Sugar Vanishes in Water Without Raising the Level
Learn the everyday evidence that matter is made of tiny particles with gaps between them, see that those particles are atoms and molecules, compare the three states by particle attraction, and classify substances by arrangement.
Where does the sugar go when it dissolves?
Into the gaps between the water particles. Stir two spoons of sugar into a full glass of water and it disappears from sight, yet the level barely rises — the sugar particles have slipped into spaces that were already there.
That single observation tells you matter is not continuous. This page covers everything in the CBSE Class 8 Science chapter's first part: evidence for particles, atoms and molecules, comparing the three states, and classifying substances.
That single observation tells you matter is not continuous. This page covers everything in the CBSE Class 8 Science chapter's first part: evidence for particles, atoms and molecules, comparing the three states, and classifying substances.
What evidence shows matter is made of tiny particles?
Several simple activities point the same way: matter is made of very small particles with spaces between them.
Dissolving. Add sugar or salt to a glass of water and stir. The solid disappears from view and the water tastes sweet or salty throughout, while the level rises far less than the volume of solid added. The particles have broken apart and spread into the spaces between the water particles.
Diffusion of colour. Drop a crystal of potassium permanganate into still water without stirring. Purple colour spreads slowly through the whole glass. The crystal has divided into particles too small to see, and they have moved on their own.
Dilution. Take a spoon of that purple solution, add it to fresh water, and repeat several times. The colour remains visible through many dilutions, which means one crystal contained an enormous number of particles.
Smell travelling. Incense lit in one corner of a room is smelt across it, and cooking reaches the next room — the particles must be moving through the air by themselves.
Compressing air. Block the nozzle of a syringe and press the plunger. The air inside squeezes into a smaller volume, so there must be spaces to squeeze out.
Two conclusions follow together, and both are needed: the particles are extremely small, and there are gaps between them. The dissolving shows the gaps, and the repeated dilution shows the smallness — which is why the syllabus asks for more than one activity.
Dissolving. Add sugar or salt to a glass of water and stir. The solid disappears from view and the water tastes sweet or salty throughout, while the level rises far less than the volume of solid added. The particles have broken apart and spread into the spaces between the water particles.
Diffusion of colour. Drop a crystal of potassium permanganate into still water without stirring. Purple colour spreads slowly through the whole glass. The crystal has divided into particles too small to see, and they have moved on their own.
Dilution. Take a spoon of that purple solution, add it to fresh water, and repeat several times. The colour remains visible through many dilutions, which means one crystal contained an enormous number of particles.
Smell travelling. Incense lit in one corner of a room is smelt across it, and cooking reaches the next room — the particles must be moving through the air by themselves.
Compressing air. Block the nozzle of a syringe and press the plunger. The air inside squeezes into a smaller volume, so there must be spaces to squeeze out.
Two conclusions follow together, and both are needed: the particles are extremely small, and there are gaps between them. The dissolving shows the gaps, and the repeated dilution shows the smallness — which is why the syllabus asks for more than one activity.
What are these particles, and why can we not see them?
They are atoms and molecules.
An atom is the smallest particle of an element. A molecule is a group of atoms joined together, and it is the smallest particle of a substance that can exist on its own and still show that substance's properties.
So a molecule of water is made of atoms of hydrogen and oxygen joined together, and it is water molecules that are present in a glass of water.
They cannot be seen because they are far too small — beyond the reach of the naked eye and of an ordinary school microscope. Even a tiny visible speck of salt contains an immense number of them, which is what the repeated dilution activity demonstrated.
This is why the particle idea is called a model. Nobody watches the particles directly; their existence is inferred from what can be observed — dissolving, diffusion, compression and smell. The evidence is indirect but consistent, and the model explains all of it at once.
The misconception to clear up early is about heating. Warming a substance does not make its particles bigger. It makes them move faster, so they push further apart — and that increased spacing, not any growth of the particles themselves, is what expansion actually is.
An atom is the smallest particle of an element. A molecule is a group of atoms joined together, and it is the smallest particle of a substance that can exist on its own and still show that substance's properties.
So a molecule of water is made of atoms of hydrogen and oxygen joined together, and it is water molecules that are present in a glass of water.
