Recent comments in /f/askscience

kdeff t1_j9bfg98 wrote

Reply to comment by Fo0ker in When something is bent (a metal ruler for example) and returns to its original shape, what is happening on the molecular level? Where is the information of the original shape stored and what forces do the unbending? by JewNugget2525

There is a difference though, between bending a paperclip back and forth a few times so it plastically deforms and breaks - and fatigue.

The former is exceeding it's ultimate strength and breaking the paperclip. Fatigue is a different phenomenon caused by cyclic loading and not related to the ultimate yield strength - slip bands form and eventually cause a crack.

Seems like a technicality but they are two different phenomenon, and material health is asses completely differently when looking at overstress vs. fatigue!

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janoc t1_j9bf08x wrote

Reply to If a water bottle is in the same vicinity as an X-ray being done, will the water bottle still be safe to drink from? by 0zMosiss

The answer would be likely - "it depends".

Water isn't likely to be affected even by high doses of powerful x-ray radiation from an industrial CT machine (orders of magnitude more powerful than a medical x-ray/CT). It could get a bit warm from the absorbed energy, though.

However with the bottle it would depend a lot on what that bottle has been made from. Glass and metal are very unlikely to be affected to any significant degree even by a strong x-ray source. Plastic - would depend on the dose (time & energy) and what kind of plastic are we talking about. Some could start decomposing/breaking down under the intense x-ray radiation and possibly leach some nasty stuff into the water.

That is very unlikely to happen with a low energy medical x-ray and one-shot exposure, though. However, if you leave a plastic bottle inside of an industrial CT-machine during a multiple hour scan using a high energy beam (e.g. because you are scanning an engine piece made out of metal), there I would be quite careful because who knows how the plastic could react.

And finally, as mentioned by others - it is not enough to be "in the vicinity" for the bottle to be affected by the rays. It would need to be directly in the path of the beam from the x-ray source (or some reflection). X-rays are very directional, the same (or even more so, given it is a shorter wavelength) as light.

Given how well the x-ray machines are shielded and enclosed to avoid accidentally exposing the operators, if your bottle outside of the machine got damaged by x-rays then you would have much much worse problems to care about than some stuff possibly leaching into water ...

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Busterwasmycat t1_j9bbltc wrote

Reply to Why are we not acidic? by stronkreddituser

Acids come in different strengths so the amount of H+ that can be released relative to the total concentration of the acid compound is highly variable. Basically, the idea is that "acid" means that there is a favorable condition for releasing an H+ from the rest of the compound. Water is an acid, even. Its pH is 7. It isn't a very strong acid (hardly lets go of the H+ unless really forced to). Some acids, like HCl, though, have almost no hold on the H+ and will lose it easily, and acidify water a lot.

All by itself, the pH of water will be (about) 7, what we call neutral. Very weak acids tend to be made from strong bases (strong bases GRAB loose H+ if it is around), and strong acids when combined with strong bases, tend to make salts and water (like HCl (strong acid) reacts with NaOH (strong base) to make H2O (water) and NaCl (table salt); a solution with a near neutral pH).

In the bulk mix of compounds that is the natural world, and the human body too, the mixture of strong acids, weak acids, strong bases, and weak bases, and other things with effectively no acid behavior at all, when all the competition is done between the various species for grabbing or releasing an H+, the system tends to end up somewhere around neutral. The system "wants" to go to neutral if it can, basically. Strong bases grab H+ just as much as strong acids release it. The end balance in most systems sees this happen, and near-neutral is the result (just as likely to see a strong base as a strong acid, and the two coming together makes a neutral salt and water).

That is the basic reason that the human body is slightly above neutral in pH. It is mimicking (trying to reproduce) the natural world, the ocean that life came from or developed in, originally, which is slightly basic too.

What is an amino acid? They are basically ammonia (NH3) where the H atoms have been replaced, at least in part, by some other large chemical group, many of which are weak acids. We call such compounds "amines" (hence they make "amino" acids), because the nitrogen is in the reduced state (filled to max with electrons, hasn't met other elements that want its electrons even more than it does itself). When reduced nitrogen does meet a strong electron grabber like oxygen, it makes nitrates (and nitrites, sort of the same thing), but that is a little off where I am going with this explanation.

The amine groups tend to be attached to weakly acidic other compounds, or rather weak acids like HCO3-, so one side of the compound is a weak acid, but the other side, or sometimes inside (if all H+ has been replaced by weak acids), it is a strong base. So, because the primary characteristic is a weak acid, the compounds are "amino acids". However, they are not strong acids at all, they are weak acids and do not lower pH by much. If they had been stronger acids, they would have reacted with ammonia to make NH4+ (would have just told ammonia, here is the H+, take it and leave us alone out here in weak base land away from you).

Amino acids are, in effect, the result of that pH-balancing process that happens in nature. The amine group is, if in its normal state of ammonia (NH3), a very strong base. It wants to make NH4+ by grabbing any H+ it can find. the end result is that it sort of does do that, by grabbing on to other compounds that have a weak grasp on an H+, changing the product into a very weak acid rather than its once powerful base. When together, these combined molecules are pretty "happy" in near-neutral conditions. The strong base (ammonia) was neutralized by reacting with acids (like carbonic acid) and now we have a happy "acid" that is not very acidic in behavior, it is very weak. And there are still some bases out there floating around, not strong enough to grab H+ from the weak amino acids, but plenty strong enough to grab H+ from some stronger acids, if they come along, and in so doing, neutralizing them.

So, the system is happy and near neutral. It is where things, taken all together, tend to go to be in balance, somewhere near neutral pH. The world likes to get into some sort of middle balance if it can.

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