Kinetic particle model of matter overview

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Kinetic particle model of matter

All 4 topics and the revision summary on one page.

States of matter

Use the arrangement, motion, forces and spacing of particles to explain the properties of solids, liquids and gases.

Particles may be atoms or molecules, depending on the substance. The kinetic particle model describes matter using these particles and their interactions. It links what we observe, such as a liquid flowing, to an explanation at a much smaller scale.

A drawing of circles is a model, not a literal microscope image. Its symbol sizes, gaps and particle counts help show differences; they do not give the actual sizes or numbers in a sample.

Use arrangement and motion to explain the properties

Circles represent particles such as atoms or molecules. The particle symbols stay the same size. These are schematic models, with selected motion indicators shown in blue.

Solid: a crystalline model

Fixed shape and approximately fixed volume. Close particles vibrate about fixed mean positions.

Liquid

Takes the container's shape; approximately fixed volume. Close particles move past one another and change neighbours.

Gas

Fills the available container and is easily compressed. Widely separated particles move randomly through the space.

Compare all four features: particle arrangement, motion, forces and spacing. The solid panel represents a crystalline arrangement. The symbols are the same size across the panels; a change of state does not mean that the atoms or molecules become bigger.

Solid

Observable properties: Usually keeps its own shape and volume under ordinary conditions. It does not flow like a liquid and is difficult to compress.

Arrangement and spacing: Particles are close together and remain near fixed positions. The regular pattern drawn here represents a crystalline solid; not every solid has that regular arrangement.
Motion: Particles vibrate about their positions. They are not motionless, even though they do not move freely through the solid.
Forces: Forces between particles keep them near their positions and resist changes of shape. Their close spacing makes a large reduction in volume difficult.

Liquid

Observable properties: Has a nearly fixed volume but takes the shape of the part of the container it occupies. It flows and is difficult to compress.

Arrangement and spacing: Particles are close together in an irregular arrangement. There is much less space between them than in a gas.
Motion: Particles move randomly and can move past their neighbours. This allows the liquid to flow and change shape.
Forces: Attractions keep the particles close while allowing them to rearrange. Liquid particles do not behave as if there are no forces between them.

Gas

Observable properties: Has no fixed shape or volume. It flows, spreads through the available container and can be compressed much more easily than a solid or liquid.

Arrangement and spacing: Particles are far apart compared with their own sizes. Much of the container is space between the particles.
Motion: Particles move rapidly in random directions through the available volume, changing direction when they collide.
Forces: In the ordinary gas model, attractions are negligible for most of the time between collisions. Particles are not held close together as in a liquid or solid.

Explain two properties of the same liquid

Water can be poured because its particles can move past one another. At the same time, it is difficult to compress because its particles are already close together. Being able to flow does not mean that there must be large empty spaces between the particles.

Worked explanation

Air and water in sealed syringes

Compare two otherwise identical sealed syringes, one containing air and the other filled with water and no trapped air. Apply comparable modest forces to their plungers.

Observation: the air volume decreases much more than the water volume.
Gas explanation: its particles are initially widely separated, so the spaces between them can become much smaller.
Liquid explanation: its particles are already close together, so there is little space to remove without strong resistance.

The air molecules themselves are not being squeezed into much smaller molecules. The large change is in their spacing. Trapped air in the water syringe would make the comparison less useful because that air can compress.

A gas can fill a larger volume without larger particles

If a fixed amount of gas is allowed into a larger container, its random motion spreads the particles through the newly available space. The number and sizes of the particles do not need to increase. Their average spacing becomes greater.

Its mass stays the same while its occupied volume increases, so its density, mass divided by volume, decreases. A particle explanation and a bulk quantity can describe the same change from different viewpoints.

Use the feature that explains the property. Motion past neighbours explains flow. Close spacing explains the difficulty of compression. Forces help explain why particles remain close or near fixed positions. Listing "particles move" alone does not explain all three states.

Optional check Water can be poured from one container into another, but a water-filled sealed syringe is difficult to compress. Which particle explanation accounts for both observations?

Temperature and particle motion

A rise in temperature corresponds to an increase in the average kinetic energy associated with the particles' random motion.

Kinetic energy is associated with motion. The particle model concerns random motion within a substance, including vibration in a solid. It is different from the whole object travelling across a room.

Consider the same gas first at 20°C and then at 50°C. In the warmer sample, the particles have greater average random kinetic energy. Their sizes and number do not increase just because the temperature rises.

Same gas, different temperatures

The particle count, symbol size and gas state are unchanged. The warmer sample has greater average random kinetic energy.

Blue arrows show particle motion. Lengths give a qualitative indication of speed; they do not give a numerical speed or temperature ratio.

20 °C

50 °C

Individual speeds vary. Higher temperature describes the average random kinetic energy of all the particles.

These models show the same gas at 20°C and 50°C. The same number of equally sized particle symbols is used. Motion arrows have varied directions and lengths in both panels; the warmer sample has greater typical speeds, not one identical speed for every particle.

Average does not mean every particle moves alike

Particles have a range of kinetic energies and continually exchange energy in collisions. A temperature rise concerns the average over all the particles, not a requirement that every individual particle speeds up at every instant.

