Wikipedia:Reference desk/Archives/Science/2019 November 14

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November 14

Enthalpy of fusion

A solid crystal (such as ice) is a more ordered state than water, so one would expect that when transitioning from water to ice extra energy is expended to order the atoms in the ice crystal lattice. When ice melts back into water, I would expect that that energy that had been bound into the system gets released, so melting would be an exothermic reaction. Yet it seems that melting a solid actually binds energy. The example in the article clearly states that it takes 333.55 kJ/kg extra energy just to move from 0C solid to 0C liquid water. Why is that so? Where is the fault in my conceptual model? 93.136.57.235 (talk) 00:09, 14 November 2019 (UTC)

In a simple form, the answer is right there in the article: ``The liquid phase has a higher internal energy than the solid phase. This means energy must be supplied to a solid in order to melt it and energy is released from a liquid when it freezes, because the molecules in the liquid experience weaker intermolecular forces and so have a higher potential energy (a kind of bond-dissociation energy for intermolecular forces). `` This is true for all solid-liquid transitions (unless quantum-mechanical effects become dominant and stabilize the liquid phase even at T = 0K, but that's irrelevant for this discussion). The volume may increase (normal transition) or decrease (anomalous transition, as water has at about 1 atm of pressure) but the key is that the internal energy of a liquid is always higher than that of a solid. Hope this helps. Dr Dima (talk) 00:55, 14 November 2019 (UTC) . To add, I don't really like the "weaker intermolecular forces" formulation in the article because, first, the enthalpy of fusion is positive for all materials undergoing first-order solid-liquid transition: metals, molecular solids, etc.; and second, because the forces are not necessarily weaker per se, but rather because the melting causes the atoms / ions / molecules to move against those forces, that is, to perform work (for which the energy must be supplied from outside), hence the positive enthalpy of fusion. Dr Dima (talk) 01:04, 14 November 2019 (UTC)

Yes I see that it has a higher internal energy, but why? It's a more disordered state. I see an analogy with a tower of bricks arranged one atop of another, which obviously has more "energy" (in this case gravitational potentional energy) than a pile of collapsed bricks, which is obviously the opposite of what is the case with states of matter. What analogy works here instead? 93.136.57.235 (talk) 01:16, 14 November 2019 (UTC)

Hmm I thought about this some more, thank you for reminding me that in a crystal atoms have to first overcome the forces holding them in stable symmetry to create the disorder that is a liquid. But when you look at it from the opposite side, the freezing, then energy is obtained from the system by binding it in this highly organized and improbable state? Why isn't that a violation of the law that entropy always increases? 93.136.57.235 (talk) 01:27, 14 November 2019 (UTC)

Its called Phase transition and water is a special case. See Liquid-liquid critical point.--Kharon (talk) 04:41, 14 November 2019 (UTC)

Are we talking about the same thing? The article Enthalpy of fusion lists positive enthalpies for a bunch of substances and only helium is mentioned as an exception (which I was taught in school never freezes). 93.136.94.213 (talk) 06:29, 17 November 2019 (UTC) (OP)

Water molecules have two positively charges hydrogen atoms and one negatively charged oxygen atom. In ice the molecules are arranged such that the hydrogen atoms of one molecule point at the oxygen atom of its neighbour. This gives lower internal energy. Our article on ice I_h has a picture attempting to show the crystal structure.

You forgot about the containment of the bricks in your brick analogy. Take a vertical tube and neatly stack the bricks inside. Now take an identical vertical tube and randomly throw the same number of bricks into it. The second pile of bricks will be higher. Or, starting from a random pile of bricks, reorder them on the same surface area in a regular pattern, like a road surface. You'll get them in a lower energy state.

