Phases

Discussion

the basics

This section has become a disaster of epic proportions.

gas & chaos have related etymology

Jan Baptist van Helmont (1577-1644) Belgium

Chaos (χαος) was used to define the most disperse and fluid state of matter, that in which no particular order could be observed. Interestingly enough, when van Helmont wanted to refer to steam-state of materials he was studying used the term chaos but in the particular Flemish accent, converting it to gas, by which the state is known to us today.

phase diagrams

Increased pressure increases the range of temperatures over which a substance can exist as a liquid. Reduced pressure reduces this range. At a certain special pressure the boiling and melting points will equal, and the substance can no longer exist as a liquid. Below this pressure, the only possible phase transition is from solid to gas (and vice versa). This phase change is called sublimation (the reverse process is called deposition or desublimation) and the temperature at which it occurs is called the sublimation point (or sublimation temperature). That's the essence of the upcoming discussion. If this is enough info for you, stop reading and jump to the next section. If you want to understand what I'm talking about then keep reading. Knowing why some phenomena occurs is often more important than knowing that it occurs. (Of course, the reverse is also true, which is why I offer you the option to read on or jump ahead.)

To a certain extent, liquids are like a minimum security prison. (Solids are like a maximum security prison in permanent lock down, but that's another matter.) The molecules within have limited freedom and can only leave infrequently or with great effort. As long as a liquid has some surface area exposed to the atmosphere, here and there a molecule within the liquid near the surface will be moving fast enough to escape the liquid prison and enjoy the freedom of a vapor molecule in the surrounding atmosphere. But rather unlike a a prison, the reverse process is also possible. From time to time, a molecule in the atmosphere will be traveling fast enough to plow its way through the tightly guarded walls of the liquid only to find itself trapped within. Both events are happening simultaneously, but not necessarily with equal probability.

The thing that makes these processes different from inmates entering and leaving a prison is that they aren't so much governed by laws of good or bad behavior, but rather by physical laws describing energy and momentum. If a molecule of a liquid has sufficient momentum in the right direction, it will escape the liquid. Likewise, if a molecule of a vapor has sufficient momentum and is traveling in the wrong direction, it will join the liquid.

Given a droplet of water on the side of a glass; when more molecules of water escape the droplet than enter from the atmosphere the droplet is said to be evaporating. When more molecules enter than escape, the water in the atmosphere is said to be condensing on the glass. When the rate at which these two events are equal, the droplet is said to be in equilibrium — or more precisely dynamic equilibrium to distinguish it from the static equilibrium of a stationary bridge or level flying airplane.

All of this blah blah is a necessary set up for the remainder of the discussion, so be patient.

What is boiling and how is it different from evaporation? Both processes involve the same liquid to gas phase transition, but where evaporation can occur at any temperature boiling occurs only at a specific temperature. Let's return to the description of evaporation just discussed.

Evaporation occurs whenever more molecules leave a liquid than enter. Condensation occurs whenever more enter than leave. These changes are driven by the concentration of liquid molecules in the atmosphere. When their concentration is low, it's more likely that molecules will leave the liquid phase than enter it, so evaporation rules. When their concentration is high, it's more likely that molecules will enter the liquid phase than leave it. When neither process dominates it must be because the atmosphere has just the right concentration of liquid molecules floating around within it — no more, no less than what it can handle. Under these circumstances the atmosphere is said to be saturated.

The most energetic vapor molecules present in the atmosphere are fighting their way into the liquid. The most energetic liquid molecules are fighting their way out into the atmosphere. There's room in the atmosphere, but it has a limit. When that's reached, evaporation stops. What we have here is a war — a war of momentum on the microscopic scale or pressure on the macroscopic scale.

I lost track of where I was going. Here's some neat pictures.


A pressure cooker reduces cooking times by forcing water to remain a liquid at temperatures much higher than would be possible in an ordinary pot.

Homer sleep now.


