# States of Matter and the Kinetic Particle Theory

> Chemistry · CIE IGCSE 0620
> Source: https://www.owlsprep.com/study/cie-0620-u1-states-of-matter-and-the/

This guide covers core properties of solids, liquids and gases, their particle arrangements, state changes, heating/cooling curves, the effect of temperature and pressure on the volume of a gas, plus extended content on diffusion for CIE IGCSE Chemistry 0620.

**Prerequisites:** [Basic understanding of pure substances and mixtures](https://www.owlsprep.com/study/cie-0620-u1-pure-substances-and-mixtures/)

## Learning objectives

- Describe the properties and particle arrangements of solids, liquids and gases
- Explain state changes using the kinetic particle theory
- Interpret heating and cooling curves for pure substances
- Describe the effects of temperature and pressure on the volume of a gas (and, for Extended, explain them using kinetic particle theory)
- Predict the state of a substance at a given temperature using melting/boiling point data
- (Extended only) Explain diffusion, and the effect of relative molecular mass on the rate of diffusion of gases, using the kinetic particle theory

## Properties and Particle Arrangements of the Three States of Matter

All matter exists in one of three common states: solid, liquid, or gas. The kinetic particle theory explains the observable properties of each state by describing the arrangement, movement, and energy of their constituent particles.

**Kinetic Particle Theory** — A model that describes all matter as being made of tiny, constantly moving particles, with varying amounts of energy depending on their state and temperature.

*Example:* Particles in a gas have much higher kinetic energy than particles in a solid at the same pressure.

| State | Shape | Volume | Particle Arrangement | Particle Movement |
| --- | --- | --- | --- | --- |
| Solid | Fixed | Fixed | Tightly packed, regular lattice | Vibrate around fixed positions |
| Liquid | Takes shape of container | Fixed | Close together, irregular arrangement | Slide past each other, move freely within the liquid |
| Gas | Fills entire container | Fills container, easily compressed | Far apart, random arrangement | Move quickly and randomly in all directions |

**Worked example:** A student has an unknown pure substance sample. The sample has a fixed volume but takes the shape of the beaker it is poured into. State the phase of the substance, and describe the movement of its particles.

1. Step 1: Match the given properties to the three states of matter. Fixed volume applies to solids and liquids; taking the shape of a container applies to liquids and gases. The overlapping state is liquid.
2. Step 2: Describe particle movement for a liquid. Particles in a liquid are close together with no regular arrangement, and slide freely past each other while moving at moderate speed.

> **Exam tip:** Always link any property of a state directly to particle arrangement or movement in structured questions to earn full marks.

## State Changes and Heating/Cooling Curves

State changes are reversible physical changes where no new substance is formed. They occur when particles gain or lose enough energy to change their movement and arrangement. Common state changes include melting, freezing, boiling, condensing, and sublimation.

**Sublimation** — A state change where a solid turns directly into a gas, or a gas turns directly into a solid, without forming a liquid phase.

*Example:* Solid carbon dioxide (dry ice) sublimes at room temperature to form carbon dioxide gas.

> **tip**
>
> The only three subliming substances you need to memorise for 0620 are iodine, solid carbon dioxide (dry ice), and ammonium chloride.

Heating curves show how the temperature of a pure substance changes as it is heated. Temperature rises steadily when a substance is in a single state, as particles gain kinetic energy. During state changes, temperature stays constant: all energy is used to overcome forces between particles, rather than increasing their speed. Cooling curves follow the reverse pattern.

**Worked example:** A pure substance has a melting point of 17°C and a boiling point of 118°C. State the phase of the substance at 25°C, and explain what happens to its temperature as it is heated from 10°C to 25°C.

1. Step 1: Compare 25°C to the given melting and boiling points. 17°C < 25°C < 118°C, so the substance is in the liquid state at 25°C.
2. Step 2: Explain temperature changes from 10°C to 17°C: the substance is solid, so temperature rises steadily as particles gain kinetic energy and vibrate faster.
3. Step 3: Explain temperature at 17°C: temperature stays constant as all energy is used to overcome forces between solid particles during melting.
4. Step 4: Explain temperature changes from 17°C to 25°C: the substance is liquid, so temperature rises again as liquid particles gain kinetic energy and move faster.

> **Exam tip:** You may be asked to draw or label parts of a heating/cooling curve in structured exams, so practice identifying melting, boiling, and single-state regions.

## Effect of Temperature and Pressure on the Volume of a Gas

Unlike solids and liquids, the volume of a fixed mass of gas changes noticeably when its temperature or pressure changes. For 0620 you must be able to describe these two effects.

> **info**
>
> Increasing the temperature of a gas at constant pressure increases its volume. Increasing the pressure on a gas at constant temperature decreases its volume.

**Worked example:** A fixed mass of gas is trapped in a sealed gas syringe. Describe what happens to the volume of the gas when (a) the syringe is warmed gently at constant pressure, and (b) the plunger is pushed in to increase the pressure at constant temperature.

