Why does sweat even want to evaporate?
June 26, 2024 6:59 AM Subscribe
So I understand that the reason sweat cools us is that it takes heat from our bodies to evaporate. But why does it do that? I mean why not just stay liquid? If i put wax on my body it doesn't take heat from my body to melt it just stays solid, right? Why is water just so eager to evaporate?
If this is a monstrously stupid question please forgive me and blame my science teachers.
If this is a monstrously stupid question please forgive me and blame my science teachers.
Try playing with this simulator
One thing to notice is that not every molecule has the same amount of kinetic energy, and that the energy is transferred between particles by collisions.
(Wax does this, too. It is just made from very heavy molecules)
posted by Acari at 7:13 AM on June 26 [3 favorites]
One thing to notice is that not every molecule has the same amount of kinetic energy, and that the energy is transferred between particles by collisions.
(Wax does this, too. It is just made from very heavy molecules)
posted by Acari at 7:13 AM on June 26 [3 favorites]
Okay so I feel somewhat stupid providing this answer since it comes from a lot of vague half-remembered things from my reading rather than scientific knowledge, but I think it has to do with:
1. water is a much better conductor of heat than wax so it's going to be better at absorbing heat from your body, whereas wax just sits there unbudged
2. water has a high specific heat capacity, in fact its specific heat capacity is double that of wax, which makes it "want" to absorb and store more heat since it's not boiling off just yet. And that kind of "backfires" on water because oops, evaporation is a thing that happens well below boiling point, all that heat it absorbed is making it turn to vapor now, byeee. (very sciencey, this explanation)
posted by MiraK at 7:27 AM on June 26 [1 favorite]
1. water is a much better conductor of heat than wax so it's going to be better at absorbing heat from your body, whereas wax just sits there unbudged
2. water has a high specific heat capacity, in fact its specific heat capacity is double that of wax, which makes it "want" to absorb and store more heat since it's not boiling off just yet. And that kind of "backfires" on water because oops, evaporation is a thing that happens well below boiling point, all that heat it absorbed is making it turn to vapor now, byeee. (very sciencey, this explanation)
posted by MiraK at 7:27 AM on June 26 [1 favorite]
All liquids evaporate.
An explanation for people like me, who only remember basic science.
posted by Tell Me No Lies at 7:36 AM on June 26 [3 favorites]
An explanation for people like me, who only remember basic science.
posted by Tell Me No Lies at 7:36 AM on June 26 [3 favorites]
Wax actually will take heat from your body to melt, it just needs more heat so it's less likely to melt - your body temp isn't hot enough to melt it completely. But if you were to hold a chunk of wax, like a candle, with a hot part of your body like your armpit, it would soften and the surface would melt a bit. Lip balm and skin moisturizing butters contain various types and amounts of wax too, and they soften and glide onto warm skin, or melt in your pocket if it's really hot.
posted by nouvelle-personne at 7:50 AM on June 26 [4 favorites]
posted by nouvelle-personne at 7:50 AM on June 26 [4 favorites]
The sweat doesn't choose to take heat from your body to satisfy some desire to evaporate. It can't help being heated by your body since it's in contact with your body. Some of the molecules gain sufficient energy by that heating that they are able to break free and evaporate. Once those high energy molecules are out there in the air and not coming into contact with other sweat molecules still on your body, the total amount of energy is reduced.
Think of it this way. You're really hot. That means the molecules in your body have a lot of energy. They're zipping around fast and banging into each other, passing energy back and forth to each other. But all that energy is staying in your body, keeping you hot. Until your body makes sweat. Your skin and other parts of your body can't easily evaporate, but that water on the surface of your skin can. As the water molecules are zipping around and banging into each other some of them get banged into so hard that they get enough energy to evaporate. They leave, taking that energy with them. Energy keeps getting passed from molecules in your body to molecules in your sweat and then being lost as those molecules go off into the air. Eventually enough energy is lost that you start to feel the cooling.
