Matter in Our Surroundings — Class 9 Physical Science

"Everything around you is matter. The air you breathe, the water you drink, the chair you sit on. But what IS matter — and why does it behave the way it does?"

1. About the Chapter

Matter is anything that has MASS and occupies SPACE (volume). This chapter explores the PARTICLE NATURE of matter and how it behaves in different states.

• Physical nature of matter — particles are tiny, in constant motion, with gaps between them
• Three states of matter — solid, liquid, gas — and their properties
• Interconversion of states — melting, boiling, condensation, freezing, sublimation
• Latent heat — the hidden energy behind phase changes
• Evaporation — a surface phenomenon with cooling effect
• Plasma and BEC — the fourth and fifth states of matter

Why This Chapter Matters

• It is the FOUNDATION of chemistry and physics — understanding matter is understanding the universe
• Everyday phenomena like sweating, drying of clothes, and cloud formation are EXPLAINED here
• AP Board frequently asks numerical questions on latent heat and evaporation

2. Physical Nature of Matter

Particles of Matter are TINY

'A single drop of water contains about 10²¹ molecules — that is 1 followed by 21 zeros. If you could count one molecule per second, it would take you TRillions of years to count them all.'

Particles of Matter are in CONSTANT MOTION

Evidence: Diffusion — the intermixing of particles on their own. A drop of ink spreads in water. The smell of food travels across the room. 'Diffusion is FASTER in gases (room-filling in seconds), slower in liquids (minutes), and EXTREMELY slow in solids (years).'

Particles have SPACE Between Them

Evidence: When you dissolve salt in water, the water level does NOT rise. The salt particles fit into the INTERMOLECULAR SPACES between water molecules.

3. States of Matter — Comparison

Examples in Daily Life

• Solid: Ice cube, stone, wood, steel
• Liquid: Water, milk, oil, mercury
• Gas: Oxygen, carbon dioxide, water vapour

'Solids are RIGID because particles cannot slide past each other. Liquids FLOW because particles can slide. Gases occupy ALL available space because particles move randomly at high speeds.'

4. Interconversion of States — Heating Curve

Interconversion means changing from one state to another by CHANGING TEMPERATURE or PRESSURE.

The Six Processes

1. Melting (Fusion): Solid → Liquid (add heat)
2. Freezing: Liquid → Solid (remove heat)
3. Boiling (Vaporisation): Liquid → Gas (add heat)
4. Condensation: Gas → Liquid (remove heat)
5. Sublimation: Solid → Gas DIRECTLY (skip liquid)
6. Deposition: Gas → Solid DIRECTLY (skip liquid)

The Heating Curve of Water

'When you heat ice at 0°C, the temperature stays at 0°C until ALL ice melts. This is the MELTING POINT. Then water heats up to 100°C. At 100°C, temperature STAYS CONSTANT until ALL water boils away. This is the BOILING POINT.'

Effect of Pressure

Increasing pressure DECREASES the melting point of ice (regelation). Increasing pressure INCREASES the boiling point of water (pressure cooker uses this).

5. Latent Heat — The Hidden Energy

Latent heat is the HEAT ENERGY absorbed or released during a phase change WITHOUT a change in temperature. 'This energy is used to BREAK (or form) intermolecular bonds — NOT to increase kinetic energy.'

Latent Heat of Fusion (L_f)

• Heat required to convert 1 kg of SOLID into LIQUID at the melting point
• For ice: L_f = 3.34 × 10⁵ J/kg (334 J/g)
• '334 J of energy is needed to melt just 1 g of ice at 0°C — this is a LOT of energy!'

Latent Heat of Vaporisation (L_v)

• Heat required to convert 1 kg of LIQUID into GAS at the boiling point
• For water: L_v = 22.6 × 10⁵ J/kg (2260 J/g)
• 'Boiling water requires about 7 TIMES more energy per gram than melting ice. This is why steam BURNS are far more dangerous than boiling water burns — steam has more latent heat to release on your skin.'

Worked Example — Latent Heat Numerical

'How much heat is required to convert 200 g of ice at 0°C into water at 0°C? (L_f = 334 J/g)'

Solution: Q = m × L_f = 200 × 334 = 66,800 J = 66.8 kJ.

'Now, how much heat to convert 200 g of water at 100°C into steam at 100°C? Q = m × L_v = 200 × 2260 = 452,000 J = 452 kJ. Notice the steam needs MUCH more energy.'

6. Evaporation

Evaporation is the process by which a LIQUID changes into GAS at ANY temperature BELOW its boiling point. It is a SURFACE PHENOMENON.

Evaporation vs Boiling

Factors Affecting Evaporation

1. Surface area: LARGER area → FASTER evaporation (clothes spread to dry faster)
2. Temperature: HIGHER temperature → FASTER evaporation (sunny day dries clothes quickly)
3. Humidity: LOWER humidity → FASTER evaporation (dry climate vs rainy season)
4. Wind speed: HIGHER wind → FASTER evaporation (fan speeds up drying)

Cooling Effect of Evaporation — Explained

'Energetic particles (with higher kinetic energy) escape from the surface. The PARTICLES LEFT BEHIND have lower average kinetic energy → temperature DECREASES → cooling.'

