Why a Gas Can Be Squeezed but Water Cannot
Learn how mass, weight and volume differ, how intermolecular space and attraction decide a state of matter, compare solids, liquids and gases, and name every change of state correctly.
Why can a gas be squeezed into a small cylinder but water cannot?
Because a gas has huge empty spaces between its particles, and squeezing simply pushes them closer. In water the particles are already almost touching, so there is nothing left to squeeze out.
That single difference — how much space lies between the particles — explains nearly everything in this chapter. This page covers everything in the ICSE Class 7 Chemistry chapter on matter: mass, weight and volume, the particulate nature of matter, comparing the three states, and the changes between them.
That single difference — how much space lies between the particles — explains nearly everything in this chapter. This page covers everything in the ICSE Class 7 Chemistry chapter on matter: mass, weight and volume, the particulate nature of matter, comparing the three states, and the changes between them.
How do mass, weight and volume differ?
Matter is anything that occupies space and has mass. Its three measurable properties are easy to confuse, so take them one at a time.
Mass is the quantity of matter contained in a body. Its SI unit is the kilogram (kg), and it is measured with a beam balance. Mass does not change with location.
Weight is the force with which the Earth attracts a body. Its SI unit is the newton (N), and it is measured with a spring balance. Weight does change with location, because gravity is not the same everywhere.
Volume is the space occupied by a body. Its SI unit is the cubic metre (), and litres and millilitres are used for liquids.
A bag of rice on a shop's beam balance is measured by mass, which is why the same bag reads the same in every shop.
The standard exam case is a body taken to the Moon. Its mass stays the same, because the amount of matter has not changed, but its weight becomes about one-sixth, because the Moon's gravitational pull is weaker. Saying "weight in kilograms" in everyday speech is convenient, but in chemistry the two quantities are separate, with separate units.
Mass is the quantity of matter contained in a body. Its SI unit is the kilogram (kg), and it is measured with a beam balance. Mass does not change with location.
Weight is the force with which the Earth attracts a body. Its SI unit is the newton (N), and it is measured with a spring balance. Weight does change with location, because gravity is not the same everywhere.
Volume is the space occupied by a body. Its SI unit is the cubic metre (), and litres and millilitres are used for liquids.
A bag of rice on a shop's beam balance is measured by mass, which is why the same bag reads the same in every shop.
The standard exam case is a body taken to the Moon. Its mass stays the same, because the amount of matter has not changed, but its weight becomes about one-sixth, because the Moon's gravitational pull is weaker. Saying "weight in kilograms" in everyday speech is convenient, but in chemistry the two quantities are separate, with separate units.
What is matter made of, and what holds it together?
Matter is made of extremely small particles — atoms, which combine to form molecules. A molecule is the smallest particle of a substance that can exist on its own and still show that substance's properties.
Two quantities then decide which state a substance is in:
- Intermolecular space — the distance between neighbouring molecules.
- Intermolecular force of attraction — how strongly they pull on one another.
The two work in opposition. Small spaces mean strong attraction, which locks the particles into a solid. Large spaces mean weak attraction, which lets them fly apart as a gas. Liquids sit in between.
You can see the spaces exist. Dissolve sugar in a full glass of water and the level barely rises, because the sugar particles slip into gaps between the water molecules. A drop of ink spreading through still water shows the particles moving on their own.
Molecules are also in constant motion — vibrating in place in a solid, sliding past each other in a liquid, and moving freely and rapidly in a gas.
The common misconception is that heating makes molecules bigger. It does not. Heating makes them move faster, which pushes them further apart and increases the intermolecular space — and that is what expansion actually is.
Two quantities then decide which state a substance is in:
- Intermolecular space — the distance between neighbouring molecules.
- Intermolecular force of attraction — how strongly they pull on one another.
The two work in opposition. Small spaces mean strong attraction, which locks the particles into a solid. Large spaces mean weak attraction, which lets them fly apart as a gas. Liquids sit in between.
You can see the spaces exist. Dissolve sugar in a full glass of water and the level barely rises, because the sugar particles slip into gaps between the water molecules. A drop of ink spreading through still water shows the particles moving on their own.