And, yes, the same thing happens with wax. As high energy molecules from your body bang into molecules on the surface of the wax. they pass on their energy and the surface of the wax gets closer to its melting point. If it doesn't actually melt it's because it takes more energy to make wax melt than to make water evaporate.
posted by Redstart at 7:55 AM on June 26 [5 favorites]
Think of it this way. You're really hot. That means the molecules in your body have a lot of energy. They're zipping around fast and banging into each other, passing energy back and forth to each other. But all that energy is staying in your body, keeping you hot. Until your body makes sweat. Your skin and other parts of your body can't easily evaporate, but that water on the surface of your skin can. As the water molecules are zipping around and banging into each other some of them get banged into so hard that they get enough energy to evaporate. They leave, taking that energy with them. Energy keeps getting passed from molecules in your body to molecules in your sweat and then being lost as those molecules go off into the air. Eventually enough energy is lost that you start to feel the cooling.
And, yes, the same thing happens with wax. As high energy molecules from your body bang into molecules on the surface of the wax. they pass on their energy and the surface of the wax gets closer to its melting point. If it doesn't actually melt it's because it takes more energy to make wax melt than to make water evaporate.
posted by Redstart at 7:55 AM on June 26 [5 favorites]
I think you're basically asking why some liquids evaporate faster than others. You can consider alcohol, water and oil. If you put alcohol on your skin (like when you get a vaccination or give a blood sample), it evaporates very quickly, taking energy from your skin and making it feel cold quickly. Water on your skin (like sweat) evaporates more slowly, but still cools you. If you rub oil onto your skin, it basically doesn't evaporate at all and no cooling takes place.
The difference is basically how heavy the molecules of the liquid are and how much they stick to each other. Alcohol is a pretty light molecule and it only sticks to itself a little, so it evaporates quite easily. Water is very light (just two hydrogens and an oxygen), but it sticks to itself a fair bit, so it doesn't evaporate quite as easily. Oil is a much heavier molecule, so even though it doesn't stick to itself that much, it barely evaporates at all.
posted by ssg at 8:25 AM on June 26 [6 favorites]
The difference is basically how heavy the molecules of the liquid are and how much they stick to each other. Alcohol is a pretty light molecule and it only sticks to itself a little, so it evaporates quite easily. Water is very light (just two hydrogens and an oxygen), but it sticks to itself a fair bit, so it doesn't evaporate quite as easily. Oil is a much heavier molecule, so even though it doesn't stick to itself that much, it barely evaporates at all.
posted by ssg at 8:25 AM on June 26 [6 favorites]
This is such a good question and brought me back immediately to summers as a kid in Mississippi where’d we’d all be running around playing after sunset and we were sweaty and it was so humid and our t-shirts would just be draping off our shoulders because nothing evaporated.
posted by amanda at 8:52 AM on June 26 [1 favorite]
posted by amanda at 8:52 AM on June 26 [1 favorite]
amanda's response made me think that another very likely reason why sweat evaporates but wax doesn't melt on your skin is that the AIR around us can be dry or humid. When you have a sheen of sweat on your skin, the relative dryness of the air around you encourages the high-humidity surface of your skin to give some moisture up. It's the same principle as osmotic pressure causing water to get sucked in or out of animal cells depending on the cell's level of hydration.
This is why amanda's drenched tshirt would stick to the skin on a very humid day - the sweat can't evaporate because the air is already saturated with water vapor.
This doesn't apply to wax, since wax molecules are solid and nowhere near its melting or vapor point and there's no saturation/deficit of wax in the environment around it. If, in contrast, you had a liquid pool of melted hot wax already and you dropped another bit of wax into it, THEN the molecules in your bit would also melt and do their brownian motion thing and get all mixed up in the big pool of wax melt. Or if you put your bit of wax in a solution which contained *some* wax but not enough to saturate the solution, your wax would get all dissolved and mixed up quickly.
posted by MiraK at 11:26 AM on June 26 [3 favorites]
This is why amanda's drenched tshirt would stick to the skin on a very humid day - the sweat can't evaporate because the air is already saturated with water vapor.