[magnify]


[magnify]


Frozen carbon dioxide is also known as dry ice since it cannot exist as a liquid under normal pressures. Dry ice doesn't melt, it sublimates.

bridge text

phases & solutes

dissolved substances (solutes)


Salt depresses the melting point of water making it an effective deicer at moderately low temperatures.

unorganized thoughts

glass

Glass is not a material like steel or concrete. It's the name given to a phase of matter equal in importance to that of the solid, liquid, gas, or plasma phases. Glasses combine the properties of both liquids and solids but are also distinctly different from each. In one sense, glasses are supercooled liquids, but one should not take this description too literally. While glasses share an equal level of disorder with liquids they flow about as much as solids do; which is to say, glasses don't flow at all. In another sense, glasses are just disordered solids, but again one should not take this description too literally. Glasses are no more disordered than are liquids; which is to say, glasses are semi-ordered (or semi-disordered for you pessimists).

The atoms of a glass are held firmly in place like they are in a solid but are arranged randomly like those in a liquid. This gives glasses some mechanical strength, but not as much as a true solid. When glasses fail, they fracture along curved surfaces that reveal the flow patterns of a liquid frozen in time. Glasses are usually formed by melting crystalline materials at very high temperatures and then cooling them "quickly" before the atoms get a chance to settle into a nice, well-ordered arrangement.

Glasses share properties with both solids and liquids.
solid glass liquid
long range atomic order short range atomic order short range atomic order
when solids break, they fracture along planar surfaces when glasses break, they fracture along curved surfaces liquids can't withstand shear forces
very little atomic mobility very little atomic mobility limited atomic mobility
retain their shape for long periods of time and return to their original shape after stresses have been removed retain their shape for long periods of time and return to their original shape after stresses have been removed take the shape of their container up to the limit of their volume, which is more or less fixed

Indefinite melting point. No plateau on heating/cooling curve.

Glasses are nonequilibrium materials, so their physical properties depend on the process used to make them.

Commercial glasses …

The first glass containers were not molded or blown, but were sculpted from glass blocks 4000 years ago in the eastern Mediterranean (Mesopotamia, Egypt, and surrounding territories).

  These glass object were formed well over 3000 years ago in Egypt, yet they have not changed shape in any appreciable way. That's because glasses are not liquids. They do not flow at temperatures far below their melting point.

Pliny the Elder. Historia Naturalis. (I can't believe the Latin and English texts match.)

Pars Syriae, quae Phoenice vocatur, finitima Iudaeae intra montis Carmeli radices paludem habet, quae vocatur Candebia. ex ea creditur nasci Belus amnis quinque milium passuum spatio in mare perfluens iuxta Ptolemaidem coloniam. lentus hic cursu, insaluber potu, sed caerimoniis sacer, limosus, vado profundus, non nisi refuso mari harenas fatetur; fluctibus enim volutatae nitescunt detritis sordibus.   In Syria there is a region known as Phœnice, adjoining to Judæa, and enclosing, between the lower ridges of Mount Carmelus, a marshy district known by the name of Cendebia. In this district, it is supposed, rises the river Belus, which, after a course of five miles, empties itself into the sea near the colony of Ptolemaïs. The tide of this river is sluggish, and the water unwholesome to drink, but held sacred for the observance of certain religious ceremonials. Full of slimy deposits, and very deep, it is only at the reflux of the tide that the river discloses its sands; which, agitated by the waves, separate themselves from their impurities, and so become cleansed. It is generally thought that it is the acridity of the sea-water that has this purgative effect upon the sand, and that without this action no use could be made of it. The shore upon which this sand is gathered is not more than half a mile in extent; and yet, for many ages, this was the only spot that afforded the material for making glass.
 
tunc et marino creduntur adstringi morsu, non prius utiles. quingentorum est passuum non amplius litoris spatium, idque tantum multa per saecula gignendo fuit vitro. fama est adpulsa nave mercatorum nitri, cum sparsi per litus epulas pararent nec esset cortinis attollendis lapidum occasio, glaebas nitri e nave subdidisse, quibus accensis, permixta harena litoris, tralucentes novi liquores fluxisse rivos, et hanc fuisse originem vitri.   The story is, that a ship, laden with nitre, being moored upon this spot, the merchants, while preparing their repast upon the sea-shore, finding no stones at hand for supporting their cauldrons, employed for the purpose some lumps of nitre which they had taken from the vessel. Upon its being subjected to the action of the fire, in combination with the sand of the sea-shore, they beheld transparent streams flowing forth of a liquid hitherto unknown: this, it is said, was the origin of glass.

