Wikipedia:Reference desk/Archives/Science/2017 June 19

= June 19 =

Why does the squirty hand soap dribble in hot weather?
There are no stupid questions, right? :D So: I have a squirty hand soap, i.e. plastic bottle with a vertical tube that dispenses the soap when the top is pressed down. Recently the weather has been hot (for the UK: high 20s ºC; and that part of my house is a heat trap), and I keep noticing that some of the soap has drooled out on its own. Is there some heat-related explanation for this? Equinox ◑ 00:58, 19 June 2017 (UTC)


 * Thermal expansion. A decrease of viscosity may play a role too. Shock Brigade Harvester Boris (talk) 01:06, 19 June 2017 (UTC)


 * I doubt expansion matters here. Likely the viscosity is rising with the temperature and the dispenser starts leaking because its not build to keep such highly viscose content. Easy to test. Put some of that soap into a glass and put the glass into hot water. Then check if it becomes more "fluid" when heated up. --Kharon (talk) 03:41, 19 June 2017 (UTC)
 * This is what I was thinking ... but take note that high viscosity means a high resistance to flow, while low viscosity liquids are what seem more fluid. Wnt (talk) 11:19, 19 June 2017 (UTC)


 * Agreed. The viscous friction normally keeps the thick fluid from the last use in the tube, but when it becomes thin, then gravity can pull it down. StuRat (talk) 04:22, 19 June 2017 (UTC)


 * Rihgt! That's easy to test: replace the liquid soap with water and see how the device behaves (in normal temperature and in heat; when pumped and when left alone). Then replace water with glue or with plasticine and see again. Correlate results with temperature and with viscosity. --CiaPan (talk) 12:30, 19 June 2017 (UTC)


 * I don't think that test would work, because with water it would flow completely out of the tube right away, as opposed to being thick enough to stay in there initially, and later becoming liquid enough to dribble out. StuRat (talk) 17:29, 19 June 2017 (UTC)


 * Using water (with food coloring over a paper towel) will prove that after pumping completes, there is still water in the tube. It drips out. If water remains in the tube after pumping, then we have to decide if it is reasonable that soap remains in the tube after pumping. Add gelatin to the water to get it between the consistency of the liquid soap and plain water. Does it still remain in the tube and dribble out? Keep increasing the consistency of the liquid. If you prove that soap remains in the tube after pumping (which makes sense), then you put the soap on a small cold dish with a thermometer and slowly heat it up. At what temperature does it start to get runny? Is it low enough that the ambient warm air could cause it to be runny? If so, you've demonstrated that soap remains in the tube and it is runny in the warm air. So, there is no need for gas expansion to explain the observation. There could be gas expansion - but it is not necessary and, in reality, gas expansion is a one-time thing. It would expand, dribble out some soap, and then be done. It won't dribble out soap every time you use the container. 209.149.113.5 (talk) 13:53, 20 June 2017 (UTC)

I put it down to thermal expansion, try loosening the pump from the top of the bottle (undo the thread a bit) and it will stop doing it! works every time for me here in Brisbane. 49.197.176.179 (talk) 07:33, 19 June 2017 (UTC)


 * Moka Animation.gif Seconded: it's a matter of pressure, caused by thermal expansion of gas. The same way the Moka pot works. See the animation.
 * In a coffe pot the hot vapour presses water below, which escapes upwards through the internal vertical tube. The same way hot air in a soap bottle presses the liquid soap which escapes upwards and drips outside. --CiaPan (talk) 10:13, 19 June 2017 (UTC)


 * There you're talking about boiling temps, and we aren't considering temps as hot as that here. The expansion created when a liquid boils to become a gas is far greater than that due to thermal expansion of liquids.  See boiling liquid expanding vapor explosion.  StuRat (talk) 17:30, 19 June 2017 (UTC)
 * The coffee pot is an analogy, not an exact parallel. Try reading CiaPan's last sentence: air expands considerably when warmed, easily enough to create in the sealed, partially air-filled bottle a mild overpressure sufficient to force some soap through the pump mechanism. {The poster formerly known as 87.81.230.195} 94.12.79.194 (talk) 18:24, 19 June 2017 (UTC)


