Know the terms for the changes in state between solids, liquids and gases (gas to solid and solid to gas transfers are not required)

2.1.1 States of Matter

Learning Objective

By the end of this unit students will be able to:

• Identify the macroscopic properties that distinguish solids, liquids and gases.
• Describe each state of matter using the particle‑model (arrangement, motion and intermolecular forces).
• Explain how particle motion is related to temperature and to the concept of absolute zero.
• State the origin of pressure in gases (and, briefly, in liquids).
• Recognise the experimental evidence for the kinetic model (Brownian motion).
• Write the symbolic form, common name and energy change for the five principal phase changes that involve solids, liquids and gases (solid ↔ gas transitions are not required).

1. Distinguishing Macroscopic Properties

2. Particle‑Model Overview

• Solids – Particles are tightly packed in a regular (often crystalline) pattern. They vibrate about fixed positions. Strong intermolecular forces keep the particles together.
• Liquids – Particles remain close together but are free to slide past one another. Intermolecular forces are weaker than in solids, allowing the liquid to flow and take the shape of its container.
• Gases – Particles are far apart and move rapidly in straight lines until they collide with each other or with the walls of the container. Intermolecular forces are negligible for an ideal gas.

3. Temperature, Kinetic Energy & Absolute Zero

Temperature measures the average kinetic energy of the particles in a substance.

• Higher temperature  →  higher average kinetic energy  →  faster particle motion.
• At absolute zero (0 K, –273 °C) the average kinetic energy is at its theoretical minimum; particles are essentially at rest (quantum‑mechanical zero‑point motion is ignored at the IGCSE level).

4. Origin of Pressure

• Gases – Pressure is produced by frequent, elastic collisions of fast‑moving particles with the walls of the container. The pressure increases if the temperature (collision speed) rises or if more particles are present (higher density).
• Liquids – Pressure also results from particle collisions, but because the particles are much closer together the effect is much smaller. In practice we refer to hydrostatic pressure, which depends on depth and the weight of the liquid above.

5. Evidence for the Kinetic Model – Brownian Motion

The erratic, jittery movement of microscopic particles (e.g., pollen grains) suspended in a liquid is called Brownian motion. It provides direct visual evidence that liquid particles are in constant random motion and collide with the suspended particles.

6. Terminology for Phase Changes (solid ↔ gas not required)

• Melting – solid → liquid
• Freezing (Solidification) – liquid → solid
• Evaporation – liquid → gas (occurs at any temperature below the boiling point; surface phenomenon)
• Boiling – rapid bulk evaporation when a liquid reaches its boiling point; bubbles form throughout the liquid.
• Condensation – gas → liquid

7. Phase‑Change Summary Table

8. Energy Considerations During a Phase Change

While a substance changes phase its temperature remains constant. The heat supplied or released is called latent heat and is calculated by:

Q = m L

• Q – heat energy (J)
• m – mass of the substance (kg)
• L – appropriate latent heat (L_f for fusion, L_v for vapourisation)

9. Real‑World Examples

1. Ice melting at 0 °C → solid → liquid (melting).
2. Water freezing at 0 °C → liquid → solid (freezing).
3. Water evaporating from a puddle on a warm day → liquid → gas (evaporation).
4. Steam condensing on a cold window → gas → liquid (condensation).
5. Water boiling at 100 °C (1 atm) → liquid → gas (boiling).

10. Suggested Diagram

11. Common Misconceptions

• “Evaporation only occurs at the boiling point.” – Evaporation can happen at any temperature; it is a surface process.
• “Boiling and evaporation are the same.” – Boiling is rapid, bulk vapour formation at a fixed temperature; evaporation is a slow, surface phenomenon.
• “Heat is ‘used up’ during a phase change.” – The energy is stored as latent heat within the material and is released when the reverse change occurs.

12. Quick Revision Checklist

• Can you list the macroscopic properties that distinguish solids, liquids and gases?
• Can you describe the particle arrangement and motion for each state?
• Do you know how temperature relates to particle kinetic energy and what absolute zero represents?
• Can you explain the origin of pressure in gases (and briefly in liquids)?
• Are you able to write the symbolic form, name and energy change for each of the five phase transitions?
• Can you give a real‑world example for melting, freezing, evaporation, boiling and condensation?
• Do you understand why the temperature remains constant during a phase change?

13. Alignment with Cambridge IGCSE Physics 0625 (Core)