Until the mid Nineteenth Century there were only two kinds of commercial glass: soda-lime, crown glass and lead-containing, flint glass. Then in 1884, Otto Schott …

Most of this was adapted from the Corning Museum of Glass
http://www.cmog.org/page.cfm?page=279
A smaller portion came from British Glass
http://www.britglass.co.uk/aboutglass/types.htm

Types or categories based on chemical composition.

  1. Soda-lime glass [crown glass] is the most common (90% of glass made), and least expensive form of glass. It usually contains 60-75% silica [silicon dioxide SiO2], 12-18% soda [sodium oxide Na2O], 5-12% lime [calcium oxide CaO]. Resistance to high temperatures and sudden changes of temperature are not good and resistance to corrosive chemicals is only fair. [Transmits visible light, but absorbs ultraviolet. Used in windows and bottles.]

    {Crown glass}, a variety of glass, used for making the finest plate or window glass, and consisting essentially of silicate of soda or potash and lime, with no admixture of lead; the convex half of an achromatic lens is composed of crown glass; — so called from a crown-like shape given it in the process of blowing.
  2. Lead glass [flint glass] has a high percentage of lead oxide [PbO] (at least 20% of the batch). It is relatively soft, and its refractive index gives a brilliance that may be exploited by cutting. It is somewhat more expensive than soda-lime glass and is favored for electrical applications because of its excellent electrical insulating properties. Thermometer tubing and art glass are also made from lead-alkali glass, commonly called lead glass. This glass will not withstand high temperatures or sudden changes in temperature. [Denser with higher refractive index than soda-lime glass. Lead oxide instead of lime? Used in CRT-TV screens to block out x‑rays.]

    Flint glass \Flint" glass`\ (Chem.) A soft, heavy, brilliant glass, consisting essentially of a silicate of lead and potassium. It is used for tableware, and for optical instruments, as prisms, its density giving a high degree of dispersive power; — so called, because formerly the silica was obtained from pulverized flints. Called also {crystal glass}. Cf. {Glass}. Note: The concave or diverging half on an achromatic lens is usually made of flint glass.
  3. Borosilicate glass [Pyrex] is any silicate glass having at least 5% [typically 10-25%] of boric oxide [B2O3] in its composition [and a small amount of Al2O3]. It has high resistance to temperature change and chemical corrosion. Not quite as convenient to fabricate as either lime or lead glass, and not as low in cost as lime, borosilicate's cost is moderate when measured against its usefulness. Pipelines, light bulbs, photochromic glasses, sealed-beam headlights, laboratory ware, and bake ware are examples of borosilicate products.

    Boron. Nonmetallic element, fifth in atomic table, distinguished by the symbol B, atomic weight 10.8. Boron does not occur free in nature but always in combination with oxygen and other elements, notably sodium and/or calcium. Borax. Natural or refined sodium tetraborate with ten molecules of water, i.e., Na2B4O7.10H2O. Borax also has pentahydrate (five molecules of water) and anhydrous forms. Borate. Generic term for boron compounds containing oxygen. Boric acid. An acid formed form reaction of borates with H2SO4. Formerly called boric acid, its formula is H3BO3. Borosilicate. General term referring to a range of glasses containing silica (typically over 60%) and boric oxide (5-20%). Two specific glasses cover most applications: low expansion Pyrex-type glass; and highly durable 'neutral' glass. Glass fibers[?], ceramic glazes, and vitreous enamels are also borosilicates.
  4. Aluminosilicate glass [?] has aluminum oxide in its composition. It is similar to borosilicate glass but it has greater chemical durability and can withstand higher operating temperatures. Compared to borosilicate, aluminosilicates are more difficult to fabricate. When coated with an electrically conductive film, aluminosilicate glass is used as resistors for electronic circuitry