This doesn't apply to wax, since wax molecules are solid and nowhere near its melting or vapor point and there's no saturation/deficit of wax in the environment around it. If, in contrast, you had a liquid pool of melted hot wax already and you dropped another bit of wax into it, THEN the molecules in your bit would also melt and do their brownian motion thing and get all mixed up in the big pool of wax melt. Or if you put your bit of wax in a solution which contained *some* wax but not enough to saturate the solution, your wax would get all dissolved and mixed up quickly.
posted by MiraK at 11:26 AM on June 26 [3 favorites]
Also, water is good at evaporating at the temperatures we generally have here on Earth. If we had evolved on a planet with a different climate, either much hotter or much colder assuming such things are possible, we might find that we'd be using some other material better suited for it. Or the need to sweat to deal with excess heat might not be an issue at all.
posted by any portmanteau in a storm at 1:15 PM on June 26
posted by any portmanteau in a storm at 1:15 PM on June 26
If you think about what is happening to the water at the molecular or atomic level, it helps to make some sense of it.
A water molecule consists of 3 atomic nuclei (2 hydrogen & 1 oxygen) surrounded by a swarm of electrons. The nuclei are tiny little things that only interact with other nuclei & such when they get very, very close to them. And the electron swarm is positively h-u-g-e in comparison - like think of the size of the sun compared with the orbits of the outer planets. It's on a scale something like that.
So nuclei only get close enough to each other to interact under very extreme conditions, and the main interactions of one water molecule interacting with another molecule (or generally, any two atoms or molecules interacting with each other) are all due to the properties of the electron cloud.
This is true of essentially all chemical & physical reactions & properties of matter that we are familiar with - everything from snow melting to salt dissolving to the color of an ear of corn. Those are all results of the structure of the electron cloud surrounding the various molecules involved.
If you want to dive a little deeper into the structure of water & water molecules that what you usually see in say, 6th grade science or even Physics 101, take a look at this nice paper in Chemical Reviews. It has some cool diagrams of water molecules' electron clouds in various states. It also talks about various interesting states that water can get into - for example, water can get into a crystalline state even when in liquid form! - and also the reasons for some of waters unusual properties, such as ice at certain temperatures being less dense that liquid water.
Back to your question: In very simplified form, we often think that water boils at 100 degree celsius. That is the point where it turns from liquid to vapor.
At the molecular level, however, it is quite a lot more complex and interesting than that. A (still quite simplified) version of that might go:
- A water molecule in liquid water is in a state where the electron cloud has a moderate amount of energy - more than ice, but less than vapor. The liquid water molecule has an electron cloud that is in a state where it can easily join with other water molecules but also fairly easily break that bond and slide on past the other molecules.
- If a photon of the right energy strikes the molecule, it will pop up into a higher energy state.
- This higher energy state gives the electron cloud a different shape and also different chemical properties. For example it might switch from having a high affinity for bonding with nearby water molecules to having a higher affinity for bonding with nearby oxygen molecules (or some other constituents of air).
- If this molecule is completely surrounded by other water molecules, maybe this won't lead to anything. Maybe it will just emit a photon after a while and return to its previous energy state after a while. (And maybe that photon will strike another water molecule somewhere else nearby, repeating the same process.)
- However, if this molecule is near the surface of the water, maybe this molecule come into contact with a nearby oxygen molecule and in the new, more receptive state of its electron cloud, fits in better with this oxygen molecule than its current neighboring water molecules. So it hops on over to the oxygen, making a (fairly weak) bond with it.
- From our point of view that water molecule has just become "water vapor". From the molecule's point of view it has just released its bond from its nearby water molecules and created a bond with a nearby air molecule, or perhaps a group of such molecules (air typically consists of oxygen, water, hydrogen, nitrogen, a bunch of other stuff. Typically we think of these as individual free-floating atoms and such, but at a more detailed level they are going to have different forms of weak bonds with each other in much the same way liquid water molecules do with each other.)
- The water vapor molecule that has bonded with that air molecules is then part of a gas, meaning that it has a lot more freedom to move around. As a rule it will move off - for one reason because water vapor is a bit lighter than the average density of air, so it tends to rise. Meanwhile, the liquid water molecules are far more tightly bound to each other (see the same article for a nice discussion of surface tension etc) and so stay where they are.
