1.5 Phase Transitions
Phase transitions (changes) are physical changes a substance undergoes to transition into another state of matter.
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Figure 1.19: Phase transitions Source: Wikipedia
These transitions are dependent on things like pressure, temperature, etc.
For example, water undergoes vaporization (i.e. transitions from a liquid to a gas)
at 100 °C at sea level where the pressure is 1 bar. This is referred to
the normal boiling point as this is the boiling point temperature measured
at 1 bar.
However, water also boils at 50 °C if the pressure is about 100 mbar. In fact,
water has many boiling points but only one of which is referred to as the normal
boiling point.
This terminology also applies to the freezing point. The normal freezing point
of water is 0 °C which indicates that the pressure is 1 bar. However, water also
freezes at other temperatures depending on the pressure.
Phase transitions can occur by changing the temperature of a substance under constant pressure conditions. Freezing, deposition, condensation, and recombination are all exothermic processes whereas sublimation, melting (also known as fusion), vaporization, and ionization are all endothermic processes.
Normal refers to the provided temperature measured with the substance under a pressure of 1 bar.