2.2.1a The three states of matter

States of matter: temperature and energy decide whether a substance is solid, liquid or gas

1. The three states of matter are solid, liquid and gas.
2. A change of state is a physical change, so no new substance forms.
3. Melting changes a solid into a liquid.
4. Freezing changes a liquid into a solid.
5. Melting and freezing take place at the substance's melting point.
6. Boiling changes a liquid into a gas throughout the liquid.
7. Condensing changes a gas into a liquid.
8. Boiling and condensing take place at the substance's boiling point.

The particle model: arrangement and movement distinguish the three states

1. The particle model represents particles as small solid spheres.
2. The spheres may represent atoms, molecules or ions, depending on the bonding and structure of the substance.
3. Particle theory links the arrangement, movement and energy of these particles to the state of the substance.
4. Solid
   1. The particles are packed closely together in a fixed, regular arrangement.
   2. The particles vibrate around fixed positions but do not move past one another.
   3. A solid has a fixed shape and a fixed volume.
5. Liquid
   1. The particles are close together in an irregular arrangement.
   2. The particles move past one another while staying close together.
   3. A liquid has a fixed volume but takes the shape of its container.
6. Gas
   1. The particles are far apart with no fixed arrangement.
   2. The particles move rapidly in random directions.
   3. A gas has no fixed shape or volume and spreads out to fill its container.

Changes of state: energy transfers alter how particles move and arrange

1. Heating transfers energy from the surroundings to a substance.
2. Cooling transfers energy from a substance to the surroundings.
3. Melting
   1. Heating makes the particles in a solid vibrate more.
   2. At the melting point, the particles gain enough energy to overcome some of the forces holding them in fixed positions.
   3. The particles can then move past one another, so the solid becomes a liquid.
4. Boiling
   1. Further heating increases the energy of the particles in the liquid.
   2. At the boiling point, the particles gain enough energy to overcome the forces holding them close together.
   3. The particles separate and move freely, so the liquid becomes a gas.
5. Freezing
   1. Energy is transferred from the liquid to the surroundings.
   2. The particles move less and become held in fixed positions by the forces between them.
6. Condensing
   1. Energy is transferred from the gas to the surroundings.
   2. The particles move less and come close enough for the forces between them to hold them together as a liquid.
7. During a change of state, the temperature of a pure substance stays at its melting point or boiling point until the change is complete.
8. The transferred energy changes the arrangement of the particles rather than raising the temperature during the change.

Bonding and structure: stronger forces give higher melting and boiling points

1. The energy needed to melt or boil a substance depends on the strength of the forces between its particles.
2. Stronger forces need more energy to overcome, so the substance has a higher melting point and boiling point.
3. Weaker forces need less energy to overcome, so the substance has a lower melting point and boiling point.
4. The particles and forces involved depend on the type of bonding and the structure of the substance.
5. Simple molecular substances
   1. The particles are molecules held by strong covalent bonds inside each molecule.
   2. Weaker intermolecular forces act between separate molecules.
   3. Melting and boiling overcome the intermolecular forces without breaking the covalent bonds inside the molecules.
   4. Simple molecular substances therefore often have low melting and boiling points.
6. Ionic substances
   1. The particles are oppositely charged ions arranged in a giant structure.
   2. Strong electrostatic attractions act between the positive and negative ions.
   3. A large amount of energy is needed to overcome these attractions, so ionic substances have high melting and boiling points.
7. Giant covalent substances
   1. The particles are atoms joined by many strong covalent bonds.
   2. A large amount of energy is needed to overcome these bonds, so giant covalent substances have very high melting points.
8. Metals
   1. Metals contain positive metal ions and delocalised electrons that can move through the structure.
   2. Strong metallic bonding acts between the positive ions and the delocalised electrons.
   3. Metals therefore usually have high melting and boiling points.

Predicting state from data: compare the temperature with both change points

1. You can predict a substance's state by comparing the given temperature with its melting point and boiling point.
2. Below the melting point
   1. The substance is a solid.
3. Between the melting point and boiling point
   1. The substance is a liquid.
4. Above the boiling point
   1. The substance is a gas.
5. At the melting point, solid and liquid may both be present while melting or freezing takes place.
6. At the boiling point, liquid and gas may both be present while boiling or condensing takes place.

Bulk properties: many particles and their structure create material behaviour

1. Bulk properties include a material's state, hardness, density, melting point and boiling point.
2. An individual atom does not have the bulk properties of the material that contains it.
3. Bulk properties arise from the bonding, forces and arrangement of very large numbers of particles.
4. A single carbon atom is not hard like diamond or soft like graphite, because those properties come from the different arrangements and bonding of many carbon atoms.
5. Explain a material's properties using its particles, bonding and structure, rather than assigning those properties to one atom.

2.2.1b Limitations of the simple particle model

The particle model: a handy picture that leaves out how particles really behave

1. The simple particle model represents matter as small spheres and shows how their arrangement, spacing and movement differ between solids, liquids and gases.
2. A model is a simplified representation, so the spheres are not literal pictures of what particles look like.
3. The model has three main limitations: it includes no forces between particles, it draws every particle as a sphere, and it treats those spheres as solid and inelastic.
4. Because of these simplifications, the model shows useful patterns but cannot explain every detail of a change of state.

Missing forces: why the model cannot show the energy needed to change state

1. Real particles feel forces of attraction that pull them towards one another.
2. In a solid these attractions hold the particles in fixed positions, so they can only vibrate.
3. In a liquid the particles stay close together but have enough energy to slide past one another.
4. In a gas the particles have broken away from most of these attractions and move freely.
5. Melting or boiling needs energy to overcome some of the forces of attraction between particles.
6. Freezing or condensing releases energy as attractions pull the particles closer together or into fixed positions.
7. A model with no forces between particles can show the arrangement changing but cannot explain why energy is needed for the change.

Solid inelastic spheres: what the drawing hides about each particle

1. Spherical shape: the model draws every particle as a sphere, even though many particles, including molecules, are not spherical.
   1. So the diagram cannot show the true shapes of different particles.
2. Solid spheres: each particle is shown as a tiny filled ball of material.
   1. Real atoms and molecules are not miniature lumps of the bulk substance, so the picture reveals nothing about their internal structure.
3. Inelastic spheres: the model oversimplifies collisions because it does not fully show how particles rebound and pass on their movement energy.
   1. This limits how well it can describe changes in particle movement when a substance is heated or cooled.