- Because only the higher-energy molecules are in a state that allows them to join the air molecules and float away, while lower-energy molecules are in a state where they strongly prefer to stick where they are, this is a sort of automatic sorting mechanism, where higher-energy molecules are preferentially removed from the sweat and float away, while lower-energy molecules preferentially stay right where they are. Upshot is, a bunch of energy leaves the sweat in the form of water vapor, while lower energy molecules (ie, those that are cooler - meaning at a lower temperature) remain. So the average temperature of the sweat becomes lower over time.
Above, I mentioned that the inciting event for all of this is when a photon is absorbed by a water molecule, bumping the water molecule's electron cloud into a higher state of energy.
At the chemical level, these are the two basic forms energy can take: A photon (which is electromagnetic radiation, like a beam of light or heat energy from the sun, or from a light bulb, or infrared radiation that might emanate from a warm body) OR a increased state of energy within the electron swarm surrounding a molecule.
So, for example, if a photon is absorbed by a water molecule, the electron swarm around the molecule bumps up to a higher state of energy. It will be in a different, higher energy state, and generally it will be larger. The molecule can emit a photon, which means the electron swarm drops to a lower energy state, while the same amount of energy travels out from the molecule in the form of a photon.
All energy is transmitted and stored in this fashion. For example, folks upthread have mentioned that when the water has more energy, one molecule will "bump into" another, transferring the energy to the second molecule. What is actually happening there involves the first molecule emitting a photon (thus dropping to a lower energy state) and the other molecule absorbing it (thus moving to a higher energy state in its electron cloud).
Even things we typically think of as "physical" movement is mediated by photo exchange. For example, if you clap your hands against each other, the two hands don't pass through each other - rather, they bounce off. That whole exchange of momentum is mediated by a veritable cloud of photon exchanges when the molecules of your two hands get very, very close to one another.
Angela Collier has a nice explanation of this in a recent Youtube video.
My point is, if you think of vague metaphors like "water molecules have a lot of energy as they get warmer, and all that energy makes them move around a lot and bump into each other" then it is very easy to get confused.
If on the other hand you think of the energy gained as the water gets warmer in terms of energy states of the molecules (that is, different states of the electron cloud surrounding the molecule, which expand and contract and, generally, take different specific shapes, depending on their energy level) and photons that carry energy from one molecule to another - well then, it is easier to understand what is going on and harder to be led astry by fuzzy thinking.
When you add in the additional important bit of information that the chemical properties of a molecule are completely determined by the state of its electron cloud then you are in an even better position.
TL;DR: Evaporating water cools because molecules receiving energy via a photon are in a higher energy state that makes them chemically better able to detach from their surrounding water molecules and attach to surround air molecules. Whereas the water molecules that gave up that electron are in a lower energy state that makes them stick to the surrounding water molecules more tightly.
End result is, high energy (warm) molecules tend to leave while low energy (cool) molecules tend to stay.
Now apply this to wax: Wax is a solid, so when molecule there receives some more energy in the form of a photon, it's likely to move towards melting (liquid form) but in liquid form it still remains quite tightly bound to its surrounding "wax" molecules and doesn't go anywhere. None of the energy or heat has floated off to somewhere else - it's all still right there.
Now, the constituent molecules of wax will evaporate at some point - a relatively high temperature & energy level. If they do evaporate in this way, they can have the same cooling effect. But the chances of a wax molecule reaching this relatively high energy level when, say, place on your arm, are much, much, much smaller than the chances of a water molecule doing the same.
(In chemistry terms the partial pressure of wax at room temperature is about 0.1 kPa whereas water is about 3.2kPa. So water is about 30X more likely to evaporate at that temperature.)
Thus, very, very, very few wax molecules are evaporating in this fashion, and the amount of cooling resulting from this is pretty much indetectable.
posted by flug at 3:25 PM on June 26 [4 favorites]
A water molecule consists of 3 atomic nuclei (2 hydrogen & 1 oxygen) surrounded by a swarm of electrons. The nuclei are tiny little things that only interact with other nuclei & such when they get very, very close to them. And the electron swarm is positively h-u-g-e in comparison - like think of the size of the sun compared with the orbits of the outer planets. It's on a scale something like that.
So nuclei only get close enough to each other to interact under very extreme conditions, and the main interactions of one water molecule interacting with another molecule (or generally, any two atoms or molecules interacting with each other) are all due to the properties of the electron cloud.
