Principles of Chemistry

States of Matter and Particle Model

Edexcel IGCSE Chemistry Revision Notes

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Key Definitions and Terminology

• **State of matter**: One of the three physical forms in which a substance can exist — solid, liquid, or gas.
• **Particle model (kinetic theory)**: A model that describes matter as being made up of tiny particles (atoms, ions, or molecules) that are in constant motion, with the nature of that motion depending on the state of matter.
• **Melting point**: The temperature at which a solid changes state to a liquid at standard pressure.
• **Boiling point**: The temperature at which a liquid changes state to a gas throughout the bulk of the liquid at standard pressure.
• **Diffusion**: The net movement of particles from a region of higher concentration to a region of lower concentration, due to their random motion.
• **Brownian motion**: The random, erratic movement of small visible particles (e.g., smoke particles) caused by collisions with invisible gas or liquid molecules, providing evidence for the kinetic particle model.

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Main Concepts

1. Arrangement, Movement, and Energy of Particles in Three States

| Property | Solid | Liquid | Gas |

|---|---|---|---|

| Arrangement | Regular, closely packed pattern (lattice) | Irregular, closely packed but no fixed pattern | Random, widely spaced |

| Movement | Vibrate about fixed positions | Move around each other; slide past one another | Move rapidly in random directions |

| Relative energy | Lowest kinetic energy | Moderate kinetic energy | Highest kinetic energy |

| Spacing | Very close together | Close together (slightly further apart than solid) | Very far apart |

| Forces between particles | Strong forces hold particles in place | Weaker forces than solids; particles can move | Negligible forces between particles |

2. Changes of State

• Changes of state are **physical changes** — no new substances are formed, and the changes are **reversible**.
• The particle themselves do not change during a change of state; only the arrangement, spacing, and movement of particles change.

The key transitions:

| Change of State | Name | Energy Change |

|---|---|---|

| Solid → Liquid | Melting | Energy absorbed (endothermic) |

| Liquid → Gas | Boiling / Evaporation | Energy absorbed (endothermic) |

| Gas → Liquid | Condensation | Energy released (exothermic) |

| Liquid → Solid | Freezing | Energy released (exothermic) |

| Solid → Gas (directly) | Sublimation | Energy absorbed (endothermic) |

• During melting or boiling, the temperature remains **constant** even though energy is being supplied. This is because the energy is used to **overcome the forces of attraction between particles**, not to increase their kinetic energy.

3. Heating and Cooling Curves

• A **heating curve** is a graph of temperature against time when a substance is heated steadily.
• **Flat (horizontal) sections** on a heating curve represent changes of state — the temperature stays constant while energy breaks intermolecular forces.
• The first plateau corresponds to **melting**; the second corresponds to **boiling**.
• A **cooling curve** shows the reverse process, with plateaux at the condensation and freezing points.

4. Explaining Properties Using the Particle Model

• **Solids** have a fixed shape and volume because particles are held in fixed positions by strong forces.
• **Liquids** have a fixed volume but take the shape of their container because particles can slide over each other.
• **Gases** have no fixed shape or volume and fill any container because particles move rapidly in all directions with negligible forces between them.
• Gases are **compressible** (particles are far apart with lots of empty space); solids and liquids are virtually **incompressible**.

5. Diffusion and Evidence for the Particle Model

• Diffusion occurs in **liquids and gases** (not in solids, because particles in solids cannot change position).
• The rate of diffusion is **faster in gases** than in liquids because gas particles move much more quickly and have larger spaces between them.
• **Lighter particles diffuse faster** than heavier particles at the same temperature (they have a higher average speed).
• Brownian motion (e.g., observing smoke particles under a microscope jerking randomly) provides direct evidence that gas molecules are in constant, random motion.

6. Predicting States from Given Temperatures

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

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Worked Examples and Real-World Applications

Worked Example 1: Predicting the State of a Substance

Question: Ethanol has a melting point of −114 °C and a boiling point of 78 °C. What is the state of ethanol at 25 °C?

Answer:

• 25 °C is **above** the melting point (−114 °C) and **below** the boiling point (78 °C).
• Therefore, ethanol is a **liquid** at 25 °C. ✓

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Worked Example 2: Interpreting a Heating Curve

Question: The heating curve below shows a pure substance being heated from −20 °C to 120 °C. There are flat sections at 0 °C and 100 °C.

(a) What changes of state occur at 0 °C and 100 °C?

(b) Why does the temperature remain constant at these points?

Answer:

(a)

• At 0 °C: **melting** (solid → liquid) ✓
• At 100 °C: **boiling** (liquid → gas) ✓

(b)

• The energy being supplied is used to **overcome the forces of attraction (intermolecular forces) between particles** ✓ rather than increasing the kinetic energy of the particles ✓, so the temperature does not rise.

*(This substance is water.)*

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Worked Example 3: Real-World Application — Ammonia and HCl Diffusion Experiment

Scenario: A cotton wool pad soaked in concentrated ammonia solution (NH₃, Mr = 17) is placed at one end of a glass tube, and a pad soaked in concentrated hydrochloric acid (HCl, Mr = 36.5) is placed at the other end. A white ring of ammonium chloride (NH₄Cl) forms inside the tube, closer to the HCl end.

Explanation:

• Both ammonia and hydrogen chloride molecules diffuse along the tube as gases.
• Ammonia molecules are **lighter** (Mr = 17) than hydrogen chloride molecules (Mr = 36.5), so ammonia particles move **faster** and diffuse further in the same time.
• The white ring therefore forms **closer to the HCl end** of the tube. ✓
• This demonstrates that the rate of diffusion depends on the **relative molecular mass** of the gas — lighter molecules diffuse faster.

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Exam Technique Tips (Edexcel-Specific)

> Tip 1: Use precise Edexcel mark scheme language for changes of state.

> When explaining why temperature remains constant during a change of state, always write that energy is used to "overcome the forces of attraction between particles" (or "overcome intermolecular forces"). Do not say "break bonds" — Edexcel mark schemes penalise this because covalent/ionic bonds within particles are not broken during changes of state. The forces between molecules (intermolecular forces) are what are overcome. This is one of the most common mistakes and frequently costs marks.

> Tip 2: For "describe" vs "explain" questions on particle diagrams.

> If asked to describe the differences between states, focus on arrangement, spacing, and movement (e.g., "particles are closely packed in a regular pattern and vibrate about fixed positions"). If asked to explain a property (e.g., why gases are compressible), you must link the particle description to the property (e.g., "gas particles are far apart with large spaces between them, so when pressure is applied the particles can be pushed closer together"). Edexcel examiners award marks for the link, not just the description — always connect the particle behaviour to the observable property.

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*End of revision notes — States of Matter and Particle Model*