Question

Among other things, phase diagrams help us predict those conditions under which a substance will exist as a supercritical fluid. Supercritical CO2 is especially useful as it is very effective at dissolving nonpolar oils which constitute many of the flavouring or odor-causing compounds in foods. For example, food companies use supercritical CO2 to extract caffeine from coffee.

Pressure (atm) Temperature (°C)

a) Use the following data to label the following points on the phase diagram for CO2: Triple point -57°C, 5.1 atm, Normal sublimation point -78°C, Critical point 31°C, 75 atm

b) Indicate regions of solid, liquid and gas on the phase diagram.

c) Why is there no information provided concerning the normal boiling point of CO2?

d) Consider the data shown in this phase diagram. Speculate why food industries would favour the use of supercritical CO2 instead of supercritical H2O (critical point 374 °C, 218 atm).

e) Describe the phase change(s) that occur(s) when the temperature of CO2 is increased from -100°C to 200°C at a pressure of 25 atm.

f) Does the density of the solid increase or decrease as the temperature rises? Briefly explain.

Answer 1

The number of phases that exist together when the system is in equilibrium depends upon the conditions of temperature and pressure or conditions of temperature and composition ,pressure kept constant. these conditions are determined experimentally and plotted in temp-pressure diagram. the diagram so obtained giving the conditions of equilibria between various phases of a system is called phase diagram.

the phase diagram of CO₂ showing various phases is shown in the picture.

OB^' is the sublimation curve [ ]

SSG

OC is the melting curve [ ]

SSL

OA is the vaporization curve [   ]

GYL

triple point is the point on the phase diagram where all three phases of liquid , solid , gas co-exist.

P: Phase diagrom of cozi A 75 atm ------- - - -- - - - LIQUID SOLID Pressure -> GAS Tlatm - 78°C -57°C 31°C - Temperature Point O: Triple point Point A : Critical point Point B : Normal sublimation point

c)  normal boiling point would be the temperature at which CO₂ would boil at pressure of 1 atm. now as it is clear from the diagram that CO₂ can not boil below pressure of 5.1atm(as no liquid state can be achieved below 5.1 atm) hence there is no mention of normal boiling point in the phase diagram.

d) supercritical CO₂ is used in food industry to extract components for e.g caffeine from coffee beans without compromising on other important flavours. now critical fluid CO₂ is obtained at and above 31.1^o C which is just a little above room temperature but in case of supercritical water the temperature required is 374^oC which is very high. hence supercritical water is not used in food industries.

e) phase changes that occur : At 25 atm and -100^o C carbon dioxide is present in solid phase. keeping the pressure constant at 25 atm and increasing the temperature, solid carbon dioxide will change into liquid phase at around temperature of (59-60)^o C. on further increase in temperature it will change into gaseous phase. at 25 atm and 200^oC it will be in gas phase.

f) the density of solid usually decreases with increase in temperature, because on increasing the temperature of solid ,the kinetic energy of molecules (constituting the solid) increases. hence the volume of solid increases which in turn decreases the density.

( as

)

mdss density = volume