This is true of essentially all chemical & physical reactions & properties of matter that we are familiar with - everything from snow melting to salt dissolving to the color of an ear of corn. Those are all results of the structure of the electron cloud surrounding the various molecules involved.
If you want to dive a little deeper into the structure of water & water molecules that what you usually see in say, 6th grade science or even Physics 101, take a look at this nice paper in Chemical Reviews. It has some cool diagrams of water molecules' electron clouds in various states. It also talks about various interesting states that water can get into - for example, water can get into a crystalline state even when in liquid form! - and also the reasons for some of waters unusual properties, such as ice at certain temperatures being less dense that liquid water.
Back to your question: In very simplified form, we often think that water boils at 100 degree celsius. That is the point where it turns from liquid to vapor.
At the molecular level, however, it is quite a lot more complex and interesting than that. A (still quite simplified) version of that might go:
- A water molecule in liquid water is in a state where the electron cloud has a moderate amount of energy - more than ice, but less than vapor. The liquid water molecule has an electron cloud that is in a state where it can easily join with other water molecules but also fairly easily break that bond and slide on past the other molecules.
- If a photon of the right energy strikes the molecule, it will pop up into a higher energy state.
- This higher energy state gives the electron cloud a different shape and also different chemical properties. For example it might switch from having a high affinity for bonding with nearby water molecules to having a higher affinity for bonding with nearby oxygen molecules (or some other constituents of air).
- If this molecule is completely surrounded by other water molecules, maybe this won't lead to anything. Maybe it will just emit a photon after a while and return to its previous energy state after a while. (And maybe that photon will strike another water molecule somewhere else nearby, repeating the same process.)
- However, if this molecule is near the surface of the water, maybe this molecule come into contact with a nearby oxygen molecule and in the new, more receptive state of its electron cloud, fits in better with this oxygen molecule than its current neighboring water molecules. So it hops on over to the oxygen, making a (fairly weak) bond with it.
- From our point of view that water molecule has just become "water vapor". From the molecule's point of view it has just released its bond from its nearby water molecules and created a bond with a nearby air molecule, or perhaps a group of such molecules (air typically consists of oxygen, water, hydrogen, nitrogen, a bunch of other stuff. Typically we think of these as individual free-floating atoms and such, but at a more detailed level they are going to have different forms of weak bonds with each other in much the same way liquid water molecules do with each other.)
- The water vapor molecule that has bonded with that air molecules is then part of a gas, meaning that it has a lot more freedom to move around. As a rule it will move off - for one reason because water vapor is a bit lighter than the average density of air, so it tends to rise. Meanwhile, the liquid water molecules are far more tightly bound to each other (see the same article for a nice discussion of surface tension etc) and so stay where they are.
- Because only the higher-energy molecules are in a state that allows them to join the air molecules and float away, while lower-energy molecules are in a state where they strongly prefer to stick where they are, this is a sort of automatic sorting mechanism, where higher-energy molecules are preferentially removed from the sweat and float away, while lower-energy molecules preferentially stay right where they are. Upshot is, a bunch of energy leaves the sweat in the form of water vapor, while lower energy molecules (ie, those that are cooler - meaning at a lower temperature) remain. So the average temperature of the sweat becomes lower over time.
Above, I mentioned that the inciting event for all of this is when a photon is absorbed by a water molecule, bumping the water molecule's electron cloud into a higher state of energy.
At the chemical level, these are the two basic forms energy can take: A photon (which is electromagnetic radiation, like a beam of light or heat energy from the sun, or from a light bulb, or infrared radiation that might emanate from a warm body) OR a increased state of energy within the electron swarm surrounding a molecule.
So, for example, if a photon is absorbed by a water molecule, the electron swarm around the molecule bumps up to a higher state of energy. It will be in a different, higher energy state, and generally it will be larger. The molecule can emit a photon, which means the electron swarm drops to a lower energy state, while the same amount of energy travels out from the molecule in the form of a photon.
All energy is transmitted and stored in this fashion. For example, folks upthread have mentioned that when the water has more energy, one molecule will "bump into" another, transferring the energy to the second molecule. What is actually happening there involves the first molecule emitting a photon (thus dropping to a lower energy state) and the other molecule absorbing it (thus moving to a higher energy state in its electron cloud).