The Second Law only tells us that entropy can never decrease in a closed system. A bucket of water being frozen isn't a closed system. Heat is flowing out of it, along with entropy. PiusImpavidus (talk) 11:09, 14 November 2019 (UTC)

Yeah, I forgot that entropy can decrease in one place while increasing in another, of course that makes sense. I've come up with a way of conceptualizing the positive heat of solidification: as the avg kinetic energy drops, at some temperature particles encountering a crystal formation don't have enough energy to escape being bound into it, and that energy is when moving a good bit faster than zero speed (which would be necessary to get stuck if there were no binding forces). But this drop into a lower energy state means some heat must be created, maybe particles knocking into the crystal transfer their momentum while being bound into it and increase its own temperature so that little crystals quickly blow apart if the temperature is a little too high. On the other hand big crystals maybe dissipate energy from collisions amongst its particles better so take extra energy input in form of even faster-moving than freezing-speed particles to melt (kinda like launching something out of a gravity well, you need to make it move faster than the speed you want it to move at when it's out of the well, which is here the enthalpy of fusion).

Is my model realistic? That would explain seed crystals phenomenon and why for example water freezes quickly but snow melts slowly at the same ΔT from freezing point. 93.136.94.213 (talk) 06:29, 17 November 2019 (UTC)

Your "model" is not too wrong, but it is not entirely correct either. At the molecular level there is no such thing as "dissipating energy from collisions" (i.e. inelastic collisions), and there is no "collision" in solids except for a very weird definition of the term.

As for the kinetics of melting/freezing, that is mostly due to nucleation problems (see subcooling and supercooling). The basic model for supercooling is that small crystals are unstable because they have a small volume to surface ratio hence surface tension effects (which do not like the solid/liquid interface) are higher than the energy win from a better organization in the bulk; hence you need to go through an energy barrier to create a "germ" i.e. a crystal above the critical volume. (Yet more nitpicking: critical germ size is usually about a few hundred atoms so "surface tension" is not really a thing at this scale either.) Tigraan^{Click here to contact me} 10:56, 18 November 2019 (UTC)

OK, thanks, good to know I'm on the right track. I know I "explained away" some things to keep it simple and easy to portray in the mind's eye. 93.136.31.83 (talk) 04:12, 19 November 2019 (UTC)

Separate question
So water boiling to steam (at boiling point) and melting of ice (at room temp) are both endothermic? And water is a rare exception? What other compounds/molecules fit this criteria? 67.175.224.138 (talk) 15:31, 16 November 2019 (UTC).

• Melting#As_a_first-order_phase_transition says that melting is endothermic except for two pathological cases (helium close to absolute zero), and so is boiling. Water is no exception here. Water does have a pathological phase diagram (as the article says: Water is an exception which has a solid-liquid boundary with negative slope so that the melting point decreases with pressure., which is not usual - but the enthalpy of fusion is still positive). Tigraan^{Click here to contact me} 10:37, 18 November 2019 (UTC)

Relativistic jets and black holes

Reading the article on relativistic jets, I get the impression that they can be produced by black holes. But black holes absorb all matter with nothing escaping. What's the relationship between relativistic jets and black holes? How are relativistic jets produced? 5225C (talk) 00:58, 14 November 2019 (UTC)

They are created by the accretion disk, which is outside the event horizon of the black hole (the point of no return). SinisterLefty (talk) 03:23, 14 November 2019 (UTC)

My layman's impression is that it's a situation similar to the gravity assist. Matter is attracted from somewhere else to the vicinity of the black hole, does a pirouette outside the event horizon and gets accelerated and focused in a narrow beam by some mechanism. You'll have to ask someone else what that particular mechanism is. I suspect it's extracting work from the rotation in case of a rotating black hole, but no idea for when the hole isn't rotating. 93.136.57.235 (talk) 03:53, 14 November 2019 (UTC)

Obviously some black holes dont absorb everything. Quasars have already been found in the 1960s to typically develope these jets. So science knows for 60 years now that black holes dont absorb everything. --Kharon (talk) 05:14, 14 November 2019 (UTC)

