Particulate Nature of Matter | CBSE Class 8 Science Notes

This note covers the particles that make up matter, spaces and attractions between particles, the properties of solids, liquids and gases, melting, boiling, evaporation, compression, dissolving, particle movement and the effect of heating.

What does the particulate nature of matter mean?

Matter, the material of which substances and objects are made, consists of extremely small particles. Its particulate nature means that a larger piece of material is built from a very large number of these tiny units.

Definition: A constituent particle is a basic unit making up a larger piece of a substance or material. Constituent particles are extremely small and cannot be seen even through an ordinary microscope.

What does breaking chalk show?

A chalk stick can be broken into smaller pieces and then ground into a fine powder. A magnifying glass, which makes small objects appear larger, reveals tiny grains in the powder. Each visible speck is still chalk, rather than a new substance.

1. Take a chalk stick and break it into two pieces.
2. Continue breaking the pieces until further breaking by hand becomes difficult.
3. Grind the small pieces using a mortar and pestle, the bowl and grinding tool used to crush material.
4. Observe the resulting powder with a magnifying glass and identify its grains as chalk.

Grinding is a physical change: the pieces become smaller without producing a new substance. The visible powder grains are not the smallest constituent units. They themselves contain many smaller particles.

Imagine continuing this subdivision until reaching the basic units of chalk. This imagined continuation helps explain how a whole chalk stick can consist of many constituent particles. It does not mean that the basic particles have been seen through the magnifying glass.

Are sand grains the smallest pieces of rock?

Rocks gradually break down through erosion, the wearing away of rock. Rivers carry rock pieces and break them down further into pebbles, stones and sand. Even fine grains of sand and clay contain many constituent particles.

The important distinction is between a grain that can be observed and the extremely small units that compose it. Breaking a visible object into smaller visible pieces does not by itself reveal its constituent particles.

How does dissolving sugar reveal particles too small to see?

Dissolving is the process in which a substance such as sugar separates into its constituent particles and spreads among water particles. The resulting sugar-containing water is a solution. The sugar is present even when its particles can no longer be seen.

How is the sugar activity carried out?

Use drinking water and sugar for this particular activity. Perform it under the supervision of a teacher or an adult. Never eat or drink anything during an activity unless asked to do so.

1. Fill a glass tumbler with drinking water.
2. Add two teaspoons of sugar and initially leave the water unstirred.
3. Under the stated supervision, compare the taste of a small spoonful from the top layer before stirring.
4. Stir until the sugar has dissolved completely, then compare the taste of a small spoonful from the top layer again.

After the sugar dissolves, the top layer tastes sweet. This observation shows that sugar has reached that layer and remains in the water. Its disappearance from view does not mean that it has ceased to exist.

Each tiny sugar grain contains millions and millions of constituent particles. On dissolving, these separate from one another. They are too small to be seen, but their presence can be detected by the sweetness of the solution.

Where do the sugar particles go?

Interparticle spaces are the spaces between constituent particles. The separated sugar particles occupy available spaces between water particles. Water therefore has a particulate structure even though it looks continuous to the unaided eye.

The chalk and sugar activities provide related evidence. Chalk shows that visible pieces can be subdivided while remaining the same substance. Sugar shows that a substance can remain present after its particles become too small to observe.

Note: The tasting step belongs only to the supervised drinking-water-and-sugar activity. It is not a method for identifying other materials used in experiments.

How do attractions between particles affect the state of matter?

Interparticle attractions are the attractive forces holding constituent particles together. Their strength depends on the nature of the substance and the interparticle distance, meaning the distance between its particles.

Even a slight increase in this distance decreases the attractive forces drastically. These attractions help determine the physical state of a substance: solid, liquid or gas.

A solid has a definite shape and volume; a liquid has definite volume but no fixed shape; a gas has neither fixed shape nor fixed volume. Volume means the amount of space occupied.

How are spacing, movement and attraction connected?

Particles packed closely together experience strong attractions that restrict their movement. In a solid, they remain about fixed positions. In a liquid, the attractions are slightly weaker than in solids, allowing particles to move around within a limited space.

In a gas, particles move freely in all directions and attractions are negligible. Negligible here means very small in the explanation of their behaviour. Gas particles can therefore spread through all the space available to them.

Why does heating matter?

