States of Matter (Edexcel 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

• 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

• The amount of energy needed to change state from solid to liquid and from liquid to gas depends on the strength of the forces between the particles
  • The stronger the forces of attraction, the more energy that is needed to overcome them for a state change to occur
  • Therefore, the stronger the forces between the particles the higher the melting point and boiling point of the substance

• When matter changes from one state to another due to changes in temperature or pressure, the change is called an interconversion of state
• It is a physical change involving changes in the forces between the particles of the substances, 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 

Melting

• Melting is when a solid changes into a liquid
• The process requires heat energy which transforms into kinetic energy, allowing the particles to move
• It occurs at a specific temperature known as the melting point which is unique to each pure solid 

Boiling

• Boiling is when a liquid changes into a gas
• This requires heat which causes bubbles of gas to form below the surface of a liquid, allowing for liquid particles to escape from the surface and from within the liquid
• It occurs at a specific temperature known as the boiling point which is unique to each pure liquid 

Freezing

• Freezing is when a liquid changes into a solid
• This is the reverse of melting and occurs at exactly the same temperature as melting, hence the melting point and freezing point of a pure substance are the same
  • Water for example freezes and melts at 0 ºC

• It requires a significant decrease in temperature (or loss of thermal energy) and occurs at a specific temperature which is unique for each pure substance 

Evaporation

• When a liquid changes into a gas
• Evaporation occurs only at the surface of liquids where high energy particles can escape from the liquids surface at low temperatures, below the boiling point of the liquid
• The larger the surface area and the warmer the liquid/surface, the more quickly a liquid can evaporate
• Evaporation occurs over a range of temperatures, but heating will speed up the process as particles need energy to escape from the surface

Condensation

• When a gas changes into a liquid, usually on cooling
• When a gas is cooled its particles lose energy and when they bump into each other, they lack energy to bounce away again, instead grouping together to form a liquid

Sublimation

• When a solid changes directly into a gas
• This happens to only a few solids, such as iodine or solid carbon dioxide
• The reverse reaction also happens and is called desublimation or deposition

Interconversion between the three states of matter

• The physical state of a substance under certain conditions can be predicted from a given set of data.
• Normally you are given melting and boiling point data for a substance and asked to predict its physical state in specified conditions.
• At temperatures below the melting point:
  • The substance is will be in the solid state

• At temperatures above the melting point but below the boiling point:
  • The substance will be in the liquid state

• At temperatures above the boiling point:
  • The substance will be in the gaseous state.