If you google "black hole accretion disk magnetic field" you can find lots of articles and images and mathematical models of the magnetic fields around a black hole. The black hole itself is expected to have little to no innate magnetic field, but the accretion disk around the black hole can have a strong one. These magnetic fields are what accelerate charged particles, successfully launching some of them away. Someguy1221 (talk) 06:01, 14 November 2019 (UTC)

To elaborate: matter only gets "absorbed" into a black hole if it passes the event horizon. Outside the event horizon, a black hole behaves just like any other massive object; matter can orbit the black hole, or fall towards it and get flung away, as long as its trajectory does not cross the event horizon. Black holes aren't "cosmic vacuum cleaners", any more than stars are. --47.146.63.87 (talk) 11:01, 14 November 2019 (UTC)

Last and maybe least - there is a maximum rate to a black hole matter consumption. The maximum input is a single neutron layer with in the density of a neutron star, all around the event horizon. anything beyond this will either accumulate or flung away. אילן שמעוני (talk) 15:14, 14 November 2019 (UTC)

Yes, think of it like water trying to go down a drain, or the The Three Stooges all trying to get through the same doorway at once [1]. :-) SinisterLefty (talk) 17:07, 14 November 2019 (UTC)

The saying is that "black holes are messy eaters". [2] --Amble (talk) 17:57, 14 November 2019 (UTC)

Like so [3]. :-) SinisterLefty (talk) 18:02, 14 November 2019 (UTC)

"medical oxygen"

What's the concentration level of oxygen in a standard medical oxygen gas which is normally found in ambulances or hospitals? Should it contain some percent of nitrogen too? 93.126.116.89 (talk) 20:14, 14 November 2019 (UTC)

I'm an EMT and administer oxygen to patients routinely. We always treat it, and do calculations, as if it's 100% oxygen. Of course, minor amounts of nitrogen or other harmless gases wouldn't matter. Oxygen used in various chemical operations probably has to meet different specifications. Jc3s5h (talk) 20:17, 14 November 2019 (UTC)

Note that the percentage of oxygen in the tank isn't the same as the percentage which reaches the lungs. Even in a healthy individual there would be mixing of the old air and the new, with carbon dioxide continuously being released into the lungs by the body. But in somebody with breathing problems (an obstruction, fluid in the lungs, not breathing, etc.), the proportion reaching the alveoli could be considerably less. SinisterLefty (talk) 20:37, 14 November 2019 (UTC)

Prolonged exposure to high partial pressure of oxygen can cause oxygen toxicity. I think medical oxygen in ambulances is close to 100% oxygen, but on the understanding that it will only be administered undiluted to a patient for a short period. Dolphin (t) 21:05, 14 November 2019 (UTC)

See Explanation On PSA Medical Oxygen Generator Purity 93±3%. It seems that nobody minds a bit of argon but halogen is a bad idea. Alansplodge (talk) 21:56, 14 November 2019 (UTC)

Thank you all for the answers, but I don't understand it yet properly. in the last reference it's mentioned that "United States Pharmacopoeia (USP) Oxygen 93% Monograph has specifications as “not less than 90.0% and not more than 96.0%” for PSA oxygen". So EMTs use 100% against it? or maybe I miss something? 93.126.116.89 (talk) 01:17, 15 November 2019 (UTC)

Also, if the standard medical oxygen which found in ambulances is 100% concentration, so how could it be that AHA (see here paragraph No.6.2.1) states: "In term and late-preterm newborns (≥35 weeks of gestation) receiving respiratory support at birth, the initial use of 21% oxygen is reasonable". How could it be? or maybe it's a specific oxygen found in hospitals in neonates department? 93.126.116.89 (talk) 01:18, 15 November 2019 (UTC)

21% oxygen is just normal air. EMTs are trained to avoid giving 100% (or whatever the concentration of medical oxygen is) to newborns whenever possible. You will notice the guidelines you quoted indicated to start with 21% oxygen, and titrate to achieve acceptable oxygen saturation in the blood, measure with pulse oximetry. Jc3s5h (talk) 02:47, 15 November 2019 (UTC)

