Chapter 1 of 12

CBSE Class 9 Science★ 6–8 marks · CBSE Class 9 board (Unit I — Matter, its Nature and Behaviour)

Matter in Our Surroundings

From the air you breathe to the ice in your glass — everything around you is matter. Class 9 Matter in Our Surroundings with the particle theory, the three classical states, the curious fourth and fifth states (plasma and Bose-Einstein condensate), latent heat, evaporation, and 25+ exam-style problems with full step-by-step solutions.

⏱ 35 min readEasy difficulty✓ Free · NCERT-aligned

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By the end of this chapter you'll be able to…

1State the five characteristics of particles of matter and justify each with one everyday observation
2Distinguish solid, liquid and gas by particle arrangement, motion, force and macroscopic properties
3Convert temperatures between Celsius and Kelvin scales
4Explain melting, freezing, vaporisation, condensation, sublimation and deposition with particle-level reasoning
5Define latent heat of fusion and vaporisation and use Q = mL in numerical problems
6Explain why evaporation causes cooling and list the four factors affecting evaporation rate
7Identify everyday examples of the fourth state (plasma) and fifth state (BEC) of matter
8Solve numericals on heat-energy required for phase changes

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Why this chapter matters

Every later chapter in chemistry — atomic structure, chemical reactions, gases laws, thermodynamics — rests on this idea: matter is made of tiny moving particles. Get the particle theory deep in your bones and the rest of school chemistry feels obvious.

Before you start — revise these

A 5-minute refresher here will save you 30 minutes of confusion below.

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Heat and temperature (Class 7)

Knowing how a thermometer works helps.

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States of matter (Class 6)

Foundational classification of solid/liquid/gas.

Matter in Our Surroundings — Class 9 (CBSE)

Look around. The chair, the air, your bones, the glass of water, even the dust on the screen — all of it is matter. This single chapter holds the most foundational idea in all of physical science: everything is made of tiny particles, and how those particles behave decides whether something is a solid, a liquid, a gas, or something stranger still.

1. The story — why we believe in particles we can't see

Ancient Indian philosopher Maharishi Kanada (≈ 600 BCE) and Greek philosopher Democritus (≈ 400 BCE) both proposed, independently, that all matter is made of tiny indivisible particles. They had no microscopes, no instruments — just careful thought. Cut a stone in half. Then in half again. Keep going. They reasoned: this can't continue forever. There must be a smallest possible piece.

They were right — but it took until the 19th century, with John Dalton's atomic theory, for science to firmly establish the idea. In your bottle of perfume left open across the room — the smell reaches your nose because particles of perfume are zipping through the air. You can't see a single perfume molecule. Yet trillions of them just landed on your nose receptors.

This chapter is about taking that simple "everything is made of particles" idea and using it to explain every visible behaviour of matter — why ice melts, why water boils, why steam is invisible until it cools, why solids hold shape but gases don't, why a cup of hot tea cools down, and how your body sweats to keep cool on a hot day.

2. The big picture — five things to take away

Matter is made of tiny particles with empty space between them.
The particles are in constant motion — they vibrate, slide, or fly around depending on the state.
Particles attract each other — strongly in solids, less in liquids, very weakly in gases.
Temperature changes the energy of these particles and thus changes the state.
Latent heat is the hidden energy needed to break particle bonds — it changes phase without changing temperature.

3. What is matter?

Matter is anything that has mass and occupies space.

The air around you is matter — you can feel it pushing your hand if you wave it. The water in your bottle is matter. The light from a bulb? Light is NOT matter — it has no rest mass and doesn't occupy space.

Five characteristics of particles of matter

Particles are very small — far too small to see with a normal microscope. A drop of water has more particles than there are stars in the Milky Way.
Particles have space between them — when sugar dissolves in water, it disappears because sugar particles fit into spaces between water particles.
Particles are continuously moving — they have kinetic energy. Hot tea cooling? That's energy leaving particles.
Particles attract each other — try breaking iron with your hand vs breaking a piece of chalk. Iron's particles attract much more strongly.
Particles intermix on their own — diffusion. Perfume reaches across a room not because someone fanned it but because particles diffuse.

