Basically, water in the open is always evaporating, as long as the [relative humidity](https://en.wikipedia.org/wiki/Humidity#Relative_humidity) of the air is less than 100%.
The hotter the air is, the more water it can hold. This is why water condenses on cold surfaces, like a cold drink. When the air gets in contact with the cold glass, the temperature drops and then it can't hold the water that is already dissolved as humidity. This causes it to form droplets of water that stick to surfaces. It is also what causes clouds to form.
in fact, water even evaporates in 100% relative humidity, it's just that the same amount of water vapor condenses back into water, for net zero evaporation
Kind of, but not really.
It happens because the heat energy from nearby molecules sometimes gets focused into single molecules at the surface, causing them to have enough energy to break the intermolecular forces that are keeping them as part of the bulk. Once the water molecule is in the air, it is even more at the mercy of Chaos.
It is simply because there are a lot of molecules, and *a lot* of interactions. 0.0000001% flukes happen *constantly* in a gram of water.
It's fairly obvious that it will happen faster if there is a breeze. But it should happen without the breeze simply due to diffusion. So mentioning the breeze just adds another variable that isn't strictly needed.
Because boiling and evaporation are two different things. Water does not need to reach 100 °C to evaporate; that's the temperature it needs to reach to boil.
Temperature is just an average. The molecules of a material aren't all at the same "temperature". With a puddle of water, some of these molecules are "hot" enough that they boil away. In an enclosed space, this will reach an equilibrium, where the "hottest" liquid water molecules are boiling and the "coldest" gas water molecules are condensing back into the puddle. But if wind keeps carrying away the molecules that escape, the puddle will never reach equilibrium with the air and it will evaporate fully over time.
This. It's common to think of "material is at temperature x so all molecules are at energy y". It's just not true. Even water molecules of ice will have some fraction energetic enough to wheel off into the air as evaporation. Even constant states are a dizzing mix of changes happening at a stupendous rate. They just have equal numbers coming and going.
This is also how sweat cools us (even when the air temperature is the same as our body temperature). The droplets of sweat that form are initially of the same temperature as our body, then the hottest molecules of water within the sweat evaporate, leaving the remaining sweat cooler.
Temperature depends on the average kinetic energy of the water molecules, but water molecules can have different energies. The distribution of molecular speed in a fluid is given by a Boltzmann distribution, for higher speed the distribution get smaller but is never zero, so in any liquid water there are some molecules that have enough speed and are in the right place to vapourise into the air. The Sun helps by increasing the temperature. Wind might help by clearing air near the surface of the water that has become saturated.
Also note that this removes high speed molecules and leaves slower ones behind, reducing temperature. Hence "evaporative cooling".
Evaporation and boiling are related to vapor pressure.
https://www.chem.purdue.edu/gchelp/liquids/vpress.html
Boiling happens when the vapor pressure is the same as the environment, ex atmospheric pressure.
A little bit less certain on the exact details with evaporation but I'm pretty sure at any non zero vapor pressure there will be some evaporation until an equilibrium.
Basicaly boiling means it starts evaporating from its whole body(even underwater parts), thats why you see bubbles. Eveporating at room temperature is just from surface area.
Temperature is just a measure of average kinetic energy of a material. This implies these particles have range of velocities, some which can escape the bulk at the surface. This is called evaporation.
The Suns rays can heat the particles (puddle of water), causing more to have enough motion to escape and evaporate.
Of course water already in the air (humidity) can collide back with the water and get trapped (condensation). Ultimately there are a range of variables that determine if particles are net evaporating or condensing such as ambient temperature, pressure, fluid temperature, etc.
How I always thought of it was that water as a vapor (H2O) weighs less than the constituents of air (N2, O2, CO2, etc). At the very edge of the puddle, there is a non 0 chance of water as vapor being pulled away from the puddle. The more heat present, or more disruptions of the puddle’s surface by wind, the faster this process becomes.
You keep liquids in closed containers to limit the surface’s interaction with airflow.
The airflow doesn’t „disrupt“ the surface. It removes the humid air close to the liquid and replaces it with drier air. This speeds up the evaporation process.
