Particulate Nature of Matter | Science Grade 8 CBSE

Play with it

The six ideas behind every particle

Matter looks solid and still, but it is really tiny particles with space around them, always moving and attracting each other. Tap each term to see what it means and how it links to the rest.

Explore · Particles of mattertap a term

Learn

The three big ideas

Matter is made of extremely tiny particles. They are far too small to see — a single grain of salt holds many millions of them.
There are spaces between the particles. That is why a spoon of sugar dissolves in a glass of water with almost no rise in level — the sugar particles slip into the gaps between the water particles.
The particles are always moving. They are in constant random motion, and they move faster as the temperature rises.
The particles attract one another. This force of attraction is strongest in solids, weaker in liquids and almost absent in gases.

Worked example. A few crystals of purple potassium permanganate are dropped into water. The water is diluted again and again — yet it stays faintly coloured. What does this show?

Each crystal must contain a huge number of unimaginably small particles, which spread out evenly through a large volume of water. It shows particles are tiny, have spaces to spread into, and are in constant motion.

Solid: particles closely packed with strong attraction. They only vibrate in fixed positions, so a solid has a fixed shape and a fixed volume and is almost impossible to compress.
Liquid: particles held more loosely, with moderate attraction. They slide past one another, so a liquid has a fixed volume but takes the shape of its container and can flow.
Gas: particles very far apart with almost no attraction. They move fast and freely, so a gas has no fixed shape or volume, fills any container, and can be compressed a great deal.

Common mistake: thinking liquids can be squeezed like gases. Gases compress easily because of their large empty spaces (this is how cooking gas is stored in cylinders); liquids and solids hardly compress at all.

Diffusion is the spreading of the particles of one substance through another on their own — like the smell of food drifting across a house. It is fastest in gases (most space, fastest particles) and slower in liquids.
Temperature changes the state. Heating gives particles more energy: a solid melts to a liquid, and a liquid vaporises (boils or evaporates) to a gas. Cooling reverses this — a gas condenses and a liquid freezes.
At sea level ice melts at 0 °C and water boils at 100 °C.

Worked example. A tea bag colours hot water far faster than cold water. Why?

In hot water the particles have more energy and move faster, so the colour and flavour diffuse out of the bag more quickly. Higher temperature → faster particle motion → faster diffusion.

Common mistake: confusing melting with dissolving. Melting is a solid turning to liquid on heating; dissolving is a substance spreading into a liquid to form a solution — two different changes.

Where you'll meet it

Particles at work

Drying clothes

Wet clothes dry because water particles with enough energy escape into the air as vapour. Spreading the clothes out on a warm, breezy day gives more particles the energy and space to leave — so they dry faster.

Cooking gas in cylinders

Gases are squeezed into small steel cylinders because their particles have huge gaps between them. This is how cooking gas and vehicle fuel are stored and carried in a compact form.

The smell of cooking

The aroma of food being cooked in the kitchen soon reaches every room. The scent particles diffuse among the air particles — and on a hot day the smell travels even faster.

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.

Score 0/12