    According to British Glass: "For textiles, an alumino-borosilicate glass with very low sodium oxide content is preferred because of its good chemical durability and high softening point. This is also the type of glass fibre used in the reinforced plastics to make protective helmets, boats, piping, car chassis, ropes, car exhausts and many other items."
    "A small, but important type of glass, aluminosilicate, contains 20% aluminum oxide (alumina-Al2O3) often including calcium oxide, magnesium oxide and boric oxide in relatively small amounts, but with only very small amounts of soda or potash. It is able to withstand high temperatures and thermal shock and is typically used in combustion tubes, gauge glasses for high-pressure steam boilers, and in halogen-tungsten lamps capable of operating at temperature as high as 750 oC."
  5. Ninety-six percent silica glass [?] is a borosilicate glass, melted and formed by conventional means, then processed to remove almost all the non-silicate elements from the piece. By reheating to 1200 ℃ the resulting pores are consolidated. This glass is resistant to heat shock up to 900 ℃.
  6. Fused silica [optic fibers] glass is pure silicon dioxide in the non-crystalline state. It is very difficult to fabricate, so it is the most expensive of all glasses. It can sustain operating temperatures up to 1200 ℃ for short periods. [Transparent to a wide range of visible wavelengths.]
  7. Alkali-barium Silicate Glass [CRT glass]
    "Without this type of glass watching TV would be very dangerous. A television produces x‑rays that need to be absorbed, otherwise they could in the long run cause health problems. The x‑rays are absorbed by glass with minimum amounts of heavy oxides (lead, barium or strontium). Lead glass is commonly used for the funnel and neck of the TV tube, while glass containing barium is usually used for the screen." From British Glass.

Composition

  1. Formers are the basic ingredients. Any chemical compound that can be melted and cooled into a glass is a former. (With enough heat, 100% of the earth's crust could be made into glass.) Most commercial glass is made with sand that contains the most common former, Silica. Other formers include …
    • Silica (sand, quartz pebbles) silicon dioxide SiO2
    • Anhydrous Boric Acid
    • Anhydrous Phosphoric Acid
  2. Fluxes help formers to melt at lower, more practical to achieve temperatures. But melting sand by itself is too expensive because of the high temperatures required (about 1850 ℃, or 3360 ℉). Therefore fluxes are added, which let the former melt more readily and at lower temperatures (1300 ℃, or 2370 ℉). These include …
    • Soda (soda ash, natron, marine plant ashes) (Na2CO3): sodium oxide Na2O
    • Potash (ashes of inland plants) potassium oxide K2O
    • Lithium Carbonate
  3. Stabilizers combine with formers and fluxes to keep the finished glass from dissolving, crumbling, or falling apart. But fluxes also make the glass chemically unstable, liable to dissolve in water or form unwanted crystals. Therefore stabilizers are added to make the glass uniform and keep its special structure intact. These include …
    • Lime (chalk, limestone (CaCO3)) calcium oxide CaO
    • Litharge
    • Alumina
    • Magnesia
    • Barium Carbonate
    • Strontium Carbonate
    • Zinc Oxide
    • Zirconia
  4. Additives?
    • Lead (oxidized lead metal) lead oxide PbO
    • Boron (modern mineral) boric oxide B2O3
    • Magnesium (impurity) — MgO
    • Aluminum (impurity) — Al2O3
    • Iron (impurity) level must be reduced to less than 1 ppb in optic fibers
    • Copper — CuO copper oxide makes glass green
    • Iron — Fe2O3 iron oxide makes glass green (brown?)
      • brown bottle glass with from the addition of a sulfur-iron colloidal solution
      • brown beer bottle glass is made from brown sand containing iron impurities
    • Cobalt oxide CoO — deep blue cobalt glass
    • Chromium — green bottle glass contains small amounts of chromium

Natural glasses …

Candy glasses …

Candies
crystalline
solid-like		 noncrystalline
glass-like
rock candy with large visible crystals, cream candies (fondant, fudge) with very small homogenous, but imperceptible crystals hard candy (lemon drop, clear mint, butterscotch, lollipop), caramel, taffy, peanut brittle, marshmallow, divinity, nougat
sucrose glucose & fructose (invert sugar)

Water has a glass phase. Glassy water can be produced by …

Just like ordinary window glass, water glass requires annealing to relax into a metastable state. Depending on pressure and temperature conditions, one of three glassy states are currently known: low-density amorphous ice (LDA), high-density amorphous ice (HDA), and very-high-density amorphous ice (VHDA). These glassy states differ microscopically in structure and macroscopically in properties like density. The commonly accepted temperature for water's glass phase transition 136 K at one atmosphere.

polymorphs

An element that can exist in two or more forms is said to be allotropic, the different forms are called allotropes, and the existence of these other forms as a phenomena called allotropy. Allotropes exist when there is more than one way for the atoms of a particular element to combine with each other to form molecules or a crystalline array.

