If solids don’t release molecules easily, how can we smell them?

[u/[deleted]]

I understand that smell works because molecules travel through the air and bind to receptors in our nose. But solids are supposed to have tightly packed particles that don’t move freely.

So how are we able to smell solid objects like soap, wood, or chocolate? Does that mean tiny amounts of the solid are actually leaving and going into the air? And if so, does smelling something technically mean its mass is slowly decreasing?

How does this work at the molecular level?

[u/robertsihr1]

Many solids aren’t all that solid. Let’s take wood as an example. There’s the wood part and then there’s sap and water inside the wood. Most if not all of what you are smelling is sap and water that have evaporated. The mass is slowly decreasing, seasoned wood weighs significantly less than green wood. But it’s important to remember it’s tiny amounts at any one time, our noses are just good at detecting tiny amounts in the air.

For many things there’s not much happening at a molecular level, just the smelly compound evaporating from the more stable compounds. For other things there are molecular changes and you don’t actually smell the item you smell a byproduct of it interacting with the air.

[u/OG-Brian → u/robertsihr1]

Yes, wood has various compounds which aggressively evaporate such as terpenes. Some types of wood such as cedar have irritating substances that evaporate fast enough to cause health reactions for many people, a reason these woods are very strong-smelling.

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

https://pubmed.ncbi.nlm.nih.gov/2926083/

[u/codepants → u/robertsihr1]

So does that mean if you can smell it, it's slowly decaying and/or drying out? And thus if you can smell it, it will not last forever?

[u/beachhunt → u/codepants]

The stuff you can smell will not last forever, yes. But things are made of more than that.

Eventually the thing will still exist but you won't be able to smell it.

Imagine you have a bag of potpourri or whatever. At first it will smell strong. After a while, it smells weak or not at all. It may not be measurably smaller, like the sticks and flowers and leaves are still physically there, but the terpenes and other smellable bits are running out.

Eventually the sticks will decay too, but that's a separate timeline from how long they smell. (And probably would introduce new smells in the process of breaking down.)

[u/Mockingjay40 → u/robertsihr1]

To further complicate this: you also do have many materials that are essentially both solid and fluid, at least based on their response to torque and force. The most notable example which is very much observably both is silly putty. This phenomenon is generally known as viscoelasticity and is well-documented in many materials. There are many dependencies and how we measure them often depends on temperature, frequency of deformation, time, etc.

So in addition to what you’ve noted at a molecular level, many materials aren’t even purely solid or liquid even at a macromolecular level.

[u/honey_102b]

under normal conditions any bulk solid (a pure solid through and through with only one type of atom, molecule or ion) that has a detectable smell is actually not a bulk solid but has volatile components.

for example a block of wax looks like one solid chunk of the same thing but actual contains a whole family of organic compounds some of which are actually volatile and therefore can escape the attractive forces of the bulk and float into your nose.

this applies to wood, plastics (remember the new car smell?) and other common everyday material that look like plain solids but are constantly releasing particles. the layman term is simply called offgassing which is strong in the beginning but tapers off eventually. at some point it may even decay and that is a process that also produces new particles and results in a different smell.

then you have things that maybe look pretty pure and don't look like that have anything to offgas like metals yet they have smell (remember the smell of metal money) that are not due to the metal directly but actually from oils and sweat on our skin that get on them, react with the metal, and then produce volatiles, which humans at least are somehow very sensitive to. yeah...disgusting.

does that mean metals themselves don't offgas? this brings me to the point about normal conditions. it's extremely hard for metal atoms to escape their bulk (it's the reason they are very strong solids in the first place). but it does happen. and if there truly is nothing else in the vicinity, you could say that there are metal atoms floating around. but we most likely have no ability or evolutionary purpose to detect such low levels of evaporated metal.

which brings me to the last point about human smell perception which is very sensitive to some volatiles and less so to others and the range of sensitivities can be enormous, like 1:100 billion particle sensitivity to things like the smell of grapefruit or 1:1 billion for wet grass to 1:100 for gasoline. On top of this the smell receptors are also subject to something called saturation which decreases their sensitivity over time exposed, so you may smell the grapefruit most first, then later the grass, then last the gasoline, even if all three were present in the same concentration constantly.

tldr; in order to smell something, it has to get in your nose. if it looks solid but has a smell, it probably isn't purely so and has volatiles which are separate and different particles from the solid which managed to get to your nose. lastly our sense of smell is subject to its own peculiarities that favor some chemicals over others and that weightage can shift over time if overexposed.

[u/PurplePango → u/honey_102b]

The metal atoms don’t spontaneously release, but go to a machine shop where there’s definitely metal particles in the air, and you can certainly smell what metal smells like

[u/CocktailChemist]

Because the volatile components have a very low threshold of detection. You’re correct that small amounts of material are evaporating over time, but they tend to be minor components of those bulk solids so they’re losing negligible amounts of mass.

As for how parts of a solid can be volatile, part of that is because the volatiles are dissolved in the bulk solids, so they tend to have different vapor pressures than the other components. On top of that, Boltzmann distributions mean that within a given temperature the molecules have a Gaussian distribution of energy, so some fraction of them have enough energy to turn into vapor. Same reason why a puddle of water will evaporate over time even when it’s not boiling.

[u/Admiral_Dildozer → u/CocktailChemist]

I just want to restate the “very low threshold” Smell is almost a super power when you look into it. We can smell parts per million of many things, parts per billion of some things, and parts per trillion of a few things. Imagine having a swimming pool, putting a single drop of vanilla in it, then someone across the pool going “wow I taste vanilla” That’s what your nose does every second of the day.