They cannot be seen because they are far too small — beyond the reach of the naked eye and of an ordinary school microscope. Even a tiny visible speck of salt contains an immense number of them, which is what the repeated dilution activity demonstrated.
This is why the particle idea is called a model. Nobody watches the particles directly; their existence is inferred from what can be observed — dissolving, diffusion, compression and smell. The evidence is indirect but consistent, and the model explains all of it at once.
The misconception to clear up early is about heating. Warming a substance does not make its particles bigger. It makes them move faster, so they push further apart — and that increased spacing, not any growth of the particles themselves, is what expansion actually is.
How do solids, liquids and gases compare?
The three states differ in the space between their particles and the strength of attraction holding them together.
Solid
- Shape — definite and fixed
- Volume — definite
- Interparticle space — very small
- Force of attraction — very strong
- Particles are closely packed in a regular arrangement and only vibrate about fixed positions
- Examples: ice, iron, wood, stone
Liquid
- Shape — no definite shape; takes the shape of its container
- Volume — definite
- Interparticle space — larger than in a solid
- Force of attraction — moderate
- Particles slide over one another, so a liquid can flow
- Examples: water, oil, milk
Gas
- Shape — no definite shape
- Volume — no definite volume; fills the whole container
- Interparticle space — very large
- Force of attraction — negligible
- Particles move freely and rapidly in all directions
- Examples: air, oxygen, cooking gas
Half a litre of water poured into a jug, a bottle or a bowl is still half a litre but a different shape each time — while the same amount of air released from a cylinder spreads through the whole room.
The liquid's properties are the ones most often misstated. A liquid has a definite volume but no definite shape, and writing "neither definite" confuses it with a gas — so pair the two words carefully for each state.
Solid
- Shape — definite and fixed
- Volume — definite
- Interparticle space — very small
- Force of attraction — very strong
- Particles are closely packed in a regular arrangement and only vibrate about fixed positions
- Examples: ice, iron, wood, stone
Liquid
- Shape — no definite shape; takes the shape of its container
- Volume — definite
- Interparticle space — larger than in a solid
- Force of attraction — moderate
- Particles slide over one another, so a liquid can flow
- Examples: water, oil, milk
Gas
- Shape — no definite shape
- Volume — no definite volume; fills the whole container
- Interparticle space — very large
- Force of attraction — negligible
- Particles move freely and rapidly in all directions
- Examples: air, oxygen, cooking gas
Half a litre of water poured into a jug, a bottle or a bowl is still half a litre but a different shape each time — while the same amount of air released from a cylinder spreads through the whole room.
The liquid's properties are the ones most often misstated. A liquid has a definite volume but no definite shape, and writing "neither definite" confuses it with a gas — so pair the two words carefully for each state.
How do you classify a substance from its particle arrangement?
Ask two questions: does it keep its own shape, and can it be squeezed?
- Keeps its shape and cannot be squeezed — solid, with closely packed particles and strong attraction.
- Flows and takes the container's shape but keeps its volume — liquid, with moderate spacing and attraction.
- Fills the whole container and is easily squeezed — gas, with very large spacing and negligible attraction.
Worked classifications:
- Ice — solid. Fixed shape, particles vibrating in place.
- Water — liquid. Definite volume, particles sliding past each other.
- Steam — gas. Fills the space available, particles moving freely and far apart.
- Salt and sugar — solids, even as powders. Each grain keeps its own shape; a powder only looks like it flows because the small grains slide over one another.
- Honey — a liquid, though a very thick one. It flows slowly because its particles move past each other with difficulty, but it still takes the shape of its jar.
- Cooking gas in a cylinder — stored as a liquid under high pressure, and released as a gas.
- Sponge — a solid, despite being squeezable. What compresses is the air in its holes, not the solid itself.
Those last two are the cases that separate a careful answer from a careless one. Classification depends on the particles of the substance, not on how the object behaves as a whole — which is why a sponge is a solid and a powder is not a liquid.
- Keeps its shape and cannot be squeezed — solid, with closely packed particles and strong attraction.
- Flows and takes the container's shape but keeps its volume — liquid, with moderate spacing and attraction.
- Fills the whole container and is easily squeezed — gas, with very large spacing and negligible attraction.
