1.1.1 The particle model and changes of state

All matter is made of moving, attracting particles

1. All matter, whether solid, liquid or gas, is made of tiny particles such as atoms, molecules or ions.
2. The particles are in constant motion, even in a solid.
3. Neighbouring particles pull on one another through forces of attraction.
4. The gaps between particles are empty space, with nothing in them at all.
5. The more kinetic energy the particles have, the faster they move and the more easily they overcome those attractions.
6. The state of a substance depends on the balance between the energy of its particles and the strength of the attractions between them.

How particles differ in solids, liquids and gases

1. In a solid, the particles are packed closely in a fixed, regular pattern and can only vibrate about fixed positions.
   1. Strong attractions hold solid particles in place, so a solid keeps its own shape and volume.
2. In a liquid, the particles are still touching but are arranged randomly and can slide past one another.
   1. A liquid therefore flows and takes the shape of its container, but its volume stays the same.
3. In a gas, the particles are far apart, move quickly in random directions and barely attract one another.
   1. A gas spreads out to fill any container and can be compressed, because there is so much space between its particles.

Changing particle energy causes changes of state

1. Melting happens when heating gives solid particles enough energy to break free of their fixed positions and slide past one another.
2. Boiling happens when particles throughout a liquid gain enough energy to overcome the attractions and separate as a gas.
3. Even in a cool liquid, the fastest-moving particles at the surface have enough energy to escape, which is how evaporation happens.
4. Condensing and freezing are the reverse changes, which happen when cooling removes energy so the attractions pull the particles closer together.
5. At normal atmospheric pressure, solid carbon dioxide sublimes rather than melts, so its particles go straight from fixed positions to moving freely as a gas.

Physical changes keep particles, chemical changes rearrange atoms

1. In a change of state, the particle model shows the same particles moving differently, so the substance keeps its identity.
2. After dissolving, the sugar particles in a cup of tea are still sugar particles, spread out among the water particles.
3. In a chemical change, the particle model shows atoms being rearranged, so the particles after the change are different from those before it.
   1. When magnesium burns, magnesium atoms and oxygen molecules become magnesium oxide, a white powder with properties unlike either starting substance.
4. Physical changes are usually easy to reverse, as when melted chocolate sets again, whereas most chemical changes are hard to reverse.

1.1.2 Limitations of the particle model

The particle model shows particles as solid spheres

1. Particle diagrams draw every particle of a substance as an identical solid sphere.
2. The model treats the particles as inelastic spheres, like bowling balls that are hard and never change shape.
3. The spacing of the spheres shows the state: packed in rows for a solid, touching but jumbled for a liquid and far apart for a gas.
4. The model explains why solids keep their shape, why liquids flow and why gases can be compressed.
5. Because it is a simplified picture, the model leaves out several features of real particles that matter during a change of state.

Real particles are not solid spheres drawn to scale

1. Real atoms are not solid all the way through, because each one is mostly empty space.
2. Many particles are not spheres at all, since a water molecule is bent and a carbon dioxide molecule is a straight line of three atoms.
3. Particles of different substances have different sizes, so one set of identical circles cannot represent every substance.
4. Diagrams cannot show the size of particles compared with the spaces between them.
5. In steam, the water molecules themselves take up less than a thousandth of the volume, so a diagram drawn to scale would look almost empty.

The model leaves out forces of attraction

1. Real particles attract their neighbours, but the spheres in a diagram have no forces drawn between them.
2. These forces of attraction are what hold a solid in shape and keep a liquid together.
3. The strength of the attractions varies from one substance to another, which a set of identical circles cannot show.
4. Oxygen boils at $-183{}^{\circ }C$ while water boils at $100{}^{\circ }C$, a difference caused by how strongly their particles attract.
5. The attractions act between particles, so melting and boiling overcome them without breaking the particles themselves apart.

The model cannot show energy in changes of state

1. A particle diagram is a still picture, so it cannot show how fast the particles are moving.
2. Heating a substance gives its particles more kinetic energy, which a diagram can only hint at with arrows.
3. During melting and boiling, the energy supplied goes into overcoming attractions between particles, and no sphere diagram shows where that energy goes.
4. During condensing and freezing, energy is released as the attractions pull the particles closer together, which the diagram cannot show either.
5. Two diagrams side by side show the arrangement before and after a change of state, but not the energy transfer that caused it.