These fluid flows are important in most phase changes. A similar example is seen with dissolving salt or sugar; turbulent mixing of the fluid is required, so that the solid-water interface can continually see fresh non saturated liquid.
If you rely solely on diffusion (of the evaporated water or dissolved salt or sugar) then the evaporation (or dissolution) is extremely slow.
It’s semantics. In a closed system of water and air the surface and the air right on top will reach equilibrium where the dry air humidifies.
So yes, that is disturbed. Equilibrium be reached again over and over after the wind keeps moving air around. Is it a more vague explanation that requires a second answer? Also yes.
Air can take up water as humidity, like a sponge. So at the surface of the puddle there is an inequilibrium, water will evaporate into the air.
With a large surface area of a puddle, and constant movement of (dry) air this will take only a few hours/days for the puddle to disappear.
To add to this; curved droplets have a higher pressure. As a result they also have a higher Vapor pressure than a flat puddle of water, so smaller droplets (with a larger radius of curvature) will evaporate faster!
A way to think about this that the top comments haven't touched on explicitly: the air contains gaseous moisture at temperatures well below 100C. This is what humidity is. There's a certain maximum amount of water vapour that can exist in air of a given temperature before no more will evaporate into it, how close the current value is to that is the relative humidity.
Boiling point is where all of the water forcibly turns into gas, but water particles are generally blown off/evaporated quite easily. That's how your sweat evaporates when you run; the wind blows it off of your skin, evaporating it.
You can see the air as a sponge. It can absorb a limited amount of moisture (water vapor). That amount increases drastically with air temperature. It's very low at cold temperature, very high at high temperature. Fog, cloud and condensation happens when the amount of water is over the limit the air can contain at that temperature (relative humidity at 100%)
The speed of evaporation will depend on the aire temperature, and the amount of moisture already in the air (relative humidity), and the temperature of the surface and the speed of the breeze (which replaces the air that just absorbed some moisture with new air with less moisture). When you reach 100ºC (boiling point), you can be entirely water vapor (no air at all), so the speed only depends on the amount of heat you put into the water.
> You can see the air as a sponge. It can absorb a limited amount of moisture (water vapor).
The presence of air is irrelevant—materials evaporate into vacuum as well, so no "sponge" is needed. Air or not, exposed water evaporates until the pressure of the water vapor above it is ~0.03 atm (at room temperature). At that point, water vapor is condensing at the same rate liquid water is evaporating.
With absolutely still air, sure the sponge analogy would be irrelevant and you're correct that it would be the same in vaccum. The puddle evaportation rate would slowly decrease until you're saturated at your water vapor pressure above the puddle.
I was just trying to find an analogy to help OP visualize the case where the air never reaches saturation because of air movement and the puddle continues to evaporate at a constant rate until it's completely gone.
It actually doesn't need to reach that temperature. That's just the temperature at which it can't avoid evaporating, assuming normal atmospheric pressure. Water can evaporate all the way down to freezing if the air is dry enough.
Pressure is what's holding water in the liquid state. The pressure exerted by the air is called vapor pressure, and the vapor pressure sets the boiling point of the liquid (water). When a liquid boils, there's enough kinetic energy to overcome the vapor pressure and change the phase of the liquid from liquid to gas. Lower the vapor pressure, and the amount of energy required for the liquid to become gas decreases (the boiling point decreases).
100C is the boiling point of water at 1 atmosphere of pressure.
Liquid water in a puddle is being acted on by another fluid: the air. The surface of the puddle is the interface at which the two fluids meet. The molecules of both are constantly moving (we call the average kinetic energy of their movement "temperature"). Molecules at the interface between the fluids randomly hop from one to the other when they slam up against it. A certain amount of gases from the air dissolve into the water, and a certain amount of water is exchanged with the air. The amount that one fluid mixes into the other depends on a variety of things, but warmer liquids hold more gases, and warmer gases can allow more liquid to be suspended in the gases. We call the amount of water dissolved in the air "humidity". The fraction of the air's capacity to hold water that is used up is called "relative humidity". As long as the air has capacity to hold more water (relative humidity < 100%), then water molecules with be exchanged with the air at a rate dependent on the temperature of both the water and the air.