 * The Moka pot effect is due to the change in volume of the contained gas, not the liquid. It's easy to calculate the change in pressure, using the ideal gas law.  Assuming the container is air tight and doesn't change its volume, the pressure of the gas is proportional to the temperature, expressed in Kelvin.  So, for example, if the temperature increases from 15 C to 32 C (288 K to 305 K), the pressure increases by 6% (17/288).  At standard atmospheric pressure of 14.7 psi, the increased pressure would be about 0.9 psi.  If the soap tube is 3/8 inch in diameter like the one I just measured, it has a cross sectional area of 0.11 square inches, so the force on the liquid in the tube would be about 0.4 Newtons (1.6 ounces).  That seems like sufficient force to move the gelatinous soap.  How much would actually be extruded from the nozzle would depend on the volume of the contained gas, since the pressure decreases as the air expands into the tube.  CodeTalker (talk) 23:08, 19 June 2017 (UTC)
 * Actually, the air won't expand into the tube. In the top-plunger design described by the OP, the tube extends (almost) to the bottom of the (partially) soap-filled bottle, so until the soap is (almost) entirely expended, the end of the tube will always be submerged in soap, while the (heat-expanded) air is exerting overpressure on the soap's top surface.
 * Sorry, I expressed that poorly. I meant the pressure decreases as the volume of the trapped air expands, due to the soap (not air) being pushed into the tube.  CodeTalker (talk) 15:41, 20 June 2017 (UTC)
 * A possible contributory factor is that there must be something of a one-way valve effect allowing air into the bottle, otherwise as soap is expelled the air in the bottle would develop underpressure/partial vacuum and work against the pump mechanism, and the bottle (if flexible plastic) would tend to contract. Fluctuating ambient temperatures will then tend to "pump up" the internal pressure. {The poster formerly known as 87.81.230.195} 94.9.80.133 (talk) 14:49, 20 June 2017 (UTC)
 * It seems barely worth mentioning Kipp's apparatus, though it's not that relevant. Wnt (talk) 12:06, 23 June 2017 (UTC)

Why does Earth keep getting nearer and nearer to the edge of the "habitable zone"?
The idea of a habitable zone, a part of a solar system where liquid water can exist, is deceptively simple. After all, habitable for what kind of planet? But I cannot for the life of me figure out the NASA graphics with the green habitable zones that keep putting Earth nearer and nearer to the edge. Like Figure 6 -- I mean, their bright green non-"optimistic" habitable zone would seem to have Mars more squarely in it than Earth! (I think this estimate comes from here, but I don't see that much explanation of the logic... while liquid water may have flowed on Mars once, under what conditions was that?)

I just don't get it. From first principles, I'd think a non-optimistic habitable zone would be about the width of Earth on the diagram. Maybe you can broaden it on the basis that a planet might be a bit bigger or smaller than Earth, have a bit more or less CO2, a bit more or less water and so on. It's true that the Sun was fainter before (faint young Sun paradox), but it's also thought to be the case that there was a snowball Earth as recently as 600 million years ago, and that this lack of insolation was something that held back development of life. We ourselves came out of an Ice Age, and our culture seems to think it can take a six-degree-C increase in temperature over the next century and scarcely even complain. So where does the asymmetry come from? Does a habitable zone include underground Martian brine? Because I'd think there's a chance you could find brine in a cryovolcano on Pluto, so is that habitable zone? Why wouldn't we be able to postulate a planet in the position of Venus with a thinner atmosphere and a high albedo that is no warmer than Earth, but still has seas? Wnt (talk) 21:48, 19 June 2017 (UTC)


