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📚 Class 9 · Science · Chapter 1

Matter in Our Surroundings

Everything around you — air, water, stone, fire, even you — is matter. Unlock the invisible world of particles and discover how matter changes its form!

🎯 CBSE / NCERT / SEBA / AHSEC 📖 Class 9 ⚗️ Chemistry 🏆 Exam Ready

📋 Quick Navigation

📝 Summary 🧊 States of Matter 📊 Properties 🔄 Changes of State ⭐ Important Notes 💡 Highlights ❓ Q&A ✏️ Practice

📝 Chapter Summary

What is Matter? Matter is anything that has mass and occupies space. Everything visible and invisible — from a grain of sand to the air we breathe — is made of matter. Matter is composed of tiny particles called atoms and molecules that are constantly in motion.

🔬 Physical Nature of Matter

• Matter is made up of tiny particles; these particles are too small to be seen with the naked eye.
• Particles of matter have spaces between them — this explains why a liquid fills a container.
• Particles of matter are continuously moving (possess kinetic energy).
• Particles of matter attract each other — the force of attraction holds them together.
• Evidence of particle motion: Diffusion — mixing of ink in water, smell of perfume spreading across a room.

Particle Arrangement in Three States

Solid

Liquid

Gas

Blue = Solid  |  Teal = Liquid  |  Purple = Gas

Diffusion: The intermixing of particles of two different types of matter on their own is called diffusion. It occurs faster in gases, slower in liquids, and very slow in solids.

Brownian Motion: The random zig-zag movement of particles suspended in a liquid or gas. First observed by botanist Robert Brown in 1827.

🧊 The Three States of Matter

🪨

SOLID

Definite shape & volume. Tightly packed particles. Negligible compressibility. Very high density. Eg: Ice, Stone, Wood

💧

LIQUID

Definite volume, no fixed shape. Particles less tightly packed. Low compressibility. Eg: Water, Mercury, Milk

💨

GAS

No fixed shape or volume. Particles move freely. Highly compressible. Very low density. Eg: Oxygen, Air, Steam

📊 Comparison of Properties

Property	Solid	Liquid	Gas
Shape	Definite	Takes container shape	No fixed shape
Volume	Definite	Definite	No fixed volume
Compressibility	Negligible	Very low	High
Fluidity	Cannot flow	Can flow	Can flow
Density	High	Medium	Very Low
Inter-particle space	Very less	More than solid	Very large
Kinetic energy	Lowest	Medium	Highest
Inter-particle force	Strongest	Weaker than solid	Weakest

🌊 Plasma — The 4th State of Matter

Plasma is a superheated state of matter where electrons separate from nuclei, forming an ionised gas. Found in the Sun, stars, fluorescent lights, and lightning. It is the most abundant state of visible matter in the universe!

❄️ Bose-Einstein Condensate (BEC) — 5th State

Predicted by S. N. Bose and Albert Einstein in 1924–25. Formed at temperatures very close to absolute zero (−273.15 °C). Particles lose their individual identities and behave as a single quantum entity.

🔄 Changes of State of Matter

Matter can change from one state to another by changing temperature or pressure.

SOLID

Heating → Melting

↓

x

LIQUID

Heating → Vaporisation

↓

GAS

Reverse: Cooling → Condensation / Freezing / Deposition

Process	Transition	Heat Change
Melting (Fusion)	Solid → Liquid	Absorbed (Endothermic)
Solidification (Freezing)	Liquid → Solid	Released (Exothermic)
Vaporisation (Evaporation/Boiling)	Liquid → Gas	Absorbed
Condensation	Gas → Liquid	Released
Sublimation	Solid → Gas (directly)	Absorbed
Deposition	Gas → Solid (directly)	Released

🌡️ Key Definitions

Melting Point: The temperature at which a solid starts to melt (convert to liquid) at standard atmospheric pressure. Melting point of ice = 0°C (273 K).

Boiling Point: The temperature at which a liquid starts to boil (convert to gas) at standard atmospheric pressure. Boiling point of water = 100°C (373 K).

Latent Heat of Fusion: Amount of heat energy absorbed when 1 kg of a solid converts to liquid at its melting point without any change in temperature. For ice: 3.34 × 10⁵ J/kg.

