Nature of matter

Gases are highly compressible because their particles are separated by relatively large spaces. In air, the particles themselves are very small compared with the total volume occupied by the gas. When the plunger of a closed syringe is pushed, the particles are forced closer together, so the same amount of air occupies less space. The particles do not disappear or normally break apart; their average separation changes. This explains why the volume decreases while the syringe remains closed.

Option A states exactly this particle-level reason. Option B is irrelevant because air is a mixture of gases, and whether it is an element does not explain compression. Option C concerns a supposed colour, not the arrangement of particles. Option D is also incorrect because compression does not mean that the particles break. The empty space between gas particles is therefore the key idea behind the answer.

An iron rod is a solid. Its particles are closely packed and have very little empty space between them. Therefore, pressing it by hand cannot bring the particles much closer, so its volume does not decrease easily. Option D concerns solubility, not compressibility. Exam tip: solids are nearly incompressible because they have very little intermolecular space.

Gases have large spaces between particles so they are highly compressible. Exam tip: compressibility is a key gas property.

Gas particles spread and mix which is called diffusion. Exam tip: smell spreading is a simple example of diffusion.

Diffusion is fastest in gases because particles are free and fast. Exam tip: choose by particle motion.

The governing concept is the particle theory of matter. Diffusion occurs because particles move randomly and intermingle from a region of higher concentration toward a region of lower concentration. In gases, particles are widely separated, experience relatively weak intermolecular attraction, and move rapidly, so they spread quickly. In liquids, particles are much closer together and interact more strongly; their movement is possible but less free, so intermixing takes longer. Therefore option A gives the correct reason. Options B, C, and D are scientifically false: liquids contain particles, are not always solids, and may be metallic or non-metallic.

A liquid can have a free surface and takes the container shape. Exam tip: connect surface and flow with liquids.

A gas does not remain at a fixed surface and fills the container. Exam tip: absence of free surface indicates gas.

The governing concept is the relationship between particle arrangement and the physical properties of matter. Rigidity means resistance to a change in shape. In a solid, particles are closely packed and held by comparatively strong attractive forces. They mainly vibrate about fixed mean positions, so the solid retains its own shape and resists deformation. Liquids have a definite volume but their particles can move past one another, allowing them to flow and take the shape of a container. Gases and vapours have particles far apart and possess neither a fixed shape nor a fixed volume. Therefore option A, solid, is correct. The remaining choices describe fluids, which do not normally show rigidity as a characteristic bulk property.

Particles in a liquid are close together but can slide past one another. Therefore, a liquid can flow and is called a fluid. A liquid has a definite volume, but that is not the reason for its fluidity; it does not have a definite shape. Exam tip: substances that can flow, such as liquids and gases, are called fluids.

The intermolecular force between gas particles is very weak, so the particles move freely and the gas can flow. Because it can flow, a gas is classified as a fluid; it takes the shape of its container but has neither a fixed shape nor a fixed volume. Exam tip: both liquids and gases are fluids, whereas solids generally are not.

The governing concept is particle arrangement in different states of matter. In a crystalline or ordinary solid, particles occupy closely spaced positions in an ordered arrangement and mainly vibrate about those positions. A gas has no fixed arrangement: its particles are far apart, move rapidly in different directions, and continually change their relative positions. This produces the greatest degree of randomness among the listed choices. Therefore option B, gas, is correct. Crystalline solids are especially ordered, and metallic solids also possess an organized lattice, so options C and D cannot be correct. Option A is also wrong because solids generally retain an ordered or comparatively fixed arrangement.

In liquids attraction is less than solids and more than gases. Exam tip: treat liquid as the middle state.

Ice is solid water. On heating, it reaches its melting point and first melts to form liquid water, so liquid is correct. It becomes a gas only after the liquid water is heated further and evaporates or boils. Exam tip: remember the usual sequence: solid → liquid → gas.

Water is a liquid. On sufficient heating, it boils and changes into water vapour, which is the gaseous state. Plasma requires extremely high energy and ionisation; ordinary heating of water does not produce plasma. Exam tip: the change from liquid to gas is called vaporisation; boiling is vaporisation at the boiling point.

When water vapour is cooled, its particles lose energy, come closer together, and form droplets of liquid water. This change from a gas to a liquid is called condensation. Ice forms only when liquid water is cooled further to its freezing point. Exam tip: remember that gas → liquid is condensation.

A liquid has fixed volume so it does not fill the whole container. Exam tip: distinguish liquid and gas by volume.

Air is a gas whose particles move continuously and randomly. They collide with the inner walls of the balloon, producing outward pressure; the elastic rubber then stretches and the balloon changes shape. Option B is incorrect because filling a balloon with air does not turn its rubber into a solid; rubber is already a solid elastic material. Exam tip: gas pressure is caused by collisions of gas particles with the walls of their container.

A bottle may look empty, but it contains air. Air is a mixture of gases; it has mass and occupies space, so it is matter. The presence of light alone does not make a bottle contain matter because light is not matter. Exam tip: matter has mass and occupies space.

The governing concept is the effect of particle spacing on the volume of matter. Gas particles are separated by large empty spaces and experience relatively weak attractive forces. Consequently, a gas expands to fill any available container and can be compressed substantially when pressure is applied; its volume therefore changes most easily. Solids have closely packed particles and a nearly fixed volume, while liquids have particles that remain close together and show only slight compressibility, although they can change shape. A crystal is a solid with an ordered structure and similarly resists volume change. Thus option C, gas, is correct; the other options describe condensed states with much less freedom of volume.

In a solid, particles are packed very closely, so the space between them is the least. Therefore, solids have very low compressibility. Particles in liquids are slightly farther apart than in solids, while gases and smoke have much more space between particles. Exam tip: less interparticle space generally means lower compressibility.

Gas has large distance between particles. Exam tip: more empty space means more compressibility.

Particles in solids are very close together, so the force of attraction between them is greatest. In liquids, particles are comparatively farther apart; therefore, the attraction is less than in solids but greater than in gases. Gas particles are farthest apart, so their attraction is minimum. Hence, the correct order is solid > liquid > gas. Option C is incorrect because it places liquids above solids. Exam tip: as the distance between particles increases, the force of attraction generally decreases.

Direct answer: B, solid, liquid, gas. Freedom of particle motion means how freely particles can move from place to place. In a solid, particles are packed closely and mainly vibrate about fixed positions, so their freedom is lowest. In a liquid, particles remain close but can slide past one another, so freedom is greater. In a gas, particles are far apart and move rapidly in many directions, so freedom is greatest. Therefore the increasing order is solid < liquid < gas, which is option B. Option A gives gas, liquid, solid, the reverse order, so it is decreasing rather than increasing. Option C puts liquid before solid, although liquid particles move more freely than solid particles. Option D puts gas before liquid and is also not the correct increasing order. Memory cue: more space and weaker attraction mean more freedom; solid → liquid → gas.

During melting particles become freer and distance increases. Exam tip: remember heating increases particle separation.