States of Matter (AQA GCSE Combined Science: Synergy: Life & Environmental Sciences): Revision Note

States of matter

• 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 the three states of matter

	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

• Solids have the highest density

  • Their particles are packed closely together

  • This means that more particles fit into a given volume

• Gases have the lowest density

  • Their particles are far apart

  • This means that very few particles occupy the same volume

Changing states of matter

• 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 between the particles, the more energy that is needed to overcome them 

• Therefore, the stronger the forces between the particles the higher the melting point and boiling point of the substance

• Changing states is a physical change

  • This is different from a chemical change, in which new substances are formed and the change is not reversible

  • In a physical change, the substance can be returned to its original state by reversing the process

Melting

• Melting is when a solid changes into a liquid

• Heat / thermal energy absorbed by the particles is transformed into kinetic energy

• This causes the particles to vibrate more and start to move / flow

• Melting happens at a specific temperature, known as the melting point (m.p.) 

Boiling

• Boiling and evaporation are both when a liquid changes into a gas

  • However, there is a key difference between boiling and evaporation

• In boiling, heat / thermal energy causes bubbles of gas to form inside the liquid, allowing for liquid particles to escape from the surface and within the liquid

• Boiling happens at a specific temperature, known as the boiling point (b.p.)

Freezing

• Freezing is when a liquid changes into a solid

• This is the reverse of melting and occurs at the same temperature as melting

  • So, the melting point and freezing point of a pure substance are the same

  • For example, water freezes and melts at 0 ºC

• Freezing needs a significant decrease in temperature (or loss of thermal energy) and occurs at a specific temperature 

Evaporation

• Evaporation occurs over a range of temperatures

  • It can happen at temperatures below the boiling point of the liquid

• Evaporation occurs only at the surface of liquids where high energy particles can escape from the liquid's surface at low temperatures

• The larger the surface area and the warmer the liquid surface, the more quickly a liquid can evaporate

Condensation

• Condensation occurs when a gas changes into a liquid on cooling and takes place over a range of temperatures

• When a gas is cooled its particles lose energy and when they bump into each other they lack the energy to bounce away again, instead, they group 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

Changing states of matter

State changes require a change in the energy of the particles

Predicting physical state

• 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 will be in the solid state

• At temperatures between the melting point and the boiling point:

  • The substance will be in the liquid state

• At temperatures above the boiling point:

  • The substance will be in the gas state