Lesson 5: Matter and Its States

Everything you can touch, taste or smell — the steel tiffin in your bag, the monsoon rain, the LPG in your kitchen — is matter. This NCERT lesson explores what matter is, its three states, how matter changes state, and the kinetic theory that explains it all. Chemistry begins here: with the particle model that makes sense of melting, boiling, evaporation and diffusion. Indian kitchens, with their pressure cookers and tadka pans, are excellent chemistry laboratories.

What is matter? The particle model

Matter is anything that occupies space and has mass — solids, liquids, gases, and the plasma of lightning and stars. The particle model: all matter consists of tiny particles (atoms and molecules) that are always moving, attract each other, and have spaces between them. Three postulates explain everything: particles are incredibly small (a drop of water contains ~10²¹ molecules), they have space between them (gases compress easily, solids barely), and they attract each other (the force is strongest in solids, weakest in gases). Evidence is everywhere: diffusion — the smell of frying pakoras spreading through the house, ink spreading in water, a drop of Dettol dispersing in a bucket — shows particles moving and mixing on their own. Brownian motion (pollen grains jiggling in water, seen under a microscope) shows invisible particles bombarding visible ones. Dissolving sugar in chai: the sugar particles occupy spaces between water particles — the volume barely increases. The kinetic theory adds energy: particles' motion is their kinetic energy, which rises with temperature. This single idea explains thermal expansion (particles move more, push apart — the sagging of power lines in May heat), diffusion rates (faster when hot — tea brews faster in boiling water), and eventually all changes of state. For board answers, always link observations to particles: "what the particles are doing" is the explanation.

Solids, liquids and gases

Solids: steel utensils, ice, salt crystals. Liquids: water, milk, oil — they flow and can be poured. Gases: air, LPG, water vapour — they exert pressure by bombarding walls. Explaining properties from the particle model is the skill: "gases are compressible because large spaces between particles can be squeezed" earns marks; "because they are gases" does not. Diffusion is fastest in gases (large spaces, fast particles), slowest in solids. Plasma — the fourth state, ionised gas in lightning, fluorescent tubes, stars — and Bose-Einstein condensates (Satyendra Nath Bose's prediction, realised in 1995) complete the picture. Bose, the great Indian physicist, shows in this very topic: Indian science is part of the story. For Class 9, know plasma exists; the detail belongs to higher classes.

Change of state — melting, boiling, evaporation

Melting: solid to liquid at the melting point (ice: 0°C/273 K). Heat energy overcomes particle attraction; temperature stays constant during melting (latent heat of fusion, 3.34 × 10⁵ J/kg for ice) — the energy goes into loosening particles, not raising temperature. That is why ice-water stays at 0°C until all ice melts, and why ice is so effective in a cooler: it absorbs huge heat without warming. Boiling: liquid to gas at the boiling point (water: 100°C/373 K at sea level), with bubbles forming throughout the liquid; latent heat of vaporisation (22.5 × 10⁵ J/kg) — steam burns are worse than boiling-water burns because condensing steam releases this extra energy. Pressure affects boiling: high in the Himalayas, water boils below 100°C (lower air pressure) — food takes longer to cook, which is why pressure cookers (raising pressure, raising boiling point) are Himalayan kitchen essentials. Evaporation: slow surface change at any temperature — puddles dry, wet clothes dry, sweat cools the body. Factors: temperature (hotter, faster), surface area (spread clothes dry faster), wind (removes vapour — the loo's winds dry laundry quickly), humidity (dry air evaporates faster — desert coolers work best in dry heat). Evaporation causes cooling (fastest particles leave, average energy drops) — the principle of earthen matkas cooling water, desert coolers, and perspiration. Distinguish evaporation (surface, any temperature, slow) from boiling (bulk, fixed temperature, fast) — a classic board distinction.

Activity — measuring evaporation

Four identical shallow dishes, each with 100 g of water, are left for one day: (A) in sun and wind, (B) in sun but still air, (C) in shade with wind, (D) in shade and still air. The water lost: A 18 g, B 12 g, C 9 g, D 5 g. Reading the data: compare A with B — same sun, wind alone adds 6 g of evaporation (wind sweeps vapour away). Compare A with C — same wind, sun alone adds 9 g (heat supplies particle energy). Compare C with D — wind matters even without sun (9 g vs 5 g). And the matka principle follows: water that evaporates through the earthen wall carries energy away, cooling what remains. This is how CBSE activity data should be handled — change one factor at a time, subtract, conclude.

Sublimation, condensation, freezing — and the water cycle

Sublimation: solid directly to gas — camphor (used in aarti and mothballs) slowly vanishes, naphthalene balls shrink, dry ice (solid CO₂) fumes without melting. Deposition is the reverse (frost forming on winter windows in North India). Condensation: gas to liquid — dew on morning grass, water droplets on a cold glass of nimbu pani, clouds forming. Freezing: liquid to solid — water to ice at 0°C, ghee solidifying in winter. The interconversion map: melting, freezing, vaporisation, condensation, sublimation, deposition — learn it as a diagram with arrows. Each has a latent heat and a particle explanation: heating increases particle energy until attraction is overcome; cooling reverses it. Temperature plateaus during every change of state — the energy goes into rearranging particles, not speeding them. The water cycle is these changes on a planetary scale: evaporation from oceans, rivers and fields; transpiration from plants; condensation into clouds; precipitation as the monsoon; collection in rivers and groundwater; and back again. The Indian monsoon — the mightiest water-cycle engine on Earth — delivers ~80% of India's rainfall in four months, deciding harvests, economies and lives. Chemistry's particle model thus scales up to the subcontinent's destiny.

