Phase Transitions
In general, each of the three states of matter can change into either of the other two. The following table summarizes the different kinds of phase changes.
| Phase Transition | Name | Example |
| Solid ===> liquid | Melting or fusion | Melting of ice |
| Solid ===> gas | Sublimation | Sublimation of dry ice |
| Liquid ===> solid | Freezing | Freezing of water |
| Liquid ===> gas | Vaporization | Evaporation of water |
| Gas ===> liquid | Condensation or liquefaction | Liquefaction of CO2 |
| Gas ===> solid | Condensation or deposition | Formation of snow |
Vapor Pressure
If we place a liquid in a closed container with a space above it, initially there will be a decrease in the amount of liquid and an increase in the partial pressure of the liquid substance in the space above the liquid. After a while the pressure above the liquid becomes constant, and it appears that nothing further is happening. If we could watch the process just described at the molecular level, we would see first the liquid vaporizing, next as the rate of the liquid vaporization slowed we would see vapor condensing back to liquid. Eventually the rate of the two opposing processes would become equal, and no further net vaporization would occur. This process is called dynamic equilibrium. We can show the process with an equation.
The vapor pressure of a liquid is the pressure exerted by the vapor in dynamic equilibrium with a liquid at a constant temperature.
Boiling Point
A special vaporization phenomenon occurs when we heat a liquid in an open container. When a certain temperature is reached, vaporization occurs throughout the liquid. Vapor produced in the interior of the liquid forms bubbles that rise to the surface of the liquid and escape. The liquid is said to boil. The boiling point of a liquid is the temperature at which its vapor pressure becomes equal to the prevailing atmospheric pressure. Boiling cannot occur at temperatures below the boiling point because the pressure ot the atmosphere would cause any vapor bubbles in the liquid to collapse. We should also note that the temperature of a boiling liquid cannot rise above the boiling point. The normal boiling point of a liquid is defined as the temperature at which its vapor pressure is equal to 1 atm.
The Critical Point
If an adequate quantity of a liquid is heated in a closed container, boiling does not occur. What happens is as we raise the temperature the density of the liquid decreases while the density of the vapor increases. The boundary between the liquid and vapor becomes less distinguished and disappears leaving only the gaseous state. The critical temperature is the highest temperature at which a liquid can coexist in equilibrium with its vapor. The vapor pressure at this temperature is the critical pressure. The point corresponding to the critical temperature and critical pressure is called the critical point.