 * Life has existed for approximately 4 billion years, but we're near the end "In about one billion years, the solar luminosity will be 10% higher than at present. This will cause the atmosphere to become a "moist greenhouse", resulting in a runaway evaporation of the oceans. As a likely consequence, plate tectonics will come to an end, and with them the entire carbon cycle." Count Iblis (talk) 22:55, 19 June 2017 (UTC)
 * We almost missed having stable liquid water, actually: if you look at circumstellar habitable zone, you will notice that even at 0.98 AU calculations (see the recent Kopparapu 2013 paper) show that an Earthlike world (which would need to have similar quantities of liquid water – that seems a plausible general assumption) would receive enough insolation to slowly boil off the oceans. Then the water vapour in the air would be split into hydrogen (lost) and oxygen by solar ultraviolet radiation, and the entry of carbon dioxide into the atmosphere ends up going unchecked. Venus followed this route, and if the Sun was a little bit brighter (as it soon will be) we would too.
 * Mars is far enough away from the Sun that this problem does not apply. Instead, its problem is that it is too small and lacks a strong magnetic field, so its original atmosphere was stripped away by the solar wind and the water all froze. An Earthlike world at Mars' orbit may not have such a big problem. In general, the obstacles at the hotter side are more difficult to surmount than those on the colder side: the Kopparapu paper places the upper limit around the orbital distance of Mars. Double sharp (talk) 07:53, 20 June 2017 (UTC)
 * An additional factor is that Mars is apparently too small to have maintained tectonic activity, which apparently ceased there around around 2 billion years ago. On the Earth this causes atmospheric gases and liquids that have been chemically combined into the surface rock to be subducted and then recycled via vulcanism: on Mars, however, the rock-absorbed gases and liquids remain 'entombed' in the now-static crust. {The poster formerly known as 87.81.230.195} 94.9.80.133 (talk) 15:06, 20 June 2017 (UTC)
 * Venus has infact a much higher albedo (0.75) then earth (0.3). Nevertheless its around 450 °C there. Its much to close to our sun. Also a habitable zone orbital position does not guarantee a habitable planet. It only implies a good chance for one. Many other properties can make it unhabitable. From toxic gases, vulcanic activity to an eliptic orbit around the sun, allot can be wrong. We need much more than the right temperature and some water. I dont understand what edge you refere to in the nasa diagram. As noted below our 3 planets are only put there as reference. --Kharon (talk) 10:24, 20 June 2017 (UTC)
 * Yes, a lot more can go wrong. The border at about 0.99 AU merely alludes to how it can go wrong in one specific way (becoming a Venus), and the habitable zone merely states where things might not go wrong. It is quite possible that the border just outside the orbit of Mars is overoptimistic; at the very least it shows that planetary size is another way in which things might go awry. Double sharp (talk) 10:34, 20 June 2017 (UTC)

Here is the paper I have been referring to. While it is true that we appear closer to the inner edge than we might really be, there is generally much less wiggle room at the inner edge than the outer edge. Earth would then look like a perilously borderline case, while Mars would almost certainly have been habitable had it been Earth-sized. Double sharp (talk) 10:52, 20 June 2017 (UTC)
 * Thanks for pulling up the paper! Albeit I find myself inhospitable to its content.  So far, I see that they assume a 6.5 atm surface pressure, a fixed assumed stratospheric temperature of 200 K, and ignore the effect of clouds entirely ... then come up with the notion of a "moist greenhouse" that depletes all the water via a very sharp-edged set of curves for stratospheric content if surface temperatures get just slightly higher than on Earth.  Now this is indeed alarming in that they show a curve where at a nearly constant Seff only slightly above 1, the surface of the planet gets more than 50K hotter - it suggests utter catastrophe awaits us with global warming, for example.  But a problem I have with that theory is that we did that with the Paleocene-Eocene Thermal Maximum and nothing very significant happened.  I mean, it wasn't long enough for a "moist greenhouse" to do much, so we don't know that, but we know the surface temperature didn't go completely insane and roast everything.
 * So I'm tending to reject the validity of their model to some degree; yet there is one thing I will admit... yes, the Earth has all the water it can have.  I mean, I'd think it seems possible there was even more water at some point but it raised the level enough that some of it got lost to space - fine tuning us right smack dab on the inner edge of the "habitable zone" where Earth can exist.  And by the same token, if Earth started losing water to "moist greenhouse" until it was 1/3 ocean and 2/3 land, surely the level of water vapor in the stratosphere would go back down again, no?  Also, what about the magnetic field?  Couldn't that drastically expand a habitable zone against slow atmospheric loss, just as its lack put Mars, it would seem, inside the inner edge in reality?
 * Comparing Venus and Mars seems to understate the importance of carbon dioxide. Earth has 0.0004 atmospheres of CO2; Venus has 90.  It's hotter than Mercury on average, but I'm not convinced it has to be that way.  I don't know what the CO2 level on Mars was when it had liquid water flow - if that was ever water and not a dense brine.  So I'm just not buying all of this ... but the drastic moist greenhouse definitely seems worth keeping track of anyway, just in case they have something there. Wnt (talk) 11:44, 20 June 2017 (UTC)