1. Part (a): Warming the gas at constant pressure increases its volume, so the plunger is pushed outwards and the gas occupies a larger volume.
2. Part (b): Increasing the pressure at constant temperature decreases the volume, so the gas is squeezed into a smaller space.

> **Extended Content Only**
>
> Extended candidates (Papers 2 and 4) must also EXPLAIN these effects using the kinetic particle theory. Core candidates only need to describe the effects.

(Extended) When a gas is heated at constant pressure, its particles gain kinetic energy and move faster. They collide with the container walls harder and more often, so the gas expands to occupy a larger volume until the pressure is balanced again. When the pressure on a gas is increased at constant temperature, the particles are forced closer together into a smaller space, so the volume of the gas decreases.

> **Exam tip:** Always state which quantity is being kept constant (temperature or pressure) when describing the effect on gas volume, or you may lose marks.

## Extended Only: Diffusion

> **Extended Content Only**
>
> This section is only for Extended candidates taking Papers 2 and 4. Core candidates taking Papers 1 and 3 can skip this section entirely.

**Diffusion** — The net random movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, until particles are evenly spread.

*Example:* The smell of perfume spreading across a room is caused by diffusion of gas particles.

At the same temperature, gas particles with a smaller relative molecular mass have a higher average speed, so they diffuse faster than gas particles with a larger relative molecular mass. This is a qualitative comparison only: lighter gas particles diffuse faster than heavier ones.

**Worked example:** Explain why brown bromine gas spreads upwards to fill an inverted gas jar placed on top of a jar of bromine, even though bromine gas is denser than air.

1. Step 1: Link the observation to kinetic particle theory. Bromine gas particles and air particles both have high kinetic energy and move randomly in all directions.
2. Step 2: Explain movement despite higher density. The random motion of the particles is strong enough to overcome the effect of gravity, so bromine particles diffuse upwards into the upper jar, and air particles diffuse down into the lower jar until the mixture is uniform pale brown in both jars.

> **Exam tip:** For diffusion questions, always mention the random movement of particles and the concentration gradient if applicable to earn full marks.

## Common pitfalls

- **Wrong:** Stating that particles in a solid do not move at all.
  - Why it fails: All particles have kinetic energy, even in solids: they vibrate around fixed positions, they do not stay completely stationary.
  - Correct: State that solid particles vibrate around fixed positions, as they only stop moving at absolute zero (a concept not tested in 0620).
- **Wrong:** Confusing boiling and evaporation as the same process.
  - Why it fails: Boiling happens at a fixed boiling point throughout the liquid, while evaporation only happens at the liquid surface at any temperature between melting and boiling point.
  - Correct: Clearly distinguish the two processes if asked, linking each to particle behaviour and temperature requirements.
- **Wrong:** Stating that temperature rises during melting or boiling.
  - Why it fails: During state changes, all heat energy is used to overcome inter-particle forces, so temperature stays constant for pure substances.
  - Correct: Mention that temperature remains constant during state changes, and explain why using energy use for breaking inter-particle forces.
- **Wrong:** (Extended) Stating that diffusion only happens in gases.
  - Why it fails: Diffusion also occurs in liquids (e.g. potassium manganate(VII) spreading in water) but is slower because liquid particles move more slowly than gas particles.
  - Correct: State that diffusion occurs in both gases and liquids, but is faster in gases due to higher particle speed and more space between particles.

## Cheatsheet

| Concept | Core Required Knowledge | Extended Required Knowledge |
| --- | --- | --- |
| Solid Properties | Fixed shape/volume, particles vibrate in regular lattice | Same as core, plus explain density relative to other states |
| Liquid Properties | Fixed volume, takes container shape, particles slide past each other | Same as core, explain why liquids are incompressible |
| Gas Properties | Fills container, compressible, particles move randomly fast | Same as core, link to pressure from particle collisions with container walls |
| State Changes | Name all 6 changes, interpret heating/cooling curves, know melting/boiling point definitions | Same as core, explain state changes in terms of energy changes and inter-particle forces |
| Gas Volume (Temperature/Pressure) | Describe: higher temperature (constant pressure) increases volume; higher pressure (constant temperature) decreases volume | Explain both effects using kinetic particle theory (particle speed, collisions, spacing) |
| Diffusion | Not required for core | Define diffusion, explain observations using kinetic particle theory, and state that lighter (smaller Mr) gases diffuse faster (qualitative only) |

## What's next

Now that you have mastered the kinetic particle theory and states of matter, you are ready to move on to more foundational chemistry concepts for CIE IGCSE Chemistry 0620. The next core topic is pure substances and mixtures, where you will learn how to separate different mixtures using physical methods, building on your understanding of particle behaviour and state changes. Extended candidates will also apply the kinetic particle theory later when studying rates of reaction, where collision theory builds on the particle movement concepts you have learned here. This topic is the foundation for all other physical chemistry content in the syllabus, so make sure you have memorised the key properties and can explain observations using the particle model before moving forward. Use the cheatsheet above for quick revision before mock exams or the final assessment.

---

From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/cie-0620-u1-states-of-matter-and-the/