The variation of a liquid's equilibrium vapor pressure with temperature was described in the previous module.
Plots of vapor pressure versus temperature represent how the boiling point of the liquid varies with pressure.
The heating and cooling curves were used to determine a substance's melting point.
A phase diagram is a graphical representation of the data that may be presented.
The diagrams show the physical states that exist under certain conditions of pressure and temperature.
A phase diagram shows the physical state of a substance.
The pressure and temperature axes on this phase diagram of water are not drawn to constant scale in order to show important properties.
The phase diagram can be used to identify the physical state of a sample of water.
Water is only a solid in these conditions.
The "water" region has a pressure of 50 kPa and a temperature of 50 degC.
Water can only be found in the gaseous state at 25 kPa and 200 degC.
The H2O phase diagram does not have the pressure and temperature axes drawn to a constant scale in order to allow for illustrations of important features.
The boiling point for water is provided by the "liquid-vapor" curve of the phase diagram.
The boiling point is 100 degrees.
The liquidvapor curve terminates at a temperature of and a pressure of 218 atm, indicating that water cannot exist as a liquid above this temperature.
Water's physical properties are intermediate between its liquid and gaseous phases.
The topic of a supercritical fluid will be described in the next section of this module.
If we place a frozen sample in a vacuum with a pressure less than 0.20 kPa, ice will form.
This ice cream is dehydrated at pressures below the triple point for water.
The solid-liquid curve shows the temperatures and pressures at which ice and liquid water are in equilibrium.
The curve shows a slight negative slope, which indicates that the melting point for water decreases as pressure increases.
Most substances exhibit an increase in melting point with increasing pressure, and water is an exception.
The weight of the glacier can melt some of the ice, forming a layer of liquid water on top of the glacier, which can be easier to slide.
A layer of water that provides lubrication is created by partial melting of glaciers.
The edge of the glacier is shown in the satellite photograph.
The pressure and temperature represent the equilibrium of the three phases of water.
Water can't be a liquid at pressures lower than the triple point.
The phase diagram for water shows the state of water at each temperature and pressure.
The positive slope of the solid-liquid curve indicates that the melting point for CO2 increases with pressure as it does for most substances.
The triple point indicates that carbon dioxide can't be a liquid under pressure.
The deposition of carbon dioxide into the solid state can be achieved by cooling it at 1 atm.
Solid carbon dioxide does not melt at 1 atm pressure, but it does yield gas.
The critical point for carbon dioxide is that the OpenStax book is available for free at http://cnx.org/content/col11760/1.9 observed at a relatively modest temperature and pressure in comparison to water.
The pressure and temperature axes on this phase diagram of carbon dioxide are not drawn to constant scale in order to illustrate several important properties.
We can determine the state of CO2 at each temperature and pressure by using the phase diagram.
If we place a sample of water in a sealed container at 25 degC, remove the air, and let the vaporization-condensation equilibrium establish itself, we are left with a mixture of liquid water and water vapor at a pressure of 0.03 atm.
There is a boundary between the more dense liquid and the less dense gas.
The liquid-gas curve in the phase diagram for water shows that the pressure of the water Vapor increases as the temperature increases.
The boundary between liquid and vapor phases can be lost if the temperature goes up to 372 degC.
The physical properties of the water in the container are intermediate between those of the gaseous and liquid states.
No matter how much pressure is applied, a gas cannot be liquefied.
The critical pressure is the amount of pressure needed to liquefy a gas.
Table 10 contains critical temperatures and critical pressures of some substances.
The reestablishment of separate liquid and gaseous phases occurs when the supercritical fluid is cooled.
There are different densities between the liquid, gaseous, and supercritical fluid states.
Take a look at the for carbon dioxide.
The density of a supercritical fluid is much greater than that of a gas, which is why it is available for free in the OpenStax book.
Similar to liquids, these fluids are capable of dissolving nonvolatile solutes.
They have no surface tension or viscosities that are high, so they can penetrate very small openings in a solid mixture and remove components.
The properties of supercritical fluids make them useful in a wide range of applications.
Supercritical carbon dioxide is a very popular solvent in the food industry, being used to decaffeinate coffee, remove fats from potato chips, and extract flavor and fragrance compounds from citrus oils.
It is inexpensive and not considered to be a pollutant.
The CO2 can be easily recovered if the pressure is reduced and the gas is collected.
Liquid CO2 can be heard in the cylinder of a fire extinguisher if we shake it.
On a hot summer day, the cylinder appears to have no liquid.
Liquid CO2 is present in the cylinder because the temperature of the CO2 is below the critical temperature.
The temperature of the CO2 is greater than its critical temperature on a hot day.
No liquid CO2 exists in the fire extinguisher because there is no pressure to liquefy CO2 above this temperature.
Oxygen cannot be liquefied under these conditions.
The critical temperature of ammonia is higher than the room temperature.
Oxygen cannot be liquefied at room temperature because of the critical temperature.
Coffee is the second most traded commodity.
People all over the world love coffee's smell and taste.
Coffee is one component of coffee that we depend on to help us get going in the morning or stay alert in the afternoon.
Coffee can keep you awake late in the day, so you may want to drink decaffeinated coffee in the evening.
Many methods have been used to decaffeinate coffee.
All depend on the physical and chemical properties of caffeine.
Coffee is a polar molecule and can be dissolved in water.
The smell and taste of decaffeinated coffee can be adversely affected by hot water decaffeination processes, since many of the other 400+ compounds that contribute to coffee's taste and aroma also dissolved in H2O.
Dichloromethane (CH2Cl2) and ethyl acetate (CH3CO2C2H5) are both very effective for the removal of caffeine, but they also remove some flavor and aroma components, and their use requires a lot of time.
Health concerns have been raised about the effect of residual solvent remaining in the decaffeinated coffee.
Carbon dioxide is being used as a more effective and eco-friendly method of decaffeination.
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