Change of State

Matter ordinarily exists in three states -- solid, liquid and gas -- and moving from one of these to another is a CHANGE OF STATE, driven by an exchange of heat between the substance and its surroundings. The two most familiar changes are solid-to-liquid (and back) and liquid-to-gas (and back).

A simple activity makes the process concrete: ice cubes in a beaker are heated slowly and continuously on a steady heat source, with their temperature recorded (and the contents stirred) once every minute, right through complete melting and then on until vapour starts appearing; plotting temperature against time gives the heating curve of Fig. 7.9. Reading the curve from its start: between points A and B the temperature stays perfectly FLAT at 0 °C, even though heat keeps flowing in continuously -- all of that heat is going into changing the state from solid ice to liquid water (melting), not into raising the temperature. Between B and C the temperature RISES steadily as the now-liquid water heats up from 0 °C towards 100 °C. Between C and D the temperature is flat again, this time near 100 °C, while the water boils away into vapour -- once again, all the incoming heat goes into the change of state (vaporisation) rather than a temperature rise.

This immediately gives two useful definitions: melting is the solid-to-liquid change of state, solidification the reverse; the melting point of a solid (equivalently, the freezing point of the corresponding liquid) is the temperature at which its solid and liquid states can coexist in thermal equilibrium, and it is called the NORMAL melting/freezing point when measured at one standard atmosphere. Similarly, vaporisation is the liquid-to-vapour change of state, condensation the reverse; the boiling point is the temperature at which liquid and vapour coexist in equilibrium, again "normal" boiling point at one atmosphere. At standard atmospheric pressure, the freezing point of water/melting point of ice is 0 °C (32 °F).

It is worth carefully distinguishing EVAPORATION from BOILING, since both involve liquid turning to vapour but are otherwise quite different. In a liquid, molecules move about with a spread of speeds; a molecule with unusually high kinetic energy near the surface can escape the attractive pull of its neighbours and leave as vapour -- this can happen at ANY temperature, and is called evaporation. It occurs only from the exposed SURFACE of the liquid, and its rate increases with the exposed surface area and with temperature (more molecules are fast enough to escape). Because it is precisely the FASTEST molecules that escape, the average kinetic energy -- and hence the temperature -- of the liquid left behind DROPS: evaporation has a cooling effect, which is why a wet sari dries faster spread out (more exposed area) and why dabbing spirit on skin before an injection feels cool. Boiling, by contrast, happens throughout the WHOLE body of the liquid, but only at one fixed temperature for a given pressure, unique to each liquid. …