When matter changes from one state (i.e. a solid) to another state (i.e. a liquid), we call that a change of state or a phase transition. For matter to change from a solid to a liquid, or a liquid to a gas, requires additional energy (often in the form of temperature, or heat) and results in increased molecular motion (see Fig. 5-3).
While most substances, under normal pressure conditions, will melt from a solid to a liquid, and vaporize to a gas with increasing temperature (for instance water), Figure 5-4 shows that sometimes matter can go directly from a solid to a gas (bypassing the liquid state). We call this sublimation. Sublimation happens when a solid, such as dry ice (which is solid carbon dioxide (CO2)) changes directly from dry ice into CO2 gas (See Fig. 5-5A).
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Fig. 5-5. (A) Dry Ice sublimating. (B) A dry ice maker, the cylinder stores liquid carbon dioxide under pressure, the liquid is transferred to a dry ice maker to make solid carbon dioxide.
Influence of Pressure and Temperature
In figure 5-3, phase changes are brought on by an increase in temperature. Pressure also plays a role in the state of a substance. Dry ice is produced from liquid carbon dioxide (Fig. 5-5B). Dry ice sublimates at normal atmospheric pressure, or 1 atm, but very cold conditions (about -78.5˚C, Fig. 5-6). In order to get liquid dry ice, you need both additional pressure (at least 5.11 atm) and temperature (at least -56.4˚C, Fig. 5-6).
Fig. 5-6. Carbon dioxide phase diagram.
Fig. 5-7. Water phase diagram (M. Pt. = Melting point, B. Pt. = Boiling Point, T. Pt. = Triple Point, 100kPa = 1 atm).
Water can sublimate as well (see Fig. 5-7). When we freeze dry food; we put frozen food into a reduced pressure chamber (also called a vacuum) and the water in the food sublimes from solid ice to a gas. The food left behind does not have water inside (Figure 5-8). When you are camping and want to carry less weight, you can buy freeze dried food, which is light, as it has no water, and rehydrate it when you reach your destination. Freeze dried food stays fresh for a long time, so can also be used for emergency food supplies at home. Water also sublimates at normal atmospheric pressure, but very slowly, which we will discuss later in this unit.
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Fig. 5-8. (A) Freeze dried ice cream and (B) Beef stew