Mixtures, solutions and separation

Pure substances (elements, compounds) versus mixtures: air, milk, chai, and brass are mixtures — physically combined, separable, with variable composition. Homogeneous mixtures (solutions: sugar in water, air) look uniform; heterogeneous (sand in water, oil in water) do not. Alloys (brass, steel), amalgams, and aerosols complete the classification. Solutions have a solvent (the dissolver, usually water) and solute (the dissolved); concentration measures solute per solvent — dilute versus concentrated, saturated (no more dissolves) versus unsaturated. Separation techniques — each exploiting a property difference: filtration (insoluble solids from liquids — filtering chai, cleaning water), evaporation (salt from seawater — the salt pans of Kutch and Tamil Nadu), distillation (liquids by boiling point — purifying water), fractional distillation (petroleum refining at Jamnagar), chromatography (ink dyes, forensic analysis), sublimation (camphor from salt), magnetic separation (iron from waste), and centrifugation (cream from milk — the dairy industry's workhorse; blood tests). Match technique to mixture in board questions: "How would you separate salt from seawater?" (evaporation); "cream from milk?" (centrifugation); "dyes in ink?" (chromatography). The reasoning — "because the components differ in X, use the method exploiting X" — is the mark-earning step. Water purification chains these methods: sedimentation, filtration, chlorination — the municipal water reaching Indian taps.

Physical versus chemical change

Physical changes alter form, not substance — melting ice, tearing paper, dissolving sugar: reversible, no new substance. Chemical changes make new substances — burning wood, rusting iron, cooking an egg, digesting food, setting curd: usually irreversible, with energy changes. Tests: is a new substance formed? Is it reversible? The curd-setting (Lactobacillus fermenting milk) is chemical — you cannot get milk back; melting ghee is physical. Board questions probe the boundary: dissolving sugar is physical (recover it by evaporation); burning sugar is chemical (caramel, then carbon — new substances). Rusting needs both air and water (the classic nail experiment); galvanisation and painting prevent it by exclusion. Cooking is applied chemistry: the Maillard reaction browns rotis, fermentation leavens idli batter, and pressure cooking uses the boiling-point–pressure relationship. Everyday Indian chemistry: the pressure cooker (physics + chemistry), the tadka (hot oil extracting flavours — solubility), soap cleaning (emulsification), and the antacid tablet fizzing (chemical reaction neutralising acidity). NCERT's message: chemistry is not confined to the laboratory — the kitchen, the field and the factory are chemical systems. Observe them as a chemist.

Common mistakes

• Saying the temperature rises during melting or boiling — during a change of state the temperature is constant; the heat is latent.
• Confusing evaporation with boiling — evaporation is surface-only at any temperature; boiling is bulk change at one fixed temperature.
• Explaining a property without particles — "gases expand because they are gases" earns nothing; say what the particles do.
• Calling dissolving a chemical change — no new substance forms; the solute is recoverable.
• Pairing the wrong separation technique — always name the property difference the method exploits (boiling point, solubility, particle size, magnetism).
• Forgetting that sublimation skips the liquid state — camphor does not melt first.

Exam Experience

MCQ 1. 2 kg of ice at 0 °C is converted completely to steam at 100 °C. Which single step needs the most heat? (L_ice = 3.34 × 10⁵ J/kg, c = 4,200 J/kg °C, L_steam = 2.26 × 10⁶ J/kg) (a) Melting the ice (b) Heating water 0→100 °C (c) Vaporising the water (d) All three need equal heat Key: (c). Melt: 6.68 × 10⁵ J; heat: 8.4 × 10⁵ J; vaporise: 4.52 × 10⁶ J — vaporisation dominates (total ≈ 6.0 × 10⁶ J). MCQ 2 (Assertion–Reason). Assertion (A): Evaporation causes cooling. Reason (R): During evaporation, the particles with the highest kinetic energy escape, lowering the average kinetic energy of the remaining liquid. (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 but R is false (d) A is false but R is true Key: (a). Selective escape of energetic particles is exactly why the residue cools — the principle behind earthen pots and desert coolers. Short answer (3 marks). In an evaporation activity, identical shallow bowls of water were kept for 6 hours: A in still shade lost 18 g, B in still sunlight lost 30 g, C in shade with a fan lost 24 g, D in sun with a fan lost 33 g. (i) How much of D's loss is due to wind alone in shade conditions? (ii) State two conclusions from the data. *Marking scheme:* (i) Wind effect in shade = C − A = 24 − 18 = 6 g (1). (ii) Sunlight raises evaporation (B − A = 12 g) (1); wind raises evaporation, and the sun's effect is the larger single factor here (1). Long answer (5 marks). (a) Define latent heat of vaporisation and state its SI unit (2). (b) Why do steam burns hurt more than boiling-water burns at the same temperature? (2) (c) Name the state change and energy direction when naphthalene balls disappear from a cupboard (1). *Marking scheme:* (a) Heat required to convert 1 kg of liquid to vapour at its boiling point without temperature change (1); J/kg (1). (b) Steam first releases its latent heat on condensing, then the hot water cools — extra 2.26 × 10⁶ J/kg delivered (2). (c) Sublimation; energy absorbed from surroundings (1).