Feb 21 2011

Pique the Geek 20110220: Phase Transitions

Everyone is familiar with phase transitions even it they are not familiar with the term.  Amongst the most familiar is the melting of ice and the boiling of water to form steam.  Technically, these transitions are called fusion and vaporization, respectively.  There are more and we shall discuss some of them later.

All phase transitions are accompanied by changes in the free energy of the substance undergoing the transition, and this free energy has two components, the enthalpy of the transition and the entropy of the transition.  Unless very careful work is being done, the entropy change is often ignored because it, in many cases, is the lesser contributor.  However, it is never zero (except at absolute zero) and sometimes is the dominant factor.

For now we shall just focus on the enthalpy.  This the amount of energy (heat) required to melt a given amount of a material.  For example, to melt ice requires almost exactly 6 kilojoules per mole (18 g).  That is why ice cools drinks, not so much because it is cold, but that it absorbs energy from its surroundings when it melts.  The process also requires that the ice melts at 0 degrees C, its melting point.  At normal pressures, ice can never be warmer than its melting point.  By the way, only one other common substance has a larger enthalpy of fusion, ammonia.

If you take a sensitive thermometer and place it in container of water mixed with ice and stir it well, you will see that the temperature of the system will not exceed 0 degrees C until all of the ice has melted, regardless of the amount of heat that you add to the system.  Once all of the ice has melted, the temperature will rise until the boiling point is reached, at sea level 100 degrees C.  It takes 40.7 kJ/mol to vaporize water at its boiling point, quite a bit more than to melt the ice.  Then the temperature levels out once again until all of the water has boilt away, and the water vapor begins to increase in temperature.

The fact that even though energy is being added but that the temperature stays the same during the phase transitions has led to those quantities being called the latent heats of fusion and vaporization, respectively.

Now, when the process is reversed, the same amounts of heat are released from the water as it condenses back from steam to liquid water, and from liquid water to ice.  This is one of the reasons that steam cooks things fairly rapidly, because as the steam condenses on the food, 40.7 kJ/mol is released onto the food, thus heating it.

Now, water has more phases than just ice, liquid, and steam.  Under enough pressure, ice itself undergoes phase transitions that change its crystal structure and its volume.  Unlike normal ice, these forms are more dense than liquid water, so those ice cubes would sink in a glass!  Kurt Vonnegut, Jr.  actually wrote a novel based on that fact called Ice Nine.   In his novel, ice IX was created in a laboratory and was found to be stable once the pressure was released, and it became very popular for drinks because the cubes did not float.

Unfortunately, in the book at least, it turned out that ice IX was thermodynamically more stable than normal ice, so once freezing water was exposed to ice IX, normal ice did not form.  The ramifications of this are actually quite grave.  Once seeded, all bodies of water would freeze from the bottom up, rather than from the top down.  Eventually the oceans would freeze solid and the planet would no longer support life, because the floating layer of normal ice actually insulates the water under it from the cold, keeping it liquid.  Fortunately, at atmospheric pressures only normal ice is in fact stable.

Ice IX was actually discovered many years after the novel, and it is indeed more dense than liquid water, but is less, not more stable than normal ice Ih, the h standing for hexagonal, the crystal structure of ordinary ice.  Actually over a dozen different phases of ice are known, but almost all of them require pressures of millions or billions of pascals of pressure to form.  Normal atmospheric pressure is about 101325 pascals, so there is little chance of encountering most of the other phases.

But there is yet another phase of water!  If you heat it to 374 degrees C or higher, at a pressure of 22.1 MP (about 218 atmospheres) it becomes a supercritical fluid, with the density near that of liquid water but with almost no viscosity, much like a gas.  Its solvent properties are also very different than liquid water, in that things like salt are no longer soluble in it.  On the other hand, organic materials like oils and such become soluble it supercritical water.  This gives rise to the supercritical water oxidation process wherein things like hazardous waste can be put into a reactor, brought up to temperature and pressure, and then be treated with oxygen, thus “burning” the waste.  This process is being used more and more, but obviously involves significant costs and technology.  However, it is sometimes an alternative to incineration, particularly for extremely hazardous materials because all of the effluent can be retained rather than going into the atmosphere.