Likewise, there are often several ways to arrange the particles of a substance in the solid phase. Such substances are said to be polymorphic or polymorphous, the variations are called polymorphs, and the existence of these other forms as a phenomena is called polymorphy or polymorphism. Polymorphs exist when there is more than one way for the particles of a particular substance to arrange themselves into a crystalline array.

Allotropy vs. Polymorphy
  particles involved are … particles combine to form …
allotropy atoms of an element molecules or crystals
polymorphy atoms or molecules crystals

Allotropes of an element and polymorphs of a substance differ in their chemical and physical properties. These differences can be subtle (like the polymorphy of cocoa butter in chocolate bars) or extreme (like the differences between graphite and diamond). The rest of this section will be taken up with a discussion of examples of allotropy and polymorphism.

carbon

This discussion is so common, what can I say that hasn't been already said?

Primary Allotropes of Carbon (The Elementary Version)
diamond graphite
the hardest substance known
(10 on the mohs scale)
used as an abrasive		 among the softest substances
(0.5 on the mohs scale)
used as a lubricant
usually transparent
colorless to red or blue
used in jewelry		 always opaque
black (somewhat metallic)
used in pencils (thus the name)
a good electrical insulator
~TŒ© m resistivity		 a good electrical conductor
650 nŒ© m resistivity
high thermal conductivity
(higher than any metal)
895 W/m K		 dual thermal conductivity
1950 W/m K parallel to plane layers
5.7 W/m K perpendicular to layers

Other Stuff

Junk I've come across

Primary Allotropes of Carbon (The Advanced Version)
property diamond (I) graphite
hardness 1010 kg/m2  
hardness, mohs 10 0.5
hardness, knoop   7000
strength, tensile > 1.2 GPa  
strength, compressive > 110 GPa  
speed of sound 18,000 m/s  
density 3510 kg/m3 2250 kg/m3
young's modulus 1.22 GPa  
poisson's ratio 0.2  
thermal expansion coefficient 1.2 × 10−6 K−1 4.3 × 10−6 K−1
thermal conductivity 895 W/m K 1950 W/m K
thermal shock parameter 30 MW/m  
debye temperature 2340 K  
specific heat 472 J/kg K 715 J/kg K
optical index of refraction 2.417 opaque
optical transmissivity 225 opaque
loss tangent at 40 Hz 0.0006  
dielectric constant 5.87  
dielectric strength 10 TV/m  
electron mobility 0.22 m2/V s  
hole mobility 0.16 m2/V s  
electron saturated velocity 270 km/s  
hole saturated velocity 100 km/s  
photoelectric work function small and negative 4.8 eV
bandgap 5.45 eV  
resistivity 1011 - 1014 Œ© m 650 nŒ© m
Source: Molecule of the Month

ice

Nice summary from Physics Today

Water can exist in many different crystalline forms, 13 of which have been identified to date. Of those, nine are stable over some range of temperature and pressure--for example, at atmospheric pressure, ordinary hexagonal ice is stable between 72 and 273 K--and the other forms are metastable.

George Lucas freely admits that Star Wars is really an ancient myth cloaked behind futuristic-looking technology and I've heard Star Trek described (rather accurately in my opinion) as a "space opera" In an age where science fiction is everywhere, it seems that very little of it contains any science. Cat's Cradle is an exception in that it's a real marriage of science and fiction.

"There are several ways," Dr. Breed said to me, "in which certain liquids can crystallize — can freeze — several ways in which their atoms can stack and lock in an orderly, rigid way."

That old man with spotted hands invited me to think of the several ways in which cannonballs might be stacked on a courthouse lawn, of the several ways in which oranges might be packed into a crate.

"So it is with atoms in crystals, too; and two different crystals of the same substance can have quite different physical properties."

"Now suppose," chortled Dr. Breed, enjoying himself, "that there were many possible ways in which water could crystallize, could freeze. Suppose that the sort of ice we skate upon and put into highballs — what we might call ice-one — is only one of several types of ice. Suppose water always froze as ice-one on Earth because it had never had a seed to teach it how to form ice-two, ice-three, ice-four … ? And suppose," he rapped on his desk with his old hand again, "that there were one form, which we will call ice-nine — a crystal as hard as this desk — with a melting point of, let us say, one-hundred degrees Fahrenheit, or, better still, a melting point of one-hundred-and-thirty degrees."

ellipsis

And that old man asked me to think of United States Marines in a Godforsaken swamp.

