KEY POINTS:
- Melting a solid overcomes some forces of attraction between particles. This is an endothermic process, i.e. it requires energy.
- Vaporising a liquid overcomes all remaining forces of attraction between particles. This is also an endothermic process, and requires even more energy.
- Condensing a gas leads to the formation of some forces of attraction. This is an exothermic process, i.e. it releases energy (and explains why steam burns are so dangerous!)
- Freezing a liquid leads to the formation of even more forces of attraction. This is also an exothermic process, and releases even more energy.
- Examples of substances that sublime include iodine and ‘dry ice’ (a very misleading name for solid carbon dioxide).
THINK OUTSIDE THE BOX:
- Water is unique in that it’s solid form has a lower density that the liquid form. How does this contradict what we know about the arrangement of particles in the three major states of matter?
- What does the ‘triple point’ of water refer to? How is this possible?