Thermal energy means heat energy. The thermal energy of particles affects their movement and separation, and therefore their physical state. Heating can make a solid's particles vibrate more vigorously or increase the movement of particles in a liquid.

The explanation links energy, distance and attraction. It is not enough to say that particles simply disappear when a state changes. The particles remain, while their arrangement and freedom of movement change.

Why do solids retain their shape, and how do they melt?

A solid has a definite shape and a definite volume, the amount of space it occupies. An iron nail, rock salt, a stone, a wooden block, a key and a piece of aluminium are examples.

Solid particles are tightly packed and held by very strong interparticle attractions. These forces keep them in fixed positions and prevent them from moving freely past one another.

Are particles in a solid motionless?

Solid particles can vibrate or oscillate, meaning move to and fro about their positions. They do not move past one another as liquid particles do. A fixed particle position therefore does not mean a complete absence of movement.

When a solid is heated, these vibrations become more vigorous. Eventually, particles begin leaving their positions as the attractions weaken. The solid changes into a liquid through melting.

Three schematic particle drawings show a closely packed solid, particles with increased vibrations, and a liquid with a less ordered arrangement. These are representations of particles, not photographs of them.

See Fig. 7.4 in your NCERT textbook

What is the melting point?

The melting point is the minimum temperature at which a solid becomes a liquid at atmospheric pressure. Atmospheric pressure is the pressure exerted by the surrounding air. Temperature values below use °C, meaning degrees Celsius. In the table, S.No. means serial number.

Some solids have weak interparticle attractions and low melting points, while others have strong attractions and high melting points. The listed temperatures show that different solids melt at different temperatures.

Generally, particles in a liquid are somewhat farther apart than in the solid state. Ice is an exception: its particles are farther apart than those in liquid water. This exception must be retained when comparing solid and liquid particle spacing.

Why do liquids change shape but retain a definite volume?

A liquid has a definite volume but no fixed shape. Its particles can move around within a limited space. Their attractions are slightly weaker than in solids, but remain strong enough to keep the particles close together.

What happens when the same water is transferred?

Label three clean, dry containers of different shapes A, B and C; these letters identify the containers. Mark the 200 mL level in each. The symbol mL means millilitre, a unit used to measure volume.

1. Fill container A with water up to its 200 mL mark.
2. Transfer the water carefully from A into B without spilling it.
3. Observe the shape of the water and check its level against B's mark.
4. Transfer the same water into C without spilling, and compare its shape and volume again.

Water takes the shape of each container. In all three containers, its volume remains 200 mL. Its changing outline therefore demonstrates a change of shape without an observed change of volume.

The drawing shows water in a bottle labelled A, a jug labelled B and a tumbler labelled C. Each container has a mark labelled 200 mL.

See Fig. 7.5 in your NCERT textbook

If a container is not clean, some water may stick to its walls. After pouring, the next container's water level may then be slightly less than 200 mL. Careful transfer and clean containers matter when interpreting the observation.

Why can a finger move through water?

Moving a finger through water temporarily displaces the water. Removing the finger allows the water to return. The water is not permanently broken or cut, because its particles can move around one another.

The slightly weaker attractions explain this movement, while the remaining attraction keeps the particles together within a limited space. Both parts of the explanation are needed: liquid particles have freedom to move, but not the unrestricted spreading of gas particles.

Milk spreading when spilled and a glass tumbler retaining its shape illustrate the same distinction. Milk behaves as a liquid, while the tumbler behaves as a solid.

How do boiling and evaporation differ?

When a liquid is heated, particle movement becomes more vigorous. Particles move farther apart, interparticle attractions decrease, and particles can escape from the liquid state. The substance then forms vapour, its gaseous state.

The boiling point is the temperature at which a liquid boils and changes into vapour at atmospheric pressure. The pressure condition belongs to the definition and should be included when explaining the term.

Where does vapour form?

At the boiling point, vapour forms very rapidly both at the surface and within the liquid. Formation inside the liquid is observed as bubbles. This is different from the slower vapour formation taking place at the surface below the boiling point.

Evaporation is the slower formation of vapour at a liquid's surface. Vapour formation occurs at all temperatures, even below the boiling point. Spilled water disappearing after some time is an example of evaporation.

Why should the two processes not be confused?

A liquid need not reach its boiling point before any vapour can form. The disappearance of spilled water therefore does not show that it boiled. Boiling and evaporation both produce vapour, but differ in where and how rapidly this happens.