Latent Heat of Vaporisation: Heat energy absorbed when 1 kg of liquid converts to gas at its boiling point. For water: 22.6 × 10⁵ J/kg.

Sublimation: Direct conversion of solid to gas without passing through the liquid state. Examples: Dry Ice (CO₂), Camphor, Iodine, Ammonium Chloride, Naphthalene.

Evaporation: A surface phenomenon where liquid converts to gas at temperatures below its boiling point.

Temperature in Kelvin = Temperature in Celsius + 273
K = °C + 273

💨 Factors Affecting Rate of Evaporation

• Surface area ↑ → Evaporation ↑ (More molecules at surface)
• Temperature ↑ → Evaporation ↑ (More kinetic energy)
• Humidity ↑ → Evaporation ↓ (Air already saturated)
• Wind speed ↑ → Evaporation ↑ (Vapour carried away)

🥶 Cooling by Evaporation: Evaporation causes cooling because the particles of liquid absorb energy from the surroundings (skin) to overcome inter-particle forces and escape. This is why sweating cools us, and why we feel cold after coming out of a pool!

⭐ Important Notes

• Matter is defined as anything that has mass and occupies space.
• Particles of matter are very small, constantly moving, have spaces between them, and attract each other.
• Solids have definite shape and volume; Liquids have definite volume but no definite shape; Gases have neither.
• SI unit of temperature: Kelvin (K). Kelvin = Celsius + 273.
• Melting point of ice = 273 K (0°C); Boiling point of water = 373 K (100°C).
• Latent heat is the heat absorbed or released during change of state at constant temperature.
• Substances that sublime: Dry ice, Camphor, Iodine, Naphthalene, Ammonium Chloride.
• Evaporation is a surface phenomenon; Boiling is a bulk phenomenon.
• Diffusion is fastest in gases and slowest in solids.
• Pressure can change the state of matter. Increasing pressure converts gas to liquid.
• Liquefied Petroleum Gas (LPG) is stored in cylinders under high pressure.
• Steam causes more severe burns than boiling water because it has extra latent heat.

Remember: 0°C = 273 K and 100°C = 373 K. To convert any Celsius to Kelvin, simply add 273. Absolute zero = 0 K = −273.15°C — the coldest possible temperature!

💡 Key Highlights

🔴 Must Remember

• K = °C + 273
• MP of ice = 273 K
• BP of water = 373 K
• Steam > boiling water in burns (latent heat)
• Evaporation = cooling effect

🔵 Exam Focus

• Difference: evaporation vs boiling
• Sublimation examples
• Factors affecting evaporation
• Latent heat concept
• Why gases are compressible?

🟢 Quick Facts

• Dry ice is solid CO₂
• Plasma = 4th state
• BEC = 5th state
• Kelvin is SI unit of temp
• Gases fill entire container

🟣 Did You Know?

The Sun is made of plasma, not hot gas! Plasma makes up more than 99.9% of the visible universe. The Northern Lights (Aurora) are also caused by plasma interactions!

🏆 Most Important Questions for Exam:

1. Why does a gas exert pressure on the walls of a container?
2. Why does steam cause more burns than boiling water?
3. Explain the cooling effect of evaporation with examples.
4. What is latent heat? How does it differ from specific heat?
5. Convert 0°C, 100°C, and −40°C to Kelvin.

❓ Questions & Answers

📌 Very Short Answer Questions (1 Mark)

Q1. What is matter?

Ans. Matter is anything that has mass and occupies space.

Q2. What is the SI unit of temperature?

Ans. Kelvin (K).

Q3. Name any two substances that sublime.

Ans. Dry ice (solid CO₂) and Camphor.

Q4. What is the boiling point of water in Kelvin?

Ans. 373 K (100°C).

Q5. What is Brownian motion?

Ans. The random zig-zag movement of tiny particles suspended in a liquid or gas due to unequal bombardment by the surrounding particles.

📌 Short Answer Questions (2–3 Marks)

Q6. Why does the smell of perfume spread far from where it is sprayed?

