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The Three States of Matter (Edexcel IGCSE Chemistry: Double Science)
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 |
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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
- The particles themselves remain the same, it is just the forces between the particles which change
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
Examiner Tip
Questions may show state changes with a reversible arrow: ⇌, which means that the process can go forwards and backwards.
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