Class 9 Matter in Our Surroundings — States and Changes of State
"Matter in Our Surroundings" is the opening chapter of NCERT Class 9 chemistry, and it asks a question that sounds obvious until you have to explain it properly: what makes a solid a solid, and what actually happens, at the particle level, when ice melts or water boils? This guide covers the particle model of matter, the three states, and the numerical questions on latent heat that examiners build from this chapter.
Matter is made of tiny particles
Several everyday observations tell us matter is particulate rather than continuous: a crystal of potassium permanganate colours a large volume of water when dissolved (diffusion — the dye particles spread out and mix with water particles), and a sugar cube disappears completely into water yet the sweetness remains throughout the glass. NCERT lists three defining characteristics of these particles:
The three states of matter
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Shape | Definite | Takes the shape of the container | No definite shape |
| Volume | Definite | Definite | No definite volume |
| Compressibility | Negligible | Very low | High |
| Interparticle space | Least | More than solid | Maximum |
| Interparticle force of attraction | Strongest | Intermediate | Weakest |
| Rate of diffusion | Extremely slow | Slow | Fast |
This ordering — solid has the strongest attraction and least space, gas has the weakest attraction and most space — is what explains every change of state in this chapter; liquids sit in between on every single property.
Change of state — the effect of temperature
Supplying heat increases the kinetic energy of particles until the attractive forces are overcome enough for the substance to change state. The reverse — removing heat — allows particles to slow down and pack closer together.
Gas --(condensation)--> Liquid --(freezing/solidification)--> Solid
The temperature at which a solid melts at atmospheric pressure is its melting point; the temperature at which a liquid boils at atmospheric pressure is its boiling point. Crucially, the temperature does not rise while a substance is changing state — all the heat supplied goes into breaking the intermolecular forces, not into raising the kinetic energy (and hence temperature) of the particles. This "hidden" heat is called latent heat.
where m = mass of the substance, L = latent heat of fusion or vaporisation (specific to the substance)
Worked example 1 — heat required to melt ice
The latent heat of fusion of ice is about 3.34 × 10⁵ J/kg (334 J/g). Find the heat needed to melt 50 g of ice at 0°C into water at 0°C, with no change in temperature.
Q = m × L = 50 g × 334 J/g = 16,700 J = 16.7 kJ
Worked example 2 — heat required to boil water into steam
The latent heat of vaporisation of water is about 2.26 × 10⁶ J/kg (2260 J/g). Find the heat needed to convert 20 g of water at 100°C into steam at 100°C.
Q = m × L = 20 g × 2260 J/g = 45,200 J = 45.2 kJ
Notice this is almost three times the energy needed to melt the same mass of ice — which is why steam burns are far more severe than burns from boiling water at the same temperature: steam releases this large extra quantity of latent heat as it condenses on skin.
Kelvin scale and temperature conversion
Worked example 3 — Celsius/Kelvin conversion
Convert the boiling point of water, 373 K, to Celsius, and convert −10°C to Kelvin.
°C = K − 273 = 373 − 273 = 100°C (correct, this is water's boiling
point at 1 atmosphere)
K = °C + 273 = −10 + 273 = 263 K
Sublimation and the effect of pressure
Some solids — solid carbon dioxide (dry ice), camphor, naphthalene and ammonium chloride — change directly from solid to gas without passing through the liquid state at normal atmospheric pressure; this is sublimation. Pressure also affects boiling point: increasing pressure raises the boiling point, which is why a pressure cooker cooks food faster — water inside boils above 100°C under the raised internal pressure, transferring heat to the food more quickly.
Evaporation — a surface phenomenon, not boiling
Evaporation is the change of a liquid to vapour at any temperature below its boiling point, and it happens only at the liquid's surface. Its rate increases with more surface area, higher temperature, lower humidity and stronger wind speed. Because the fastest, most energetic particles escape first during evaporation, the average kinetic energy of the particles left behind drops — which is why evaporation causes cooling (the physical basis of sweating and of a matka keeping water cool).
Common mistakes that cost marks
- Saying temperature keeps rising during melting or boiling. It stays constant at the melting/boiling point until the change of state is complete — this is the single most-tested conceptual point in the chapter.
- Confusing evaporation with boiling. Evaporation happens only at the surface, at any temperature; boiling happens throughout the liquid, only at the boiling point.
- Forgetting units when using Q = mL — mass and latent heat must use consistent units (both in grams or both in kilograms) before multiplying.
- Calling sublimation "evaporation of a solid." It is a distinct solid-to-gas change with its own name and its own examples (dry ice, camphor, naphthalene, ammonium chloride).
Where this chapter appears in exams
| Exam | Typical use |
|---|---|
| CBSE Class 9 | Latent heat numericals, state-of-matter comparison tables, sublimation examples |
| CBSE Class 11 | Foundation for the gas laws and kinetic theory chapter |
| Everyday-science questions | Explaining cooling by evaporation, pressure cookers, dry-ice packaging |
Practise the latent-heat and temperature numericals. Use the free calculator suite to check unit conversions and arithmetic while revising this chapter.
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