Molecules are also in constant motion — vibrating in place in a solid, sliding past each other in a liquid, and moving freely and rapidly in a gas.
The common misconception is that heating makes molecules bigger. It does not. Heating makes them move faster, which pushes them further apart and increases the intermolecular space — and that is what expansion actually is.
How do solids, liquids and gases compare?
The three states differ on seven properties, and each difference follows from the spacing of the particles.
Solid — definite shape and definite volume; high density; almost incompressible; very small intermolecular space; very strong force of attraction; molecules only vibrate about fixed positions. Ice, iron, wood.
Liquid — no definite shape but a definite volume, so it takes the shape of its container; moderate density; very slightly compressible; larger intermolecular space; moderate force of attraction; molecules slide over one another. Water, oil, milk.
Gas — neither definite shape nor definite volume, filling the whole container; very low density; highly compressible; very large intermolecular space; negligible force of attraction; molecules move freely and rapidly in all directions. Air, oxygen, cooking gas.
An LPG cylinder is the everyday proof of compressibility: a large volume of gas is squeezed into a steel cylinder you can carry, something impossible to do with the same volume of water.
The property most often stated wrongly is a liquid's. A liquid has a definite volume but no definite shape — pour half a litre of water into a jug, a bottle or a bowl and it is still half a litre, but its shape changes each time.
Solid — definite shape and definite volume; high density; almost incompressible; very small intermolecular space; very strong force of attraction; molecules only vibrate about fixed positions. Ice, iron, wood.
Liquid — no definite shape but a definite volume, so it takes the shape of its container; moderate density; very slightly compressible; larger intermolecular space; moderate force of attraction; molecules slide over one another. Water, oil, milk.
Gas — neither definite shape nor definite volume, filling the whole container; very low density; highly compressible; very large intermolecular space; negligible force of attraction; molecules move freely and rapidly in all directions. Air, oxygen, cooking gas.
An LPG cylinder is the everyday proof of compressibility: a large volume of gas is squeezed into a steel cylinder you can carry, something impossible to do with the same volume of water.
The property most often stated wrongly is a liquid's. A liquid has a definite volume but no definite shape — pour half a litre of water into a jug, a bottle or a bowl and it is still half a litre, but its shape changes each time.
What are the correct names for each change of state?
Changing the temperature or the pressure converts one state into another, and each direction has its own name.
- Melting (fusion) — solid to liquid, on heating. Ice to water at .
- Freezing (solidification) — liquid to solid, on cooling. Water to ice.
- Vaporisation (boiling or evaporation) — liquid to gas, on heating. Water to steam at .
- Condensation — gas to liquid, on cooling. Steam to water; dew on a cold bottle.
- Sublimation — solid directly to gas, with no liquid stage. Camphor, naphthalene balls, dry ice, ammonium chloride.
- Deposition — gas directly to solid, the reverse of sublimation. Frost forming on a cold surface.
A camphor tablet kept in a cupboard slowly disappears without leaving any liquid behind, and naphthalene balls in a wardrobe do the same — both are sublimation in an ordinary Indian home.
Heating increases the intermolecular space, so solids become liquids and liquids become gases. Cooling reverses it. Pressure works the other way: squeezing a gas hard enough forces its molecules close enough to become a liquid, which is how cooking gas is stored as a liquid in its cylinder.
The boundary case worth remembering is that the temperature stays constant during a change of state. Ice and water together remain at until the last of the ice has melted, because the heat supplied is used to overcome the forces of attraction rather than to raise the temperature.
- Melting (fusion) — solid to liquid, on heating. Ice to water at .
- Freezing (solidification) — liquid to solid, on cooling. Water to ice.
- Vaporisation (boiling or evaporation) — liquid to gas, on heating. Water to steam at .
- Condensation — gas to liquid, on cooling. Steam to water; dew on a cold bottle.
- Sublimation — solid directly to gas, with no liquid stage. Camphor, naphthalene balls, dry ice, ammonium chloride.
- Deposition — gas directly to solid, the reverse of sublimation. Frost forming on a cold surface.
