Principles of Chemistry
States of Matter and Solubility
Pearson Edexcel International GCSE Chemistry
The three states
| State | Arrangement | Separation | Movement |
|---|---|---|---|
| solid | regularly arranged | close together | vibrate about a fixed position |
| liquid | randomly arranged | close together | move around each other |
| gas | randomly arranged | far apart | move freely and move quickly |
- Energy of the particles: lowest in a solid, higher in a liquid; gas particles have high energy.
- Solid particles vibrate about a fixed position. Never write solid particles do not move.
- Drawing a liquid: particles touching, filling the box from the bottom, randomly arranged. Never draw a regular arrangement in a liquid.
- Below its melting point a substance is a solid; between its melting point and its boiling point, a liquid; above its boiling point, a gas.
Changes of state
| Change | From | To |
|---|---|---|
| melting | solid | liquid |
| freezing | liquid | solid |
| evaporation | liquid | gas |
| condensation | gas | liquid |
| sublimation | solid | gas |
On heating, the particles gain more kinetic energy and overcome the forces between the particles, so a solid melts and a liquid evaporates.
Dissolving and diffusion
- A coloured crystal in water: the crystal is dissolving, and the colour spreads by diffusion.
- Diffusion: the particles spread out evenly throughout the liquid, because the particles move randomly.
- In warmer water the particles move faster, so the colour spreads in a shorter time.
- Dilution: adding more water spreads the same coloured particles through a larger volume, so the colour becomes paler.
Gases diffusing in a tube
- Ammonia and hydrogen chloride meet and react to form a white ring of ammonium chloride.
- The ring forms closer to the hydrochloric acid: ammonia travels further in the same time, because ammonia particles move faster.
- The higher the Mr the slower the gas diffuses.
- The ring takes time to form because gas particles move in random directions and collide with air particles.
Solutions
| Term | Meaning |
|---|---|
| solute | the substance that dissolves |
| solvent | the liquid the solute dissolves in |
| solution | the mixture formed when a solute dissolves in a solvent |
| saturated solution | a solution that contains as much dissolved solute as possible at a particular temperature |
Solubility
- Solubility: the maximum mass of solute that dissolves in 100 g of solvent at a particular temperature, in g per 100 g of water.
- Solubility depends on temperature. For most solids, the solubility decreases as the temperature decreases.
solubility = mass of solid ÷ mass of water × 100
mass of water = mass of solution − mass of solid
Solubility curves
- Two substances have the same solubility at the temperature where their lines cross.
- Mass of solid crystallising when a saturated solution in 100 g of water cools = solubility at the higher temperature − solubility at the lower temperature, both read from the graph.
- For a mass of water other than 100 g, scale in proportion: mass dissolving = solubility × mass of water ÷ 100. 1.0 cm3 of water has a mass of 1.0 g.
Method: solubility of a solid at a specific temperature
- Measure a volume of water into a beaker with a burette.
- Add the solid to the water a little at a time and stir until no more dissolves.
- Measure the temperature of the saturated solution.
- Filter to obtain the saturated solution.
- Record the mass of an empty evaporating basin.
- Pour the saturated solution into the evaporating basin and record the mass of the basin and solution.
- Heat the basin gently to remove all the water.
- Record the mass of the evaporating basin and the dry solid.
- Heat gently so the solution does not spit out. Strong heating can decompose the solid and the gas escapes, so the mass of solid remaining will be less and the value of the calculated solubility will be lower.
- A value that is too low also results when the solution was not stirred or not enough solid was added: the solution was not saturated.
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