The Particle Model (OCR Gateway GCSE Chemistry: Combined Science)

• The three states of matter are solids, liquids and gases
• A substance can usually exist in all three states, dependent on temperature (and pressure)
• State changes occur at the melting point (solid to liquid, liquid to solid) and at the boiling point (liquid to gas and gas to liquid)
  • Melting and freezing occur at the melting point
  • Boiling and condensing take place at the boiling point
  • Evaporation is also a change of state and occurs at any temperature; the maximum rate of evaporation occurs at the boiling point 

• Individual atoms themselves do not share the same properties as bulk matter
• The three states of matter can be represented by a simple model
  • In this model, the particles are represented by small solid spheres

Summary of the Properties of Solids, Liquids and Gases

	Solid	Liquid	Gas
Diagram
Arrangement of particles	Regular arrangement	Randomly arranged	Randomly arranged
Movement of particles	Vibrate about a fixed position	Move around each other	Move quickly in all directions
Closeness of particles	Very close	Close	Far apart

• A physical change in a substance is a change that does not involve a change in the identity of the substance
• A chemical change or chemical reaction is a change in which one or more substances are converted into different substances
• The substances that react in a chemical change are called the reactants
• The substances that are formed by the chemical change are called the products
• A physical change involves changes in the forces between the particles of the substances, but the particles themselves remain the same, as do the chemical properties of the substance
• Physical changes are relatively easy to reverse as no new substance is formed during interconversions of state
• The interconversions have specific terms to describe them:

A Summary of State Changes