Worked classifications:
- Ice — solid. Fixed shape, particles vibrating in place.
- Water — liquid. Definite volume, particles sliding past each other.
- Steam — gas. Fills the space available, particles moving freely and far apart.
- Salt and sugar — solids, even as powders. Each grain keeps its own shape; a powder only looks like it flows because the small grains slide over one another.
- Honey — a liquid, though a very thick one. It flows slowly because its particles move past each other with difficulty, but it still takes the shape of its jar.
- Cooking gas in a cylinder — stored as a liquid under high pressure, and released as a gas.
- Sponge — a solid, despite being squeezable. What compresses is the air in its holes, not the solid itself.
Those last two are the cases that separate a careful answer from a careless one. Classification depends on the particles of the substance, not on how the object behaves as a whole — which is why a sponge is a solid and a powder is not a liquid.
Exam tip
Exam tip: naming the space and the attraction together
Nearly every "give a reason" question in this chapter is answered by the same two quantities, so name both.
Write it as: gases are easily compressed because the interparticle space is very large and the force of attraction is negligible. An answer saying only "gas is light" earns nothing.
For a comparison question, answer property by property in the same order for all three states — shape, volume, space, attraction, motion — rather than in a paragraph.
Pair a liquid's properties correctly: definite volume, no definite shape.
When asked for evidence of particles, give at least two different activities and say what each one shows — dissolving shows the gaps, repeated dilution shows the smallness.
And never write that heating makes particles bigger. Heating makes them move faster and spread further apart.
Write it as: gases are easily compressed because the interparticle space is very large and the force of attraction is negligible. An answer saying only "gas is light" earns nothing.
For a comparison question, answer property by property in the same order for all three states — shape, volume, space, attraction, motion — rather than in a paragraph.
Pair a liquid's properties correctly: definite volume, no definite shape.
When asked for evidence of particles, give at least two different activities and say what each one shows — dissolving shows the gaps, repeated dilution shows the smallness.
And never write that heating makes particles bigger. Heating makes them move faster and spread further apart.
Did you know
Why does the water level barely rise when sugar dissolves in it?
Because the sugar does not sit on top of the water — it goes inside it.
Water particles are not packed solid against one another; there are gaps between them. When sugar dissolves, its particles separate and slip into those existing gaps, so they take up almost no extra room in the glass.
The same idea explains why a measured litre of water mixed with a measured litre of spirit gives slightly less than two litres. It is not that matter has been lost — the particles of one liquid have settled into the spaces of the other.
Water particles are not packed solid against one another; there are gaps between them. When sugar dissolves, its particles separate and slip into those existing gaps, so they take up almost no extra room in the glass.
The same idea explains why a measured litre of water mixed with a measured litre of spirit gives slightly less than two litres. It is not that matter has been lost — the particles of one liquid have settled into the spaces of the other.
Key takeaways
Particulate nature of matter: quick revision
- Dissolving, diffusion of colour, repeated dilution, travelling smells and compressing air all show that matter is made of tiny particles with spaces between them.
- The particles are atoms and molecules, far too small to see — so the particle idea is a model supported by indirect evidence.
- Heating does not enlarge particles; it makes them move faster and spread further apart.
- Solid — definite shape and volume, very small spaces, very strong attraction, particles vibrate in place.
- Liquid — definite volume but no definite shape, larger spaces, moderate attraction, particles slide over each other.
- Gas — neither definite, very large spaces, negligible attraction, particles move freely; classification follows the particles of the substance, so a sponge is a solid and a powder is not a liquid.
You will remember all of this far better after answering five questions on it than after reading it twice.
- The particles are atoms and molecules, far too small to see — so the particle idea is a model supported by indirect evidence.
- Heating does not enlarge particles; it makes them move faster and spread further apart.
- Solid — definite shape and volume, very small spaces, very strong attraction, particles vibrate in place.
- Liquid — definite volume but no definite shape, larger spaces, moderate attraction, particles slide over each other.
- Gas — neither definite, very large spaces, negligible attraction, particles move freely; classification follows the particles of the substance, so a sponge is a solid and a powder is not a liquid.
You will remember all of this far better after answering five questions on it than after reading it twice.