Wikipedia:Reference desk/Archives/Science/2014 April 10

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April 10

Nature of carbon atom

Why do carbon atom form bonds tetrahedral in shape? Why do two carbon atoms not form a quadruple bond? 117.242.108.105 (talk) 05:53, 10 April 2014 (UTC)

Only diamonds are tetrahedral, I believe. Graphite is hexagonal and in sheets. The 3 double bonds are actually shared with 6 carbon atoms. Bond order and quantum mechanics are the reason why quadruple bonds aren't common. Basically, the higher the order the less stable so mixtures with lots of carbon will reform into lower order bonds. Acetylene has a triple bond and the amount of energy released when the bond is broken is tremendous. It would be difficult or impossible to have a carbon substance with quadruple bonds, and the diatomic distance associated with it, without it reducing to paired covalent bonds at farther spacings and less potential energy. --DHeyward (talk) 06:40, 10 April 2014 (UTC)

To answer question 1) All atoms that form 4 bonds to 4 different atoms will assume a tetrahedral geometry, because that geometry maximizes the angular distance between electron pairs on the valence level of that atom. Because electron pairs will tend to repel each other, the geometry that is most stable will be the one that maximizes the distance between said pairs of electrons. See VSEPR for more details. The reason why Carbon doesn't form quadruple bonds is pretty complex, you'd need to have some background understanding of molecular orbital theory. You can read a general overview of such bonds at Quadruple bond if you wish. --Jayron32 11:08, 10 April 2014 (UTC)

Dicarbon (C₂) exists, but it doesn't have quadruple bonds as simple valence bond theory would predict. Looking at it with MO theory shows that there are two sets of paired electrons in the sigma system (one bonding and one antibonding) and two sets of pairs in a degenerate pi bonding set. In total the bond order is 2, so we have a double bond instead: and indeed the MO diagram shows two pi bonds and no sigma bonds. Confirmation of this prediction can be seen from B₂, C₂, and N₂ having increasing bond dissociation energy across period 2, indicating single, double, and triple bonds respectively.

In any case, you are not likely to find dicarbon samples lying around, as it is a very strong acid and only persists in dilution or as an adduct. Double sharp (talk) 15:15, 10 April 2014 (UTC)

Jayron's VSEPR bond-counting explanation omits an important detail (though not relevant to 4-bonded carbon specifically): it's not solely the number of bonds, because lone-pairs also count as a "direction". An atom with 4 bonds and a non-bonded pair is the geometry similar to 5 bonds (AX₄E₁) not tetrahedral. Going further, especially into the transition metals, lots of molecules have square planar molecular geometry. DMacks (talk) 15:19, 10 April 2014 (UTC)

Deep water pressure

Why Titanic and other deep water debris is not crushed flat by water pressure?--93.174.25.12 (talk) 07:34, 10 April 2014 (UTC)

It is because the pressure is basically uniform over the entire surface of the object, including the part on the seabed (yes, the part on the seabed gets more force because that force must be the sum of the debris weight in addition to the water weight, but that extra force has always been there regardless of whether there was water), so the forces could only ever act to compress the object. It would be natural to think the metal panels have compressed already, so they will compress no further.--Jasper Deng (talk) 07:43, 10 April 2014 (UTC)

(edit conflict)Because the water is on the inside as well as the outside. The solid pieces of steel are strong enough to withstand very high pressure, though they will compress very slightly so that they have internal stress to match the external pressure. Dbfirs 07:46, 10 April 2014 (UTC)

For clarity, my comment applies to solid objects, not hollow objects which have no internal stress.--Jasper Deng (talk) 08:26, 10 April 2014 (UTC)

Most solid objects don't compress much at those pressures, just like rocks on the sea floor. Styrofoam cups, however, do compress to a small fraction of their original size, under such pressures, because they contain gas bubbles inside each cell. StuRat (talk) 04:20, 12 April 2014 (UTC)

