# States of matter

> CIE A-Level Chemistry · CIE 9701
> Source: https://www.owlsprep.com/study/cie-9701-u4-overview/
> Weight: n/a

This unit explores the behaviour and properties of gases, liquids, and solids, linking microscopic particle motion and interactions to observable macroscopic properties, a core foundation for physical chemistry.

**Prerequisites:** Basic understanding of atomic structure and intermolecular forces

## Learning objectives

- Explain kinetic molecular theory and apply it to describe the three states of matter
- Use the ideal gas equation to solve physical and stoichiometric chemistry problems
- Classify crystal structures and relate their macroscopic properties to their bonding and microscopic structure

## Unit at a Glance

This unit progresses from the simplest state of matter (gases) where particle interactions are negligible, through liquids with moderate intermolecular forces, to ordered solid crystalline structures. We connect particle behaviour to measurable properties like pressure, volume, density, and melting/boiling point, building a consistent framework for understanding all physical states of matter.

This unit includes three core sub-topics that build sequentially on each other:
- [Gases and ideal gas equation](https://www.owlsprep.com/study/cie-9701-u4-gases-and-ideal-gas-equation/) — Learn kinetic molecular theory and apply the ideal gas equation to solve molar mass and stoichiometry problems.
- [Liquids and solids](https://www.owlsprep.com/study/cie-9701-u4-liquids-and-solids/) — Compare the physical properties of liquids and solids, and explain the origin of phase changes.
- [Crystal structures](https://www.owlsprep.com/study/cie-9701-u4-crystal-structures/) — Classify and describe the structure and properties of the four main types of crystalline solid.

## Common pitfalls

- **Wrong:** Assuming all real gases follow the ideal gas equation perfectly at all temperatures and pressures.
  - Why it fails: Ideal gas assumptions break down at high pressure or low temperature when intermolecular forces and particle volume become significant.
  - Correct: Only use the ideal gas approximation for gases at moderate temperatures and low pressures.
- **Wrong:** Confusing the bonding and properties of different crystal structure types.
  - Why it fails: Each crystal lattice type has distinct bonding that leads to very different macroscopic properties.
  - Correct: Always link observed crystal properties to the type of bonding holding the lattice together.

## Cheatsheet

| Concept / Formula | Key Summary |
| --- | --- |
| Ideal gas equation | $pV = nRT$ |
| Core ideal gas assumption | Gas particles have negligible volume and no intermolecular attractions |
| Ionic crystal | Held together by electrostatic attraction between oppositely charged ions |
| Metallic crystal | Bonded by electrostatic attraction between cations and delocalised electrons |
| Covalent network crystal | Continuous covalent bonding across the lattice, giving very high melting points |

## What's next

Begin your study of this unit with the first sub-topic, Gases and the ideal gas equation, to master core foundational calculations and theory for gaseous states. After you complete all three sub-topics in this unit, you will move on to the next unit covering chemical energetics.

- [Gases and ideal gas equation](https://www.owlsprep.com/study/cie-9701-u4-gases-and-ideal-gas-equation/)
- [Chemical Energetics](https://www.owlsprep.com/study/cie-9701-u5-overview/)
- [Liquids and Solids](https://www.owlsprep.com/study/cie-9701-u4-liquids-and-solids/)

---

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-9701-u4-overview/