The particle explanation connects both processes to particles leaving the liquid. It does not require calling every instance of vapour formation boiling, or describing evaporation as a process occurring throughout the liquid.

How do gases occupy the space available to them?

A gas has neither a fixed shape nor a fixed volume. Its particles move freely in all directions, and their interparticle attractions are negligible. Gases tend to occupy the entire space available inside a vessel.

What does the smoke activity show?

Use two transparent gas jars or glass tumblers, labelled A and B for this activity. Smoke from a burning incense stick makes spreading through the jars visible. Be careful when burning the incense stick.

1. Hold jar A upside down over the incense smoke so that smoke collects inside.
2. Turn jar A over and cover it with a glass plate.
3. Place jar B upside down over the plate covering A.
4. Remove the plate slowly, keeping the jars close with no gap through which smoke can escape.
5. Observe the smoke spread into B and occupy the space in both jars.

The spreading shows why a gas cannot be assigned a fixed volume independent of the vessel. It also acquires the vessel's shape. Its particles are not restricted to fixed positions or to the limited space occupied by liquid particles.

Four drawings show smoke collection in jar A, covering A with a glass plate, placing inverted jar B above A while removing the plate, and smoke present in both jars.

See Fig. 7.7 in your NCERT textbook

Are the visible smoke particles gas particles?

Smoke represents the gaseous state in this demonstration, but the visible smoke particles are not the invisible constituent particles of gases. Tiny smoke particles suspended in air are constantly struck by gas particles. Their movement helps make gas-particle motion observable.

Iodine vapour can also be used to demonstrate spreading. It forms when solid iodine is left in a closed gas jar for some time. Handle solid iodine carefully because its vapour can cause irritation.

Fluids are substances that flow and do not retain a fixed shape. Both liquids and gases belong to this group. A liquid nevertheless retains a definite volume, whereas a gas spreads through all available space.

What does a syringe show about spaces between particles?

Compression means reducing the space occupied by a substance. A syringe can demonstrate that air can be compressed. Use a syringe without a needle and its plunger, the movable part pushed into or pulled out of the syringe.

How is trapped air compressed?

1. Pull the plunger outwards to its fully extended position.
2. Cover the open end with a thumb so that the air cannot escape.
3. Push the plunger slowly and steadily inwards.
4. Observe the reduction in air volume, then stop pushing and observe the plunger move back.

Pushing the plunger brings the air particles closer together. Gas particles have a lot of space between them, and this space can be reduced by pressure applied from outside. The activity changes the distance between particles rather than showing particles leaving through the closed opening.

Three drawings show a needle-free syringe with its plunger pulled out, its opening covered by a thumb, and its plunger pushed in while the opening remains covered.

See Fig. 7.9 in your NCERT textbook

When pushing stops, the gas particles spread again and the plunger returns to its original position. This links the visible movement of the plunger to the spreading of the trapped gas.

What happens when water is used instead?

Repeating the activity with water shows that water is practically incompressible. This means its volume is not appreciably reduced in the activity. The qualification “practically” matters; it should not be replaced by an unlimited claim of absolute incompressibility.

The comparison supports a difference in particle spacing. Air has large spaces that can be reduced. Water particles are much closer together. The inability to compress water appreciably does not mean there are no spaces at all between its particles.

Closely packed solid particles also leave some space between them. These interparticle spaces are not filled with air: they contain nothing at all.

How does the water level change when sugar dissolves?

A water-level activity gives another way to investigate interparticle spaces. It compares the level before adding sugar, immediately after adding it, and after stirring until it dissolves. These are distinct stages and should be recorded separately.

What do the three level marks mean?

1. Half-fill a glass vessel with water and label the initial level A.
2. Add two teaspoons of sugar and label the new level B.
3. Stir with a glass rod until the sugar dissolves.
4. Label the level after dissolving C and compare it with B.

The letters here identify water-level marks: A is the initial level, B is the level after adding sugar, and C is the level after dissolving. They are not calculated quantities.

Initially, adding sugar raises the level. After dissolution, the level may decrease to some extent. The volume of the solution is less than the sum of the separate volumes of water and sugar, indicating space between water particles.

Dissolved sugar particles occupy these spaces. The explanation does not require sugar to disappear from the material in the vessel. Nor does the observation establish that the final level must return exactly to the original mark.

