#fun fact: a 'heavy water' ice cube will sink in a glass of regular water

[vampireonastick]

Hello. ^_^ Do you happen to have any theories about the water? Nikolai said that it should be too heavy to swim in, yet Fyodor and Chuuya managed to reach the ceiling. Do you think it's a plot hole or intentional? And if it's the latter, which side do you think set it up and what do you think the other side thinks of it, since it's impossible for it to be unnoticed by either Dazai or Fyodor?

Hello Anon!! I also thought it was odd that Fyodor and Chuuya were floating considering Nikolai’s earlier explanation, but I ended up completely forgetting about how weird it was with the insanity from chapter 101’s ending XD.

So I don’t have any theories, but you bringing this up got me interested in it again, so I’ll definitely take a crack at it!!

From my understanding, (formed through the combination of google, and my almost 5 year old knowledge of 11th grade chemistry (and part of grade 12 chemistry, before I dropped the class because the math was too hard…)), I don’t believe it’s a plot hole. I also don’t think it’s necessarily a set up from either side. I think it’s just how the ‘heavy water’ works.

If you are just interested in a short answer: I believe the density of the water forces Fyodor and Chuuya to the surface. The ‘not being able to swim in it’ part comes from the fact that since they are forced to the surface, it’s difficult/impossible to swim downwards, or beneath the surface, without being pushed right back up. 

If you are interested in a (very) long answer, with science and stuff, it’s under the cut:

What Exactly is Heavy Water Anyway??

(Spoilers up until chapter 101 of the manga!!)

So firstly, a quick look at what ‘density’ is. We measure density by multiplying an object's mass (or, how much that object weighs) by that object's volume (or, how much physical space that object occupies).

On average, humans are less dense than regular water, that’s why we can float on its surface without sinking. 

Now, let’s look at ‘heavy water’.

Heavy Water

So, ‘heavy water’ actually does exist!! (as google so graciously informed me of about 20 minutes ago, lmao I had no clue).

Though the most common formula that comes up when searching ‘heavy water’ is D2O (the formula for regular water is H2O, for reference). ‘D’ stands for ‘deuterium’, which is basically just a heavier form of hydrogen (about double the weight. Though keep in mind, hydrogen is the lightest element, with an atomic mass of 1, so double that only gives us an atomic mass of 2. But it still makes D2O heavier than regular water).

But from what I can tell, D20 isn’t what is referenced in the manga. The manga gives us this formula for it’s ‘heavy water’:

H2O18 aka ‘oxygen-heavy water’, which is 2 hydrogens bonded to an oxygen-18 atom. 

I wasn’t able to find much information on H2O18, other than the fact it exists. Though when reading about D2O, I noticed there are quite a lot of similarities between its effects on humans in real life, and the effects Nikolai attributes to ‘heavy water’ in the manga. But I’ll touch more on its effects on humans later.

D2O and H2O18 actually weigh the same, and in both cases, their combined atomic weight equals about 20u. Regular water weighs 18u (one oxygen atom that weighs 16u + two hydrogen atoms that weigh 1u each). 

So the difference in atomic weight between ‘heavy water’ and regular water is only 2u per molecule. But since there is obviously more than one molecule of ‘heavy water’ filling up the room with Fyodor and Chuuya (this is me, remembering I am writing about bsd here, and not researching for school anymore XD), exactly how much heavier is ‘heavy water’ compared to regular water?

‘Heavy water’ is about 10% heavier than regular water. 

Regular water weighs 1,000 kilogram/m3 (cubic meter) (so if you filled a cube with 1 meter dimensions with water, the amount of water it would take to fill that cube would weigh 1,000 kilograms).

Heavy water weighs 1106 kilograms/m3 (so 10% more than regular water).

So, looking back at our formula for density, where density = mass/volume, ‘heavy water’ weighs more than regular water, so it has a higher density.

“It’s heavier than regular water, so you can’t swim in it.” (Nikolai)

I think what Nikolai is saying here, is that since the ‘heavy water’ is much denser than normal water, it is impossible/incredibly difficult to swim beneath the surface. Because human’s are less dense than water, and the added density of ‘heavy water’ means floating on ‘heavy water’ takes much less effort. They can still float in the water without drowning, and they can likely swim on the surface of the water, such as swimming from one wall to another, but they cannot dive downwards to the bottom of the cell, and swim along the floor, because the water will forcibly push them back up to the surface.

But Chuuya is Sinking???

So, this would leave us with the question of why Chuuya was able to sink at the end of the chapter. If the water is dense enough to keep him afloat, it should actually be relatively difficult to sink beneath the surface.

