg of water to vaporise at 100°C without temperature change is called .
Module 4 — Match the Process

Match each process with its correct description. Click a term, then click its match.

PROCESS (Column A)
Evaporation
Sublimation
Condensation
Latent Heat
Diffusion
DESCRIPTION (Column B)
Gas changes to liquid on cooling
Liquid → vapour below boiling point, surface only
Spontaneous mixing of particles, high → low concentration
Solid → Gas directly, no liquid stage
Heat absorbed/released during state change at constant temperature
Module 5 — States of Matter Explorer

Select a state to explore all its properties.

Shape
Definite, rigid shape
Volume
Definite volume
Particle arrangement
Highly ordered, closely packed
Inter-particle space
Minimum
Inter-particle force
Maximum (very strong)
Kinetic energy
Minimum (only vibration)
Compressibility
Negligible / incompressible
Fluidity
Cannot flow
Density
High
Examples
Ice, iron, wood, rock, salt
Shape
Takes shape of container
Volume
Definite volume
Particle arrangement
Less ordered, can slide past each other
Inter-particle space
More than solid
Inter-particle force
Less than solid, still significant
Kinetic energy
Moderate (translational + vibrational)
Compressibility
Very low (nearly incompressible)
Fluidity
Flows freely
Density
Lower than solid (usually)
Examples
Water, mercury, oil, alcohol, milk
Shape
No definite shape
Volume
No definite volume — fills container
Particle arrangement
Completely random, widely spread
Inter-particle space
Very large (1000× solid)
Inter-particle force
Negligible
Kinetic energy
Maximum (rapid random motion)
Compressibility
Highly compressible
Fluidity
Flows in all directions
Density
Very low
Examples
Oxygen, nitrogen, steam, LPG, air
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ACADEMIA AETERNUM तमसो मा ज्योतिर्गमय · Est. 2025
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Matter In Our Surroundings | Science Class 9 | Academia Aeternum
Matter In Our Surroundings | Science Class 9 | Academia Aeternum — Complete Notes & Solutions · academia-aeternum.com
This post is a collection of resources for students who are looking for solutions to the exercises of Chapter 1: Matter in Our Surroundings from the Class 9 Science textbook. This chapter is a foundational topic in chemistry. The resources include comprehensive solutions for various in-text and end-of-chapter questions, such as identifying what constitutes matter from a given list [3]. The solutions are presented with step-by-step explanations, making them a valuable tool for students who are…
🎓 Class 9 📐 Science 📖 NCERT ✅ Free Access 🏆 CBSE · JEE
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    Matter in Our Surroundings — Learning Resources

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    Frequently Asked Questions

    Matter is anything that has mass and occupies space.

    The three states of matter are solid, liquid, and gas.

    Particles of matter have space between them, move continuously, and attract each other.

    Gases can be compressed because their particles are far apart and can come closer on pressure.

    Diffusion is the mixing of particles of two substances due to particle motion.

    Solid state has definite shape and fixed volume.

    Gaseous state has neither fixed shape nor fixed volume.

    Liquids take the shape of the container they are kept in.

    Because gases have very large intermolecular spaces between particles.

    On heating, particles gain energy and move faster, increasing the distance between them.

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