"What gas has the highest refractive index?" is too broad or is it?

Question

I rarely ever look through the reopen queue any more, but today I did.

I found a question that seems to be highly controversial of whether it should be open or closed:
What gas has the highest refractive index?

To give you some more perspective. The first round of 'too broad' review was initiated on 2017/11/28 and failed with 3 Leave Open and 2 Close votes (review). The votes then aged away. (It gained an answer after that.)

For reasons I cannot figure out, it entered the review queue a second time on 2017/12/6. This time it succeeded (2017/12/14) with 2 Leave Open and 4 Close votes (review).

One day later (2017/12/15) it entered the reopen review process. (Still going on.)

Obviously, since my vote is binding, I am hesitant to do anything here.

I would like to figure out the reason for it being too broad, as well as the reason for not. If some of the participating users could comment (I mean write an answer) on their reasoning; preferably how this question could be improved to better fit our scope.
Currently Max's comment offers some insight. I think, however, that (1) of these restrictions could be inserted without much trouble.

(1) For whatever gas it will surely depend on the temperature and pressure. So no real theory to predict it is a matter of looking RI for various gases at that temperature and pressure. (2) Would a supercritical fluid be within scope? For a gas there is not going to be a great density hence fairly small refractive index. With a supercritical fluid you'd have a lot more matter at the solid surface of a planet. – MaxW

I can follow that logic and agree to an extent. I'd also like to hear counter arguments.

N.B.: In general - and that is probably no news to most of you - I would rather err on the keep open side than completely preventing further answers. Always keep in mind, that this question is here to stay, there is basically no reason why it couldn't be the best version of itself.

Answer 1

I originally voted to leave it open primarily because Jon and Max had commented in a manner that I felt could result in the OP amending the query to refine the question in a suitable way. I voted to close the second time it came up for closure because, in my opinion, the question remains quite broad and the OP did not come back with criteria that would allow one to suitably answer the question.

Answer 2

OP here. I didn’t realize it had caused this much controversy, but I deliberately left it unedited and allowed it to be closed because I really think I’m in over my head. I’m not a chemist and I came here from a worldbuilding question (https://worldbuilding.stackexchange.com/questions/98955/how-dense-of-an-atmosphere-do-i-need-to-float-an-ocean-of-liquid-at-1000-ft-el/98988#98988) in hopes that it’d be a simple factoid that would add some real-world physics. Once I saw that it wasn’t going to be so simple and given the depth of chemistry apparently required, I didn’t feel like I was the right person to clarify it and was okay leaving it closed as too broad.

Answer 3

As someone that voted to close both times, I will chime in. I added a comment originally, pointing out some of the basic physics involved. Others had comments about various cases (e.g. supercritical fluids) that might be considered. As far as I can remember the question was not edited to clarify what the OP would consider as a 'gas' (would a hot dense plasma be considered a gas?). So, in spite of comments pointing out how broad the question was, there was no attempt to reduce the scope at all. It remained broad.

Further, I agree with Jan's answer - what real purpose would an answer address? Yes, there would be a fact put forth, that might not be a fact. A question on what the longest polymer known was would likely not be received well here.

Answer 4

I am always leaning towards closing the extreme-value factoid questions (‘What is the compound with the greatest/smallest whatnot?’). It is hard to get a single definite answer. There might be a largest commonly known refractive index, there might be an uncommon compound and in three weeks maybe the Journal of Random Facts will publish the article by Neil d’A. Lyfe publishing the identification of an even more extrem one.

I also see them having only little value in the long run. So what if the refractive index of invisiblane is higher than all others by 3 arbitrary units? (Yes, I know refractive indices are measured as number ratios but we all know that there is a way to calculate arbitrary units from the ratio.) It might be something better suited for the Guiness Book of Chemistry records than for our site.

I do realise to my surprise that I did not take part in either round of review. (Probably I was taking a break from the site.)