1.1 States of Matter Notes
Edexcel IGCSE Chemistry: Revision notes
Key facts
- Solids have fixed positions and vibrate; liquids slide past each other; gases move rapidly and are far apart.
- Melting and boiling need energy gained; freezing and condensing lose energy; subliming skips the liquid.
- Diffusion is net movement from high to low concentration, caused by random particle movement.
- A solute dissolves in a solvent to make a solution; a saturated solution holds no more at that temperature.
- Solubility is measured in g per 100 g of solvent and usually rises with temperature.
Solids, liquids and gases
The state depends on how the particles are arranged, how they move and how much energy they have.
In a solid, strong forces of attraction hold particles in place. In a gas the forces are almost negligible. When describing a state, give arrangement, movement and energy.
Solid
Liquid
Gas
| Solid | Liquid | Gas | |
|---|---|---|---|
| Arrangement | Regular, tightly packed | Random, close together | Random, far apart |
| Movement | Vibrate about fixed positions | Move around and slide past | Move rapidly in all directions |
| Energy | Lowest | Medium | Highest |
Solid
- Arrangement:
- Regular, tightly packed
- Movement:
- Vibrate about fixed positions
- Energy:
- Lowest
Liquid
- Arrangement:
- Random, close together
- Movement:
- Move around and slide past
- Energy:
- Medium
Gas
- Arrangement:
- Random, far apart
- Movement:
- Move rapidly in all directions
- Energy:
- Highest
Which state has particles that are randomly arranged and far apart?
Changes of state
Gaining energy makes particles move more and separate; losing energy does the opposite.
There are six changes of state. Where energy is gained, particle movement and separation increase; where it is lost, they decrease.
- 1
Melting
solid to liquid: gain energy, lattice breaks down
- 2
Boiling or evaporating
liquid to gas: forces overcome completely
- 3
Condensing
gas to liquid: lose energy, forces pull particles closer
- 4
Freezing
liquid to solid: arrangement becomes fixed
Subliming
- Solid to gas directly
- e.g. solid carbon dioxide
Deposition
- Gas to solid directly
- Particles lose energy and settle into a solid
What is the change of state from gas to liquid called?
Diffusion
Coloured substances spreading without stirring show that particles move randomly.
Diffusion is the net movement of particles from higher to lower concentration, caused by random particle movement. A coloured gas spreading through air shows gas particles moving quickly in all directions. A coloured solution spreading through water, such as potassium permanganate, shows liquid particles also move. Liquids spread more slowly because the particles are closer and the forces stronger.
Air
Coloured gas spread through air
Permanganate spread through water
Gas diffusion
- Bromine vapour fills a gas jar within minutes
- Fast: particles far apart
Liquid diffusion
- Potassium permanganate spreads through water
- Slower: particles closer, forces stronger
Why does a purple potassium permanganate crystal slowly spread colour through still water?
Solutions
A solute dissolves in a solvent to make a solution; a saturated solution can take no more.
- Solute: the substance that dissolves.
- Solvent: the liquid it dissolves in.
- Solution: the mixture formed.
- Saturated solution: no more solute dissolves at that temperature, usually with undissolved solute present.
Solubility is the mass of solute that dissolves in 100 g of solvent at a stated temperature (g per 100 g).
Solution
In salt water, which substance is the solvent?
Solubility curves
A solubility curve shows how much solute dissolves in 100 g of solvent at each temperature.
Plot solubility (g per 100 g) on the y-axis against temperature (°C). For most solids it rises with temperature. A point below the curve is unsaturated; a point above means excess solid stays undissolved.
To find solubility at a temperature, add solid in small portions to a measured mass of water at that temperature, stirring until no more dissolves, then scale to 100 g of water.
Worked example
5.0 g of a solid dissolves in 25 g of water at 40 °C before no more will dissolve. What is its solubility at 40 °C?
- 1
The solution is saturated, so 5.0 g is the maximum in 25 g.
- 2
Scale to 100 g of water: 100 / 25 = 4.
- 3
5.0 x 4 = 20.
On a solubility curve, what does a point above the curve show?
Try an exam question
Describe the arrangement and movement of the particles in a gas, and explain how a coloured gas spreads through a room without stirring.
[4 marks]
- [1]Particles are randomly arranged and far apart.
- [1]Particles move rapidly in all directions.
- [1]Diffusion: net movement from higher to lower concentration.
- [1]Caused by the random movement of the particles.
That's the notes covered.
Carry on to the next subtopic.