# States of Matter

> Chemistry · Edexcel IGCSE 4CH1 (2017)
> Source: https://www.owlsprep.com/study/edexcel-igcse-chemistry-s1-states-of-matter/

This guide covers the particle model of solids, liquids, and gases, state interconversions, diffusion and dilution explanations, solution terms, and higher-only solubility content for Edexcel IGCSE Chemistry (4CH1) S1 Topic 1.

**Prerequisites:** [Basic scientific method for practical experiments](https://www.owlsprep.com/study/edexcel-igcse-chemistry-s1-scientific-skills/)

## Learning objectives

- Describe the arrangement, movement, and energy of particles in solids, liquids, and gases
- Name and explain state interconversions and their associated particle changes
- Explain dilution and diffusion experiments using the particle model
- Define key solution terms: solute, solvent, solution, saturated solution
- [Higher only] Interpret solubility curves and calculate solubility in g per 100g solvent
- [Higher only] Plan and carry out a practical to measure solubility at a fixed temperature

## 1. Particle Model of the Three States of Matter

**Particle Model** — A model that describes all matter as made of tiny, constantly moving particles, whose arrangement, movement, and energy depend on the state of the substance.

Each state of matter has distinct particle properties:
- **Solid**: Regular, tightly packed particles in fixed positions. Particles only vibrate around their fixed positions, with low kinetic energy. Solids have a fixed shape and volume, and cannot be compressed.
- **Liquid**: Randomly arranged, closely packed particles with no fixed pattern. Particles slide past each other and move around slowly, with medium kinetic energy. Liquids have a fixed volume, take the shape of their container, and cannot be compressed easily.
- **Gas**: Randomly arranged particles that are very far apart, with large gaps between them. Particles move freely and rapidly in all directions, with high kinetic energy. Gases have no fixed shape or volume, and can be compressed easily.

**Worked example:** Describe the differences between the arrangement and movement of particles in liquid water and water vapour.

1. Compare arrangement: Liquid water particles are close together, randomly arranged, with almost no space between them. Water vapour particles are far apart, randomly arranged, with large amounts of empty space between them.
2. Compare movement: Liquid water particles slide past each other and move around slowly. Water vapour particles move freely and rapidly in all directions.
3. Compare energy: Water vapour particles have much higher kinetic energy than liquid water particles.

> **tip**
>
> When describing particles in exam answers, always explicitly mention **arrangement**, **movement**, and **energy** separately to get full marks. Avoid vague terms like 'particles move more' — specify how they move.

*Calculator:* allowed

## 2. State Interconversions

| Interconversion Name | Starting State | End State | Energy Change | Particle Change |
| --- | --- | --- | --- | --- |
| Melting | Solid | Liquid | Gained | Particles gain energy, vibrate faster, break free of fixed positions, move around each other |
| Freezing | Liquid | Solid | Lost | Particles lose energy, move slower, form regular fixed positions, only vibrate |
| Boiling | Liquid | Gas | Gained | Particles gain enough energy to break all bonds between them, move freely rapidly |
| Evaporation | Liquid | Gas | Gained | Only surface particles gain enough energy to escape into the gas phase |
| Condensation | Gas | Liquid | Lost | Particles lose energy, move slower, move close together, form a liquid |
| Sublimation | Solid/Gas | Gas/Solid | Gained/Lost | Particles gain/lose enough energy to skip the liquid phase entirely |

**Worked example:** Explain what happens to the particles in molten iron when it cools and solidifies into a solid iron block.

1. Identify the interconversion: Freezing (liquid to solid)
2. Energy change: Particles lose thermal energy as they cool
3. Movement change: Particles slow down, stop sliding past each other
4. Arrangement change: Particles form a regular, tightly packed fixed structure, and only vibrate around their fixed positions

> **Exam tip**
>
> Sublimation examples you can use in exams: iodine solid turning to purple iodine vapour, solid carbon dioxide (dry ice) turning to carbon dioxide gas, ammonium chloride heating to form a gas.

*Calculator:* allowed

## 3. Diffusion and Dilution Explanations

**Diffusion** — The net movement of particles from an area of higher concentration to an area of lower concentration, due to the random movement of particles.

Dilution of coloured solutions (e.g., potassium manganate(VII) purple solution added to water) and diffusion of gases (e.g., bromine gas mixing with air) are both explained by the random movement of particles. When solute and solvent particles move randomly, they spread evenly across the available space over time, resulting in a uniform mixture.

**Worked example:** A student places a drop of blue food colouring in a beaker of still water. After 10 minutes, the entire beaker of water is pale blue. Explain this observation using the particle model.

1. Name the process: This is dilution due to diffusion of particles
2. Describe particle movement: The blue food colouring solute particles and water solvent particles are both constantly moving randomly
3. Explain the outcome: Over time, the blue solute particles spread out evenly from the area of high concentration (the drop) to areas of low concentration (the rest of the water), until the entire solution is a uniform pale blue colour.