 * If the surface of Mars had been covered by brine, wouldn't it now be covered in salt ? StuRat (talk) 17:04, 20 June 2017 (UTC)


 * Earth's case may have been complicated by the development of life. An aspect of the Gaia hypothesis is that, due to evolutionary-like feedback mechanisms, the Biosphere has tended to modify Earth's atmosphere in ways which moderate temperature extremes, to its benefit. {The poster formerly known as 87.81.230.195} 94.9.80.133 (talk) 15:13, 20 June 2017 (UTC)
 * We have to bear in mind that cloud feedback will cool the planet's surface down, and that given the minuscule increase of Seff needed for this, it may well have been cancelled out by the minuscule increase of the Sun's luminosity since that thermal maximum. So perhaps, given how close to the edge this seems to be, we could have gone that far up then and have had things return to normal; but if we did it now, the prospect of creating a positive-feedback loop and going the way of Venus seems more likely.
 * The reason why Venus is the way it is is because the water all evaporated away even before the CO2 got there to such a degree. Actually that is the first reason; the CO2 levels were only held in check by buffer reactions when the water was still there. Then it boiled off, and a few billion years later, here we are. Double sharp (talk) 15:25, 20 June 2017 (UTC)
 * I found a chart with the temperatures in Oligocene (now included at right) - which marked the beginning of the permanent glaciation of Antarctica. It makes it apparent that the PETM was actually a small blip on a larger pattern ... why that pattern is as it is I have no idea.  I mean, common sense says there has to be an upper limit, but I'm thinking that upper limit has nothing at all to do with the model in the paper. Wnt (talk) 18:04, 20 June 2017 (UTC)
 * The idea is that with this amount of effective insolation, the cycle can go up and down without problems, as things are balanced correctly. If you go too hot, for examples, the clouds of water vapour that form will cool the planet back down through their high albedo. The habitability limit the authors impose is when the cycle breaks down and there is nothing stopping the water vapour from accumulating without end. The authors acknowledge this and note that 0.99 AU is probably a little too pessimistic. But I would note that the authors also impose a standard Venus-style greenhouse limit, and it comes out at about 0.97 AU. So even if you disagree with the conservative estimate, the habitability zone still ends up not going very much past Earth, while going quite a distance past Mars.
 * In fact, it seems that because of radiative warming from CO2 clouds (again) and the possibility of using other greenhouse gases as well, you could extend the habitability zone another 0.2 AU at least past Mars' current orbit. I wouldn't dare to include Ceres in it as a few early speculations suggested, though. Double sharp (talk) 23:33, 20 June 2017 (UTC)
 * I think you're taking these estimates a little too literally. There are so many variables and processes we don't fully understand that any of these models are ballpark guesses at best. As for what the "habitable zone" includes, I think the article is pretty clear: the area where persistent liquid water on a planet's surface is possible. As the article notes, this concept dates back to the 1950s. It's easy to forget that back then we knew practically nothing about other Solar System bodies, apart from them being rocky or gaseous. (Fans of classic science fiction will be familiar with Venus and Mars being depicted as slightly more extreme Earths. It wasn't until the '60s and '70s that probes started giving us a clear picture of what other planets and moons were like.) As we now know, there may be environments suitable for life outside of a star's "habitable zone", so maybe we should rename it the "Earthlike zone" or something else more accurate. --47.138.161.183 (talk) 00:29, 21 June 2017 (UTC)
 * Yes indeed, they are all guesses, and the only vaguely reasonable thing we can say is that at some point things get Venus-like and at some point they get Mars-like, while shrugging our shoulders on where exactly this happens. I mention Kopparapu because he is recent and stands about midway between the historical optimists and pessimists, and gives a rough ballpark of the obstacles that may be expected at the extremes, but we should take all of this with about a mole of salt. The phenomenon Wnt refers to, minus all the authoritative-looking markers, would simply be to say that it seems to be rather easy to end up like Venus, which while plausible still needs more empirical data that is of course not going to be very forthcoming.
 * I think we focus on the water habitable zone because we know for a fact that life is at least possible in it. Whereas we don't know enough about other biochemistries: it could be that methane and ammonia are acceptable substitutes, but it could also be that there is some sort of obstacle that we just don't know about yet. So the focus on water comes out of caution. Double sharp (talk) 02:39, 21 June 2017 (UTC)

