Wikipedia:Reference desk/Archives/Science/2020 May 18

= May 18 =

Neopentane
Pentane, isopentane, and neopentane are all isomers of. Their boiling points are (repectively) 36 &deg;C, 28 &deg;C, and 9.5 &deg;C, with a decreasing trend as would be expected with increased chain branching and the resultant weakening of dispersion forces. Their melting points, however are (respectively) &minus;130 &deg;C, &minus;160 &deg;C, and &minus;16.6 &deg;C. Can anyone explain or point to an explanation for the melting point of neopentane being so much higher than that of pentane and isopentane? The WP page on neopentane mentions the difference but its explanation is not clear to me. Help! 112.213.147.109 (talk) 09:00, 18 May 2020 (UTC)


 * You may find the explanation in this paper cited in the article easier to follow. Mikenorton (talk) 09:06, 18 May 2020 (UTC)


 * Hopefully matching that paper: the neopentane molecule is more symmetrical and can be neatly packed together, so crystals are easier to form. Graeme Bartlett (talk) 23:19, 18 May 2020 (UTC)


 * The density of the 3, for the liquid phase, in the same order you provided, are 0.626 g mL, 616 mg mL, and 601.172 kg/m^3. So I guess a better question is what role does density place in melting and boiling point. I wonder if density alone is the biggest single factor. (Note that I only posted the density in their liquid phases, wonder if it's the same linearity for the density at solid or gas phases.). 67.175.224.138 (talk) 05:04, 19 May 2020 (UTC).
 * It's really a question of Degrees of freedom (physics and chemistry) which is why melting point trends can be independent of boiling point trends. Vaporization (the phase transition from liquid to gas) involves the "unlocking" of translational motion by the breaking of the intermolecular forces holding the molecules together, and is a fairly straightforward relationship; for something like the alkane family, you're basically looking at London dispersion forces, and so you find a nice predictable relationship between LDFs and boiling points.  Melting point, which is the breakdown of the crystal lattice of a substance without a breakdown of the intermolecular forces, essentially depends on freeing up all of additional degrees of freedom other than translation, so that includes all of the other kinds of motion (rotational, vibrational, stretching, bending, twisting, etc) and the shape of the molecule has a particularly significant effect on such factors; i.e. n-pentane (the straight chain) has very different sorts of such motions than the neopentane (which is much more compact and round in shape) and those differences account for the difference in MP.  The paper above discusses these properties (which is closely related to various symmetries in the molecule).  -- Jayron 32 12:40, 19 May 2020 (UTC)

mask that only covers mouth, how effective for preventing spreading it to others? How effective as a protection for oneself?
Is it really needed stopping spreading, since ones mouth spreads most of the saliva, and you can pull the mask upwards if you're about to sneeze? And for protecting oneself there's a lot more barrier through the nose. It seems to me one is recirculating carbon dioxide which could be a health risk for some people.144.35.116.6 (talk) 22:24, 18 May 2020 (UTC)
 * Standard medical disclaimer. That said, the San Fran department of public health and the Mayo clinic say cover your damn nose.  Some doctors have warned that infants shouldn't wear them.
 * Speaking in terms of breathing while wearing a face mask, in my experience (I'm a preschool teacher who has to spend all day singing, dancing, and chasing toddlers while wearing a mask), if a mask makes it hard for an adult to breath, put that one over a mask that is otherwise useless. This will create a pocket inside the effective mask, giving you more room to breathe.  The mask I'm most comfortable with has very distinct inner and outer layer, with the outer layer being water-tight (just because of how tightly woven it is), but the inner layer(s) being multiple layers of a very thin material that normally wouldn't do anything. Ian.thomson (talk) 22:44, 18 May 2020 (UTC)
 * I don't understand what you mean about wearing two masks. What do you mean by useless mask? Also, I apologize for prying, but do you have heart disease?144.35.116.6 (talk) 23:17, 18 May 2020 (UTC)
 * The way the simple masks that you can make at home work, is to reduce the reach of the cloud of virus particle that you'll cough out. They only marginally reduce the total number of virus particles. So, as far as preventing from getting infected, such a mask only has a marginal effect. Also for making sure that no  people won't get infected by you, it doesn't work well for people very close to you, as the concentration of virus particles close to you may actually increase after coughing. But it does work well for people keeping a bit of distance from you.
 * Lots of people aren't coughing at all, they are just wearing it to prevent the spread of tiny saliva particles from their mouths. Im not sure if you are saying masks in general don't work well for people close to you, or masks that cover only the mouth don't work well in that situation. (what's the antecedent of the word (it) in your sentence? But maybe that is because you are talking about people who are actually coughing, who probably should be wearing more facial covering than other people, and staying home more.144.35.116.6 (talk) 23:17, 18 May 2020 (UTC)
 * These masks are then mostly useful when the number of people who carry the virus is very small. In that case we want to make sure the basic reproduction number of the virus R stays below one to prevent a second wave of the epidemic. But we also want to open the economy as much as is possible. If everyone wears face masks, then that allows for more economic activity before R becomes 1 compared to people not wearing face masks. So, we would then wear face masks not with the idea to avert a significant risk of infecting people, as that risk will be extremely low. It's to make sure that if you happen to be the one in a million infected person who can spread the virus that you'll infect less people, say one other person instead of two. Count Iblis (talk) 22:50, 18 May 2020 (UTC)
 * I'm glad to wear a face mask. I'm just a bit worried about it covering my nose, since I might be stressing my heart disease situation by reinhaling carbon dioxide.144.35.116.6 (talk) 23:17, 18 May 2020 (UTC)
 * Johns Hopkins medicine recommends a mask if you have heart disease because whatever effect you might imagine the mask might have on your heart, Coronavirus is confirmed to be far worse. Also, isolated study instead of meta-analysis, but this study concludes "Wearing a facemask appears to abrogate the adverse effects of air pollution on blood pressure and heart rate variability."  The CDC only excludes "young children under age 2, anyone who has trouble breathing, or is unconscious, incapacitated or otherwise unable to remove the mask without assistance."  So lung disease would be a concern but for heart disease the mask will at least be safer than the alternative.  Ian.thomson (talk) 10:37, 19 May 2020 (UTC)
 * Face masks are used throughout the medical field, scientific research field, construction and HVAC field, and many others, since long before this pandemic. Many of these are fields where they have to wear masks for very long stretches, hours or even significant portions of entire days. If CO2 re-circulation and lowering blood oxygen saturation was a major problem, I think we would have known about it long before this pandemic. --OuroborosCobra (talk) 19:03, 19 May 2020 (UTC)