> **warning**
>
> Do not mention Graham's law of diffusion or calculate diffusion rates in answers. Edexcel IGCSE only requires qualitative explanations of diffusion using particle movement.

*Calculator:* allowed

## 4. Solution Terms & Solubility [Higher Only]

**Saturated Solution** — A solution in which no more solute can dissolve in the solvent at a given temperature, so any extra solute added will remain as undissolved solid.

- **Solute**: Substance that dissolves in a solvent
- **Solvent**: Liquid that a solute dissolves in
- **Solution**: Homogeneous mixture of solute and solvent
- **Solubility**: Maximum mass of solute that dissolves in 100g of solvent at a fixed temperature, units = g/100g solvent

Solubility curves plot solubility (y-axis: g/100g water) against temperature (x-axis: °C). Most solid solutes have increasing solubility as temperature rises, while gas solubility decreases with temperature. You can read off solubility values at any temperature, or interpolate to find the temperature at which a given mass of solute forms a saturated solution.

Practical to measure solubility at a fixed temperature: 1. Heat 100g of water to the target temperature, 2. Add solute in small portions with stirring until no more dissolves, 3. Filter out undissolved solute, 4. Evaporate the solution to dryness, 5. Weigh the dry solute to get solubility in g/100g water.

**Worked example:** A solubility curve for potassium nitrate shows that its solubility at 60°C is 110g/100g water. What is the maximum mass of potassium nitrate that can dissolve in 250g of water at 60°C, to make a saturated solution?

1. Recall solubility is per 100g water: 110g dissolves in 100g water at 60°C
2. Calculate mass per 1g water: $\frac{110}{100} = 1.1$g per 1g water
3. Multiply by 250g water: $1.1 \times 250 = 275$g
4. Conclusion: Maximum 275g of potassium nitrate can dissolve in 250g water at 60°C.

> **Exam tip**
>
> When plotting solubility curves in exams, always label axes with correct units: y-axis = Solubility (g per 100g water), x-axis = Temperature (°C). Draw a smooth curve of best fit, not straight lines between points.

*Calculator:* allowed

## Common pitfalls

- **Wrong:** Saying particles expand or get bigger when heated
  - Why it fails: Particles do not change size when heated; only the space between them increases
  - Correct: State that particles gain kinetic energy, move faster, and the distance between particles increases, causing the substance to expand
- **Wrong:** Confusing boiling and evaporation as the same process
  - Why it fails: Boiling occurs at a fixed boiling point throughout the liquid, while evaporation occurs only at the surface at any temperature below boiling point
  - Correct: Explicitly distinguish the two processes using the points above when asked to compare them
- **Wrong:** Forgetting to state the temperature when giving solubility values
  - Why it fails: Solubility changes significantly with temperature, so values are meaningless without a stated temperature
  - Correct: Always include the temperature when writing or quoting solubility values in g/100g solvent
- **Wrong:** Describing diffusion as particles 'choosing' to move to low concentration areas
  - Why it fails: Diffusion happens due to random, constant movement of particles, not intentional movement
  - Correct: Explain that particles move randomly, so over time they spread evenly across the available space, resulting in net movement from high to low concentration
- **Wrong:** Saying gases cannot be compressed because their particles are small
  - Why it fails: Gases are compressible because there are large gaps between gas particles
  - Correct: Explain that gas particles are very far apart, so the space between them can be reduced when pressure is applied, making gases easy to compress

## Cheatsheet

| Concept | Key Details | Exam Must-Include |
| --- | --- | --- |
| Solid particles | Regular, tightly packed, vibrate around fixed positions, low energy | Arrangement, movement, energy |
| Liquid particles | Random, close together, slide past each other, medium energy | Arrangement, movement, energy |
| Gas particles | Random, far apart, move freely rapidly, high energy | Arrangement, movement, energy |
| State interconversions | Melting, freezing, boiling, evaporation, condensation, sublimation | Name, energy change, particle changes |
| Diffusion explanation | Net movement from high to low concentration due to random particle movement | Mention random movement of particles, concentration gradient |
| Solubility (Higher only) | g per 100g solvent at fixed temperature, solubility curves y = solubility, x = temperature | Include units and temperature, label axes correctly for curves |

## What's next

Now you have mastered the states of matter and particle model, you are ready to move on to the next topic in Edexcel IGCSE Chemistry Principles of Chemistry: separation techniques, where you will use your understanding of particle properties to explain how to separate mixtures of different states of matter. This foundational particle model knowledge will also be critical for later topics, including chemical bonding, rates of reaction, and gas chemistry. For higher tier students, your solubility curve skills will be used when you learn about crystallisation separation methods and chemical calculations involving solutions. Make sure you practice describing particle changes for state interconversions and explaining diffusion experiments, as these are common 2-3 mark questions in both Paper 1C and Paper 2C exams.

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