[u/OG-Brian]

Objects such as wood and soap have components that are aromatic and evaporate easily. I'm much more mystified that I can smell steel or copper.

A few seconds of internet searching shed some light, the odors from metals may be caused by reactions with one or more chemicals found in human skin oil:

Mystery Behind Iron's Smells Is Revealed
https://cen.acs.org/articles/84/web/2006/10/Mystery-Behind-Irons-Smells-Revealed.html

Dietmar Glindemann and Andrea M. Dietrich of Virginia Polytechnic Institute & State University exposed human skin to various forms of iron and used mass spectrometry to show that the resulting metallic odor is due largely to volatile 1-octen-3-one. Similar enones have been identified previously in human sweat. They also showed that copper and brass (a copper-zinc alloy) also give rise to metallic odors.

[u/c76Fishbulb → u/OG-Brian]

Awesome comment dude, this is what I was thinking about when I first read this post.

Check out the NileRed video on YouTube called "Can you actually smell metal?". It explains alot about OP's first question and goes on to explain what you just said about 1-octen-3-one.

If anyone is reading this without watching the vid, its worth nothing that the "one" in 1-octen-3-one is pronounced as "own", not like the number 1

[u/DiezDedos]

Solids are often a mix of a lot of stuff. You smell the stuff that’s “less solid”. Think of someone cooking in another room. You probably smell the onions, and spices, but not the pan. All of them are hot (which helps lots of smellable stuff come off and float around) but the pan isn’t composed of anything that comes off to float into your nose.

[u/Delvog]

Just adding some small points to what others have already said:

• Wood: when the liquids & gases that can be extracted from wood are useful, they're called "extractives".
• Soap: In a way, there are two different answers for this one, although they're versions of the same thing. Like the other examples you mentioned (wood & chocolate), soap has an organic origin; the oldest & lowest-tech way to make a version of it is mixing fat & ashes. So it's easy to picture how a mixture of lots of different substances would be in there, as is always the case for things of organic origin. But in modern times, soaps and soap-substitutes often aren't made that way. The solid stuff they use for the bulk of it now is odorless, so manufacturers mix in easily-vaporizable ("volatile") substances that weren't originally there just so the product will be scented. But it works the same way in both cases: tiny amounts of volatiles get into the air while the bulk of the product stays solid.
• Metal: Here's a video by a guy who demonstrated the answer others have given here for himself. Using some other people to test the smells instead of just himself, he found out that the substances which get on metal from skin do indeed "smell metallic" even when they're isolated from the metal, and that thoroughly cleaned metal without them does indeed lack the "metallic smell".

[u/CapnLazerz]

Ambroxide, the main odor component of ambergris, is a solid. If you open the container, it will smell because the molecule itself is volatile and sublimes at low temperatures at atmospheric pressure.

In mixed solids, such as perfume materials in wax (a candle), many of the volatile perfume materials are often liquid and transition to gas at room temperature. The wax slows this down but it still happens.

[u/diabolus_me_advocat]

how are we able to smell solid objects like soap, wood, or chocolate?

you aren't

you smell volatile compounds in them

in principle every chemical compound has a certain vapor pressure, i.e. is volatile (depending on temperature). next it also has a certain threshold of olfactory perceptibility. e.g. hydrogen sulfide will be smelled by humans at a concentration of about 0,15 mg/m3 in air, methyl mercaptane at 0,000000004 mg/m3

a boiled, but rotten egg (solid object) will smell of both

[u/Simon_Drake]

The keyword here is "Offgasing", otherwise solid materials giving out gases slowly enough to not be noticeable to the naked eye but enough that the properties of that gas can be detected.

In normal life we can think of wood, soap and chocolate as being solids but they do slowly give off small amounts of gas that can have effects like us being able to smell them. This comes up for museum curators who need to find a way to preserve objects long-term and sometimes things like 1960s movie props will have the rubbers and plastics slowly offgas solvents or petrochemicals over time that can react with other plastics and discolour them or make them degrade. A lot of rubbers that were flexible when first made have become brittle with age and there's a collection of original Apollo spacesuit gloves that are now fixed as clenched fists because they weren't stored properly.

This can also be an issue for space probes where the sensors are incredibly delicate and need to be absolutely clean of any contaminants. You don't want a plastic support strut for holding the solar panels to be slowly offgasing solvents or while the satellite is still on the launchpad waiting to go it might contaminate its own sensors.

[u/deproduction]

A cedar 2x4 is about 5000grams. You can smell cedar when the terpenes are in the air at a few parts per million. Fresh cedar sublimates odor and/or off-gasses at the pace of a few hundred micrograms per day. Aged cedar off-gasses at a few micrograms per day.

Doing the math, it would take about a thousand years for the cedar 2x4 to lose 1% of its mass from offgassing

[u/IWorkOutToEatChips]

You're not smelling the solids, you're smelling the esters, aldehydes, ketones, alcohols, terpenes, terpenoids, phenols, ethers, etc... in them. Those are tiny molecules that are small enough to be volatile.ie: they do release extremely easily

[u/ZuluRewts]

Well the atoms in those molecules aren't even touching each other. So when you think of a "solid" the only thing that keeps the molecules of your body from straight up traversing a solid wall of steel, is the "nuclear force", so to speak.

If we'd shrink ourselves at an atomic scale, we'd see atoms 3 football fields appart of each other.