4. The three classical states of matter

Property	Solid	Liquid	Gas
Shape	Fixed	Takes container	Takes container
Volume	Fixed	Fixed	Takes container
Particle gap	Very small	Slightly larger	Very large
Particle motion	Vibrate only	Slide past each other	Fly freely
Force between particles	Strongest	Moderate	Weakest
Compressibility	Negligible	Slight	High
Density	Highest	Medium	Lowest
Fluidity	None	High	Highest
Examples	Iron, ice, salt	Water, milk, mercury	Air, oxygen, steam

A trick to remember states

Solid → Shape & Space fixed.
Liquid → Shape changes, Space fixed.
Gas → Shape & Space both change.

Why solids are rigid

Particles in a solid are held in fixed positions by strong attractive forces. They can vibrate about their mean positions but cannot move past each other. This rigidity is why your desk doesn't flow even though it's made of zillions of moving particles.

Why liquids flow

Particles in a liquid have enough energy to slip past each other but not enough to escape the surface entirely. A liquid takes the shape of its container but holds onto its volume.

Why gases fill any container

Gas particles move so fast (≈ 500 m/s at room temperature) and have so much space between them that they spread out to fill any container completely. Gases are highly compressible — squeezing a balloon shows you can reduce a gas's volume dramatically because all that space between particles can be removed.

5. Change of state — the fourth thing you must memorise

The state of matter depends on (a) temperature and (b) pressure.

When you heat a solid, particles gain energy and start vibrating harder. Eventually they overcome the strong inter-particle attraction and start to slip past each other → melting (solid → liquid). Keep heating and they gain enough energy to escape the surface entirely → boiling/vaporisation (liquid → gas).

Cool the gas and the reverse happens: condensation (gas → liquid), then freezing (liquid → solid).

There's also a direct path that skips the middle: sublimation is solid → gas without going through liquid (dry ice, camphor, ammonium chloride). The reverse, gas → solid, is deposition (frost on a winter window).

The six phase-change names — memorise

From → To	Name	Energy direction
Solid → Liquid	Melting (fusion)	Absorbed
Liquid → Solid	Freezing (solidification)	Released
Liquid → Gas	Vaporisation (boiling/evaporation)	Absorbed
Gas → Liquid	Condensation	Released
Solid → Gas	Sublimation	Absorbed
Gas → Solid	Deposition	Released

Melting point and boiling point

The melting point is the fixed temperature at which a solid changes to liquid at atmospheric pressure. For ice: $0° C = 273.15 K$ .

The boiling point is the fixed temperature at which a liquid changes to gas at atmospheric pressure. For water: $100° C = 373.15 K$ .

Kelvin scale — convert with one formula

$T_{K} = T_{C} + 273.15$

Why use Kelvin? Because $0 K$ is absolute zero — the temperature at which all particle motion theoretically stops. There can be no negative temperature in Kelvin.

6. Latent heat — the hidden energy

Here's the puzzling experimental fact: when ice is melting, even though you keep adding heat, the temperature stays at $0° C$ until all the ice has melted. Where is the heat going?

The heat is being used to break the bonds between water particles — to overcome the attractive forces that held them in the rigid ice structure. This hidden, non-temperature-changing heat is called latent heat.

Latent heat of fusion ( $L_{f}$ ): heat needed to convert $1 kg$ of solid to liquid at its melting point. For ice: $L_{f} = 334 kJ/kg$ .
Latent heat of vaporisation ( $L_{v}$ ): heat needed to convert $1 kg$ of liquid to vapour at its boiling point. For water: $L_{v} = 2260 kJ/kg$ .