Pretty lakes

Why are supposedly "polluted" lakes always so full off vegetation? It seems to me that unnatural chemicals would stop plants from growing! — Preceding unsigned comment added by 2001:708:110:1004:CACB:B8FF:FE24:8A97 (talk) 08:51, 10 April 2014 (UTC)

"Unnatural" and "natural" aren't very specific words. If the waste in question contains a lot of nitrogen and/or phosphate compounds, then I'm sure it could lead to an algae bloom, for example.--Jasper Deng (talk) 09:04, 10 April 2014 (UTC)

Maybe start with eutrophication. 86.146.28.229 (talk) 09:05, 10 April 2014 (UTC)

Yes, many of the pollutants are "natural", or organic. Sewage spill-over and agricultural runoff are two common sources of such pollution. So, while turds floating in the lake are entirely "natural", and algae just loves it, we find it rather unpleasant. StuRat (talk) 16:14, 10 April 2014 (UTC)

• The growth of algae in lakes is naturally limited by the level of the least abundant limiting resource, which is usually nitrogen. The removal of this limit by adding a little fertilizer has the same effect as removing predators from a system of herbivores like rabbits or deer; a population boom followed by starvation. In this case, the decay of dead algae will consume the oxygen in the water, and kill most of the animals in the water. μηδείς (talk) 17:21, 10 April 2014 (UTC)

See Liebig's law, which expands on the issue of limiting factors. μηδείς (talk) 20:07, 10 April 2014 (UTC)

Underwater "pings" in the Indian Ocean

Recently, searchers have been hearing several pings in the Indian Ocean, as these officials search for that missing airplane, Malaysian Flight 370. I don't understand: why are they not saying "definitely" that the pings are coming from a plane's black box? What other things in the ocean might account for these pings, other than a plane's black box? Are there any other possibilities? What are they? What other items in the ocean might make such a ping? Thanks. Joseph A. Spadaro (talk) 14:36, 10 April 2014 (UTC)

False data, as appears to have been the case in the earlier Chinese reporting, is the most likely alternative. While the particular frequency characteristics of the ping functionally assure that what is being heard is man-made, there's a whole lot of man-made technology clustered around the search area (namely, all of the searchers). It is not inconceivable that something else is generating a similar audio characteristic, even if it's unlikely. Also, from a PR standpoint, it's much easier to walk back a "probably" statement than a "definitely" statement. — Lomn 15:43, 10 April 2014 (UTC)

One report said that whales might make similar sounds. I don't know if it's true, but that's what they said. StuRat (talk) 16:12, 10 April 2014 (UTC)

Whales, dolphins, or other sea life will not make a sound at a single specific frequency with a single specific repetitive interval for a sustained period of time -- something to the effect of 37 kHz for 10 ms every second. Whales might make "similar" sounds in that they make high-frequency sounds and sometimes repeat them, but that's a use of the word "similar" that is completely misleading in terms of the actual science and technology at work. Black box pings are most certainly not natural, and the reported multi-hour contacts are completely inconsistent with sea life as a possible explanation. — Lomn 18:10, 10 April 2014 (UTC)

I've heard (but cannot confirm) that a number of undersea cables have locater beacons attached to them to make them easier to find to repair. Undersea acoustics can do strange things at times, so it's possible that they're picking up long-distance transmission of a cable's beacons. --Carnildo (talk) 01:16, 11 April 2014 (UTC)

The sad thing is, if the terrorists who allegedly disappeared the plane had a chance to get one agent on one ship anywhere in the search pattern, he could have tossed a pinger overboard that produces the exact black box signal, and they'll all spend a month trying to find it. Wnt (talk) 21:30, 12 April 2014 (UTC)

What is the relation between death rates and life expectancy?

"In the nineteenth century twenty-two out of every thousand people died each year, more than 2% of the population– today only 5 out of a thousand people die each year. In the nineteenth century the average lifespan was thirty-six years – today it’s about eighty. The biggest killer in the nineteenth century? Tuberculosis. Also known as consumption, this disease was rampant, believed to be hereditary, and in spite of numerous claims of ways to cure the disease, no cure or effective treatment was available." - From a review of The Remedy on Goodreads by Watchingthewords.

