Wikipedia:Reference desk/Archives/Science/2017 July 1

Science desk
< June 30	<< Jun | July | Aug >>	July 2 >

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

July 1

Hydrogen versus acetylene as illuminating gas

The flames of the Hindenburg disaster reportedly began as yellow-red but at the moment of this black-and-white photograph the burning hydrogen produced a white cloud. However diesel fuel and steam can also have been involved. Blooteuth (talk) 23:26, 1 July 2017 (UTC)

A later view of the airship does not support the idea that the skin contributed to the conflagration but the internal duralumin spars would be burning at this stage. Blooteuth (talk) 14:45, 2 July 2017 (UTC)

If a residential light fixture was designed to burn acetylene gas with a mantle and globe, would it also provide light burning hydrogen? Would a different flow rate be needed to provide the same light? If no mantle were used and it just had a flame, would the color and brightness at the same flow rate be similar to acetylene? I realize that either gas would pose a hazard of explosion or asphyxiation and the question is a basic science question rather than a search for advice on practical lighting methods. Edison (talk) 20:51, 1 July 2017 (UTC)

Neither of these are suitable for use with gas mantles. They, especially hydrogen, work better with the older limelight method, of a bulk incandescent element.

Acetylene produces significant soot on burning. This makes its combustion inefficient, but also makes for a bright flame, as the soot is incandescent. It has long been used in simple fishtail burners for caving and some mining (but not usually coal mining). Hydrogen is generally dreadful as an illuminating gas. It usually needs to be burned with an oxygen supply, but this is often available anyway, when the hydrogen is produced electrolytically from water (see Brown's gas too). Andy Dingley (talk) 21:13, 1 July 2017 (UTC)

The Space Shuttle Main Engine burnt hydrogen with oxygen, producing a nearly invisible flame at full thrust.

Note that hydrogen gas burned with excess oxygen and nothing else is almost invisible. See illustration. As such, you would need to add something, or reduce the oxygen ratio, to use it for illumination. StuRat (talk) 21:29, 1 July 2017 (UTC)

The Hindeneburg flames, reportedly orange, also involved the doping compound used on the skin. StuRat (talk) 23:30, 1 July 2017 (UTC)

Yes a mantle will produce light from hydrogen because it is making the mantle incandescent. The flow rate for hydrogen though, would need to be higher as it has a lower calorific value than acetylene. Any flammable gas will heat the mantle to incandescence. Mostly used today is butane, propane and methane. In past time it was often petrol/gasoline vapour, paraffin/kerosene vapour, coal gas, and of course acetylene. As anyone knows, who have done any acetylene welding, acetylene only produces soot when it is too rich ( i.e., not mixed with enough air. Turn up the regulator on the acetylene welding bottle and the tadpoles of soot stop being produced -even though no oxygen from the oxidiser bottle is beind suplied. An acetylene lamp therefore, is designed to produce a jet which courses vortexes in its flow to mix in enough air to mix and proving a bright flame with no smoke. Limelight came before mantles were develop. Hydrogen burning with oxygen contains no carbon (the incandescence of which provided the bright yellow light) so will only glow blue when the heated oxygen lases down to a lower energy state. One can see this when one looks close at the wick of a candle. The inner shell of the flame is blue. The blue light is also being emitted up further up in the yellow flame but is swamped from human eyes by the brightness of the incandescent carbon. The airship skin was painted with aluminum and iron oxide. Very similar to Thermite and very flammable. Aspro (talk) 23:48, 1 July 2017 (UTC)

Is polynosic rayon particularly sensitive to chlorine bleach ?

Virtually all my clothes say not to use bleach. Advice which I completely ignore, as most (typically cotton/polyester blends) seem to be able to handle chlorine bleach just fine, giving me sterile, odor-free clothes at minimal cost. However, I have one shirt which is 70% polynosic rayon/30% polyester, and it virtually dissolved in bleach. So, I seem to have discovered that, for this one fabric, the advice to avoid chlorine bleach is legit. Is this known to be the case ? Is it all rayon or only the "polynosic" variety ? Do we have a list of which fabrics REALLY can't be bleached ? (If it includes cotton/poly blends then I know they are lying.) StuRat (talk) 21:14, 1 July 2017 (UTC)

To "lose one's shirt" is a common expression for making a bad bet. Polynosic is the trademark[1] for a type of microfiber that is a blend of advanced polyester and rayon fibers. Characteristics include luxurious soft touch with a drapeable hand.[2]. Karin Fjellman advises that bleach only works successfully with natural fabrics such as cotton, rayon or linen, (it's also ok with polypropylene fabric) while bleach affects polyester in such a way that cannot be undone or repaired. If you absolutely must whiten polyester, the best alternative to bleach is a good fabric paint. Blooteuth (talk) 23:13, 1 July 2017 (UTC)