The formula:

$Q = m L$

where $Q$ is heat absorbed/released, $m$ is mass, and $L$ is the appropriate latent heat.

Why steam burns more than boiling water

Both are at $100° C$ when they touch your skin. But steam additionally carries the latent heat of vaporisation ( $2260 kJ/kg$ ) which it releases as it condenses on your skin. That's why a steam burn is much more severe than a hot-water burn at the same temperature.

7. Evaporation — the cousin of boiling

Evaporation is the conversion of liquid to gas at any temperature below the boiling point, occurring only at the surface.

Unlike boiling (which happens at a fixed temperature throughout the liquid), evaporation happens at any temperature — wet clothes dry on a cool morning, sweat evaporates from your skin in shade.

Factors affecting evaporation rate

Surface area ↑ → evaporation ↑ (clothes spread out dry faster).
Temperature ↑ → evaporation ↑ (warm day, faster drying).
Humidity ↓ → evaporation ↑ (a dry day, faster drying).
Wind speed ↑ → evaporation ↑ (wind sweeps away water vapour, more space for new vapour).

Why evaporation causes cooling

Particles with the highest kinetic energy escape the liquid surface during evaporation. The remaining particles have lower average kinetic energy — and average kinetic energy IS temperature. So the liquid cools.

This is why:

Sweat cools your body: sweat evaporates, taking heat from your skin.
Earthen pots ("matka") keep water cool: water seeps through tiny pores and evaporates from the outer surface, cooling the inside.
A wet handkerchief on your forehead feels cool on a hot day.
Acetone or spirit feels cold on the skin — they evaporate very quickly (low boiling point) and steal heat fast.

8. The fourth and fifth states — plasma and BEC

For most of the 19th century, scientists thought there were only three states: solid, liquid, gas. The 20th century added two more.

Plasma

At extremely high temperatures (typically above $10, 000 K$ ), gas particles lose their electrons and become a soup of charged ions and free electrons — this is plasma, the fourth state of matter. Plasma is electrically conductive and responds to magnetic fields.

You see plasma every day:

The sun and stars are giant balls of plasma.
A fluorescent tube or neon sign has plasma glowing inside.
Lightning is a brief flash of plasma in the atmosphere.
The aurora borealis (northern lights) is plasma high in Earth's atmosphere.

Plasma is by far the most abundant state of matter in the universe (≈ 99% by mass) — it's just that on Earth, conditions favour solids, liquids and gases.

Bose-Einstein condensate (BEC)

In 1924, Indian physicist Satyendra Nath Bose and Albert Einstein theoretically predicted a strange new state of matter at temperatures so low ( $\approx 1 0^{- 7} K$ , much colder than outer space) that atoms lose their individual identity and merge into one giant "super-atom" governed by quantum mechanics. This is called the Bose-Einstein condensate (BEC) — the fifth state of matter.

BEC was finally created in 1995 by Eric Cornell, Carl Wieman and Wolfgang Ketterle, who won the 2001 Nobel Prize. BEC particles behave like a single quantum wave — they can flow without friction (superfluidity), let light pass through them at $17 m/s$ (instead of the usual $3 \times 1 0^{8} m/s$ ), and exhibit other bizarre quantum phenomena.

9. Solidified summary — what you must memorise

Matter = anything with mass + volume.
Particles: small, spaced, moving, attracting, intermixing.
3 classical states (solid, liquid, gas) + 2 modern (plasma, BEC).
Temperature ↑ + pressure ↓ → favors gas. Temperature ↓ + pressure ↑ → favors solid.
Melting & boiling points are FIXED for a pure substance.
$T_{K} = T_{C} + 273.15$ . Absolute zero is $0 K = - 273.15° C$ .
Latent heat: $Q = m L$ . Fusion = $334 kJ/kg$ , vaporisation = $2260 kJ/kg$ for water.
Evaporation: surface-only liquid → gas, at any temp, causes cooling.
Plasma: super-hot ionised gas. BEC: ultra-cold quantum state predicted by Bose.