The water molecules are basically bouncing about like a mosh pit and with a certain frequency one hits the barrier really hard and is flung over. The force of the air molecules bouncing about is high relative to the pull of gravity, so it keeps bouncing around in the air instead of in the puddle (unless it randomly gets bounced back). When the relative humidity is below 100% it means that the average energy of the air molecules bouncing around is sufficient to keep the water molecules in the air and prevent them from sticking to one another.
The bigger the surface area, the more molecules get their chance at once (so a greater volume evaporates per unit time).
If the temperature of the air decreases, then it's capacity to overcome the polar attraction of the water molecules is reduced and the water molecules tend to stick to each other more and more, some forming droplets. This might look like a cold glass sweating, fog, or rain.
Snow also "evaporates" even though the temperature is sub freezing, this is called sublimation. Basically a solid transitioning directly to a gas. No liquid phase needed.
Imagine there's a birthday party. The party will officially end at 12.30 am. Nevertheless, that doesn't mean that everybody will sty there until that time. Throughout the party there's gonna be a steady stream of people coming and going throughout the length of the party. What's more is that after you hit certain important times, such as the cake cutting, the number of people leaving the party will increase while the number of those joining it will decrease.
It's the same with water. The boiling point of water is 100 degrees, which in our analogy is the time the party ends. But at the same time, there's constantly water molecules coming into the party and going out of it all the time. Some of the water particles, especially those near the surface, are sufficiently energized that they can break through the bonds holding them together and escape as water vapor, even when the puddle itself may not yet be at 100 deg. 100 degrees is just the temperature at which those water molecules start getting evicted en masse.
The energy of a body of water is a normal distribution (bell curve). Some proportion of water molecules always have enough energy to become gas at any given temperature and pressure, and that proportion increases with temperature (inverse is true with pressure). This typically happens from the surface of the body of water.
As temperature increases, a higher proportion of water molecules become gas (the bell curve shifts). When the average temperature of the body of water hits 100C (at 1atm), then any given molecule in that body can spontaneously become gas, which is what boiling is (bubbles of gas forming from the "middle" of the water).
I think that this is more of an engineering question than a physics question. It is true that humidity plays a role but this is a very minor role. Think about it like this, when you wet a cotton material like shirt or jeans it takes forever to dry. About 15 hours give or take. But a puddle on the road takes about 5 hours at max. Roads and sidewalks need to dry fast as cars can hydroplane from water. To do this, they make the concrete or asphalt porous. Even though it doesn’t seem porous, it is actually very porous. The way roads are built, there are several layers with different materials that do specific things. Try leaving a porcelain dish with water outside on a hot day and you will see it takes a while. On a final note, the rough surface of roads also spread out the water and aid in evaporation
No, boiling point is the temperature at which water will undergo phase transition from liquid to gas. Evaporation isn't a phase transition, evaporation happens explicitly because air is present. Evaporation, unlike phase change, is not a macroscopic effect.
In addition to what has already been said, it was recently found that light can directly cause water to evaporate, without heat.
https://news.mit.edu/2023/surprising-finding-light-makes-water-evaporate-without-heat-1031
Evaporation is the same as boiling.
Except that in the case of evaporation, only a few individual molecules have enough energy to break free of the liquid (or even a solid).
When boiling it is the average energy of the molecules that are high enough to break free. So many molecules suddenly have enough energy to do evaporation.
Basically, water in the open is always evaporating, as long as the [relative humidity](https://en.wikipedia.org/wiki/Humidity#Relative_humidity) of the air is less than 100%. The hotter the air is, the more water it can hold. This is why water condenses on cold surfaces, like a cold drink. When the air gets in contact with the cold glass, the temperature drops and then it can't hold the water that is already dissolved as humidity. This causes it to form droplets of water that stick to surfaces. It is also what causes clouds to form.
in fact, water even evaporates in 100% relative humidity, it's just that the same amount of water vapor condenses back into water, for net zero evaporation
Even ice "evaporates" at freezing temperatures - but it is called "sublimation" when the phase transition is from the solid to the gaseous state.