2% of the population is the same as one person out of 50. So if 2% of the population dies every year, then that means that, in any given year, on average, one person out every 50 people will die. If 50 people die every year, then it will take 40 years to kill 1,000 people, which means that your average lifespan is going to be about 40 years. 40 is not that much different from 36, so maybe we are on the right track.

But then we look at today's numbers and we get something that doesn't make any sense. If only 5 people out of 1,000 die every year, then it will 200 years to kill 1,000 people, and nobody lives 200 years.

I suspect there is something wrong with this picture, but I don't know what. I suppose an expanding population could skew the numbers somewhat, but I don't see how they can skew it that much. Pergelator. — Preceding unsigned comment added by 50.43.12.61 (talk) 17:11, 10 April 2014 (UTC)

The current world's population is young, and therefore has low death rate (5 per 1000). It means that it is growing. Your conclusion that "it will [take] 200 years to kill 1,000 people, and nobody lives 200 years" is wrong because you incorrectly assumed that the population is static. Ruslik_Zero 19:45, 10 April 2014 (UTC)

And presumably we will see a much higher death rate when the Baby Boom generation starts to die off in quantity the US, and even more so when all those born before the One Child Policy in China start to die en masse. StuRat (talk) 13:10, 11 April 2014 (UTC)

Why only the US? Baby boomers lurk in lots of un-American places, Stu. -- Jack of Oz ^{[pleasantries]} 20:33, 11 April 2014 (UTC)

The article I linked to only mentions the US, Romania, and Africa, and I'm not sure if the magnitude of those other two is sufficient to warrant a mention. PS: I'm aware that Australia had a baby rabbit boom; did it have one with humans, too, or do you still need to import criminals/prostitutes to supplement your population ? :-) StuRat (talk) 23:41, 11 April 2014 (UTC)

You're not sure if a baby boom across the entire continent of Africa would be of sufficient magnitude to mention, compared to a baby boom in the United States? 85.255.232.229 (talk) 23:46, 11 April 2014 (UTC)

Nope, and the article didn't say it was across all of Africa, either. From the description, it sounded like it excluded the Arab part, and perhaps more. StuRat (talk) 23:49, 11 April 2014 (UTC)

I think you would get more of a world view if you looked at Post–World War II baby boom although even that one is still rather US centric. Richerman (talk) 20:46, 12 April 2014 (UTC)

Death rates from birth are relatively difficult to compare. Really, the best way to look at life expectancy is when a person reaches a certain age and average remaining years. Infant and child mortality dominate stats 50-100 years ago as well as 3rd world countries. But if you look at the CDC tables they report average life remaining at each year attained. Make it to 5 and your remaining life span is nearly the same as when you were born, reflecting higher death rates for young children. Make it to 80 and your remaining life is very nearly the same as it was 50-100 years ago. Whence the huge gains in life expectancy was reducing infant mortality and childhood diseases. Those lucky enough to reach 100 have only seen an improvement of a month or two. Notice how "the oldest person in the world" doesn't appear to be getting any higher. --DHeyward (talk) 04:30, 12 April 2014 (UTC)

Effective normal stress in geotechnical engineering

Normally, effective normal stress is calculated by subtracting the pore water pressure from the normal stress, in natural conditions. However, why is it that in the untrained triaxial compression test, the pore water pressure is negative, meaning it is added to the normal stress to obtain the effective normal stress? Is it because the compressing effect, creates a force which pushes in the opposite direction, thereby adding to the normal stress? 2.221.71.15 (talk) 21:19, 10 April 2014 (UTC)

The pore water pressure in an unconfined sample is negative due to the capillary effect. When a confining pressure is applied the pore water becomes positive, see for instance here. Mikenorton (talk) 22:34, 10 April 2014 (UTC)