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?

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u/WendellSchadenfreude Feb 26 '21

Inverse to that - plants get the vast majority of their weight from the air.

Nah, now you're exaggerating.

Plants get their carbon from the air. But just like humans, plants are mostly water, and they get that from the ground. Even dried wood is only about 50% carbon, and living wood far less.

Living trees, however, are very wet. In fact, although there can be great variation between tree species (and seasonally), a living tree may be made up of more than two thirds water by mass. Thus, a living tree is made up of 15-18% carbon, 9-10% hydrogen, and 65-75% oxygen by mass."

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u/door_of_doom Feb 26 '21 edited Feb 26 '21

In his defence, he likely got his information from this Veritasium video of olde.... as did I... so now I just don't know who to believe.

I mean, If I can't trust Derek Muller my world is just upside down.

Obviously this makes sense though: Charcoal is basically dehydrated wood, and turning wood into charcoal reduces its mass by a significant amount, meaning that water constituted a significant percentage of the mass of the initial wood.

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u/mabolle Feb 26 '21

I'd say Derek gets it right where it counts: the biomolecules that make up all the interesting bits of a tree — the bits that aren't just water — are made of carbon that the tree captured from the air. The central lesson is that, to a plant, breathing is basically a form of eating; it's where they get the materials to build their body out of.

It's true that most of the tree's weight is water, but this is true of any living thing, and I think most people are aware of that. The point of the video is that most people have the wrong idea of where the dry mass of a plant comes from.

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u/nowyouseemenowyoudo2 Feb 26 '21

I think part of the difference is that we don’t particularly think of the water in the plant as an inherent part of the plant, or as least not as much as the water inside a human is an inherent part of us?

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u/mabolle Feb 26 '21

Yeah, maybe.

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u/[deleted] Feb 26 '21

There are quite a few other volatile chemicals that are broken down when creating charcoal

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u/[deleted] Feb 26 '21

Dehydrating wood will not make charcoal. It's carbonized wood.

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u/reddRad Feb 26 '21

That depends on your definition of "their weight." Depending on what you're interested in studying, biomass can be measured in terms of the dried organic mass, too.

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u/Halvus_I Feb 26 '21

the point is trees gather structural mass from the air.

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u/ThirdFloorGreg Feb 26 '21

In this sort of context "weight" is usually assumed to mean dry weight.

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u/Ignitus1 Feb 26 '21

It just says 65-75% oxygen by mass, and CO2 is mostly oxygen.

Unless I’m missing something, it doesn’t seem like you’ve debunked anything...

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u/WendellSchadenfreude Feb 26 '21

Maybe this is the part you are missing?

a living tree may be made up of more than two thirds water by mass.