Now, if the materials being oxidized form salts, there can be a problem in that since they are not soluble and can clog the reactor.  To solve this, solutions of other salts are introduced into the reactor to form eutectic mixtures with the salts being formed during the oxidation.  Whilst still insoluble, the melting point of the mixtures is lower than the supercritical water, so they remain liquid and can thus flow through the reactor.  Once the temperature and pressure is let down, through a complex valving system, the now ordinary water dissolves the salts.

You have probably seen supercritical water on TeeVee.  In very deep seawater in some places there are hydrothermal vents, and the temperature and pressure is sufficient to cause the water escaping from the vents to be supercritical.  If you look very carefully at the images, you can see a disturbance caused by the change in the index of refraction of the supercritical water versus the normal seawater nearby.

Solids can undergo phase changes other than melting.  We already talked about the different phases of ice, but a very common material that all of us exhale undergoes an interesting one.  Carbon dioxide, when subjected to pressure, condenses to a liquid.  It the pressure is suddenly released, it evaporates, and some of it gets cold enough to solidify because of the expansive cooling.  This carbon dioxide “snow” is them compressed to a solid called dry ice.  At atmospheric pressure, liquid carbon dioxide can not exist, so dry ice is converted directly from the solid phase to the gaseous phase, a process called sublimation.  That is why dry ice is dry.  It is also cold enough to freeze tissue, so always use tongs or wear heavy gloves when handling it, since you can get frostbite quickly.

Carbon dioxide also forms a supercritical fluid (most things do), and it is fairly commonly used.  Years ago, the standard EPA method for determining oil and grease in samples involved dissolving the oil in grease in a chlorofluorcarbon, one of the Freons, and then evaporating the Freon away, leaving the oil and grease.  Because of the limitations of producing Freon because of its ozone depleting potential, supercritical carbon dioxide is now the standard material for extracting oil and grease in laboratories.

Supercritical carbon dioxide also finds use in the food and beverage industry.  In the old days, decaffeinated coffee and tea were made by extracting coffee beans or tea leaves with chlorinated solvents, like dichloromethane or chloroform.  Because of health concerns involved with ingestion of trace residues of the solvents, a new process was developed that replaces those solvents with carbon dioxide.  Most of the caffeine used in medicine and in energy drinks is derived from the decaffeinated coffee and tea industry.

It is easy to see a phase change like melting or boiling, but ones that take place betwixt to solid phases are more difficult to detect.  There is a way, however.  A technique called differential scanning calorimetry measures the flow of heat through a sample and a reference material using a pair of matched thermocouples.  The reference material is chosen to be one that does not have any phase transition along the temperature range being investigated.  As both materials are heated, the temperature changes are measured.  At a phase change in the sample, say a change from one crystal structure to another, heat will either be released (an exotherm) or absorbed (an endotherm depending on whether energy is released or absorbed in the process.  For example, if you were to perform DSC of a piece of ice, you would see flat line until the melting point is reached, then an endotherm, because it takes energy to melt the ice.  If you were cooling water, at the freezing point (just the same as the melting point) you would see an exotherm because energy is released during freezing.

Two final, and related points.  Sometimes it is possible to cool a liquid below its freezing point without it freezing.  This is called supercooling and usually occurs with pure liquids in very smooth containers.  An alternate process, called superheating can occur when a liquid is brought to a temperature above its boiling point without boiling.  Once again, this usually has to do with pure liquids and very smooth containers.  This can actually be quite dangerous, as when a new, and thus not scratched, tea cup is put into the microwave oven to heat the water for a tea bag.  As soon as the cup is removed or the tea bag added, all of the water erupts out of the cup and can cause a bad scalding.  I have witnessed this happen.  So, always use an old cup to heat water.  Both of these things happen because there are no nucleation sites where the liquid can either start freezing or boiling.  But bump the container or add nucleation sites and the water, already over its boiling point, erupts out of the cup.

In chemistry laboratories it is common practice to add boiling chips to liquids being heated in glassware to prevent this from happening.  In the laboratory this effect is called bumping.

Well, you have done it again!  You have wasted many perfectly good einsteins of photons reading this transitional piece.  And even though Ted Nugent acts like a normal person when he reads me say it, I always learn much more than I could possibly hope to teach by writing this series, so keep those comments, questions, corrections, and other feedback coming.  I will stay for Comment Time and come back around 9:00 PM Eastern tomorrow for Review Time.

Warmest regards,


Crossposted at Dailykos.com, Antemedius.com, and Docudharma.com

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