• It's important to distinguish between concentration and partial pressure. Humans suffer from long-term oxygen toxicity if the partial pressure is too high. This can be caused by too great a proportion of oxygen at atmospheric pressure (or, less commonly, by the same proportion at an increased hyperbaric pressure (divers have to switch to technical mixes)). But if the overall pressure is also reduced, then increased oxygen isn't a problem, even up to pure oxygen. The partial pressure is then no more than it would be for the standard atmosphere (twice the proportion at half the pressure). This is the case for military pilots, or even the first astronauts (although see Apollo 1 fire, and the risks if this is at atmospheric pressure).

Medical oxygen is either acute use, through a mask, which is pure [sic] oxygen. As the exposure is short, there's no problem. Or else long-term use is through a nasal cannula and there's a dilution effect. The masks used for long-term use have similar effects and can give oxygen levels up to about 50%. Paediatric oxygen therapy is done through incubators which are at atmospheric pressure, so again there's an issue with an excess partial pressure if the concentration would be too high. Andy Dingley (talk) 02:06, 15 November 2019 (UTC)

To explain the 100% vs. 90-96% standards (it's already been mentioned by I'll make it clear), these are based on where the oxygen came from, as this has an effect on the contaminants present. Pressure swing adsorption is a technology that can be used to generate up to 96% oxygen with 4% argon. Commercially available systems using this technology cannot presently generate 100% oxygen. Another method of obtaining high concentrations of oxygen is with a cryogenic oxygen plant. This technology has different contaminants present in the final product, and to ensure that these are at a low enough level to not cause health problems of their own, it is recommended that the oxygen be at least 99.5% pure. Someguy1221 (talk) 03:36, 15 November 2019 (UTC)

We could do with an article on cryogenic production of oxygen, why this was so common pre-WWII as the general workshop cryofluid (despite the obvious fire risk!) and why it was supplanted by the rather safer liquid nitrogen, post-WWII. Andy Dingley (talk) 10:01, 15 November 2019 (UTC)

Medical-grade oxygen is almost pure, almost-100% O₂. The only commercially-available oxygen bottles that are more pure are Aviator's Breathing Oxygen. Here's a standard, for those who need technical details: AS8010C from SAE. The chief difference is the complete, certified, removal of trace water vapor, to reduce the risk of water-ice formation when the bottle gets abnormally cold due to aviation environmental factors; and mechanical considerations for the safe storage and delivery of the breathing gas.

As such, I would posit that most medical oxygen in nearly pure oxygen with trace water-vapor as the primary contaminant - and that interested readers should talk to their oxygen vendor for specific source and quality information.

From DTIC, here is a nice archived review study: SAM-TR-76-44: Aviator's Breathing Oxygen Specifications, which gives detailed breakdown of allowable contaminant gas, and common values for various sources that can be procured industrially. With water vapor removed, the chief remaining contaminants in the breathing gas bottle appears to be methane, at extraordinarily low concentration.

Nimur (talk) 14:24, 15 November 2019 (UTC)

"Medical oxygen" has two standards for it, by use if not a formally defined standard. That produced by cryogenic fractional distillation is as you describe. But increasingly these days, especially for chronic care at home, it's produced at low pressure and not stored in bulk, by use of a PSA concentrator. These don't separate out the argon. But as argon is about as inert as it gets, that "impurity" is inconsequential. Yet that oxygen is only 90+% purity. The actual purity varies a bit depending on machine: some offer lower purity (more argon) as it's technically simpler and more efficient, and the resulting product is just as good. Andy Dingley (talk) 15:59, 15 November 2019 (UTC)

See oxygen therapy: the actual concentration of oxygen delivered to the patient's lungs depends on flow rate, method of administration, and other factors. --47.146.63.87 (talk) 04:09, 16 November 2019 (UTC)