States of Matter and Limitations of Models

This lesson covers states of matter and the limitations of bonding models as required by the AQA GCSE Combined Science Trilogy specification (8464), sections 4.2.1–4.2.3. You need to understand the three states of matter, state changes, and recognise the limitations of the models used to describe bonding and structure.

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The Three States of Matter

All substances can exist in three states: solid, liquid and gas. The state of a substance at a given temperature depends on the strength of the forces between its particles.

State	Arrangement	Movement	Spacing
Solid	Regular, ordered pattern	Vibrate in fixed positions	Closely packed
Liquid	Irregular, no fixed pattern	Move around each other freely	Close together but can slide
Gas	Random, no pattern	Move rapidly in all directions	Far apart

Particle Model Diagram

graph LR
    A["SOLID<br/>Particles vibrate<br/>in fixed positions<br/>Regular pattern"] -->|"Melting<br/>(energy in)"| B["LIQUID<br/>Particles move<br/>around each other<br/>Close together"]
    B -->|"Boiling / Evaporating<br/>(energy in)"| C["GAS<br/>Particles move<br/>rapidly, randomly<br/>Far apart"]
    C -->|"Condensing<br/>(energy out)"| B
    B -->|"Freezing<br/>(energy out)"| A

    style A fill:#3498db,color:#fff
    style B fill:#27ae60,color:#fff
    style C fill:#e74c3c,color:#fff

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State Changes

A state change occurs when a substance changes from one state to another. During a state change:

• The temperature stays constant while the change of state is happening (because the energy is being used to overcome forces between particles, not to increase kinetic energy).
• The forces between particles are overcome (melting, boiling) or formed (freezing, condensing).
• The particles themselves do not change — it is the arrangement and movement that change.

Names of State Changes

Change	Direction	Energy
Melting	Solid → Liquid	Energy absorbed (endothermic)
Boiling / Evaporating	Liquid → Gas	Energy absorbed (endothermic)
Freezing	Liquid → Solid	Energy released (exothermic)
Condensing	Gas → Liquid	Energy released (exothermic)
Sublimation	Solid → Gas (directly)	Energy absorbed (endothermic)

Exam Tip (AQA 8464): During a state change, the temperature remains constant. This is because the energy being supplied is used to overcome the forces between particles, not to increase the kinetic energy (temperature) of the particles.

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Predicting the State of a Substance

You can predict the state of a substance at a given temperature if you know its melting point and boiling point:

• If the temperature is below the melting point → the substance is a solid.
• If the temperature is between the melting point and boiling point → the substance is a liquid.
• If the temperature is above the boiling point → the substance is a gas.

Worked Example

Sodium chloride has a melting point of 801°C and a boiling point of 1413°C. At room temperature (25°C):

• 25°C is below 801°C, so sodium chloride is a solid at room temperature.

Water has a melting point of 0°C and a boiling point of 100°C. At 50°C:

• 50°C is between 0°C and 100°C, so water is a liquid at 50°C.

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Linking States of Matter to Bonding and Structure

The melting and boiling points of a substance depend on the type and strength of the forces between its particles:

Substance Type	Forces to Overcome	Melting/Boiling Point	Example
Simple molecular	Weak intermolecular forces	Low	Water (100°C), methane (−162°C)
Giant ionic	Strong electrostatic forces between ions	High	NaCl (801°C)
Giant covalent	Many strong covalent bonds	Very high	Diamond (3550°C)
Giant metallic	Strong metallic bonds	High (varies)	Iron (1538°C)
Polymer	Intermolecular forces (and cross-links)	Moderate	Poly(ethene) (~130°C)

Exam Tip: When comparing melting points of different substances, always identify the type of structure and the type of force that must be overcome.

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Limitations of Bonding Models

The models used to describe bonding and structure (dot-and-cross diagrams, ball-and-stick models, 2D diagrams) are very useful, but they all have limitations.

Limitations of Dot-and-Cross Diagrams

Limitation	Explanation
Show only outer shell electrons	Do not show the inner electron shells or the nucleus
2D representation	Real atoms and bonds exist in 3D — the diagram cannot show the true shape of the molecule or lattice
Do not show bond length or bond strength	All bonds appear the same, regardless of actual differences
Do not show the relative size of atoms/ions	Atoms appear the same size when they may be very different
Give the impression of static electrons	In reality, electrons are constantly moving in orbitals

Limitations of Ball-and-Stick Models

Limitation	Explanation
Bonds are shown as "sticks"	In reality, bonds are regions of electron density, not physical sticks
Atoms shown as solid spheres	Atoms are mostly empty space
Spacing between atoms is misleading	Balls are shown much larger relative to the sticks than atoms are relative to bond lengths
Bonds appear identical	In reality, bonds vary in length, strength and character

Limitations of 2D Lattice Diagrams

Limitation	Explanation
Only show a small section	Real lattices contain billions of particles
Flat 2D representation	Real lattices are 3D structures
Suggest a defined edge/boundary	Real lattices do not have neat edges
Cannot show the dynamic nature	Particles are constantly vibrating, not static

graph TD
    A["Bonding Models"] --> B["Dot-and-Cross<br/>Diagrams"]
    A --> C["Ball-and-Stick<br/>Models"]
    A --> D["2D Lattice<br/>Diagrams"]
    B --> B1["✓ Show electron<br/>transfer/sharing<br/>✗ 2D only, no bond<br/>length or 3D shape"]
    C --> C1["✓ Show 3D shape<br/>of molecules<br/>✗ Misleading atom<br/>sizes and bond<br/>representations"]
    D --> D1["✓ Show regular<br/>repeating pattern<br/>✗ 2D, small section,<br/>suggest neat edges"]

    style A fill:#2c3e50,color:#fff
    style B fill:#3498db,color:#fff
    style C fill:#27ae60,color:#fff
    style D fill:#e67e22,color:#fff

Exam Tip (AQA 8464): You may be asked to "describe one limitation" of a particular model. Common answers include: "The model only shows a 2D representation, but the actual structure is 3D" or "The diagram does not show the relative sizes of atoms." Be specific about which limitation applies to the model being discussed.

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Summary Comparison Table: All Bonding Types

Feature	Ionic	Simple Covalent	Giant Covalent	Metallic	Polymer
Particles	Ions	Molecules	Atoms	Metal ions + e⁻	Long chains
Bonding	Electron transfer	Electron sharing	Electron sharing	Delocalised electrons	Covalent bonds in chain
Structure	Giant lattice	Small molecules	Giant lattice	Giant lattice	Long tangled chains
Melting point	High	Low	Very high	High	Moderate
Electrical conductivity	When molten/dissolved	No	No (except graphite)	Yes	No
Example	NaCl	H₂O	Diamond	Fe	Poly(ethene)

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Common Mistakes