Even things we typically think of as "physical" movement is mediated by photo exchange. For example, if you clap your hands against each other, the two hands don't pass through each other - rather, they bounce off. That whole exchange of momentum is mediated by a veritable cloud of photon exchanges when the molecules of your two hands get very, very close to one another.
Angela Collier has a nice explanation of this in a recent Youtube video.
My point is, if you think of vague metaphors like "water molecules have a lot of energy as they get warmer, and all that energy makes them move around a lot and bump into each other" then it is very easy to get confused.
If on the other hand you think of the energy gained as the water gets warmer in terms of energy states of the molecules (that is, different states of the electron cloud surrounding the molecule, which expand and contract and, generally, take different specific shapes, depending on their energy level) and photons that carry energy from one molecule to another - well then, it is easier to understand what is going on and harder to be led astry by fuzzy thinking.
When you add in the additional important bit of information that the chemical properties of a molecule are completely determined by the state of its electron cloud then you are in an even better position.
TL;DR: Evaporating water cools because molecules receiving energy via a photon are in a higher energy state that makes them chemically better able to detach from their surrounding water molecules and attach to surround air molecules. Whereas the water molecules that gave up that electron are in a lower energy state that makes them stick to the surrounding water molecules more tightly.
End result is, high energy (warm) molecules tend to leave while low energy (cool) molecules tend to stay.
Now apply this to wax: Wax is a solid, so when molecule there receives some more energy in the form of a photon, it's likely to move towards melting (liquid form) but in liquid form it still remains quite tightly bound to its surrounding "wax" molecules and doesn't go anywhere. None of the energy or heat has floated off to somewhere else - it's all still right there.
Now, the constituent molecules of wax will evaporate at some point - a relatively high temperature & energy level. If they do evaporate in this way, they can have the same cooling effect. But the chances of a wax molecule reaching this relatively high energy level when, say, place on your arm, are much, much, much smaller than the chances of a water molecule doing the same.
(In chemistry terms the partial pressure of wax at room temperature is about 0.1 kPa whereas water is about 3.2kPa. So water is about 30X more likely to evaporate at that temperature.)
Thus, very, very, very few wax molecules are evaporating in this fashion, and the amount of cooling resulting from this is pretty much indetectable.
posted by flug at 3:25 PM on June 26 [4 favorites]
The wax does take energy from your body. If you hold wax in your hand it gets warmer and softer than if it's just exposed to cooler air.
The difference is that the warm wax doesn't go anywhere. It stays in the same lump and the heat just spreads around the rest of the wax and back into your hand, until your hand and the wax are at equilibrium temperature. Any molecule of wax is super unlikely to get enough energy to evaporate.
A water molecule can rather easily get enough energy to evaporate. When it does, it takes that heat to disperse into the atmosphere, instead of spreading it back to the rest of the sweat and your body.
posted by polecat at 5:03 PM on June 26 [1 favorite]
The difference is that the warm wax doesn't go anywhere. It stays in the same lump and the heat just spreads around the rest of the wax and back into your hand, until your hand and the wax are at equilibrium temperature. Any molecule of wax is super unlikely to get enough energy to evaporate.
A water molecule can rather easily get enough energy to evaporate. When it does, it takes that heat to disperse into the atmosphere, instead of spreading it back to the rest of the sweat and your body.
posted by polecat at 5:03 PM on June 26 [1 favorite]
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Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. A high concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when humidity affects rate of evaporation of water. When the molecules of the liquid collide, they transfer energy to each other based on how they collide. When a molecule near the surface absorbs enough energy to overcome the vapor pressure, it will escape and enter the surrounding air as a gas. When evaporation occurs, the energy removed from the vaporized liquid will reduce the temperature of the liquid, resulting in evaporative cooling.
On average, only a fraction of the molecules in a liquid have enough heat energy to escape from the liquid. The evaporation will continue until an equilibrium is reached when the evaporation of the liquid is equal to its condensation. In an enclosed environment, a liquid will evaporate until the surrounding air is saturated.
Related: sublimation
posted by LionIndex at 7:11 AM on June 26 [3 favorites]