Your comment on the natural cycles above may have some sense to it, but did they really refer back to the PETM to decide what the habitable zone was? But it gets me thinking that there ought to be a scientific way to infer how close we are to some catastrophic greenhouse effect (moist or otherwise). Basically, the Kopparapu group claim that there is a curve of Seff vs T that goes almost horizontal if S (I'll leave off the subscript from here in) goes just a little higher than it is now. So dS/dT goes practically to zero, or dT/dS to near infinity. But we ought to be able to infer those values directly from variations in the geologic data! We have the Milankovitch cycles to tell us exactly how much the insolation changes over time, and we can look at how much those cycles affected temperature of the Earth's surface when it was high versus when it was low. Based on this we should be able to integrate a real T(S) function and then fit polynomials to it to extrapolate what actually happens. I should say though that at a far lower level of analysis, just eyeballing the data figure I posted above, and assuming the variations (whatever their cause, definitely not Milankovitch cycles) are about constant, I don't see the temperature numbers getting wild and looking to explode off the top of the chart every few million years when it is hottest. Wnt (talk) 19:16, 21 June 2017 (UTC)
 * If you're referring to Figure 3(c), I think the scale makes it a bit difficult to see, but the start of the horizontal region does not appear to be close to our current 288 K on a human scale, even though with such a wide range on the x-axis of course it will look pretty bad. Figure 3(d) shows that water vapour in their model doesn't start accumulating in the stratosphere until around 340 K, which seems a lot more plausible given that we never went anywhere near that, and seems to fit the position on the graph given. But thanks to the self-reinforcing nature of the greenhouse effect, and the rate at which T ought to increase with increasing Seff, I can believe that we avoided disaster by what looks like a large amount from the temperature perspective (since we never get anywhere near 340 K), but a small amount from the distance-from-the-Sun perspective. Double sharp (talk) 05:53, 22 June 2017 (UTC)

Health effects of sulforaphane
Cruciferous vegetables are healthy and that may be because of sulforaphane. But sulforaphane is destroyed by cooking so I don't understand how one can invoke the observed health effects in people who eat more cruciferous vegetables, because most people don't eat these vegetables raw. Broccoli sprouts contain more sulforaphane, but even in that case, you need to apply the right amount of heat for the right amount of time to get the most out of it, see here. Count Iblis (talk) 23:20, 19 June 2017 (UTC)
 * Well eating them raw will preserve the substance. But there are probably other health benefits for cooked vegetable such as vitamins, and fibre. Graeme Bartlett (talk) 00:24, 20 June 2017 (UTC)


 * Adding mustard seems to help. Count Iblis (talk) 06:19, 20 June 2017 (UTC)


 * Sulforaphane may or may not be specialty healthy, our article looks rather skeptical on this. Meaning cooking may makes no difference on a close to nil effect.
 * other possibilities includes
 * Sulforaphane are produced AFTER cooking and eating, from some others components that survive cooking
 * active substance may not be sulforaphane itself, but some other substance produced when sulforaphane is destroyed, while the cause of this destruction (cooking or digestion) do not really matter

Gem fr (talk) 13:04, 20 June 2017 (UTC)