The schematic drawing places yellow circles labelled “Sugar particle” among smaller blue circles labelled “Water particle”. It represents the distribution of sugar particles in water.

See Fig. 7.11 in your NCERT textbook

Why is adding sand different?

A soluble substance dissolves in water; sugar, common salt and glucose are examples here. An insoluble substance does not dissolve in water; sand is an example.

When sand is added to water, it settles and occupies space in the container, increasing the total volume. Both sugar and sand are solids, but their behaviour in water differs. The fact that a substance is solid does not tell us that it will dissolve.

How can particle movement be observed in liquids and gases?

Particle motion can be investigated through the spread of colour in water and fragrance in air. These activities reveal the effects of constituent-particle movement even though the extremely small particles themselves cannot be seen.

Why does colour spread through water?

Potassium permanganate is the substance whose grains produce pink colour in this activity. Use a spoon or spatula to handle it; do not touch it with bare hands. Put a few grains into a tumbler containing water.

Initially, streaks of pink spread out from the grains. Over time, the whole bulk of water becomes uniformly pink. Uniform means that the water has the same pink colour throughout, without visible differences in colour intensity.

Water particles are in constant motion. They first pull potassium permanganate particles from the grain and then strike them, spreading them through the water. In many substances, particles are held too strongly for water particles to pull them out; sand is one such insoluble substance.

How does heating affect the spreading?

Compare three clean tumblers containing hot water, water at room temperature and ice-cold water. Drop a small potassium permanganate grain into each. The spreading is fastest in hot water, less quick at room temperature and slowest in ice-cold water.

Supplying heat increases particle movement. The colour spreads fastest in hot water, less quickly in water at room temperature and slowest in ice-cold water.

Why does an incense fragrance travel across a room?

After an incense stick is lit in one corner, the fragrance is first noticed nearby and shortly afterwards throughout the room. Constantly moving air particles strike the fragrance particles and help them spread. Perfume fragrance reaching someone gives a related everyday example.

Particle interactions also help explain cleaning oily clothes with soap. Numerous soap particles surround oil particles on the fabric. One end attaches to the oil while the other mixes with water, helping lift and wash the oil away.

How can the three states of matter be compared together?

A comparison of shape, volume, spacing, attraction and motion brings the particle explanation together. Properties visible at the scale of a whole object arise from the arrangement and behaviour of its constituent particles.

The spacing comparison is a general pattern. Ice remains the stated exception: its particles are farther apart than those in liquid water. The table should therefore be used together with that qualification.

How does energy connect the states?

In a solid, low thermal energy keeps particle movement restricted to small vibrations. At the melting point, energy is used to overcome attractions, allowing particles to leave fixed positions. Liquid particles can then move within a limited space.

In the gaseous state, particles have enough energy to overcome the attractions between them and move freely. This connects the three states without confusing a change of physical state with the disappearance of matter.

Does “particle” always mean a constituent particle?

Suspended Particulate Matter, abbreviated SPM, refers here to tiny dust particles suspended in air. Such dust particles are much larger than constituent particles. Each dust particle itself contains a very large number of constituent particles.

A grain of chalk, a dust particle and an invisible constituent particle therefore belong to different scales of description. Recognising the intended meaning prevents visible grains or smoke particles from being mistaken for the smallest units composing a substance.

What are atoms and molecules?

Atoms and molecules are the tiny constituent particles of matter. A molecule is a particle formed when atoms combine. Iron consists of iron atoms, and gold consists of gold atoms.

Two hydrogen atoms combine to form a stable hydrogen molecule. A water molecule contains two hydrogen atoms and one oxygen atom. Thus, the particles discussed in explanations of matter are much smaller units than visible grains or specks.

Acharya Kanad, an ancient Indian philosopher, put forward the idea of Parmanu, meaning atom. He believed matter consisted of tiny, indivisible, eternal particles. This historical idea appears in his work Vaisheshika Sutras.