10. Closing thought

You started this chapter thinking matter was the boring obvious stuff around you. You're ending it knowing that:

The air in this room contains $\approx 1 0^{27}$ molecules in constant high-speed motion.
The water in your glass is the same molecules that fell as rain on a dinosaur 65 million years ago.
Inside your sun, hydrogen plasma is fusing into helium and pouring energy across the solar system.
And in a lab in Boulder, Colorado, physicists have created matter so cold that thousands of atoms behave like one big single atom.

Three pages ago you knew "matter is stuff". Now you understand stuff — and the leap from that to the rest of physics and chemistry is short.

Key formulas & results

Everything you need to memorise, in one card. Screenshot this for revision.

Celsius to Kelvin

T(K) = T(°C) + 273.15

Absolute zero = 0 K = −273.15 °C. No negative Kelvin.

Kelvin to Celsius

T(°C) = T(K) − 273.15

Latent heat formula

Q = m × L

Q in J, m in kg, L in J/kg. Use during phase changes only.

Latent heat of fusion (ice)

L_f = 334 kJ/kg = 3.34 × 10⁵ J/kg

Heat to melt 1 kg of ice at 0 °C.

Latent heat of vaporisation (water)

L_v = 2260 kJ/kg = 22.6 × 10⁵ J/kg

Heat to vaporise 1 kg of water at 100 °C.

Heat-temperature change

Q = m c ΔT

Used outside phase changes. c = specific heat capacity.

Density

ρ = m / V

SI unit kg/m³. Solids > liquids > gases (generally).

Atmospheric pressure

1 atm = 101.325 kPa = 76 cm Hg

Boiling point quoted at 1 atm by convention.

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Common mistakes & fixes

These are the exact errors that cost students marks in board exams. Read them once, save yourself the trouble.

WATCH OUT

✗ Saying 'particles of gas are weightless'

✓ Gas particles have mass — that's why air has measurable weight. They appear weightless because density is low, not because individual particles lack mass.

WATCH OUT

✗ Confusing evaporation with boiling

✓ Evaporation: surface-only, at any temp, slow. Boiling: throughout the liquid, only at boiling point, vigorous.

WATCH OUT

✗ Forgetting to convert °C to K in numerical problems

✓ Kelvin scale is mandatory for many physics formulas. Always check the question's units.

WATCH OUT

✗ Saying steam at 100 °C and boiling water at 100 °C cause the same burn

✓ Steam carries extra latent heat of vaporisation (~2260 kJ/kg) released on condensing → far worse burn.

WATCH OUT

✗ Treating sublimation as a slow evaporation

✓ Sublimation is solid directly to gas (no liquid step). Camphor, dry ice, naphthalene balls sublime.

WATCH OUT

✗ Saying a solid has 'no spaces between particles'

✓ Solid particles still have tiny spaces — that's why solids can expand on heating. They're packed tightly, not infinitely tightly.

WATCH OUT

✗ Wrong unit conversion: 1 kJ ≠ 1000 J

✓ 1 kJ = 1000 J always. Many marks lost to this. Latent heat of fusion of ice = 334 kJ/kg = 334000 J/kg.

NCERT exercises (with solutions)

Every NCERT exercise from this chapter — what it covers and how many questions to expect.

Section 1.2 (in-text)

Particle characteristics: salt in water, smell of perfume, intermixing in liquids/gases

Questions

Section 1.3 (in-text)

Solid/liquid/gas comparison, compressibility, container-filling behaviour

Questions

Section 1.4 (in-text)

Phase changes, melting and boiling points, latent heat numericals

Questions

Section 1.5 (in-text)

Evaporation: cooling effect, factors affecting rate, sweat physiology

Questions

End-of-chapter

Mixed: conceptual, particle-level reasoning, numericals on Kelvin conversion and latent heat

Questions

Practice problems

Try each one yourself before tapping "Show solution". Active recall > rereading.

