1. What determines the state of matter?
Matter is made of particles that are constantly moving. The state of a substance depends on the balance between the particles' kinetic energy and the attractive forces between them.
Kinetic energy
Energy associated with motion. As temperature increases, the average kinetic energy of particles generally increases.
Intermolecular forces
Attractions between particles. Stronger attractions generally make it harder for particles to separate.
Core idea: A change in temperature or pressure can change how particles move and interact, causing a substance to change its state.
2. The three common states
Solid
Particles are closely packed and usually vibrate around fixed positions. Solids have a definite shape and volume.
Liquid
Particles remain close together but can move past one another. Liquids have a definite volume but take the shape of their container.
Gas
Particles are far apart relative to their size and move freely. Gases expand to fill their containers and are easily compressed.
Plasma
Plasma is an ionised state containing charged particles. It occurs in stars, lightning and some specialised lamps.
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Shape | Definite | Takes container's shape | Fills container |
| Volume | Definite | Definite | Not definite |
| Compressibility | Very low | Low | High |
| Particle movement | Mainly vibration | Particles slide past each other | Free, rapid movement |
3. Gas laws
Gas laws describe how pressure, volume and temperature are related for a fixed amount of gas. Unless stated otherwise, temperature in gas-law calculations must be measured in kelvin.
Boyle's law
At constant temperature and amount of gas, pressure is inversely proportional to volume.
Charles's law
At constant pressure and amount of gas, volume is directly proportional to absolute temperature.
Pressure law
At constant volume and amount of gas, pressure is directly proportional to absolute temperature.
Avogadro's law
At constant temperature and pressure, gas volume is directly proportional to the amount of gas in moles.
Using the laws correctly
- Convert Celsius to kelvin: T(K) = t(°C) + 273.15.
- Keep pressure and volume units consistent on both sides.
- Identify which variables remain constant before choosing a law.
4. The ideal gas equation
Combining the gas laws gives a useful equation relating pressure, volume, temperature and the amount of gas.
Ideal gas equation
- P = pressure
- V = volume
- n = amount of gas in moles
- R = universal gas constant
- T = absolute temperature in kelvin
A commonly used value is R = 8.314 J mol⁻¹ K⁻¹ when pressure is in pascals and volume is in cubic metres. Other values of R can be used with compatible units.
Remember: The units must match the chosen value of R. For example, using pressure in pascals and volume in litres without converting units will give an incorrect result.
5. Kinetic molecular theory
The kinetic molecular theory models gas particles to explain observable gas behaviour. In the ideal model:
- Particles are treated as having negligible individual volume compared with the container.
- Particles move continuously and randomly.
- Collisions between particles and container walls are elastic.
- There are no intermolecular attractions or repulsions except during collisions.
- The average translational kinetic energy depends on absolute temperature.
Average translational kinetic energy
This is the average translational kinetic energy per mole of an ideal gas. For one particle, the expression is 3kT/2, where k is Boltzmann's constant.
6. Changes of state
A physical change of state alters the arrangement and motion of particles without changing the chemical identity of the substance.
| Change | Direction | Heat transfer |
|---|---|---|
| Melting | Solid → Liquid | Absorbed |
| Freezing | Liquid → Solid | Released |
| Vaporisation | Liquid → Gas | Absorbed |
| Condensation | Gas → Liquid | Released |
| Sublimation | Solid → Gas directly | Absorbed |
| Deposition | Gas → Solid directly | Released |
Latent heat
During a phase change at a given pressure, energy can be absorbed or released without a temperature change. This energy changes particle interactions rather than increasing average kinetic energy.
Heat during a phase change
Here, q is heat transferred, m is mass and L is the specific latent heat for the relevant phase change.
7. Vapour pressure, evaporation and boiling
Evaporation
Evaporation occurs at the surface of a liquid when some particles have enough energy to escape into the gas phase. It can occur below the boiling point.
Vapour pressure
In a closed container, vapour pressure is the pressure exerted by vapour when it is in dynamic equilibrium with its liquid at a particular temperature.
Boiling point
A liquid boils when its vapour pressure equals the external pressure. Therefore, a liquid's boiling point can change when external pressure changes.
Example: Water boils at a lower temperature at high altitude because atmospheric pressure is lower than at sea level.
8. Ideal gases versus real gases
An ideal gas is a simplified model. Real gas particles have finite volume and experience intermolecular forces, so real gases can deviate from the ideal gas equation.
| Ideal gas | Real gas |
|---|---|
| Particle volume is treated as negligible. | Particles have finite volume. |
| Intermolecular forces are ignored. | Intermolecular forces can matter. |
| Follows PV = nRT exactly in the model. | May deviate from PV = nRT. |
| An approximation. | Describes actual substances. |
Real gases generally behave more like ideal gases at relatively low pressure and high temperature, when particles are farther apart and intermolecular attractions have less influence.
Compressibility factor
The compressibility factor Z measures deviation from ideal-gas behaviour. For an ideal gas, Z = 1.
9. Key ideas to remember
- The physical state depends on particle motion and intermolecular attractions.
- Solids have a definite shape and volume; liquids have a definite volume; gases fill their containers.
- Use kelvin in gas-law calculations.
- The ideal gas equation is PV = nRT.
- During a phase change, heat transfer does not necessarily change temperature.
- Boiling occurs when vapour pressure equals external pressure.
- Real gases deviate from ideal behaviour, especially at high pressure and low temperature.