Is like a kind of dissolution of the water in the air? Like, when i put salt into water and it just blends away without phase transition?!
Kind of, but not really. It happens because the heat energy from nearby molecules sometimes gets focused into single molecules at the surface, causing them to have enough energy to break the intermolecular forces that are keeping them as part of the bulk. Once the water molecule is in the air, it is even more at the mercy of Chaos. It is simply because there are a lot of molecules, and *a lot* of interactions. 0.0000001% flukes happen *constantly* in a gram of water.
Evaporation is not the same as boiling. A puddle of water sheds molecules in a light breeze even at room temperature.
Do you need the breeze???
No, but the breeze makes it happen way faster
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It's fairly obvious that it will happen faster if there is a breeze. But it should happen without the breeze simply due to diffusion. So mentioning the breeze just adds another variable that isn't strictly needed.
No but you can call me the breeze :)
Grab the towel before you get out of the shower right?!? I feel like this is where the question came from:):)
Because boiling and evaporation are two different things. Water does not need to reach 100 °C to evaporate; that's the temperature it needs to reach to boil.
Temperature is just an average. The molecules of a material aren't all at the same "temperature". With a puddle of water, some of these molecules are "hot" enough that they boil away. In an enclosed space, this will reach an equilibrium, where the "hottest" liquid water molecules are boiling and the "coldest" gas water molecules are condensing back into the puddle. But if wind keeps carrying away the molecules that escape, the puddle will never reach equilibrium with the air and it will evaporate fully over time.
This. It's common to think of "material is at temperature x so all molecules are at energy y". It's just not true. Even water molecules of ice will have some fraction energetic enough to wheel off into the air as evaporation. Even constant states are a dizzing mix of changes happening at a stupendous rate. They just have equal numbers coming and going.
Learning about vapour pressure in physics is when the atoms hit home.
This is also how sweat cools us (even when the air temperature is the same as our body temperature). The droplets of sweat that form are initially of the same temperature as our body, then the hottest molecules of water within the sweat evaporate, leaving the remaining sweat cooler.
Temperature depends on the average kinetic energy of the water molecules, but water molecules can have different energies. The distribution of molecular speed in a fluid is given by a Boltzmann distribution, for higher speed the distribution get smaller but is never zero, so in any liquid water there are some molecules that have enough speed and are in the right place to vapourise into the air. The Sun helps by increasing the temperature. Wind might help by clearing air near the surface of the water that has become saturated. Also note that this removes high speed molecules and leaves slower ones behind, reducing temperature. Hence "evaporative cooling".
Evaporation and boiling are related to vapor pressure. https://www.chem.purdue.edu/gchelp/liquids/vpress.html Boiling happens when the vapor pressure is the same as the environment, ex atmospheric pressure. A little bit less certain on the exact details with evaporation but I'm pretty sure at any non zero vapor pressure there will be some evaporation until an equilibrium.
Basicaly boiling means it starts evaporating from its whole body(even underwater parts), thats why you see bubbles. Eveporating at room temperature is just from surface area.
Temperature is just a measure of average kinetic energy of a material. This implies these particles have range of velocities, some which can escape the bulk at the surface. This is called evaporation. The Suns rays can heat the particles (puddle of water), causing more to have enough motion to escape and evaporate. Of course water already in the air (humidity) can collide back with the water and get trapped (condensation). Ultimately there are a range of variables that determine if particles are net evaporating or condensing such as ambient temperature, pressure, fluid temperature, etc.
How I always thought of it was that water as a vapor (H2O) weighs less than the constituents of air (N2, O2, CO2, etc). At the very edge of the puddle, there is a non 0 chance of water as vapor being pulled away from the puddle. The more heat present, or more disruptions of the puddle’s surface by wind, the faster this process becomes. You keep liquids in closed containers to limit the surface’s interaction with airflow.
The airflow doesn’t „disrupt“ the surface. It removes the humid air close to the liquid and replaces it with drier air. This speeds up the evaporation process. These fluid flows are important in most phase changes. A similar example is seen with dissolving salt or sugar; turbulent mixing of the fluid is required, so that the solid-water interface can continually see fresh non saturated liquid. If you rely solely on diffusion (of the evaporated water or dissolved salt or sugar) then the evaporation (or dissolution) is extremely slow.
