Sciencemadness Discussion Board

Hydrochloric Acid: Mixture or Compound?

Deathunter88 - 25-8-2015 at 23:01

Greetings,

Today I took a quiz in chemistry class and one of the problems given was to label different things as: Heterogeneous mixture, homogeneous mixture, element, or compound. For example, magnesium chloride would count as a compound and salt water could count as a homogeneous mixture.

One of the things we had to label was Hydrochloric Acid. I labeled it as a homogeneous mixture since hydrochloric acid is a solution of hydrogen chloride in water.

However, my teacher marked it wrong and when I asked her about it she said "Because it does not say that it is dissolved in water, you cannot assume that it is a solution."

Can you assume that when something says 'Hydrochloric Acid' that it is hydrogen chloride dissolved in water, or only when it says it is in solution? Is it acceptable to call 'Hydrogen Chloride" hydrochloric acid?

Thanks in advance :)

kavu - 25-8-2015 at 23:38

Your teacher is wrong and you are right. Hydrochloric acid is indeed the solution of dissociated and solvated hydrogen chloride in water, and thus counts as a homogeneous mixture.

diddi - 25-8-2015 at 23:58

I agree. your teacher should be dissolved in said acid

Tsjerk - 26-8-2015 at 00:01

It depends a bit, was the question asked in English? Then I would say hydrochloric acid is indeed HCl in water. If the question was hydrogen chloride, then it is a compound.

woelen - 26-8-2015 at 00:56

I agree with the others. Hydrochloric acid is a mixture, being an acidic liquid. Hydrogen chloride is a compound, being a colorless gas.

Volanschemia - 26-8-2015 at 02:12

Another vote of agreement from me as well. The name "Hydrochloric Acid" means a solution of Hydrogen Chloride. (And I must say, diddi's suggestion sound like a good idea :))

aga - 26-8-2015 at 09:30

Quote: Originally posted by Deathunter88  
Is it acceptable to call 'Hydrogen Chloride" hydrochloric acid?

No.

"Hydrochloric acid is a clear, colorless, highly pungent solution of hydrogen chloride (HCl) in water."

https://en.wikipedia.org/wiki/Hydrochloric_acid

Dunno who they got doing the Smelling - doesn't smell much to me at all, never mind being 'highly pungent'.

Print out the wiki and ask for the Marks to be added back, as you were right, and she was wrong.

(this doesn't make you smarter than her - she still knows a whole universe more than you do - yet you still deserve those marks.).

szuko03 - 26-8-2015 at 10:03

I kind of understand both arguments. When you talk about the acid its almost convention to remove the water in the speech as it almost becomes part of the acid. However, I agree with the OP and everyone from a technical stand point but did part of you at least know you were going to cause an issue? I would have read it like the teacher kind of stated "it doesnt say anything about water so i wont include it in my thoughts"

However my mom use to say "if you tell him to get bread on the table for dinner you better tell him to cut it or he will take what you said and do exactly what was said and not change it even if it would make sense" It causes problems in my normal life sometimes especially with the misses ;)

Praxichys - 26-8-2015 at 10:29

Typical semantics problem. I can't even tell you how many times I got marked wrong by this kind of stuff.

Ex. In a Williamson Ether Synthesis, what haloalkene is reacted with sodium phenoxide to form phenoxymethylbenzene?

IUPAC names confuse everything. Just say sodium phenolate and benzyl phenyl ether, and we all know it's benzyl chloride. WRONG. It's chlorophenylmethane.

I should have had her call up Sigma and try to order some. Would have gotten some strange looks for sure.

aga - 26-8-2015 at 14:26

Quote: Originally posted by Praxichys  
I should have had her call up Sigma and try to order some. Would have gotten some strange looks for sure.

How do you get a strange look down a telephone ?

Darkstar - 26-8-2015 at 16:44

Quote: Originally posted by aga  
Dunno who they got doing the Smelling - doesn't smell much to me at all, never mind being 'highly pungent'.


Oh, it's pungent alright. I once absent-mindedly opened a gallon of the 31.45%/10M stuff you buy at the hardware store and took a BIG whiff. Suffice it to say, I've never made that mistake again. It was so bad that I could barely even breath for several minutes afterwards.

