Matter in Our Surroundings | Science Grade 9 CBSE

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States & changes, up close

Everything around you is one of three states — and heat moves it between them. Tap each term to see how the particles are arranged and what happens when they gain energy.

Explore · States of mattertap a term

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The three big ideas

Matter is made of tiny particles — far too small to see — and there are spaces between them (sugar dissolves into the gaps in water).
The particles are constantly moving — that is why a smell or a drop of ink spreads on its own (diffusion).
The particles attract one another. This force is strongest in solids and weakest in gases, which is what holds matter together.

Solid — fixed shape and volume; particles packed tightly with strong attraction, only vibrating in place; almost incompressible.
Liquid — fixed volume but no fixed shape; particles are close but can slide past each other, so a liquid flows and takes the container's shape.
Gas — no fixed shape or volume; particles are far apart with weak attraction, move fast and fill any container; highly compressible.
The three states differ in how the particles are arranged and how much energy they have.

Heating raises particle energy — particles move faster and break free of the forces holding them: solid → liquid (melting) and liquid → gas (boiling).
Latent heat — the hidden heat absorbed during a change of state without any rise in temperature. It is why steam burns worse than boiling water, and why ice cools better than water at the same temperature.
Evaporation — liquid turning to gas at the surface, at any temperature; it draws energy from the surface, so evaporation causes cooling.

Worked example. Why does sweating cool you down?

The most energetic water particles in your sweat escape from the skin's surface (evaporation), carrying energy away with them. With the high-energy particles gone, the surface that is left behind cools down.

Common mistake: mixing up evaporation and boiling. Evaporation happens at all temperatures and only at the surface. Boiling happens at a fixed temperature (the boiling point) and throughout the whole liquid.

Where you'll meet it

Matter, put to work

Cooling by evaporation

Sweating cools your body, and water in an earthen pot (matka) stays cool, for the same reason: evaporation carries latent heat away from the surface, leaving what's left behind cooler.

Refrigerators & pressure cookers

A fridge uses a fluid that evaporates to absorb heat from inside, then is compressed back to a liquid outside. A pressure cooker raises the pressure so water boils above 100 °C, cooking food faster — both are change of state at work.

Check yourself

Competency quiz

Modelled on the competency-based pattern — MCQ, assertion–reason and a case study, testing whether you can use the ideas, not just recall them.

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