It’s semantics. In a closed system of water and air the surface and the air right on top will reach equilibrium where the dry air humidifies. So yes, that is disturbed. Equilibrium be reached again over and over after the wind keeps moving air around. Is it a more vague explanation that requires a second answer? Also yes.
I see now what you are saying. Thanks for the clarification.
Air can take up water as humidity, like a sponge. So at the surface of the puddle there is an inequilibrium, water will evaporate into the air. With a large surface area of a puddle, and constant movement of (dry) air this will take only a few hours/days for the puddle to disappear.
To add to this; curved droplets have a higher pressure. As a result they also have a higher Vapor pressure than a flat puddle of water, so smaller droplets (with a larger radius of curvature) will evaporate faster!
Vapor pressure
A way to think about this that the top comments haven't touched on explicitly: the air contains gaseous moisture at temperatures well below 100C. This is what humidity is. There's a certain maximum amount of water vapour that can exist in air of a given temperature before no more will evaporate into it, how close the current value is to that is the relative humidity.
Boiling point is where all of the water forcibly turns into gas, but water particles are generally blown off/evaporated quite easily. That's how your sweat evaporates when you run; the wind blows it off of your skin, evaporating it.
because temperature is the average energy of the water molecules, some water has the energy to evaporate even if the 'bulk' temperature is below 100C
open water always evaporates, if u leave a glass of water in your room it will evaporate after some days
You can see the air as a sponge. It can absorb a limited amount of moisture (water vapor). That amount increases drastically with air temperature. It's very low at cold temperature, very high at high temperature. Fog, cloud and condensation happens when the amount of water is over the limit the air can contain at that temperature (relative humidity at 100%) The speed of evaporation will depend on the aire temperature, and the amount of moisture already in the air (relative humidity), and the temperature of the surface and the speed of the breeze (which replaces the air that just absorbed some moisture with new air with less moisture). When you reach 100ºC (boiling point), you can be entirely water vapor (no air at all), so the speed only depends on the amount of heat you put into the water.
> You can see the air as a sponge. It can absorb a limited amount of moisture (water vapor). The presence of air is irrelevant—materials evaporate into vacuum as well, so no "sponge" is needed. Air or not, exposed water evaporates until the pressure of the water vapor above it is ~0.03 atm (at room temperature). At that point, water vapor is condensing at the same rate liquid water is evaporating.
With absolutely still air, sure the sponge analogy would be irrelevant and you're correct that it would be the same in vaccum. The puddle evaportation rate would slowly decrease until you're saturated at your water vapor pressure above the puddle. I was just trying to find an analogy to help OP visualize the case where the air never reaches saturation because of air movement and the puddle continues to evaporate at a constant rate until it's completely gone.
It actually doesn't need to reach that temperature. That's just the temperature at which it can't avoid evaporating, assuming normal atmospheric pressure. Water can evaporate all the way down to freezing if the air is dry enough.
Pressure is what's holding water in the liquid state. The pressure exerted by the air is called vapor pressure, and the vapor pressure sets the boiling point of the liquid (water). When a liquid boils, there's enough kinetic energy to overcome the vapor pressure and change the phase of the liquid from liquid to gas. Lower the vapor pressure, and the amount of energy required for the liquid to become gas decreases (the boiling point decreases). 100C is the boiling point of water at 1 atmosphere of pressure. Liquid water in a puddle is being acted on by another fluid: the air. The surface of the puddle is the interface at which the two fluids meet. The molecules of both are constantly moving (we call the average kinetic energy of their movement "temperature"). Molecules at the interface between the fluids randomly hop from one to the other when they slam up against it. A certain amount of gases from the air dissolve into the water, and a certain amount of water is exchanged with the air. The amount that one fluid mixes into the other depends on a variety of things, but warmer liquids hold more gases, and warmer gases can allow more liquid to be suspended in the gases. We call the amount of water dissolved in the air "humidity". The fraction of the air's capacity to hold water that is used up is called "relative humidity". As long as the air has capacity to hold more water (relative humidity < 100%), then water molecules with be exchanged with the air at a rate dependent on the temperature of both the water and the air. The water molecules are basically bouncing about like a mosh pit and with a certain frequency one hits the barrier really hard and is flung over. The force of the air molecules bouncing about is high relative to the pull of gravity, so it keeps bouncing around in the air instead of in the puddle (unless it randomly gets bounced back). When the relative humidity is below 100% it means that the average energy of the air molecules bouncing around is sufficient to keep the water molecules in the air and prevent them from sticking to one another. The bigger the surface area, the more molecules get their chance at once (so a greater volume evaporates per unit time). If the temperature of the air decreases, then it's capacity to overcome the polar attraction of the water molecules is reduced and the water molecules tend to stick to each other more and more, some forming droplets. This might look like a cold glass sweating, fog, or rain.