Ans. The particles of perfume (gas) are in constant motion and have large inter-particle spaces. They mix rapidly with air particles (diffusion) and spread through the room. Since gases diffuse fastest, the smell travels quickly even to far corners.

Q7. Why is ice at 0°C more effective in cooling than water at 0°C?

Ans. Ice at 0°C absorbs the latent heat of fusion (3.34 × 10⁵ J/kg) from the surroundings while melting, in addition to absorbing sensible heat to raise the temperature. Water at 0°C can only absorb sensible heat. Therefore, ice is more effective as a coolant.

Q8. Explain why solids are rigid and cannot flow.

Ans. In solids, the inter-particle forces are very strong and the particles are tightly packed with minimal space between them. The particles can only vibrate about their fixed positions but cannot move from one place to another. This gives solids a definite shape and makes them rigid.

Q9. Why does a gas completely fill the container it is stored in?

Ans. Gas particles have the highest kinetic energy and the weakest inter-particle forces. The large spaces between particles allow them to move freely in all directions, spreading out to fill the entire volume of any container.

📌 Long Answer Questions (5 Marks)

Q10. What is evaporation? List the factors affecting the rate of evaporation. How does evaporation cause cooling?

Ans.

Evaporation: Evaporation is the process by which a liquid changes to vapour (gas) at temperatures below its boiling point. It is a surface phenomenon — only particles at the surface with sufficient kinetic energy escape into the atmosphere.

Factors affecting rate of evaporation:

1. Surface area: Greater surface area → faster evaporation (more particles escape)
2. Temperature: Higher temperature → more kinetic energy → faster evaporation
3. Humidity: Lower humidity → faster evaporation (air can hold more vapour)
4. Wind speed: Higher wind speed → faster evaporation (vapour removed quickly)

Cooling by evaporation: During evaporation, the particles of liquid absorb energy from the surroundings (e.g., skin) to overcome inter-particle attraction and convert into vapour. This absorption of energy lowers the temperature of the surroundings, producing a cooling effect.
Example: We feel cool when we sweat, when we apply alcohol/acetone on skin, or when we come out of a swimming pool.

Q11. Explain the interconversion of states of matter with the role of temperature and pressure.

Ans.

States of matter can be interconverted by changing temperature and/or pressure.

Effect of Temperature:
— On heating a solid, its particles gain kinetic energy, vibrate faster, and eventually break free → Melting (Solid → Liquid).
— On further heating, liquid particles gain enough energy to overcome all attraction and escape as gas → Vaporisation (Liquid → Gas).
— Reverse happens on cooling: Condensation (Gas → Liquid) and Freezing (Liquid → Solid).

Effect of Pressure:
— Increasing pressure compresses gas particles closer together → can cause liquefaction of gases.
— Example: LPG (Liquefied Petroleum Gas) is stored in cylinders under high pressure in liquid state.
— Decreasing pressure can lower the boiling point of a liquid (used in pressure cookers in reverse logic).

Sublimation: Some solids directly convert to gas on heating without passing through liquid state. Eg: Dry ice, Iodine, Camphor, Naphthalene.

📌 Multiple Choice Questions (MCQs)

Q12. The temperature at which a solid melts to become a liquid at atmospheric pressure is called:

(a) Boiling point ✓ (b) Melting point (c) Freezing point (d) Sublimation point

Q13. Which of the following has the strongest inter-particle forces?

✓ (a) Solid (b) Liquid (c) Gas (d) Plasma

Q14. Convert 25°C to Kelvin.

(a) 248 K ✓ (b) 298 K (c) 273 K (d) 348 K

Q15. Which of the following is an example of sublimation?

(a) Melting of wax (b) Boiling of water ✓ (c) Conversion of dry ice into CO₂ gas (d) Freezing of water

📌 Assertion & Reason Questions

Instructions: (a) Both A and R are true, and R is the correct explanation of A. (b) Both A and R are true, but R is not the correct explanation of A. (c) A is true, R is false. (d) A is false, R is true.

Q16. Assertion (A): Gases are highly compressible. Reason (R): Particles in gases have very large inter-particle spaces.

Ans. (a) Both A and R are true, and R is the correct explanation of A. Because gas particles are far apart, applying pressure easily reduces the volume by pushing them closer together.