Everyday examples: Sweating cools the body. Water in an earthen pot (matka) stays cool because water seeps through pores and evaporates. 'Desert coolers work on this principle — hot air passes over water-soaked pads, evaporation cools the air.'

AP Context — Coastal Andhra

In coastal AP (Kakinada, Visakhapatnam), high humidity SLOWS evaporation. This is why sweat does not dry easily in summer near the coast. Inland areas (Anantapur, Kurnool) have lower humidity → faster evaporation.

7. Sublimation

Sublimation is the direct change of SOLID to GAS without passing through the liquid state.

Common Examples

• Camphor (Karpooram) — AP households use camphor in pooja; it SUBLIMES leaving no residue
• Naphthalene balls (mothballs) — used in cupboards; they shrink over time by sublimation
• Dry ice (solid CO₂) — sublimes at −78.5°C; used for stage fog and preserving frozen food
• Ammonium chloride: Sublimes on heating

Deposition (Reverse Sublimation)

Gas → Solid directly. Examples: Formation of FROST on cold surfaces. 'In winter, water vapour in air directly turns to ice crystals on grass — this is deposition, NOT freezing.'

8. Kelvin Scale — The Absolute Scale

'Kelvin is the SI unit of temperature. It has NO negative values — 0 K is ABSOLUTE ZERO, the lowest possible temperature where particles have ZERO kinetic energy.'

Conversion Formula

K = °C + 273 (exact value: K = °C + 273.15)

Key Temperatures

'When solving gas law problems in higher classes, ALWAYS convert Celsius to Kelvin. 0°C = 273 K, NOT 0 K! This is a VERY common mistake.'

9. Plasma and Bose-Einstein Condensate (BEC)

Plasma — The Fourth State

'Plasma is a SUPERHOT gas where atoms are stripped of electrons (ionised). It is the MOST COMMON state of matter in the UNIVERSE.'

• Stars (including the Sun) are made of plasma
• Neon signs, lightning, and fluorescent tubes contain plasma
• Temperature: 10⁴ to 10⁸ K

BEC — The Fifth State

'BEC is achieved by cooling a dilute gas of bosons to NEAR ABSOLUTE ZERO (a few billionths of a degree above 0 K). At this temperature, atoms lose their individual identity and behave as a SINGLE quantum entity.'

• Predicted by Bose and Einstein in 1924-25
• First created in a lab in 1995 (Cornell, Wieman, Ketterle — Nobel Prize 2001)
• Used in quantum computing research

10. Common Mistakes to Avoid

1. '0°C means no heat energy' — At 0°C, water and ice can coexist. The heat supplied goes into LATENT HEAT (breaking bonds), not raising temperature.
2. 'Steam is visible' — What you see above boiling water is NOT steam — steam is INVISIBLE. The visible white cloud is tiny water DROPLETS formed by condensation of steam in cool air.
3. 'Evaporation only occurs at boiling point' — WRONG. Evaporation occurs at ALL temperatures, even below 0°C (ice sublimes slowly).
4. 'Kelvin scale uses degrees' — Kelvin is written as K, NOT °K. We say 300 kelvin, NOT 300 degrees kelvin.
5. 'Gas particles are stationary' — Gas particles move randomly at VERY high speeds (≈500 m/s for oxygen at room temperature).

11. AP SSC Exam Focus

Key Tip

'In latent heat problems, first identify WHICH phase change is happening. Melting or freezing: use L_f. Boiling or condensation: use L_v. Then check the units — if L is given in J/g, mass must be in grams.'

12. Quick Self-Test

1. Why does the temperature stay constant at 0°C while ice melts? Answer: The heat energy is used to OVERCOME the intermolecular forces of attraction (latent heat of fusion) — not to increase kinetic energy. Temperature is proportional to average kinetic energy.

2. 'An earthen pot (matka) keeps water cool in summer. Explain using evaporation.' Answer: Water seeps through the porous walls of the matka and evaporates from the outer surface. Evaporation causes cooling, which keeps the water inside cool.

3. 'Convert 25°C to Kelvin. Convert 350 K to Celsius.' Answer: 25°C = 25 + 273 = 298 K. 350 K = 350 − 273 = 77°C.

4. 'Why does a desert cooler work better in summer than in the rainy season?' Answer: In summer, air humidity is LOW, so evaporation from the cooler pads is FAST. In the rainy season, humidity is HIGH, so evaporation is SLOW — the cooling effect is reduced.

5. 'What is the difference between boiling and evaporation?' Answer: Boiling occurs at a FIXED temperature (boiling point) throughout the liquid (bulk). Evaporation occurs at ANY temperature, only at the SURFACE. Boiling is rapid; evaporation is slow.

6. 'Give two examples each of sublimation and deposition from daily life.' Answer: Sublimation — camphor disappearing, naphthalene balls shrinking. Deposition — frost formation on grass in winter, formation of snow in clouds.