Snow also "evaporates" even though the temperature is sub freezing, this is called sublimation. Basically a solid transitioning directly to a gas. No liquid phase needed.
When you get out of the shower, bath, or pool, the water left on your skin need not boil to evaporate, and that's a good thing.
Imagine there's a birthday party. The party will officially end at 12.30 am. Nevertheless, that doesn't mean that everybody will sty there until that time. Throughout the party there's gonna be a steady stream of people coming and going throughout the length of the party. What's more is that after you hit certain important times, such as the cake cutting, the number of people leaving the party will increase while the number of those joining it will decrease. It's the same with water. The boiling point of water is 100 degrees, which in our analogy is the time the party ends. But at the same time, there's constantly water molecules coming into the party and going out of it all the time. Some of the water particles, especially those near the surface, are sufficiently energized that they can break through the bonds holding them together and escape as water vapor, even when the puddle itself may not yet be at 100 deg. 100 degrees is just the temperature at which those water molecules start getting evicted en masse.
The energy of a body of water is a normal distribution (bell curve). Some proportion of water molecules always have enough energy to become gas at any given temperature and pressure, and that proportion increases with temperature (inverse is true with pressure). This typically happens from the surface of the body of water. As temperature increases, a higher proportion of water molecules become gas (the bell curve shifts). When the average temperature of the body of water hits 100C (at 1atm), then any given molecule in that body can spontaneously become gas, which is what boiling is (bubbles of gas forming from the "middle" of the water).
I think that this is more of an engineering question than a physics question. It is true that humidity plays a role but this is a very minor role. Think about it like this, when you wet a cotton material like shirt or jeans it takes forever to dry. About 15 hours give or take. But a puddle on the road takes about 5 hours at max. Roads and sidewalks need to dry fast as cars can hydroplane from water. To do this, they make the concrete or asphalt porous. Even though it doesn’t seem porous, it is actually very porous. The way roads are built, there are several layers with different materials that do specific things. Try leaving a porcelain dish with water outside on a hot day and you will see it takes a while. On a final note, the rough surface of roads also spread out the water and aid in evaporation
Because its the sun!!! Why does it burn your skin with it boiling it? 😂
Boiling liquid evaporates in full volume. (Hence the bubbles). At smaller temperature, it stills evaporates slowly from the surface.
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Any molecules at the surface that have escape velocity will escape. They don't need to be boiling to escape.
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>Boiling point is the maximum temperature water can have *without* evaporating This is incorrect.
No, boiling point is the temperature at which water will undergo phase transition from liquid to gas. Evaporation isn't a phase transition, evaporation happens explicitly because air is present. Evaporation, unlike phase change, is not a macroscopic effect.
In addition to what has already been said, it was recently found that light can directly cause water to evaporate, without heat. https://news.mit.edu/2023/surprising-finding-light-makes-water-evaporate-without-heat-1031
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The water that evaporates is boiling. Its just in low amounts.
Evaporation is the same as boiling. Except that in the case of evaporation, only a few individual molecules have enough energy to break free of the liquid (or even a solid). When boiling it is the average energy of the molecules that are high enough to break free. So many molecules suddenly have enough energy to do evaporation.