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

• 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