"Their trucks and tanks and howitzers are wallowing," he complained, "sinking in stinking miasma and ooze."

He raised a finger and winked at me. "But suppose, young man, that one Marine had with him a tiny capsule containing a seed of ice-nine, a new way for the atoms of water to stack and lock, to freeze. If that Marine threw that seed into the nearest puddle … ?"

"The puddle would freeze?" I guessed.

"And all the muck around the puddle?"

"It would freeze?"

"And all the puddles in the frozen muck?"

"They would freeze?"

"And the pools and the streams in the frozen muck?"

"They would freeze?"

"You bet they would!" he cried. "And the United States Marines would rise from the swamp and march on!"

cocoa butter

Chocolate is a preparation made from the seeds of the cacao bush (Theobroma cacao). These are often mistakenly called cocoa beans, but they are neither made of cocoa nor are they beans. Cacao is a small evergreen tree, not a legume — the seeds of which are fermented, roasted, husked, pulverized, and pressed. Cocoa is the dry residue that separates from the fat (cocoa butter) during the pressing stage. If pulverized cacao is heated to the point of liquefaction (conched) and then cooled (tempered) instead of pressed the resulting product is called chocolate. Sugar is also almost always added to the mix. Straight chocolate is far too bitter for most people.

During the cooling process the cocoa butter in chocolate can solidify into one of six different polymorphs identified with roman numerals in order of their melting point. Form V is the polymorph found in good quality, well-tempered chocolate confections. The other forms feel too sticky or thick in the mouth or are associated with fat bloom — a condition where the cocoa butter separates out during storage. Chocolate bars suffering from fat bloom look dusty or cloudy, taste bland, and melt too easily. Although form V is the best tasting polymorph of cocoa butter, form VI is the most stable. Well-tempered and well-processed chocolate can transform into the stable but undesirable form VI if it is stored in a warm place or kept on the shelf for too long. This is why chocolate should be stored somewhere cool and dark and eaten soon after it is bought. The first condition is the responsibility of the producers, distributors, and retailers. The second condition is the responsibility of the consumer. Due to chocolate's inherent power over people, it is highly unlikely that it will ever last in the hands of the consumer long enough to undergo fat bloom. Most people buy and eat their candy bars and such within a few days or hours after they are purchased.

Food scientists in the chocolate industry seek to develop techniques that encourage cocoa butter to solidify in the desirable form V polymorph. One of the more interesting techniques involves irradiating molten cocoa butter with low level x‑rays. This is done not to produce an x‑ray photograph like a doctor would, but to see the resulting interference pattern generated. The technique is called x‑ray diffraction and will be discussed in another chapter. An expert technician can interpret the pattern and infer the configuration of the atoms or molecules in the solid lattice. Different heating, cooling, and stirring regimes are tested until an optimum process evolves.

Interestingly, form V cocoa butter has a melting point of 34 to 36 ℃; slightly less than the interior of the human body — 37 ℃. This is one reason why chocolate melts so well in the mouth. This property is also exploited by pharmaceutical companies in the preparation of suppositories. A suppository is a bullet-shaped drug delivery system consisting of cocoa butter mixed with a medication that is then inserted into an orifice other than the mouth. (Think about what this last phrase is saying for a second.) As the cocoa butter melts away the drug is released gradually. The drugs may remain near the surface (as is the case for hemorrhoidal suppositories) or they may diffuse into the capillaries and spread to the rest of the body via the circulatory system (as is the case with narcotic suppositories). Suppositories are also made from other compounds, but when they are made from cocoa butter, form V is still the desired polymorph.

Polymorphs of Cocoa Butter
polymorph melting point (℃) comments
form I 17.3 Produced by rapid cooling of melt. Successive polymorphs are then obtained sequentially by heating at 0.5 ℃/min.
form II 23.3 Produced by cooling melt at 2 ℃/min or rapid cooling of melt followed by storing from several minutes up to one hour at 0 ℃. This form is stable at 0 ℃ for up to 5 hours.
form III 25.5 Produced by solidification of melt at 5-10 ℃ or transformation of form II by storing at 5-10 ℃.
form IV 27.3 Produced by solidification of melt at 16-21 ℃ or transformation of form III by storing at 16-21 ℃.
form V 33.8 Produced by tempering (cooling then reheating slightly while mixing). The most desirable form with good gloss, texture, and "snap".
form VI 36.3 The transformation of form V after spending 4 months at room temperature. Leads to the white, dusty appearance of "bloomed" chocolate.
Source: The Materials Science of Chocolate

metals

Shiny.