My idea is that it likely has something to do with how much Chuuya was struggling in the water, compared to Fyodor.

These are two panels of the two, at approximately the same time:

Fyodor doesn’t look too pleased in this panel, but he is calm. The water around him is relatively still. And his expression isn’t strained from the effort it would take to swim in such dense water, so he is likely just floating (maybe kicking his feet a little to tread water, but not enough to tire himself).

Chuuya, on the other hand, isn’t looking too hot. The water around him is much more displaced. Those little droplets are flying everywhere. This, and the struggling look on his face, suggests he is fighting against the water. He’s likely kicking his legs and flailing his arms rapidly beneath the surface, despite the fact he’ll float fine if he just relaxes a little bit. But he’s likely panicking, at least a little. 

He is also tired. We know this. He was panting, dragging those guards across the floor at the beginning of the chapter, and Dazai tells us the metal of the prison walls is suppressing his ability. 

So I believe the reason we see Chuuya sinking in this panel comes from a multitude of reasons.

While ‘heavy water’ is denser than regular water, and thus, much easier to stay afloat in, it does require some effort on behalf of Chuuya to stay afloat. Since he is tired, he has likely stopped trying to float, and has started to sink. (he likely won’t weigh enough to sink to the bottom, but if he doesn’t keep his head above water, he will still drown regardless).

With the amount of water he was splashing everywhere, he has likely swallowed quite a lot of it. Swallowing the water adds mass to his body, making him heavier, so he will sink easier.

In the above panel, we see bubbles forming on the surface of the water, likely from Chuuya letting air out of his lungs. Holding air in our lungs makes us lighter, like balloons. So expelling air from his lungs will also make Chuuya sink easier.

Toxicity of the ‘Heavy Water’

I’d also like to touch briefly on this point. In the manga, Nikolai describes the ‘heavy water’ as:

“Harmful to the human body. You will die if you drink too much of it.”

Again, I tried looking for information on ‘oxygen-heavy water’ (the H2O18 one the manga specifically mentions), but I wasn’t able to find anything about it being toxic.

In fact, I found one post that claims that since the oxygen atoms are the ones that hold the extra mass in H2O18, there isn’t much of a difference between it and regular water, and it is actually safe to drink (here is the link if anyone is interested, it’s a very cool article imo).

On the other hand, the other form of ‘heavy water’, D2O, is not safe to drink. A little bit is fine, but too much will cause problems. Something about how the bonds formed by deuterium (that heavier form of hydrogen that makes up D2O) are stronger than normal hydrogen bonds. This causes problems in the body, specifically, it prevents cell division. 

In order to experience any serious or deadly side effects, a person would have to consume a lot of ‘heavy water’. Like a very high amount. From what has been observed so far in mammals, consuming enough ‘heavy water’ to replace 25% of your body’s water will cause sterilization. If a person consumes enough to replace 50% of their body’s water, they’ll die. So while ‘heavy water’ is dangerous in large quantities, it isn’t nearly as deadly as Nikolai makes it out to be in the manga. (here is another link for the information I mentioned in this paragraph).

Thoughts and Ideas?

But like I said, those negative effects are for D2O, and it was H2O18 that is mentioned in the manga. So I’m wondering if maybe Asagiri meant to write D2O? Because from what I’ve read, D2O is the deadly one, H2O18 is relatively harmless. 

Also, if what’s filling the room is actually D2O, I wonder if a plot point will come up about D2O’s effects on cell division? I’m not sure how in the hell this could possibly work, but maybe, since Bram became a vampire due to his cells being mutated through an ability, his vampire infection works through cell mutation too?

If Chuuya drank enough water to affect cell division, could that stop his vampire mutation? (highly unlikely, he literally just swallowed that water, I am unsure of the timeframe it would take for ‘heavy water’ to have any effects, but a couple minutes seems like way too little for any of this to happen). Not to mention stopping cell production wouldn’t reverse any damage that the vampire mutation had already done to his cells. So overall, not likely. But I am wondering if Asagiri is planning on using the toxic effects of ‘heavy water’ as a plot point moving forward.

------- So yeah, this was my attempt to explain my (admittedly poor) understanding of science, so, consider this a warning not to take my word here as law or anything like that. There’s a good chance I am understanding this wrong, so if anyone more qualified would like to correct me, please feel free!!

Thank you for this ask Anon!! I had a lot of fun reading about all this stuff and writing this response!! It's been a few months now since I've had to focus my mind on anything academic, and turns out I missed it??? Imagine that. 16 years of school and apparently I'm still not sick of it XD