IGCSE Chemistry 0620: Particles, States and Diffusion - Study Guide PDF with Answers

Use particles to explain what you observe

A particle model connects visible properties to motion and spacing. Solid particles are not motionless: they vibrate around fixed positions. In a liquid, particles remain close but can move past one another. Gas particles are much farther apart. A gas is easy to compress because there is substantial space between its particles; compression does not usually mean that the particles themselves shrink.

Changes of state do not change particle identity

Melting changes a solid into a liquid; freezing reverses it. Boiling and evaporation change a liquid into a gas, while condensation reverses that change. In a physical change, the substance keeps its chemical identity. Liquid water and water vapour both contain water molecules. The particles gain or lose energy and their arrangement changes. Do not say that particles 'melt' or 'expand': describe changes in their motion, separation and attractions.

Temperature, energy and pressure

Higher temperature means greater average kinetic energy. During melting or boiling of a pure substance at constant pressure, the temperature can remain constant while energy is supplied. That energy changes the arrangement against attractive forces rather than increasing average kinetic energy. In a fixed-volume container, heating a gas increases pressure because collisions with the walls become more frequent and more forceful. If pressure is instead kept constant with a movable boundary, the gas can expand.

Diffusion is random motion with a net effect

Particles continually move in random directions. When one region has a higher concentration, there is initially a net movement from that region towards a lower-concentration region. Diffusion continues until the distribution is more uniform; individual particles still move after that. Higher temperature generally speeds diffusion. When comparing gases at the same temperature, lower relative molecular mass is associated with faster diffusion. Compare one variable at a time: temperature differences can make a simple mass-only comparison misleading.

1. Explain compression

A sealed syringe contains air. Its outlet is blocked and the plunger is pushed in slowly. Why can its volume decrease?

1. The syringe contains a gas, whose particles are far apart compared with particles in a liquid.
2. Pushing the plunger reduces the space between particles; the amount of gas remains the same.
3. The particles do not disappear or become substantially smaller. At similar temperature, more frequent wall collisions account for the higher pressure.

2. Read a heating plateau

A pure liquid is heated steadily at constant atmospheric pressure. Its temperature stays at its boiling point for several minutes. Explain why.

1. Energy is still transferred to the liquid, so a constant temperature does not mean heating has stopped.
2. Energy is used in the change of state, separating particles against attractions.
3. Average kinetic energy does not rise during the idealised plateau. After boiling is complete, further heating can raise the gas temperature.

3. Compare diffusion fairly

Gases X and Y have relative molecular masses 18 and 44. They start at the same temperature. Which would be expected to diffuse faster?

1. Temperature is controlled, allowing the difference in molecular mass to be considered.
2. X has the lower relative molecular mass, so it is expected to diffuse faster under comparable conditions.
3. Do not conclude that its particles are 'more concentrated' or that diffusion stops once gases mix. No numerical rate law is needed here.