I suppose it's technically the undissolved HCl vapors hovering above the solution that are "pungent," not so much the solution itself. The disassociated HCl molecules in solution probably don't contribute much to its smell, which is kind of the main idea behind certain gas traps.

blogfast25 - 26-8-2015 at 17:18

Quote: Originally posted by Darkstar  


I suppose it's technically the undissolved HCl vapors hovering above the solution that are "pungent," not so much the solution itself. The disassociated HCl molecules in solution probably don't contribute much to its smell, which is kind of the main idea behind certain gas traps.


Smelling hydrochloric vapours gives an idea of what pure hydrogen chloride must smell like, bearing in mind that the vapours are still much diluted compared to pure hydrogen chloride.

Camroc37 - 26-8-2015 at 17:25

I believe HCl is a gas if it is 100%, so you wouldn't have a solution. HCl is usually 31.45% in a solution with water.

mayko - 26-8-2015 at 18:02

Quote: Originally posted by Darkstar  
Quote: Originally posted by aga  
Dunno who they got doing the Smelling - doesn't smell much to me at all, never mind being 'highly pungent'.


Oh, it's pungent alright. I once absent-mindedly opened a gallon of the 31.45%/10M stuff you buy at the hardware store and took a BIG whiff. Suffice it to say, I've never made that mistake again. It was so bad that I could barely even breath for several minutes afterwards.


aga, are you sure your HCl(aq) is 30%? I'd always found the smell to be quite noticeable, but a year or two ago I was at the hardware store and the standard muriatic acid was sold out, but a 'low fume' variety was in stock. Customer service assured me it was the same concentration, and I've never had need to measure it or had trouble with it for my purposes, but other customers have found it to be too dilute for their needs

Darkstar - 26-8-2015 at 20:48

Quote: Originally posted by Deathunter88  
Is it acceptable to call 'Hydrogen Chloride" hydrochloric acid?


Technically, yes. Just to play Devil's advocate here, your teacher has a point, and wasn't wrong in marking your answer as incorrect. Sure, she should have worded the question more clearly if she specifically meant hydrogen chloride gas. "Hydrochloric acid" has historically implied an aqueous solution; however, the key word here is "implied." The name itself says nothing about it being a solution, and is based on an older naming system that many common acids were named using. The prefix "hydro-" means that the molecule consists of only a hydrogen and one other atom. The suffix "-ic" means that the anion, in this case chloride, ends in "-ide." The "acid" part, under the Brønsted-Lowry definition, simply means that it has an acidic proton that can easily be removed. Thus, under that naming system, HCl becomes hydrochloric acid.

With that said, calling anhydrous hydrogen chloride gas "hydrochloric acid" is then no more incorrect than calling anhydrous HNO3 or anhydrous H2SO4, "nitric acid" or "sulfuric acid," respectively. The anions of the latter two both end in "-ate" (nitrate and sulfate) so they both get the "-ic" suffix as well. ("-ate" and "-ide" both become "-ic") And since these are named following the same rules as hydrochloric acid, if it's not considered incorrect to call their pure forms the same name as their aqueous solutions, it shouldn't be incorrect for HCl, either.

Anyway, just looking at this from a different angle than most in this thread. But I do agree that your teacher should have just said "hydrogen chloride" if that's what she meant, or at least specified that it wasn't aqueous. And if you really want to get nice and proper, according to IUPAC rules, the correct systematic name for a hydrochloric acid solution would actually be "aqueous hydrogen chloride."

[Edited on 8-27-2015 by Darkstar]

woelen - 26-8-2015 at 22:34

If you really want to be sure, the pure gas HCl should be called chlorane, that is the real IUPAC name.

Sulaiman - 27-8-2015 at 00:32

deleted due to better answer above

[Edited on 27-8-2015 by Sulaiman]

nezza - 27-8-2015 at 11:09

Absolutely. Pure hydrogen chloride is a covalent, undissociated gas. To be called hydrochloric acid it must be a solution in water.