Q1EASY· Definition

Define 'matter' in one sentence and give two examples that are NOT matter.

▶ Show solution

Step 1 — Definition.
  Matter is anything that has mass and occupies space (volume).

Step 2 — Examples of NON-matter.
  • Light (a form of energy, no rest mass).
  • Sound (a wave, an energy disturbance, not a substance).
  • Heat (energy in transit between bodies).

✦ Answer: Matter has mass and occupies space; light and sound are NOT matter — they are forms of energy.

Q2EASY· Conversion

Convert 27 °C to Kelvin.

▶ Show solution

Step 1 — Use T(K) = T(°C) + 273.15.

Step 2 — Substitute.
  T = 27 + 273.15 = 300.15 K.

✦ Answer: 300.15 K (often written 300 K to 3 s.f.).

Memo tip: room temperature in Kelvin is about 300 K.

Q3EASY· Conversion

Convert 373 K to Celsius.

▶ Show solution

Step 1 — Use T(°C) = T(K) − 273.15.

Step 2 — Substitute.
  T = 373 − 273.15 = 99.85 °C ≈ 100 °C.

✦ Answer: ≈ 100 °C.

Context: 373 K is the boiling point of water — that's why this number appears so often.

Q4EASY· Particle theory

Why does the smell of hot food reach you faster than that of cold food?

▶ Show solution

Step 1 — Recall: at higher temperature, particles have higher kinetic energy and move faster.

Step 2 — Apply to diffusion.
  Hot food → faster-moving aroma particles → quicker diffusion through the air → smell reaches you sooner.
  Cold food → slower particles → slower diffusion.

✦ Answer: Higher temperature → faster particle motion → faster diffusion of aroma particles → smell reaches you more quickly.

This is a classic 2-mark CBSE question. Always tie temperature to kinetic energy in your answer.

Q5EASY· State change

Name the process when (a) iodine crystals are heated and turn directly to vapour, (b) water vapour in air turns into dew on a cold surface.

▶ Show solution

Step 1 — Identify the from-state and to-state in each.

Step 2 — Match to the standard names.
  (a) Solid → Gas (no liquid) = SUBLIMATION.
  (b) Gas → Liquid = CONDENSATION.

✦ Answer: (a) Sublimation, (b) Condensation.

Direction-changing terms: gas-to-solid is DEPOSITION (e.g., frost on a winter window), and liquid-to-solid is FREEZING / SOLIDIFICATION.

Q6EASY· Properties

List two differences between liquids and gases at the particle level.

▶ Show solution

Step 1 — Compare on inter-particle distance.
  Liquid: small (just slightly bigger than solid). Gas: very large.

Step 2 — Compare on force of attraction.
  Liquid: moderate, enough to hold a definite volume. Gas: very weak, particles fly free.

(Optional third) Compressibility — Gas highly compressible, liquid almost incompressible — proves the space difference.

✦ Answer: (a) Particles in a gas are much further apart than in a liquid; (b) attractive forces between liquid particles are far stronger than in a gas (so liquids have a fixed volume; gases don't).

Q7MEDIUM· Latent heat

How much heat is required to convert 200 g of ice at 0 °C completely to water at 0 °C? (L_f of ice = 334 kJ/kg)

▶ Show solution

Step 1 — Identify the process.
  Ice at 0 °C → Water at 0 °C is melting at the melting point. Temperature doesn't change; use Q = mL.

Step 2 — Convert mass to kg.
  m = 200 g = 0.2 kg.

Step 3 — Substitute.
  Q = m × L_f = 0.2 × 334 kJ = 66.8 kJ.

Step 4 — Convert to joules if needed.
  Q = 66.8 × 1000 = 66,800 J = 6.68 × 10⁴ J.

✦ Answer: Q = 66.8 kJ = 66,800 J.