For a solid, particles continue vibrating about their positions. Warming makes the average kinetic energy of that vibration greater; it does not require particles to stream freely through the solid.

The Celsius readings establish which temperature is higher, but their ratio is not an energy ratio. For example, 50°C is not evidence for 2.5 times the average kinetic energy at 20°C. A cooler sample at these temperatures still contains moving particles.

Measure the sample's temperature

Temperature may be represented by θ (theta) or T. The symbol θ commonly denotes a Celsius temperature in °C, while T commonly denotes a temperature in kelvin (K). These letters name the quantity; °C and K name the units. Always retain the stated scale and unit with a reading.

A thermometer measures temperature through a property that changes as its sensing region responds to the sample. Place and read it so that it represents the sample you intend to measure.

Choose a suitable range and resolution. A thermometer covering -10°C to 110°C can include the 20°C and 50°C readings above. With divisions every 1°C, the scale does not justify reporting many decimal places.
Immerse the sensing region correctly. Follow the instrument's immersion marking or instructions. A liquid-in-glass thermometer needs its bulb in the sample; do not leave the bulb mainly in the air above it.
Avoid contact with the vessel. Keep the sensing region clear of the base and sides, especially while the vessel is being heated. Gentle stirring, where suitable, can reduce temperature differences within a liquid.
Allow time for the response. For a sample held at a steady temperature, wait until the reading settles. If the sample is continuously warming, a slow thermometer can lag behind its changing temperature.
Read the scale properly. View the top of a liquid-in-glass thermometer's column at eye level to reduce parallax. Record the unit and the relevant time when following a change.

Measurement reasoning

Two readings of the same warming liquid

A thermometer touches the heated base of a beaker. That region may be hotter than the bulk liquid, so its reading can be too high for the intended liquid temperature. Moving the sensor into the liquid, away from the base, addresses the placement problem.

A second thermometer is correctly placed but responds slowly. While the liquid warms, its reading can be below the liquid's current temperature. Repeating the same timing does not remove this response lag; use a suitable sensor and account for its response time.

Repeated readings can reveal variation, but they do not correct a sensor touching the wrong object or an unsuitable response time. A finely divided display also does not guarantee that the measurement is accurate.

Compare this temperature relationship with internal energy, which includes total random kinetic energy and particle potential energy.

Optional check The same gas warms from 20 degrees C to 50 degrees C. What does this establish about its particles?

03 / Pure

Brownian motion: evidence and explanation

The irregular movement of a small suspended particle provides evidence for the random motion of the surrounding fluid's molecules.

A fluid is a liquid or gas. In the particle model, its molecules move randomly. A Brownian-motion experiment links that model to an observable effect.

What the smoke-cell experiment shows

A transparent smoke cell holds small smoke particles suspended in air. Light shines into it from the side, and a microscope is focused on the illuminated region. Light scattered by the smoke particles makes them appear as small bright specks.

Observe a speck over time. It moves irregularly, with changing direction and speed, rather than following one smooth straight path. This irregular motion of a small particle suspended in a fluid is Brownian motion.

Keep the observation separate from its explanation

1. Apparatus: illuminate the smoke from the side

2. Observation: bright specks move irregularly

This is a schematic trace. In the experiment, look for irregular jiggling, and distinguish it from a shared drift of many specks.

3. Model: unseen air molecules collide with the speck

The explanation uses molecules too small to see in the experiment. The large speck and small molecules are not drawn to a true size ratio.

Keep the observation separate from the explanation. The microscope view shows suspended smoke particles. The model explains one particle's motion through changing collisions from much smaller, unseen air molecules. Any drawn trace illustrates successive positions; it is not a recorded air-molecule trajectory.

Visible specks are not air molecules

The bright objects are smoke particles, each much larger than an air molecule. Individual air molecules are not seen in this microscope view. Their motion is inferred from what happens to the suspended particles.

Air molecules move randomly and continually collide with a suspended smoke particle.
At a particular instant, the collisions are not exactly balanced on all sides, so there is a resultant force on the smoke particle.
The imbalance changes from one instant to the next, changing the smoke particle's motion and producing the irregular observed path.

Worked explanation

A speck turns without meeting another visible speck

Observation: the smoke speck changes direction, although no other visible smoke particle touches it.

Explanation: unseen air molecules still bombard it. A changing imbalance in these collisions changes the force and the speck's motion. A visible collision between two smoke specks is not required.

Inference: the air molecules are moving randomly. The experiment gives evidence through their effects; it does not show their individual paths directly.

Small particles suspended in water can also show Brownian motion. In that case, changing unequal collisions from water molecules explain the suspended particles' irregular movement. The inference concerns random molecular motion in a liquid rather than in a gas.

Distinguish jiggling from a common drift

Focus sharply enough to follow individual illuminated specks, and observe several over time. A speck moving out of focus has not necessarily stopped moving; it may have left the focused region.

If many specks travel together steadily in one direction, the fluid itself may be flowing. That common drift alone does not establish the random-collision explanation. Reduce bulk movement and look for irregular changes superimposed on any drift.