Are you sure you don't have rayon and polyester confused ? My observations are the exact opposite of what you describe. The only other explanation is that the labels on most of my clothes list the wrong materials. StuRat (talk) 23:50, 1 July 2017 (UTC)

You may check my sources and you would still have a shirt if you had obeyed this: "Machine wash cold with like colors. Tumble dry low. Do not bleach." Blooteuth (talk) 14:24, 2 July 2017 (UTC)

Rayon is a sort of reassembled cellulose where the individual sugar units have been split up with carbon disulfide, then put back together again in new fibers. Online sources [3] suggest that it can be bleached, though direct exposure to bleach can "weaken" the fibers. Given that just about any organic compound seems prone to free radical reactions (this is much of the basis of macrophages killing stray bacteria or liver cells detoxifying chemicals) I don't expect many things to totally laugh at bleach. The site above also mentions that many rayon fibers are not washable because they shrink and have to be dry cleaned, with "modal" being one of the exceptions; our article says Modal is a brand name for a kind of polynosic rayon, where polynosic rayon is a high wet modulus rayon. I have not sorted out contradictions with the above yet.

Now polyester is harder to sort out. Polyester is a fabric with ... a lot of ester bonds. All sorts of different things can be between those bonds, though there are a few common types. So even though I read quite a few people saying they successfully bleached a polyester garment online, I don't know that there isn't some specific kind out there that disintegrates on contact. Wnt (talk) 11:19, 2 July 2017 (UTC)

Why not use Hydrogen Peroxide instead. Work out the cost/befit ratio. If you are a high paid company executive or salesman and must always have pristine presentability, then you are only going to get 20 washes out of a shirt anyway -before it starts looking shabby. Trump probably follows suit from other presidents that don't wear washed shits and if you look closely at interviews with Russian government officials they always appear to be wearing brand new out of the box silk shirts. Hydrogen Peroxide is only a 'little' more expensive and over 20 washes costs less than that of a new shirt. The labels 'not to use bleach' is just the same legal disclaimer that manufactures use like 'only dry-clean'. When in fact, washed properly, some labeled 'only dry-clean' cloths come out OK. The secret is often no more, than using good old fashioned soaps rather than modern detergents. But that has to make the housewife think and understand, that the (heavily advertised) stuff she normally buys from the Mall is not actually suitable for everything -as is often claimed on TV. Same for shirts. Aspro (talk) 13:31, 2 July 2017 (UTC)

Bleach and hydrogen peroxide are pretty similar chemically. Sodium hypochlorite is the salt of HO-Cl, chlorine gas is Cl-Cl, and hydrogen peroxide is HO-OH. These are all isoelectronic (if you count the hydrogen as an "H+"; I'm not sure that term strictly applies). The two halves of each one have the potential to become leaving groups if negatively charged, while homolytic cleavage produces free radicals. There are, of course, real chemical differences, but at a first approximation, I wouldn't assume that peroxide is safe for a fabric when bleach isn't. Wnt (talk) 00:45, 3 July 2017 (UTC)

Actually, I have considered that, and would use hydrogen peroxide, if it was less expensive. It costs me about US$4 for a 3% solution gallon vs US$1 for a 3% solution gallon of bleach, but I don't believe it has an equal cleaning ability. I'd estimate I would need to use maybe 10x as much, bringing the cost differential to 40x. The main advantage of the switch would be avoiding the more toxic bleach fumes. StuRat (talk) 06:35, 3 July 2017 (UTC)

Not surprised that 3% cost so much - as your paying for the transportation of mostly water. 35% would make it cheaper than Mall bought household chlorinated bleaches (which also have a premium added to the cost price of manufacture, to pay for advertising of these branded products), And what is the chlorinated bleach strength (probably not more that 18%). You can do the maths in your head to dilute down before adding to your whites. Also think, the cleavage of bonds in synthetic fibers may be greater for chlorine (can't off the top of my head give an immediate reference). After all, the production of PVC (Polyvinyl chloride) depends on this. PVC fibres would be useless for clothing fibres. It has flexibility in large cross section ( i.e. cable insulation) but not the necessary tensional strength that clothing fibres need. Clothes made from for chlorinated fibres would just fall apart – which may be the very phenomena that you may be describing. The shirt manufactures don't care – it causes you to buy more of their shirts. The bleaching companies don’t care because you continue to buy their products as their advertising suggest that only you have a limited choice. Yet, the choice really is yours. Red pill or blue pill. Personally, I like the tertiary purple pill (half way between the two (wouldn’t have made such a good movie if they had included that one). Thus, leaving the extremists to pout, pontificated and ague until the cows come home. Whilst purple people take full advantage of both marketing ideologies so as to choose the best practical stuff for their own use at a cost that suits them. As an aside: Does anybody remember Sheb Wooley's hit - 'Purple People Eater' in the late 1950's. Aspro (talk) 12:46, 3 July 2017 (UTC)

I added a / to the second small tag to ensure it was closed. Nil Einne (talk) 14:17, 3 July 2017 (UTC)