iron and steel

Iron undergoes three solid state phase transitions as its temperature increases from room temperature to 1535 ℃ where it melts. The three phases are identified by the lowercase greek letters alpha (α), gamma (γ), and delta (δ). The alpha and delta phases are also called ferrite or ferritic iron, which is derived from the latin word for iron ferrum. The gamma phase is also called austenite or austenitic iron in honor of the English metallurgist William Chandler Roberts-Austen (1843-1902). Austenitic iron is the harder of the two. Since iron changes its magnetic properties within the range of temperatures over which it is solid, the alpha phase was originally split into an alpha magnetic phase and a beta (β) nonmagnetic phase. Since this magnetic transition has nothing to do with polymorphism, the beta phase has been phased out so to speak.

The crystal structure of the two ferritic phases is body centered cubic (bcc), while the γ austenitic phase is the more tightly packed face centered cubic (fcc). The transition to the fourth phase epsilon (ε) occurs when pressures exceed 13 GPa (130,000 times atmospheric pressure). The crystal structure of this tightly packed phase is the appropriately named hexagonal close packed (hcp) arrangement. The earth's core is thought to consist of a very large single crystal of epsilon-phase, iron-nickel alloy. The hcp lattice structure gives the core a density greater than 10,000 kg/m3. Compare this to 7,900 kg/m3 for iron and 8,900 kg/m3 for nickel.

Polymorphs of Iron (under ordinary pressures*)
∆T (°C) phase crystal structure magnetism hardness
< 770 α alpha ferritic body centered cubic magnetic soft
770–910 α alpha** ferritic body centered cubic nonmagnetic soft
910–1390 γ gamma austenitic face centered cubic nonmagnetic hard
1390–1535 δ delta ferritic body centered cubic nonmagnetic soft
1535–2860 liquid   nonmagnetic  
> 2860 gas   nonmagnetic  
  * hexagonal close packed epsilon (ε) when P > 13 GPa
** formerly beta (β)

Iron is an element. Steel is an alloy of iron — a mixture of iron with other elements in a metallic matrix. The first steels were probably created accidentally when iron sword blanks were heated in charcoal forges and some carbon worked its way into the iron matrix. With carbon, steel is harder and has a higher tensile strength than iron, but is also more brittle. Steel flexes under stresses that would bend iron, but steel is also more likely to break before iron will.

Iron and carbon can also form the ceramic compound Fe3C, called iron carbide by chemists and cementite by metallurgists. Both names are evocative of the hard, brittle, and very un-metallic nature of this compound. Like all solvents, iron has a limited capacity to dissolve carbon. At high to very high concentrations, cementite crystals will begin to form within the matrix of α ferrite. When the mixture crystallizes into alternating layers of Fe and Fe3C, the resulting solid has a luster resembling mother of pearl and is called pearlite. A similar mixture with round crystals is called spheroidite. I've found other references to mixtures of Fe and Fe3C, namely cast iron, bainite, and ledeburite, but I can't find any way to succinctly describe what they are. Consult a metallurgist for more details

Carbon Steels and Related Compounds
C (%) name
0 α ferrite, pure iron (Fe)
0.05-0.16 low carbon steel
0.16-0.30 mild steel
0.30-0.60 medium carbon steel
0.60-1.00 high carbon steel
1.00-2.00 very high carbon steel
  pearlite: layered mixture of α ferrite and cementite crystals
  spheroidite: mixture of spherical α ferrite and cementite crystals
  cast iron, bainite & ledeburite: mixtures of α ferrite and cementite
6.67 cementite, iron carbide (Fe3C)
> 6.67 cementite mixed with leftover carbon

Classic steels are a mixture of a lot of iron with a little bit of carbon, but modern steels may also contain other metals such as chromium, nickel, manganese, molybdenum, or vanadium. Chromium is added to produce stainless steel. The resulting alloy resists oxidation (commonly known as rusting) and corrosion (the eating away of a material due to chemical action). Nickel and manganese are added to increase hardness by keeping iron in a phase that shouldn't exist.