Glossary

• Constituent particle — An extremely small basic unit that makes up a larger piece of a substance or material.
• Particulate nature — The description of matter as being composed of a very large number of extremely small particles.
• Interparticle space — The space between constituent particles, present even when particles are closely packed in a solid.
• Interparticle attraction — An attractive force between constituent particles that helps hold them together within a substance.
• Vibration — To-and-fro movement about a position, shown by particles held in fixed positions within a solid.
• Solid — A state with definite shape and volume, whose closely packed particles vibrate about fixed positions.
• Liquid — A state with definite volume but no fixed shape, whose particles move within a limited space.
• Gas — A state without fixed shape or volume, whose particles move freely through all available space.
• Melting point — The minimum temperature at which a solid changes into a liquid at atmospheric pressure.
• Boiling point — The temperature at which a liquid boils and turns into vapour at atmospheric pressure.
• Evaporation — The slower formation of vapour at a liquid's surface, occurring even below its boiling point.
• Fluid — A substance that flows and does not retain a fixed shape; liquids and gases are fluids.
• Compression — Reduction in the space occupied by a substance, demonstrated when trapped air is squeezed inside a syringe.
• Thermal energy — Heat energy associated with particles, affecting their movement, separation and the physical state of matter.
• Insoluble in water — Describes a substance, such as sand, whose particles do not separate and dissolve in water.

Common errors and misconceptions

• Misconception: Sugar ceases to exist when it dissolves. Correct: Its constituent particles remain among water particles, and the solution tastes sweet.
• Misconception: The particles of a solid are completely motionless. Correct: They vibrate about fixed positions but cannot move past one another.
• Misconception: Liquid particles are always farther apart than solid particles. Correct: This is generally the pattern, but ice particles are farther apart than those in liquid water.
• Misconception: Water is absolutely incompressible under every condition. Correct: Water is practically incompressible in the syringe activity.
• Misconception: Dissolving sugar must return the water level exactly to its original mark. Correct: After initially rising, the level may decrease to some extent after dissolution.
• Misconception: Interparticle spaces contain air. Correct: These spaces contain nothing at all; air itself consists of particles.
• Misconception: Vapour forms only when a liquid boils. Correct: Evaporation forms vapour at the surface even below the boiling point.
• Misconception: Visible smoke particles are the constituent particles of gases. Correct: Invisible gas particles strike the suspended smoke particles, whose movement helps reveal gas-particle motion.

Exam-style questions with model answers

Q1. A chalk stick is broken and ground into fine powder. Each observed grain remains chalk. Identify the type of change and explain whether the visible grains are constituent particles. [2 marks]

1. Grinding chalk is a physical change because it reduces the size of the pieces without forming a new substance.
2. The visible grains are not individual constituent particles. Each grain contains many extremely small constituent particles.

Q2. Sugar is completely dissolved in drinking water under adult supervision. The top layer tastes sweet although no sugar grains are visible. Explain the observations using particles. [3 marks]

1. The sweet taste shows that sugar remains present in the water and has reached the top layer rather than ceasing to exist.
2. During dissolving, sugar separates into extremely small constituent particles. These cannot be seen even though the solution contains them.
3. The sugar particles occupy available spaces between water particles, producing a solution in which the dissolved sugar is no longer visible as grains.

Q3. A pupil transfers 200 mL of water successively through three clean, dry containers of different shapes without spilling. State the expected shape and volume observations, explain them, and describe what may happen if a container is not clean. [4 marks]

1. The water takes the shape of each container because a liquid has no fixed shape of its own.
2. Its volume remains 200 mL in each clean container when the same water is transferred without spilling.
3. Liquid particles move around within a limited space. Their attractions are slightly weaker than in solids but still keep them close together.
4. If a container is not clean, some water may stick to its walls, leaving slightly less than 200 mL in the next container.

Q4. In a needle-free syringe, air is trapped by covering the opening. Pushing the plunger reduces its volume; stopping allows the plunger to return. Repeating with water causes no appreciable volume reduction. Explain these observations in three points. [3 marks]

1. Air particles have large spaces between them. Pushing the plunger brings them closer together, reducing the volume occupied by the trapped air.
2. When pushing stops, the gas particles spread again. This makes the plunger move back to its original position.
3. Water is practically incompressible in this activity. Its particles are much closer together than gas particles, so its volume is not appreciably reduced.

