States of matter
Solids, liquids and gases
The describes the three states of matter:
| Solid | Liquid | Gas | |
|---|---|---|---|
| Arrangement | Regular, touching | Random, touching | Random, far apart |
| Movement | Vibrate in fixed positions | Move around each other | Move quickly in all directions |
| Energy | Lowest | Medium | Highest |
| Shape | Fixed | Takes the container's shape | Fills the container |
Solids and liquids cannot be compressed because their particles are already touching. Gases can be compressed because there is space between the particles.
Key terms
Changing state
Changing state is a : no new substance is made.
- Melting (solid → liquid) and freezing (liquid → solid) happen at the .
- Boiling (liquid → gas) and condensing (gas → liquid) happen at the .
To melt or boil, energy must be transferred to the substance to overcome the forces between the particles. The stronger those forces, the more energy is needed, and the higher the melting and boiling points.
Boiling happens throughout a liquid, with bubbles of gas forming inside it. Evaporation happens only at the surface and can take place below the boiling point.
Key terms
Predicting the state
Compare the temperature with the substance's melting and boiling points:
- Below the → solid
- Between the melting point and boiling point → liquid
- Above the → gas
Example: bromine melts at −7 °C and boils at 59 °C, so at room temperature (20 °C) it is a liquid.
In equations, show each substance's state: (s) solid, (l) liquid, (g) gas and (aq) aqueous — dissolved in water.
Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)
Key terms
Limitations of the particle model (Higher tier only)
The simple particle model is useful, but it isn't the whole truth:
- It shows particles as solid spheres, but real atoms are mostly empty space, and molecules are not always spherical.
- It shows no forces between the particles, but the strength of these forces decides the melting and boiling points.
- It treats all particles as the same size and doesn't show the bonds within molecules.
- It doesn't show that particles in each state move at a range of different speeds.