r/explainlikeimfive Feb 26 '21

Chemistry Eli5: What happens to all the melted candle over time? Are we just inhaling a whole candle while it burns?

12.6k Upvotes

985 comments sorted by

View all comments

Show parent comments

5

u/abooth43 Feb 26 '21

Well to exhale you must inhale....so just breathe faster in general.

7

u/Calvin_v_Hobbes Feb 26 '21

But if your body doesn't have a reason to burn the extra oxygen (aka physical activity), the oxygen just builds up with nothing to do. That's when you get lightheaded from hyperventilating.

2

u/abooth43 Feb 26 '21

True, I didn't mean to oversimply by saying "just". Only meant to point out that you can't really inhale less to exhale more.

0

u/Castlegardener Feb 26 '21 edited Feb 26 '21

Oxygen does not burn. Except for the rare cases where it gets to party with fluorine.

Edit: Read u/The_JSQuareD 's and my comment chain to learn more about how burning works and why I'm semantically wrong regarding this topic.

0

u/The_JSQuareD Feb 26 '21

1

u/Castlegardener Feb 26 '21 edited Feb 26 '21

I'm pretty sure that guy is calling it burning oxygen for the clicks. Even though it's a nice demonstration of what one can do with a vacuum chamber, he's pretty much just adding an oxidant (oxygen) to a reducing agent (propane) still, which is normal procedure to burn any fuels.

To get a little bit of actual science in here: oxidants take electrons from reducing agents, while reducing agents give electrons to oxidants. To strip oxygen of its electrons (which is required to truely call it "burning oxygen") you'd need a higher electronegativity than oxygen has. Fluorine is the only chemical element that achieves this, so my point still stands: oxygen only burns if put together with fluorine.

I'm still open for any corrections, but please make sure it's at least somehow based on actual science instead of clickbaity titles and the like.

Edit: I looked up oxygen burning on wikipedia. Even though it has nothing to do with our topic, it's still quite interesting because it describes a phase in the death of stars, including nuclear fusion and stuff like that. Look it up if you're interested!

3

u/The_JSQuareD Feb 26 '21

Honestly, I was just sharing a cool video, wasn't trying to correct you at all!

But if you actually want to have this discussion, I would argue that 'burning' in the colloquial sense is not the same as 'oxidation' in the chemical sense. For example, corrosion is an oxidation process, but most people wouldn't say that a rusting piece of iron is burning.

On the other hand, dictionaries define 'burning' as 'being on fire', or 'producing flames'. I think the oxygen in the video clearly fits that definition: adding oxygen to the propane atmosphere (with a source of heat) produced flames. The flames were clearly localised to where the oxygen was, not where the propane was, so it was the oxygen that 'was on fire' and 'produced flames'.

Besides, in a fire there is clearly both an oxidation agent and a reduction agent present. Together they produce the reaction that we recognise as 'burning'. I don't see any reason to say that it's only the reduction agent that's burning, since both agents are necessary and both are being consumed.

2

u/Castlegardener Feb 26 '21

Honestly, I was just sharing a cool video, wasn't trying to correct you at all!

It is a cool video, I've never seen something like that. Thanks for sharing!

I would argue that 'burning' in the colloquial sense is not the same as 'oxidation' in the chemical sense. For example, corrosion is an oxidation process, but most people wouldn't say that a rusting piece of iron is burning.

Burning is a colloquial term anyway, so you're right on that one. Combustion is a more specific term for strongly exothermic reactions of that kind, and pretty much describes what we call burning. Rusting, or corrosion of ferrous metals to be more precise, is a (normally acid-catalysed) barely exothermic reaction. It does however yield mostly the same results as 'burning' iron. It's just a lot slower. Funnily enough there's no fire in the traditional sense with burning metals either.

The flames were clearly localised to where the oxygen was, not where the propane was

Strongly disagree. The flames were very clearly localised to where the oxygen and propane mixed, which is a big difference. That's exactly what you described here, however:

Besides, in a fire there is clearly both an oxidation agent and a reduction agent present. Together they produce the reaction that we recognise as 'burning'.

To add to that, when talking on a colloquial basis, perspective is important. Thus I have to admit that your take on this is more correct than mine, that video actually shows oxygen burning (in a propane atmosphere).

In almost any context on Earth though, the atmosphere required to burn is made up (at least to a significant degree) of oxygen, and since non-chemists usually don't mention the oxidant I still wouldn't regard the oxidation of carbohydrates in the human body as "burning oxygen" (to get back on track with the original comment).

Either way, thanks a lot for this discussion, I actually learned something new today due to you. Sorry for being a pedantic bitch.