Say what is seen and what is inferred. The suspended specks are observed. Random molecular motion is inferred. A drawing or animation can illustrate the model but is not itself experimental evidence.

Optional check Through a microscope, a bright smoke speck changes direction without meeting another visible speck. Which conclusion distinguishes observation from inference?

04 / Pure

Gas pressure from particle motion

Moving gas particles continually strike the container walls. Their collisions produce an average force per unit area: the gas pressure.

Pressure is force divided by area, measured in Pa. Temperature relates to average random particle kinetic energy. Use the conditions of the container before predicting a pressure change.

From a collision to a pressure

A gas particle approaching a wall changes its motion when it collides and rebounds. The wall exerts a force on the particle during the collision; the particle exerts an opposite force on the wall.

Many particles strike a wall at different times. Their combined effect is an average force acting perpendicular to its surface. Divide this force by the wall area to obtain the pressure.

Random motion takes particles towards every wall, including the sides and the top. Gas pressure is therefore not a downward force caused only by the gas's weight. A pressure force has the direction normal to the particular surface being considered.

A fixed amount of gas in the same rigid container

The container is sealed and its volume stays fixed. Warming the gas increases its average random kinetic energy and raises its pressure.

Blue arrows show particle motion. Lengths give a qualitative indication of speed; they do not give a numerical speed or temperature ratio.

20 °C

50 °C

A gas particle collides with a wall

Blue arrows: particle motion. Red arrow: force exerted on the wall. Collisions occur at every wall; their average force per unit area is the gas pressure.

At fixed volume, faster particles collide with a wall more frequently and give greater force contributions. The average force per unit area increases.

The two containers have the same volume and number of particles. The warmer gas has greater average kinetic energy. The collision inset separates arrows showing particle motion from the force exerted on the wall.

Warm a fixed amount in a rigid sealed container

Controlled comparison

The same gas at 20°C and 50°C

The container is rigid, so volume is unchanged. It is sealed, so the number of gas particles is unchanged. Warm the gas from 20°C to 50°C.

The higher temperature means greater average random kinetic energy, with greater typical particle speeds.
Particles reach a given wall more frequently, and their collisions make greater force contributions.
The average force on a given wall area increases, so pressure increases.

The molecules do not become larger. There is no need to add particles to explain the increase. The Celsius readings do not give a pressure ratio of 50 / 20.

The fixed conditions matter. If the gas can expand or escape, the volume or number of particles may change too, so warming does not justify the same pressure prediction by itself.

Compress the same gas while keeping its temperature unchanged

Now consider a sealed syringe containing a fixed amount of gas. Compress it slowly while keeping the temperature unchanged. The particles have the same average random kinetic energy, but occupy a smaller volume.

With particles closer together in the reduced space, the rate of collisions on each unit area of wall increases. The average force per unit area therefore increases. The explanation uses the changed volume and collision rate, without assuming that the molecules shrink or become faster.

Separate motion arrows from force arrows. An arrow showing a particle's velocity is not an extra force. The force on a wall comes from collisions and acts perpendicular to that wall.

Optional check A sealed syringe contains a fixed amount of gas. It is compressed slowly while its temperature is kept unchanged. Why does its pressure increase?

Revision summary

Connect the property to the particle model

Solid: Particles are close and vibrate near fixed positions. Forces keep them near those positions, giving a fixed shape and volume under ordinary conditions. Close spacing makes compression difficult.
Liquid: Particles are close and irregularly arranged; attractions keep them close while they move past one another. The liquid flows and changes shape but keeps nearly the same volume and is difficult to compress.
Gas: Widely separated particles move randomly through the available volume. Attractions are negligible for most of the time between collisions. Large spaces make compression easy compared with liquids and solids.

Particles may be atoms or molecules. A crystalline model's regular arrangement is not a claim about every solid. Gas expansion changes particle spacing, not the size of each molecule.

Temperature and measurement

Temperature symbols include θ (theta) and T, commonly used for Celsius and kelvin temperatures respectively. State the unit, °C or K; a symbol or number alone does not give a complete temperature reading.

A temperature rise means increased average random kinetic energy of all the particles. Individual speeds remain varied; solid particles vibrate. Ratios of Celsius readings are not particle-energy ratios.

Choose a thermometer with a suitable range and resolution. Immerse its sensing region correctly, keep it clear of the vessel, allow for response time and read an analogue scale at eye level. Repetition does not correct poor sensor placement or response lag.

Pure: observation and inference

Illumination and a microscope reveal smoke specks moving irregularly. The specks are suspended particles, not individual air molecules. Changing unequal molecular collisions explain their movement and provide evidence for random molecular motion. A common directional drift alone may be fluid flow.

Pure: wall collisions and pressure

Gas particles collide with walls and exert forces. Their average force per unit wall area is pressure, acting on every wall. At fixed volume and particle number, higher temperature gives greater typical speeds, more frequent wall collisions and greater force contributions, so pressure rises.

At unchanged temperature, compressing a fixed quantity increases the wall-collision rate per unit area and raises pressure without requiring faster or smaller molecules. State the conditions before making a prediction.

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