Explaining Properties with the Particle Model

Introduction

A model is only useful if it explains what we observe. This page tests the particle model against the everyday properties of solids, liquids and gases: why a stone keeps its shape, why water takes the shape of a glass, why air fills a room, and why you can squeeze a syringe of air but not a syringe of water. Each explanation follows directly from the arrangement, spacing, motion and attractions of particles.

Core explanation

Observed properties of the three states.

Property Solid Liquid Gas --- --- --- --- Shape Fixed Takes the shape of the container Takes the shape of the container Volume Fixed Fixed Fills the whole container Flow Does not flow Flows Flows Compressibility Almost none Almost none Easily compressed

Solids keep their shape and volume because their particles are held in fixed positions by strong attractions. They can only vibrate, so the whole arrangement — and hence the shape — stays the same.

Liquids flow and take the container's shape because their particles can move past each other. Gravity pulls the particles down, and they slide into the lowest available positions, filling the bottom of the container. But the attractions keep them close together, so the volume stays fixed and the liquid does not spread to fill the whole container.

Gases fill their container because their particles move quickly in all directions with only very weak attractions. Nothing holds them together, so they spread out until they fill the whole space available.

Compressibility. Gases are easily compressed because there are large spaces between the particles, which can be reduced by pushing the particles closer. Solids and liquids are almost incompressible because their particles are already touching; pushing them closer is resisted by strong repulsion.

Flow in gases and liquids. Both are called fluids because their particles can move past each other. Gases flow more easily than liquids because their particles are far apart and interact very little.

Step-by-step reasoning

To explain why water takes the shape of any glass it is poured into, but does not fill the glass completely:

1. Water particles are close together but not in fixed positions. 2. They can slide past each other, so the water flows. 3. Gravity pulls them to the bottom, where they fill every corner — taking the glass's shape. 4. Attractions keep the particles close, so the volume is fixed. 5. So the water fills only the bottom part of the glass, with a level surface.

Visual explanation

Three syringes, each sealed at the tip: one full of sand-like particles (solid), one full of liquid particles and one full of widely spaced gas particles. Arrows show the plunger pushed in: it moves a long way only for the gas, where the particles are squeezed closer together. In the other two, the particles are already touching and the plunger barely moves.

Real-world analogy

A box of tightly packed tins (solid) keeps its shape if you tip it over. A box of marbles (liquid) spills and spreads out, but the marbles still stay in a heap together. A room of bouncing balls in zero gravity (gas) would spread to every corner.

Real-world example

Car brakes use hydraulic fluid — a liquid — because liquids are almost incompressible. Pressing the brake pedal pushes on the fluid, which transmits the force directly to the brakes at each wheel. If air got into the brake lines, pressing the pedal would just compress the air, and the brakes would feel "spongy" and fail to work properly.

Why?

Why does a gas not settle at the bottom of its container like a liquid? Gravity does act on gas particles, but they are moving so fast (hundreds of metres per second) and interact so weakly that gravity has only a tiny effect over the height of a normal container. The particles keep spreading throughout the whole space.

Common misconception

"Liquids have no fixed volume because they change shape." Changing shape is not the same as changing volume. Pour 100 cm³ of water from a tall glass into a wide bowl and it looks different, but it still occupies 100 cm³.

Worked example

Question: Explain, using particles, why a bicycle pump outlet sealed with a finger can be pushed in partway when it contains air, but not when it contains water.

Reasoning: Air particles are far apart; pushing the plunger moves them closer together, so the air compresses. Water particles are already touching, so they cannot be pushed much closer.

Answer: Air is compressible because its particles have large spaces between them; water is almost incompressible because its particles are already close together.

Quick check

1. Why does a solid keep its shape? Answer: Its particles are held in fixed positions by strong attractions and can only vibrate.

Exam focus

Structure answers as: observation → particle arrangement/spacing → particle motion/attractions → conclusion. Key phrases: "fixed positions", "can move past each other", "far apart with large spaces", "strong/weak attractions". Avoid saying particles change size or shape.

Advanced insight

Some materials show behaviour between solid and liquid. Custard-like mixtures of cornflour and water flow slowly when stirred gently but resist sudden impacts, behaving almost like solids. Such "non-Newtonian fluids" show that particle interactions can depend on how fast forces are applied.

Summary

The particle model explains the properties of the three states. Solids have a fixed shape and volume because particles are held in fixed positions. Liquids have a fixed volume but flow and take the container's shape because particles move past each other while staying close. Gases fill their containers and are compressible because particles are far apart, fast and weakly attracted.

Practice questions

1. Why do liquids have a fixed volume but not a fixed shape? Answer: Their particles stay close together because of attractions (fixed volume) but can move past each other (no fixed shape). 2. Why is a gas used in car airbags rather than a liquid? Answer: Gases can expand rapidly to fill the bag and can be compressed as the passenger hits it, cushioning the impact. 3. Explain why a gas spreads to fill a room. Answer: Gas particles move quickly and randomly in all directions with very weak attractions, so they spread out until they fill the available space. 4. Why must there be no air bubbles in hydraulic brake fluid? Answer: Air is compressible, so pressing the pedal would squash the air instead of transmitting the force to the brakes.