CLASS 8 SCIENCE · CHAPTER 7 · STUDY NOTES
Particulate Nature of Matter
Understand particulate nature of matter through short explanations, labelled models and original practice. Read one topic, explain it in your own words, then check.
EXPLORE
A grain still contains tiny particles
Distinguish a visible grain from the particles that make it up.Zoom below the grain
- Constituent particle
- A tiny unit making up a substance, such as an atom or molecule.
Grinding chalk makes smaller pieces of chalk; it does not create a new substance. Even a visible grain contains enormous numbers of much smaller particles.
Sugar molecules can spread through water when sugar dissolves. The crystals disappear from view, but the sugar has not vanished. Never taste substances in a science laboratory.
Take a moment. What do you think?
PAUSE & TRY · think first, then check with a free account
Question
A sugar crystal dissolves. Has the sugar ceased to exist?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 2, 3, 4.
EXPLORE
Attraction and motion act together
Explain why particles stay together without being perfectly packed.Two substances mix at particle level
- Interparticle attraction
- Attractive interaction between the particles of a substance.
- Interparticle space
- Space between constituent particles.
Particles attract each other and also move. Their arrangement depends on the substance and conditions such as temperature. Gaps in a particle model are not automatically filled with air.
When a solute dissolves, its particles mix among solvent particles. Volumes need not add exactly because particle interactions and arrangement change.
PAUSE & TRY · think first, then check with a free account
Question
Must the blank space between the dots in a particle drawing represent air?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 4, 11, 12, 15.
EXPLORE
Solid particles vibrate near fixed positions
Use arrangement and motion to explain a solid’s shape.Vibration is movement too
- Solid
- A state with a definite shape and volume under the stated conditions.
- Melting point
- The temperature at which a substance melts at a specified pressure.
In a solid, particles remain close to relatively fixed positions, but they vibrate. They are not completely motionless. Heating can make vibrations more vigorous and eventually allow melting.
Most solids are denser than their corresponding liquids, but ice is an important exception. Do not treat every solid diagram as a universal density rule.
PAUSE & TRY · think first, then check with a free account
Question
A solid keeps its shape. Does that mean its particles never move?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 5, 6, 12, 15.
EXPLORE
Liquids change shape but retain volume
Explain a liquid through particles that move past one another.Different shape, same volume
- Liquid
- A state with a definite volume but no fixed shape under the stated conditions.
- Fluid
- A substance that can flow, including liquids and gases.
Liquid particles remain close but can move past one another. A liquid therefore takes its container’s shape. Pouring the same amount into another container does not create more liquid.
The surface level can change because container shape changes. Compare volume markings rather than height alone.
PAUSE & TRY · think first, then check with a free account
Question
The same water reaches a higher level in a narrow glass. Has its volume increased?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 7, 8, 15.
EXPLORE
Gas spreads into available space
Explain why gases have no fixed shape or volume.Remove the barrier
- Gas
- A state whose particles spread through the available container space.
Gas particles move freely in many directions with much larger typical separations than liquid particles. A gas fills available space rather than remaining in one small corner.
Smoke contains tiny suspended particles and is not a pure gas. Fragrance can spread by particle motion, while air currents can also carry it.
PAUSE & TRY · think first, then check with a free account
Question
A partition is removed in a sealed container holding gas on one side. What happens over time?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 8, 9, 14, 15.
EXPLORE
Compression brings particles closer
Explain why gas compresses much more than water.Change the gaps, not the dots
- Compression
- Reducing the volume occupied by a substance through an applied pressure.
A needle-free sealed syringe can compress trapped air because gas particles have large gaps. Water is much less compressible in an ordinary classroom comparison.
Particles do not have to shrink to explain a smaller gas volume. The gaps between them become smaller.
PAUSE & TRY · think first, then check with a free account
Question
When trapped air is compressed, do the individual particles become half their size?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 10, 11, 12.
EXPLORE
Heating changes particle motion
Distinguish faster motion from a new kind of particle.Watch colour spread
- Diffusion
- Spreading and mixing caused by the random motion of particles.
Particles move constantly. Increasing temperature generally increases their average motion, so a dissolved colour can spread faster under otherwise similar conditions.
Melting and boiling change arrangement and freedom of motion. Evaporation occurs at a liquid’s surface below its boiling point; boiling involves vapour bubbles within the liquid.
PAUSE & TRY · think first, then check with a free account
Question
A drop of food colour spreads faster in warmer water. Does that show the colour particles became a new substance?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 13, 14, 15.
EXPLORE
Use particle models with clear limits
Explain atoms and molecules without confusing them with dust.One water molecule is a bonded unit
- Atom
- A basic unit of an element.
- Molecule
- Two or more atoms bonded together as a unit.
Ancient thinkers such as Kanad considered whether matter had tiny building blocks. Modern atomic and molecular models are supported and refined by experiments.
A water molecule contains two hydrogen atoms and one oxygen atom. A dust grain contains enormous numbers of atoms or molecules. The ordinary word “particle” can mean different sizes in different contexts.
PAUSE & TRY · think first, then check with a free account
Question
Is a visible dust speck one atom?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 4, 18.