LEARNING OBJECTIVES
By the end of this chapter, you should be able to:
- compare the properties and particle structures of solids, liquids and gases
- describe changes of state and interpret heating and cooling curves
- explain how temperature and pressure affect gas volume
- explain diffusion and the effect of relative molecular mass
THE BIG IDEA
Use the kinetic particle model to explain solids, liquids, gases, changes of state and diffusion.
The particle model connects observations we can see to particles too small to see directly. Separation, arrangement, motion and energy explain why each state behaves differently.
Changes of state are physical changes: the particles remain the same substance, but their energy and arrangement change.
SECTION 01
Solids, liquids and gases
Solid particles are closely packed in a regular arrangement and vibrate about fixed positions. A solid therefore has a fixed shape and volume and is difficult to compress.
Liquid particles remain close together but move around one another. A liquid has a fixed volume, flows and takes the shape of its container.
Gas particles are far apart and move rapidly in random directions. A gas fills its container and is easily compressed because there is much empty space between particles.
KEY IDEAS
- Heating raises the average kinetic energy of particles.
- Gas pressure is caused by particle collisions with container walls.
- Particles themselves do not expand; their average separation changes.
SECTION 02
Changes of state and energy
Melting changes solid to liquid, freezing changes liquid to solid, boiling and evaporation change liquid to gas, and condensation changes gas to liquid.
During melting or boiling, supplied energy weakens or overcomes attractive forces instead of raising temperature. This creates a flat section on a heating curve.
Evaporation occurs at the surface below the boiling point. The fastest particles escape, so the remaining liquid has a lower average kinetic energy and cools.
KEY IDEAS
- Boiling occurs throughout a liquid at a fixed temperature for a pure substance.
- Condensation and freezing release thermal energy to the surroundings.
- A pure substance has sharp melting and boiling points.
SECTION 03
Gases and diffusion
Heating a gas at constant pressure makes particles move faster and spread farther apart, increasing volume. Increasing pressure at constant temperature pushes particles closer and decreases volume.
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration because of random motion.
Lighter gas particles generally diffuse faster than heavier particles at the same temperature. Ammonia therefore travels faster than hydrogen chloride in the classic diffusion tube.
Locating the ammonium chloride ring
- Ammonia has relative molecular mass 17 and hydrogen chloride has relative molecular mass 36.5.
- Ammonia particles move faster on average and travel farther in the same time.
- The gases meet and form solid ammonium chloride.
Answer: The white ring forms closer to the hydrogen chloride end.
QUICK CHAPTER SUMMARY
The ideas to carry forward
- Particle arrangement and movement explain the three states.
- State changes alter energy and arrangement, not chemical identity.
- Gas pressure comes from collisions with container walls.
- Diffusion is faster for lighter gas particles.