States of Matter and Kinetic Theory
States of matter and kinetic theory
This domain tests the properties of solids, liquids and gases, the kinetic particle model, changes of state and heating or cooling curves. GCE O-Level questions often ask you to explain a plateau on a graph or a pressure change using particles.
Core rules
- In a solid, particles are close together in a regular lattice and vibrate about fixed positions. A liquid has closely packed irregular particles that can slide past one another. A gas has widely spaced particles moving rapidly and randomly.
- The kinetic theory assumes gas particles are in constant random motion and collide with each other and the container walls. Gas pressure comes from these collisions per unit area.
- During melting or boiling, energy is used to overcome forces between particles, not to raise temperature. This produces a plateau on a heating curve.
- Evaporation occurs at the surface below boiling point; boiling occurs throughout the liquid when vapour pressure equals external pressure.
- Increasing pressure raises the boiling point because more energy is needed for vapour pressure to equal the external pressure.
Worked example
A fixed mass of gas at 300 K and 100 kPa occupies 300 cm3. It is compressed to 150 cm3 at constant temperature. Calculate the new pressure.
Reasoning: at constant temperature, Boyle's law gives p1V1 = p2V2. So p2 = p1V1/V2 = (100 kPa x 300 cm3)/150 cm3 = 200 kPa. The volume halves, so the pressure doubles. Particle collisions with the walls become twice as frequent per unit area.
Traps that cost marks
Temperature stays constant during a change of state even though energy is supplied. Average kinetic energy depends on temperature, not on the amount of substance. Do not say particles in a liquid have no motion. Steam at 100 degrees Celsius causes worse burns than water at 100 degrees Celsius because it releases latent heat on condensing.
Key points
- Solid, liquid and gas differ in particle spacing, arrangement and motion.
- Gas pressure arises from particle collisions with walls.
- Melting and boiling use energy to overcome forces, so temperature is constant.
- Boyle's law applies at constant temperature.
- Boiling point rises when external pressure increases.