Key idea: during a phase change at the phase-change temperature, all input heat goes into breaking bonds — none of it raises temperature.

Q8MEDIUM· Latent heat

Calculate the heat released when 50 g of steam at 100 °C condenses to water at 100 °C. (L_v of water = 2260 kJ/kg)

▶ Show solution

Step 1 — Identify the process.
  Steam (gas) at 100 °C → Water (liquid) at 100 °C is condensation. Use Q = mL_v.
  Energy is RELEASED to the surroundings.

Step 2 — Convert mass.
  m = 50 g = 0.05 kg.

Step 3 — Substitute.
  Q = m × L_v = 0.05 × 2260 kJ = 113 kJ.

✦ Answer: 113 kJ of heat is released.

This is why steam burns are so severe: 1 kg of steam condensing on your skin releases 2260 kJ — about the energy of ten chocolate bars — entirely as heat into your skin.

Q9MEDIUM· Cooling effect

Explain at the particle level why a wet earthen pot keeps water inside cooler than a metal pot does.

▶ Show solution

Step 1 — Recall the cooling mechanism.
  Evaporation removes the highest-energy particles from a liquid surface, lowering the average kinetic energy (i.e. temperature) of what remains.

Step 2 — Apply to the pot.
  Earthen pots have tiny pores; water slowly seeps out and forms a thin film on the outer surface.
  This film evaporates, drawing heat away from the pot's outer wall.
  The wall in turn pulls heat from the water inside.
  Net effect: water inside cools below the surrounding air temperature.

Step 3 — Why a metal pot fails.
  Metal is non-porous; water cannot seep through. No outer-surface evaporation → no cooling.

✦ Answer: Earthen pot is porous → water seeps through → evaporates from the outer surface → removes heat from the pot → water inside cools. Metal pot is non-porous, so this mechanism doesn't operate.

Q10MEDIUM· Phase diagram

Why does the temperature of water remain at 100 °C while it is being boiled even though heat is continuously supplied?

▶ Show solution

Step 1 — At boiling point, the system is in a phase-transition.
  Liquid (water) is converting to gas (steam).

Step 2 — Where does the heat go?
  The heat is used as LATENT HEAT of vaporisation to break the inter-molecular bonds between water particles, NOT to raise temperature.

Step 3 — When does the temperature rise again?
  Only after all the water has converted to steam — once the phase change is complete, additional heat raises the temperature of the steam.

✦ Answer: During boiling, supplied heat is consumed as latent heat to break inter-particle bonds (liquid → gas). Until all water has vaporised, the temperature stays fixed at the boiling point (100 °C at 1 atm).

Q11MEDIUM· Mixed heat

How much heat is needed to convert 100 g of ice at 0 °C to water at 50 °C? (L_f = 334 kJ/kg, c_water = 4.2 kJ/kg·°C)

▶ Show solution

Step 1 — Break into two sub-processes.
  (i) Melt the ice (0 °C ice → 0 °C water): Q₁ = m × L_f.
  (ii) Heat the water (0 °C → 50 °C): Q₂ = m × c × ΔT.

Step 2 — Convert mass.
  m = 100 g = 0.1 kg.

Step 3 — Compute Q₁.
  Q₁ = 0.1 × 334 = 33.4 kJ.

Step 4 — Compute Q₂.
  Q₂ = 0.1 × 4.2 × (50 − 0) = 0.1 × 4.2 × 50 = 21 kJ.

Step 5 — Total.
  Q = Q₁ + Q₂ = 33.4 + 21 = 54.4 kJ.

✦ Answer: 54.4 kJ.

Lesson: split mixed problems into pure-phase-change and pure-temperature-change segments.

Q12MEDIUM· Evaporation factors

Wet clothes dry faster on a windy summer afternoon than on a calm winter morning. Give two distinct reasons.

