The Three States of Matter (Oxford AQA IGCSE Chemistry)

The Three States of Matter

States of matter

• The three states of matter are solids, liquids and gases

• The particles for each state of matter are arranged and move differently

Solids

• Solids have a fixed volume and shape

• They have a high density

• The atoms vibrate in position but can’t change location

• The particles are tightly packed in a fixed and regular pattern

Liquids

• Liquids also have a fixed volume but adopt the shape of the container

• They are generally less dense than solids (an exception is water), but much denser than gases

• The particles move and slide past each other which is why liquids adopt the shape of the container and also why they can flow freely

Gases 

• Gases do not have a fixed volume, and, like liquids, take up the shape of the container

• Gases have a very low density

• Since there is a lot of space between the particles, gases can be compressed into a much smaller volume

• The particles are far apart and move randomly and quickly (around 500 m/s) in all directions

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

Inter-conversion processes

• Inter-conversion processes are more commonly known as state changes

Interconversion of solids, liquids and gases

• Each state change requires a change in the energy, arrangement and movement of the particles

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.) 

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 

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.)

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

• Sublimation occurs when a solid changes directly into a gas

• This only happens to a few solids, such as iodine or solid carbon dioxide

• The reverse reaction also happens and is called desublimation or deposition