Q17. Assertion (A): Steam produces more severe burns than boiling water at 100°C. Reason (R): Steam contains extra energy in the form of latent heat of vaporisation.

Ans. (a) Both A and R are true, and R is the correct explanation of A. When steam (100°C) touches skin, it first releases 2260 J/g of latent heat during condensation to water, and then that water at 100°C cools further, releasing more heat.

📌 Higher Order Thinking (HOTS)

Q18. A diver can compress oxygen in a small cylinder. Is this possible for a solid piece of iron? Explain why.

Ans. No, a solid piece of iron cannot be compressed. In gases like oxygen, particles are far apart with very large inter-particle spaces, so applying pressure can push them closer — compression is possible. In solids like iron, particles are already very close together with negligible inter-particle space, so there is no room to compress them further. The strong inter-particle forces in solids resist compression.

Q19. Why does the rate of evaporation increase on a hot, windy day?

Ans. On a hot day, particles have higher kinetic energy and more surface particles can overcome inter-particle forces and escape. Wind continuously removes the vapour formed near the surface, preventing the air above from becoming saturated, thus allowing more liquid to evaporate. Both factors — higher temperature and wind — simultaneously increase the rate of evaporation.

✏️ Practice Activities

📝 Fill in the Blanks

1. Matter is defined as anything that has and occupies .
2. The SI unit of temperature is .
3. Melting point of ice is K.
4. The direct conversion of solid to gas is called .
5. The process of intermixing of particles is called .
6. Liquefied petroleum gas is stored under pressure.
7. Boiling point of water is °C.
8. The random motion of particles in liquid is motion.

Answers: mass / space; Kelvin; 273; sublimation; diffusion; high; 100; Brownian

✅ True or False

1. Solids can be easily compressed. [False]
2. Diffusion is fastest in gases. [True]
3. 0°C = 273 K. [True]
4. Evaporation is a bulk phenomenon. [False]
5. Camphor sublimes on heating. [True]
6. Gases have a definite volume. [False]
7. Steam at 100°C has more energy than water at 100°C. [True]
8. Latent heat changes the temperature of the substance. [False]

🔗 Match the Following

A. Solid CO₂	1. Boiling point of water
B. 373 K	2. Dry ice / Sublime
C. Evaporation	3. Surface phenomenon
D. Plasma	4. 4th state of matter
E. Brownian motion	5. Random zig-zag movement
F. Latent heat	6. Heat at constant temperature

Answers: A-2, B-1, C-3, D-4, E-5, F-6

⚡ Quick Quiz

1. Name the 5th state of matter.
2. What happens to boiling point when pressure is increased?
3. Which has higher density — solid or gas?
4. Give one example of a substance that shows Brownian motion.
5. Convert −23°C to Kelvin.
6. Why does a sponge compress when squeezed?
7. What type of phenomenon is boiling?
8. Who first observed Brownian motion?

Answers: BEC; Increases; Solid; Pollen grains; 250 K; Air pockets in sponge; Bulk; Robert Brown

📋 Revision Worksheet — One-Line Answers

#	Question	Answer
1	Define matter	Anything with mass and volume
2	State with definite shape & volume	Solid
3	K = ?	°C + 273
4	Boiling point of water	100°C / 373 K
5	What is sublimation?	Solid → Gas directly
6	Fastest diffusion in?	Gases
7	Evaporation occurs at surface or bulk?	Surface
8	Effect of humidity on evaporation	High humidity decreases evaporation
9	4th state of matter	Plasma
10	Why does sweating cool us?	Evaporation absorbs heat from skin

🗺️ Concept Map Summary

MATTER IN OUR SURROUNDINGS

Physical Nature

Particles · Spaces · Motion · Attraction

→

States

Solid · Liquid · Gas · Plasma · BEC

→

Changes of State

Melting · Boiling · Freezing · Sublimation

→

Evaporation

Surface · Cooling · Factors

📚 Matter in Our Surroundings — Class 9 Science
CBSE · NCERT · SEBA · AHSEC · All State Boards  |  Prepared for Class 9 Students

These notes are for educational purposes. Study well, think deeply, and stay curious! 🌟