At room temperature, the natural state of iron is the relatively soft, body centered cubic, α-ferritic phase. Raising the temperature to 910–1390 °C shifts the atoms into the harder, face centered cubic, γ-austenitic phase, but this hardness will disappear the moment the iron cools back down. Adding a medium amount of carbon to the iron along with a healthy dose of nickel and a dash of molybdenum gives us a mixture with special properties. When heated above the transition temperature and allowed to cool properly, the γ-austenitic, face centered cubic structure stays locked in place. In a sense, the nickel and molybdenum atoms work defense. They keep the iron atoms from reaching their goals in the body centered cubic lattice. Add some chromium for corrosion resistance and you've got the most widely used group of stainless steels in the world.

Austenitic stainless steels are found in cookware, flatware, kitchen sinks, food processing equipment, piercing jewelry, surgical hardware, and (because they retain the nonmagnetic properties of the γ-austenitic phase) floppy disk shutters. (You remember floppy disks.) Austenitic stainless steels are often identified by the proportions of chromium and nickel added to the iron. The most common types are 18/10 (18% chromium, 10% nickel) and 18/8 (18% chromium, 8% nickel).

Austenitic steels are noted for their hardness, but they are not the hardest family of steels. That distinction belongs to martensitic steel — named in honor of the German metallurgist Adolf Martens (1850–1914). Martensitic steel is used in cutlery, scalpels, wrenches, turbines and any other application where hardness is important. It is magnetic.

The recipe for martensite starts with a basic iron, carbon, chromium mixture heated until the iron enters the γ-austenitic phase. This is then followed by a rapid cooling process called quenching, where the hot steel is dipped in water, brine or oil. The drop in temperature is so precipitous that the crystal goes from face centered cubic (in which the lattice unit is a cube) to body centered tetragonal (rectangular box shaped). Iron atoms in a solid under normal pressures want to exist in one of two arrangements: body centered cubic or face centered cubic. The tetragonal arrangement is unnatural and only exists because the material was cooled too quickly to allow the atoms to move into their "proper" positions. The resulting arrangement is unstable, but long-lived — it is metastable. The martensitic structure can easily be permanently destroyed by the addition of heat. When it's controlled, this process is called tempering. The uncontrolled addition of heat can have catastrophic consequences.

Stainless Steels (per cent composition by mass)
C (%) Cr (%) Ni (%) name crystal structure magnetism
< 0.08 12-26   ferritic body centered cubic magnetic
< 0.12 17-25 7–20 austenitic face centered cubic nonmagnetic
< 1.2 12-18   martensitic body centered tetragonal magnetic

Taking steel through its various phases to arrive at the one with the desired properties is largely accomplished by the appropriate amounts of heating and cooling. Heating pure iron, as I said at the beginning of this section takes it from a soft phase (α) to a hard phase (γ) and back to a soft phase (δ). Similar changes occur in steel.

On 11 September 2001, the twin towers of the World Trade Center in New York City were each struck by commercial jetliners in the worst attack in history on US territory. The impact of each plane caused major structural damage that was serious but not catastrophic. The towers survived. Each jet was filled with sufficient jet fuel to fly from one coast to another. When ignited it was hot, but not sufficiently so to melt the steel beams inside. The towers continued to survive as they were designed to. Both planes impacted so high up that fire fighters were unable to extinguish or even reduce the flames. The horizontal steel floor beams got hotter and hotter and hotter, but they did not melt. What they did instead was turn plastic and sag. This pulled them away from the vertical support beams causing the floor to collapse. After enough of these beams had separated a catastrophic collapse ensued, one floor pancaking down onto the next one below it until the whole tower had collapsed. This was repeated twice, once for each of the two 110 story towers, after roughly an hour of exposure to the burning jet fuel. The fire then spread through a common basement into a nearby 40 story building (World Trade Center 7). This weakened several interior vertical columns. When one failed, the others nearby were unable to compensate and a third catastrophic collapse was initiated approximately 7 hours later.

The softening of steel beams in structural fires is common. The photo below shows an abandoned cargo pier of the former New York Central Railroad yards on Manhattan's Upper West Side (now demolished).

INSERT COMIC TIMES PHOTO

more, so many more

plasma

low density and high temperature prevents electrons from returning to their homes

plasmas on earth

plasmas comprise 99.99% (um, verify please) of the visible matter of the universe

man-made plasmas

The Physics Hypertextbook

  • No condition is permanent.