Q5. Water in a vessel has an initial level A. Two teaspoons of sugar are added and the raised level is marked B. After stirring until dissolution, level C is marked. Explain the level changes and particle arrangement, and contrast the result with adding sand. [5 marks]

1. Adding the sugar initially raises the water level from A to B. This first observation is made before the sugar has completely dissolved.
2. After dissolution, the level may decrease to some extent from B. The description does not require C to coincide exactly with the original level A.
3. Sugar separates into extremely small constituent particles during dissolution. These particles occupy available spaces between the constituent particles of water.
4. The solution volume is less than the sum of the separate sugar and water volumes, supporting the presence of spaces between water particles.
5. Sand does not dissolve in water. Its particles settle and occupy space in the vessel, increasing the total volume instead of forming a dissolved solution.

Q6. Compare solids, liquids and gases under these five headings: shape, volume, particle spacing, attractive forces and movement. Include the ice-water exception in the spacing comparison. [5 marks]

1. A solid retains a definite shape. A liquid takes its container's shape, and a gas also acquires the shape of its vessel.
2. Solids and liquids have definite volumes. A gas has no fixed volume and tends to occupy the whole space available to it.
3. Particles are closely packed in solids, generally somewhat farther apart in liquids, and farthest apart in gases. Ice particles are farther apart than liquid-water particles.
4. Attractions are very strong in solids, slightly weaker in liquids, and negligible in gases. Their strength helps explain the different freedom of movement.
5. Solid particles vibrate about fixed positions; liquid particles move within a limited space; gas particles move freely in all directions through available space.

Q7. Small potassium permanganate grains are placed in hot, room-temperature and ice-cold water. Pink colour spreads through all three, fastest in hot water and slowest in ice-cold water. Explain the spread and the temperature comparison. [3 marks]

1. Water particles constantly move. They pull potassium permanganate particles out of the grains and strike them, distributing them through the water.
2. Water particles move faster in hot water, so the potassium permanganate spreads fastest there. Providing heat increases particle movement.
3. Movement is slower at room temperature and slowest in ice-cold water. The colour therefore spreads less quickly at room temperature and slowest in ice-cold water.

Q8. One sample of water forms bubbles throughout the liquid while boiling at atmospheric pressure. Another sample, spilled on a table, disappears slowly below its boiling point. Name and distinguish the two processes. [2 marks]

1. The first process is boiling, with rapid vapour formation inside the liquid and at its surface at the boiling point.
2. The second is evaporation, the slower formation of vapour only at the surface, occurring even below the boiling point.

Key takeaways

• Matter consists of extremely small constituent particles; visible grains of chalk, sugar and dust themselves contain many such particles.
• Particle spacing, attractive forces and freedom of movement together explain the different properties of solids, liquids and gases.
• Solid particles vibrate about fixed positions; liquid particles move within a limited space; gas particles move freely throughout available space.
• Melting and boiling involve changes in particle movement and attractions, and their temperature definitions include atmospheric pressure.
• Evaporation forms vapour slowly at a liquid's surface, even below the boiling point; boiling also forms vapour within the liquid.
• Air can be compressed because of its large interparticle spaces, while water is practically incompressible in the syringe activity.
• Dissolved sugar occupies spaces between water particles; the level may decrease to some extent after the sugar has dissolved.
• Heating increases particle movement, so potassium permanganate spreads fastest in hot water and slowest in ice-cold water.
• Generally, liquid particles are somewhat farther apart than solid particles, but ice particles are farther apart than those in liquid water.

Test yourself

Why is a fine grain of chalk not necessarily a constituent particle?

The visible grain itself contains many much smaller constituent particles, which cannot be seen even through an ordinary microscope.

What does the sweet taste of a clear sugar solution establish?

Sugar remains present as extremely small particles even though visible sugar grains can no longer be observed.

What does a solid particle do while remaining about a fixed position?

It vibrates or oscillates to and fro about that position without moving past neighbouring particles.

Which property do liquids and gases share that makes them fluids?

Both liquids and gases can flow and do not retain a fixed shape of their own.

What occupies the spaces between constituent particles?

The interparticle spaces contain nothing at all; they are not filled with air.

What is the important exception when comparing particle spacing in solids and liquids?

Ice particles are farther apart than those in liquid water, contrary to the general solid-liquid spacing comparison.

Why does fragrance from an incense stick spread across a room?

Constantly moving air particles strike fragrance particles and help distribute them throughout the room.

Why should the smoke activity not be described as directly seeing gas particles?

The visible smoke particles are suspended in air and struck by invisible gas particles. Their motion provides evidence of gas-particle movement.