Carbon dioxide sublimation

Leave a full ice cube tray in your freezer for a very long time and you ll notice the ice cubes will shrink and eventually disappear.This is because the ice is sublimating, or going directly to a gas.You may have seen sublimation on Halloween. Dry ice, which is frozen carbon dioxide, sublimates to a gas, forming a spooky-looking fog. 

Carbon dioxide (C02, melting point -56.6°C at 76 psi - 527 kPa, density 1.9769 gL at 0°C) is a colorless, odorless gas. Solid carbon dioxide sublimes at -79°C (critical pressure 1073 psi, 7397 kPa, critical temperature 31°C). High concentrations of the gas do cause stupefaction and suffocation because of the displacement of ample oxygen for breathing. Carbon dioxide is soluble in water (approximately 1 volume carbon dioxide in 1 volume water at 15°C), soluble in alcohol, and is rapidly absorbed by most alkaline solutions. 

The low-pressure phase diagram of carbon dioxide, shown in Fig. 10, is different from that of water in a number of respects. Carbon dioxide sublimates at l.Oatm, leading to solid C02 being called dry ice. The triple point is at 5.1 atm... 

For carbon dioxide the enthalpy of fusion Afus// is 8.3 0.1 kJ mol while the enthalpy of vaporisation Avap// is 16.9 0.2 kJ mol. In fact, carbon dioxide sublimes with the enthalpy of sublimation AsubT/ given by... 

The horizontal line at 101.3 kPa intersects the vapor pressure curve for the solid at -78.5°C. Therefore, solid carbon dioxide sublimes at this temperature. This sublimation point is equivalent to the normal boiling point of a liquid such as water. Because dry ice is at equilibrium with carbon dioxide gas at -78.5°C, it is frequently used to provide this low temperature in the laboratory. 

C. sublimation of carbon dioxide]—Sublimation is a change of state operation, which is a type of physical change. 

Dry ice—solid carbon dioxide—sublimes, which means that it changes directly from solid to gas. Assuming that the temperature of the system remains constant, which has higher energy, (1) the dry ice or (2) the gaseous carbon dioxide ... 

C Dry ice (solid carbon dioxide) subliming to form gaseous carbon dioxide... 

Phase diagrams can provide important information for substances. For example, the phase diagram for carbon dioxide in Figure 12.30 shows why carbon dioxide sublimes at normal conditions. Find 1.0 atm on the carbon dioxide graph and follow the dashed line to the yellow line. The graph shows that carbon dioxide changes from a solid to a gas at 1 atm. If you extend the dashed line past the yellow line, the graph shows that carbon dioxide does not liquefy as temperature increases. 

The phase diagram for carbon dioxide (Fig. 16.58) differs significantly from that for water. The solid/liquid line has a positive slope, since solid carbon dioxide is more dense than liquid carbon dioxide. The triple point for carbon dioxide occurs at 5.1 atm and -56.6°C, and the critical point occurs at 72.8 atm and 31°C. At a pressure of 1 atm, solid carbon dioxide sublimes at -78°C, a property that leads to its common name, dry ice. No liquid phase occurs under normal atmospheric conditions, making dry ice a convenient coolant. 

At temperatures and pressures below the triple point, carbon dioxide may be either a solid (dry ice) or a gas, depending upon temperature conditions. Solid carbon dioxide at a temperature of -109.3°F ( 78.5°C) and 1 atmosphere transforms directly to a gas (sublimes without passing through the liquid phase). Lower temperatures will result if solid carbon dioxide sublimes at pressures less than atmospheric. Thermodynamic properties of saturated carbon dioxide in the solid, liquid, and vapor phases are given in Tables 1A and IB. 

The triple point of carbon dioxide is at -57°C and 5.1 atm (Figure 11.12 top). Therefore, the solid sublimes if warmed at any pressure below 5.1 atm. This is why solid carbon dioxide sublimes at normal atmospheric pressure (1 atm). Above 5.1 atm, however, the solid melts if warmed. Sulfur has a more complicated phase diagram (Figure 11.12 bottom). It displays three triple points, one of them involving two different solid forms of sulfur (called rhombic sulfur and monoclinic sulfur), as well as the vapor. 

In this play, the fog was carbon dioxide, subliming from lumps on the stage floor. 

Carbon dioxide sublimes. What does that mean ... 

We can also start to understand why some substances, such as soHd carbon dioxide, sublime to a vapor without first forming a liquid. There is no fundamental requirement for the three lines to lie exactly in the positions we have drawn them in Fig. 3.3 the Hquid line, for instance, could lie where we have drawn it in Fig. 3.4. Now we see that at no temperature (at the given pressure) does the liquid phase have the lowest molar Gibbs energy. Such a substance converts spontaneously directly from the solid to the vapor. That is, the substance sublimes. 

Dry ice is solid carbon dioxide. Instead of melting, solid carbon dioxide sublimes according to the equation ... 

When carbon dioxide sublimes, the gaseous CO2 is cold enough to cause water vapor in the air to condense, forming fog. 

Recall from Section 11.8 that CO2 has a triple point at —57 °C and 5.1 atm. At atmospheric pressure, therefore, the liquid phase of CO2 does not exist. Solid carbon dioxide sublimes directly to the gas phase when heated, which is why solid CO2 is called dry ice. ... 

In a process called sublimation, the particles on the surface of a solid change directly to a gas with no temperature change and without going through the liquid state. In the reverse process called deposition, gas particles change directly to a soUd. For example, dry ice, which is solid carbon dioxide, sublimes at —78 °C. It is called dry because it does not form a liquid as it warms. In extremely cold areas, snow does not melt but sublimes directly to water vapor. 

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