MrChemCoach

Digital Molecular Lab

Cambridge IGCSE
SECTION 1
CORE PRACTICAL Interactive Simulation

The Particulate Nature of Matter

States of Matter • Changes of State • Particle Theory

Aligned with 2026 syllabus
Learning Objectives
Describe the arrangement, movement and relative energies of particles in solids, liquids and gases
Explain changes of state (melting, freezing, boiling, condensing, sublimation) using particle theory
Relate temperature to the kinetic energy of particles and strength of inter-particle forces
Understand how particle model explains macroscopic properties (shape, volume, compressibility, flow)
Interactive Particle Simulation Real-time
Reset
Pause
Temperature
20.0
-40°C
140°C
Current State
SOLID
Melting point 0 °C
Boiling point 100 °C
Phase Controls
Compression Test
Demonstrate compressibility
64 particles
Solid
Liquid
Gas
Particle Behaviour
Particles are arranged in a regular fixed lattice. Strong attractive forces hold them in position. They vibrate about fixed points.
Macroscopic Properties
Shape Fixed
Volume Fixed
Compressibility Very low
Can flow? No
Kinetic energy Low
Substance: Water (H₂O)
Particles: 64
View full particle theory notes
Shape & Volume Comparison See the difference

Click on a state below to see animated behavior of particles in different containers. This clearly shows the concept of shape and volume.

Tall Narrow Container
Solids keep shape
Liquids take shape
Gases fill it
Short Wide Container
Solids keep shape
Liquids spread out
Gases fill it
Rectangular Container
Solids unchanged
Liquids adapt
Gases expand fully
IGCSE-Style Assessment Questions Test understanding
1. A solid is heated until it melts. Which statement correctly describes the change in the particles?
A
Particles gain kinetic energy, vibrate more strongly and eventually overcome the forces holding the lattice together so they can slide past each other.
B
Particles lose energy and move closer together while staying in fixed positions.
C
Particles remain completely still but the forces between them weaken.
D
Particles move freely throughout the container with very high speeds.
2. Which of the following best explains why gases are easily compressed compared to solids and liquids?
A
Gas particles are very far apart with weak forces between them, so they can be pushed closer together easily.
B
Gas particles have no mass so they take up no space.
C
Gas particles are arranged in a rigid lattice structure.
D
Gas particles move in straight lines at constant speed.
3. When steam condenses to form water, what happens to the particles and the energy?
A
Particles slow down, lose kinetic energy and move closer together as attractive forces become significant again.
B
Particles gain energy and move further apart.
C
Particles vibrate more but stay in fixed positions.
D
Particles break apart into atoms.

What this experiment shows

Solids, liquids and gases differ in how their particles are arranged, how strongly they are held and how they move. Heating supplies energy that first makes particles vibrate or move faster, and then, at a change of state, breaks the forces between them - which is why the temperature stays still on a heating curve while melting or boiling is happening. That flat part is the single most examined feature of the graph.

solid → liquid → gas (melting, then boiling)

What you need

  • A model on screen - or, in the lab, ice, a beaker, a thermometer and a stopclock
  • A Bunsen burner and tripod
  • Graph paper for the heating curve

How it is done

  1. Start with ice below 0 °C and record the temperature every 30 seconds while heating.
  2. Keep heating right through melting, warming and boiling.
  3. Plot temperature against time.
  4. Mark the two flat sections and say what is happening in each.

What you should see

  • The temperature rises, then holds at the melting point, rises again, then holds at the boiling point.
  • During a flat section the energy goes into breaking forces, not into raising temperature.
  • Gas particles are far apart and move randomly at speed - which is why gases diffuse and can be compressed.
  • Diffusion is faster at higher temperature and faster for lighter particles.

Where marks are lost

  • Saying the temperature stops rising "because the heat is turned off". It is not - the energy is breaking forces between particles.
  • Calling evaporation and boiling the same thing. Evaporation happens at any temperature, from the surface only.
  • Saying particles "melt". Particles do not melt; the arrangement between them changes.
  • Mixing up sublimation with evaporation.

Questions students ask

Why is the heating curve flat while ice melts?
All the energy going in is being used to overcome the forces holding the particles in the lattice, not to make them move faster. Temperature measures the movement, so it holds still.
What is the difference between evaporation and boiling?
Evaporation happens only at the surface, at any temperature. Boiling happens throughout the liquid, at one fixed temperature.
Why do gases diffuse faster when hot?
The particles have more kinetic energy, so they move faster and spread through one another more quickly.

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Written for Cambridge IGCSE Chemistry 0620 and CBSE Class 9-12 by Ajay Shekhawat, founder of MrChemCoach. Run the simulator above, then check yourself against the questions.