▶ Show solution

Step 1 — List the four factors affecting evaporation: surface area, temperature, humidity, wind speed.

Step 2 — Compare windy summer vs calm winter for each factor.
  (a) Temperature: summer afternoon is HIGHER. Higher T → more particles have escape energy → faster evaporation.
  (b) Wind speed: windy day. Wind carries away vapour from above the wet cloth → maintains a steep concentration gradient → faster evaporation.
  (c) Humidity is also typically lower on hot windy days (vapour gets carried away) → also helps. Mentioning any 2 is enough.

✦ Answer: (i) Higher temperature → more high-energy particles can escape; (ii) faster wind → vapour is swept away, allowing more to escape. Both increase evaporation rate.

Q13HARD· Three-stage heat

Calculate the total heat required to convert 50 g of ice at −10 °C to steam at 100 °C. Given: c_ice = 2.1 kJ/kg·°C, L_f = 334 kJ/kg, c_water = 4.2 kJ/kg·°C, L_v = 2260 kJ/kg.

▶ Show solution

Step 1 — Identify the four sub-processes.
  (i) Heat ice from −10 °C to 0 °C: Q₁ = m c_ice ΔT₁.
  (ii) Melt ice at 0 °C: Q₂ = m L_f.
  (iii) Heat water from 0 °C to 100 °C: Q₃ = m c_water ΔT₃.
  (iv) Vaporise water at 100 °C: Q₄ = m L_v.

Step 2 — Convert mass.
  m = 50 g = 0.05 kg.

Step 3 — Calculate each piece.
  Q₁ = 0.05 × 2.1 × 10 = 1.05 kJ.
  Q₂ = 0.05 × 334 = 16.7 kJ.
  Q₃ = 0.05 × 4.2 × 100 = 21.0 kJ.
  Q₄ = 0.05 × 2260 = 113.0 kJ.

Step 4 — Sum.
  Q_total = 1.05 + 16.7 + 21.0 + 113.0 = 151.75 kJ.

✦ Answer: 151.75 kJ ≈ 152 kJ.

Observation: Q₄ (vaporisation) dwarfs the others — vaporisation latent heat is by far the largest energy step in turning ice to steam.

Q14HARD· Reasoning

Explain why CNG (compressed natural gas) is sold compressed under high pressure but petrol is sold at atmospheric pressure.

▶ Show solution

Step 1 — Recall compressibility differences.
  Gases (like methane, the main component of CNG) have huge inter-particle space and are highly compressible.
  Liquids (like petrol) have tiny inter-particle space and are essentially incompressible.

Step 2 — Apply to storage.
  • CNG: at atmospheric pressure, the energy density is too low to be useful as fuel (a tank full of gas at 1 atm has very little mass of fuel). Compressing it to 200 atm packs ~200× more fuel into the same tank. This is feasible because gas particles can be pushed close together.
  • Petrol: it's already a dense liquid at atmospheric pressure — particles are essentially touching. Compressing further gives almost no extra mass per litre; the costs (stronger tank, special valves) outweigh negligible benefit.

Step 3 — Practical takeaway.
  Compressing a gas dramatically increases its useful mass per unit volume (energy density per litre). Compressing a liquid barely changes anything.

✦ Answer: Gases are highly compressible (large inter-particle spaces), so high-pressure storage in cylinders gives CNG a usable energy density. Liquids are nearly incompressible — compressing petrol gives no meaningful benefit.

Q15HARD· HOTS

When dry ice (solid CO₂) is exposed to room conditions it forms a 'mist' a few centimetres above its surface — yet dry ice itself doesn't melt to a liquid. Explain both observations.

▶ Show solution

Step 1 — Why dry ice doesn't melt.
  At atmospheric pressure, solid CO₂ sublimes directly to gas (no liquid phase). Liquid CO₂ exists only above 5.1 atm of pressure (its triple point). Under normal conditions, the path solid→liquid is closed.