A camphor tablet kept in a cupboard slowly disappears without leaving any liquid behind, and naphthalene balls in a wardrobe do the same — both are sublimation in an ordinary Indian home.
Heating increases the intermolecular space, so solids become liquids and liquids become gases. Cooling reverses it. Pressure works the other way: squeezing a gas hard enough forces its molecules close enough to become a liquid, which is how cooking gas is stored as a liquid in its cylinder.
The boundary case worth remembering is that the temperature stays constant during a change of state. Ice and water together remain at until the last of the ice has melted, because the heat supplied is used to overcome the forces of attraction rather than to raise the temperature.
Exam tip
Exam tip: explaining a state by its space and force
Nearly every "give a reason" question in this chapter is answered by naming the intermolecular space and the force of attraction.
So: gases are highly compressible because the intermolecular space is very large and the force of attraction is negligible. Two clauses, full marks. An answer that only says "because gas is light" earns nothing.
For mass and weight, always give the unit with the quantity — kilogram for mass, newton for weight — and name the instrument if asked: beam balance for mass, spring balance for weight.
Use the exact term for a change of state. Writing "solid to gas" where the question expects sublimation loses the mark, as does confusing sublimation with evaporation.
And in comparison questions, answer property by property in the same order for all three states. A jumbled paragraph is much harder to award marks to than three parallel lines.
So: gases are highly compressible because the intermolecular space is very large and the force of attraction is negligible. Two clauses, full marks. An answer that only says "because gas is light" earns nothing.
For mass and weight, always give the unit with the quantity — kilogram for mass, newton for weight — and name the instrument if asked: beam balance for mass, spring balance for weight.
Use the exact term for a change of state. Writing "solid to gas" where the question expects sublimation loses the mark, as does confusing sublimation with evaporation.
And in comparison questions, answer property by property in the same order for all three states. A jumbled paragraph is much harder to award marks to than three parallel lines.
Did you know
Why does a camphor tablet vanish without ever becoming wet?
Because its molecules escape straight from the solid into the air, skipping the liquid stage altogether.
In most solids the forces of attraction must be loosened into a liquid first, and only then broken fully into a gas. In camphor those forces are weak enough that molecules at the surface can leave directly, so no puddle ever forms.
That is sublimation, and it is put to work in the laboratory too: a mixture of common salt and ammonium chloride can be separated by heating it, because only the ammonium chloride sublimes away.
In most solids the forces of attraction must be loosened into a liquid first, and only then broken fully into a gas. In camphor those forces are weak enough that molecules at the surface can leave directly, so no puddle ever forms.
That is sublimation, and it is put to work in the laboratory too: a mixture of common salt and ammonium chloride can be separated by heating it, because only the ammonium chloride sublimes away.
Key takeaways
Matter and its composition: quick revision
- Matter occupies space and has mass; mass is measured in kilograms on a beam balance, weight in newtons on a spring balance, and volume in cubic metres.
- Mass stays the same everywhere, while weight changes with gravity — about one-sixth on the Moon.
- Matter is made of atoms and molecules, and the intermolecular space together with the force of attraction decides the state.
- Heating does not enlarge molecules; it makes them move faster and pushes them further apart.
- Solids have definite shape and volume and are incompressible; liquids have definite volume but no definite shape; gases have neither and are highly compressible.
- Melting, freezing, vaporisation, condensation, sublimation and deposition name the six changes — and the temperature stays constant while any change of state is in progress.
You will remember all of this far better after answering five questions on it than after reading it twice.
- Mass stays the same everywhere, while weight changes with gravity — about one-sixth on the Moon.
- Matter is made of atoms and molecules, and the intermolecular space together with the force of attraction decides the state.
- Heating does not enlarge molecules; it makes them move faster and pushes them further apart.
- Solids have definite shape and volume and are incompressible; liquids have definite volume but no definite shape; gases have neither and are highly compressible.
- Melting, freezing, vaporisation, condensation, sublimation and deposition name the six changes — and the temperature stays constant while any change of state is in progress.
You will remember all of this far better after answering five questions on it than after reading it twice.