I am currently watching the celebrity special edition of Who Wants To Be A Millionaire (the UK version). The last question was ‘What gas is commonly found in light bulbs’ and the possible answers were:
a) Argon
b) Chlorine
c) Methane
d) Hydrogen
The correct answer was Argon but I was wondering if Chlorine might not also be a valid answer. How Stuff Works only mentions that halogen bulbs use a gas from the halide group, not which one(s) are used in practice.
So, would Chlorine have been an alternate correct answer?
Well, a halogen light generally has an inert gas like argon, and a halogen gas, like bromine or iodine. I couldn’t find any references to chlorine being used as the halogen gas in a halogen light, so absent some more knowledgeable doper coming in and showing where to get halogen lights which contain chlorine gas, I’d say that chlorine would not have been an alternate correct answer.
Even if some halogen lamps use chlorides rather than iodides, “Chlorine” would not be a correct answer - the bulbs might contain the element chlorine, but they don’t contain chlorine gas as such.
NO! Not If you are considering ordinary incandescent bulbs as was asked on the TV program.
Lets not mix apples and oranges.
Halogen lamp bulbs use one and maybe more of the following Halogen gasses to prolong the life and increase the temperature and hence the light output.
Fluorine * Chlorine * Bromine * Iodine and * Astatine
Astatine? The rarest of all elements, one of the most radioactive? I doubt it, somehow. Indeed, have more than a few ug of At-211 ever been present in the same place at the same time?
Therefore, chlorine is an incorrect answer in any case.
What about the other halogens?
Chlorine and fluorine, in addition to being toxic and reactive, would be unlikely to work in any known design of halogen lamp. Being the most electronegative elements, fluorine and chlorine would bind too strongly to the tungsten vapor and and therefore disrupt the re-deposition process, causing the bulb to fail quickly. (also they’re highly toxic and reactive, which makes them less attractive for consumer applications).
The only remaining halogen candidate would be astatine. This wouldn’t work because it is not only one of the rarest elements on earth, but also radioactive. Its longest-live isotope has a half life of just a few hours.
So, that’s why halogen lamps only use iodine or bromine.
N.B. - so are bromine and iodine, just slightly less so. Trust me, you get a snoutful of any halogen gas, you’re going to be regretting it for days at least. (Can’t vouch for astatine for reasons already discussed). In lamps I’m guessing they consider it safe enough because it’s in a very lean mix with an inert gas.
And, while we’re on the subject - why can’t you touch them with your bare hands? What happens if you do? I appreciate it’s because the bulb is made of quartz rather than glass, but what are the consequences of getting fingerprints on it?
Group I metals, including sodium ions in sweat from fingers, act as network modifiers in glass which makes the molten glass less viscous and melt at a lower temperature, reducing manufacturing costs. This is why why most glass items are made of soda glass. Soda glass also undergoes much greater thermal expansion so quartz glass is used in halogen lamps which operate at higher temperatures. If you get salt from your fingers on the glass you create a region where thermal expansion is greater than the rest which leads to cracking.
The TV show did not specify ‘ordinary’ bulbs, just light bulbs. I was wondering if this could have been ambiguous given the four possible answers. It appears not (thanks everybody), but not because halogen bulbs were excluded by the question as posed.
It appears that some do, some don’t, and some have a combination. I had a heck of a time digging up a reference. Here, apparently, is the one site on the entire internet that isn’t content to hand-wave over the magic “halogen gas” (link )
This is also true of the incandescent bulbs used in stage lighting – you don’t touch the glass of the bulb with bare hands, because the oil from your hands left on the glass can cause the bulb to burn out much faster than normal.
Regarding the handling of the lamps, I don’t know exactly who says that or why. I do know that glass is transparent, fingerprints are not. So compared to the clean glass adjacent to it, the fingerprinted glass would get hotter and undergo different rates of expansion. Over time this could cause accumulation of micro-cracks and eventually speed up the failure of the bulb. I know this is a concern for high-energy laser optics, I’m just guessing it applies to halogen bulbs as well.
Indeed, have more than a few ug of At-211 ever been present in the same place at the same time?