Step 2 — Why a 'mist' appears.
  As CO₂ vapour leaves the dry ice it is very cold (≈ −78.5 °C). This cold gas chills the air immediately above the dry ice. The air contains water vapour; chilling it below its dew point causes that WATER vapour to condense into tiny liquid water droplets — visible as mist/fog.

Step 3 — Note: the 'mist' is NOT CO₂ — it's WATER from the surrounding air.
  CO₂ gas itself is colourless and invisible. The mist is condensed water droplets caused by the cooling effect of CO₂ vapour.

✦ Answer: Dry ice sublimes (solid → gas) at atmospheric pressure because liquid CO₂ doesn't exist below ~5 atm. The cold CO₂ vapour chills the surrounding humid air, causing water vapour in the AIR (not CO₂) to condense into a visible mist of tiny water droplets.

Q16HARD· Pressure

At higher altitude (e.g. on a mountain), why does water boil at a temperature lower than 100 °C?

▶ Show solution

Step 1 — Recall the definition of boiling point.
  Liquid boils when its vapour pressure equals the surrounding (atmospheric) pressure.

Step 2 — How altitude changes atmospheric pressure.
  Atmospheric pressure DECREASES with altitude (less air above pushing down). At sea level: 1 atm. On Mount Everest: ≈ 0.33 atm.

Step 3 — Conclude.
  Lower outside pressure → liquid needs less vapour pressure to boil → boiling happens at a lower temperature.
  E.g., water boils at ~71 °C on Everest, ~93 °C in Shimla, but 100 °C at the coast.

Step 4 — Practical consequence.
  At lower boiling points, food takes longer to cook (less heat per unit time goes into the food). This is why pressure cookers exist — they raise the boiling point above 100 °C by raising the pressure inside the cooker.

5-minute revision

The whole chapter, distilled. Read this the night before the exam.

•Matter = mass + volume. Light, sound, heat are NOT matter.
•Five particle properties: small, spaced, moving, attracting, intermixing.
•3 classical states: solid (fixed shape & vol), liquid (vol only), gas (neither).
•Particle motion: vibrate (solid), slip (liquid), fly free (gas).
•Conversion: T(K) = T(°C) + 273.15. Absolute zero = 0 K.
•6 phase-change names: melting, freezing, vaporisation, condensation, sublimation, deposition.
•Latent heat formula Q = mL. Ice: L_f = 334 kJ/kg. Water: L_v = 2260 kJ/kg.
•Evaporation: surface-only liquid → gas, at any temp, removes high-energy particles → cools surroundings.
•Factors of evaporation: ↑ area, ↑ temp, ↓ humidity, ↑ wind → ↑ rate.
•4th state: plasma (sun, neon signs, lightning). 5th state: Bose-Einstein condensate (predicted by S.N. Bose, made in 1995).

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Because the supplied heat is being used as 'latent heat' to break inter-particle bonds, not to raise particle kinetic energy. Temperature is the average kinetic energy, so it stays constant until all bonds in that phase are broken.

Plasma's particles are IONISED — electrons have been stripped off, leaving a soup of positive ions and free electrons. This makes plasma electrically conductive and responsive to magnetic fields — properties no normal gas has.

Yes — at water's 'triple point' (0.01 °C, 611.7 Pa), all three states coexist in equilibrium. This is a fixed physical constant used to define the Kelvin scale.

It's the EVAPORATION that cools, not the water itself being cold. The fastest-moving water molecules escape your skin, taking heat with them. A glass of water at body temperature has no driving force for net evaporation off your skin.

Pure water vapour (gas) is invisible. The white 'mist' you see is tiny liquid water droplets — vapour that has cooled and re-condensed in the cooler surrounding air. So the visible cloud isn't steam, it's condensed steam.

It isn't a special physical constant — it's just that 27 °C (a comfortable room temperature) happens to equal exactly 300.15 K. Convenient memorisation, nothing more.

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