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When boiling water, how is evaporation rate affected by the surface area of the container?
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+ Thermodynamics
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Mark Harder
When boiling water, how is evaporation rate affected by the surface area of the container?
Boiling water
When you boil water on a stove, the temperature on the bottom of the pot is higher than the boiling point. Steam bubbles are generated, and they help to heat the water at the top of the pot (convection also helps, and conduction of the pot itself). Once you have a rolling boil, the rate is determined by how quickly you can supply more heat (almost all of it now goes into the phase transition). If you use the same burner, and mostly heat the pot (so no tiny pot on a big burner), at that stage the rate of boiling off should be the same.
Evaporating water
When the temperature of the water is below boiling, the mechanism is different. Rather than steam displacing the air above the water, single water molecules break away from the liquid and mix with the air. Evaporation rates depend on the partial pressure of water just above the surface (this influences condensation, i.e. the reverse reaction), the surface area, and the vapor pressure of water in the liquid state. Anything that lowers the partial pressure of water (such as "wind" bringing in dry air), increases the surface area (different pot geometry) or increases the vapor pressure (temperature) will increase net evaporation rates.
Beaker vs Erlenmeyer
Tall vs. wide pot changes the contact area with the stove top and the area of the liquid:gas interface at the same time. If you compare a beaker with an Erlenmeyer (same bottom area), it should show similar boiling rates (maybe a big mess with overboiling for the Erlenmeyer) but very different evaporation rates.
When you boil water on a stove, the temperature on the bottom of the pot is higher than the boiling point. Steam bubbles are generated, and they help to heat the water at the top of the pot (convection also helps, and conduction of the pot itself). Once you have a rolling boil, the rate is determined by how quickly you can supply more heat (almost all of it now goes into the phase transition). If you use the same burner, and mostly heat the pot (so no tiny pot on a big burner), at that stage the rate of boiling off should be the same.
Evaporating water
When the temperature of the water is below boiling, the mechanism is different. Rather than steam displacing the air above the water, single water molecules break away from the liquid and mix with the air. Evaporation rates depend on the partial pressure of water just above the surface (this influences condensation, i.e. the reverse reaction), the surface area, and the vapor pressure of water in the liquid state. Anything that lowers the partial pressure of water (such as "wind" bringing in dry air), increases the surface area (different pot geometry) or increases the vapor pressure (temperature) will increase net evaporation rates.
Beaker vs Erlenmeyer
Tall vs. wide pot changes the contact area with the stove top and the area of the liquid:gas interface at the same time. If you compare a beaker with an Erlenmeyer (same bottom area), it should show similar boiling rates (maybe a big mess with overboiling for the Erlenmeyer) but very different evaporation rates.
for an example how phase transitions help in heat transfer.More
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There are multiple effects which differ in both cases.
Firstly, convection (e.g. "wind") in the air will be much more efficient in the wide container. The tall container will limit the quantity of air flowing just above the surface. This, in turns, means that the water that does evaporate will remain for longer just above the surface of the water, and has more chances of returning to solution.
Secondly, the container itself will act as condenser. As the metal is typically cooler than the water vapour, the metal can cool down the vapour and turning it back into water, which conveniently flows back into the pot. The more exposed surface you have around the boiling water, the more important this effect will be.
You might also consider the power of the stove itself. This is hard to account for, but briefly, there might be some differences in the heat diffusion in the water.
There are multiple effects which differ in both cases.
Firstly, convection (e.g. "wind") in the air will be much more efficient in the wide container. The tall container will limit the quantity of air flowing just above the surface. This, in turns, means that the water that does evaporate will remain for longer just above the surface of the water, and has more chances of returning to solution.
Secondly, the container itself will act as condenser. As the metal is typically cooler than the water vapour, the metal can cool down the vapour and turning it back into water, which conveniently flows back into the pot. The more exposed surface you have around the boiling water, the more important this effect will be.
You might also consider the power of the stove itself. This is hard to account for, but briefly, there might be some differences in the heat diffusion in the water.
See my comment. It is really tricky and engineering oriented. Water in a baking tin might even not boil, and in a very extended ideal one can fast evaporate. It is really complicated, at least in my opinion. Obviously there are parts of truth in your answer, at least listing the parameters, or some of the parameters, involved.More
Boiling water
When you boil water on a stove, the temperature on the bottom of the pot is higher than the boiling point. Steam bubbles are generated, and they help to heat the water at the top of the pot (convection also helps, and conduction of the pot itself). Once you have a rolling boil, the rate is determined by how quickly you can supply more heat (almost all of it now goes into the phase transition). If you use the same burner, and mostly heat the pot (so no tiny pot on a big burner), at that stage the rate of boiling off should be the same.
Evaporating water
When the temperature of the water is below boiling, the mechanism is different. Rather than steam displacing the air above the water, single water molecules break away from the liquid and mix with the air. Evaporation rates depend on the partial pressure of water just above the surface (this influences condensation, i.e. the reverse reaction), the surface area, and the vapor pressure of water in the liquid state. Anything that lowers the partial pressure of water (such as "wind" bringing in dry air), increases the surface area (different pot geometry) or increases the vapor pressure (temperature) will increase net evaporation rates.
Beaker vs Erlenmeyer
Tall vs. wide pot changes the contact area with the stove top and the area of the liquid:gas interface at the same time. If you compare a beaker with an Erlenmeyer (same bottom area), it should show similar boiling rates (maybe a big mess with overboiling for the Erlenmeyer) but very different evaporation rates.
Boiling water
When you boil water on a stove, the temperature on the bottom of the pot is higher than the boiling point. Steam bubbles are generated, and they help to heat the water at the top of the pot (convection also helps, and conduction of the pot itself). Once you have a rolling boil, the rate is determined by how quickly you can supply more heat (almost all of it now goes into the phase transition). If you use the same burner, and mostly heat the pot (so no tiny pot on a big burner), at that stage the rate of boiling off should be the same.
Evaporating water
When the temperature of the water is below boiling, the mechanism is different. Rather than steam displacing the air above the water, single water molecules break away from the liquid and mix with the air. Evaporation rates depend on the partial pressure of water just above the surface (this influences condensation, i.e. the reverse reaction), the surface area, and the vapor pressure of water in the liquid state. Anything that lowers the partial pressure of water (such as "wind" bringing in dry air), increases the surface area (different pot geometry) or increases the vapor pressure (temperature) will increase net evaporation rates.
Beaker vs Erlenmeyer
Tall vs. wide pot changes the contact area with the stove top and the area of the liquid:gas interface at the same time. If you compare a beaker with an Erlenmeyer (same bottom area), it should show similar boiling rates (maybe a big mess with overboiling for the Erlenmeyer) but very different evaporation rates.
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There are multiple effects which differ in both cases.
Firstly, convection (e.g. "wind") in the air will be much more efficient in the wide container. The tall container will limit the quantity of air flowing just above the surface. This, in turns, means that the water that does evaporate will remain for longer just above the surface of the water, and has more chances of returning to solution.
Secondly, the container itself will act as condenser. As the metal is typically cooler than the water vapour, the metal can cool down the vapour and turning it back into water, which conveniently flows back into the pot. The more exposed surface you have around the boiling water, the more important this effect will be.
You might also consider the power of the stove itself. This is hard to account for, but briefly, there might be some differences in the heat diffusion in the water.
There are multiple effects which differ in both cases.
Firstly, convection (e.g. "wind") in the air will be much more efficient in the wide container. The tall container will limit the quantity of air flowing just above the surface. This, in turns, means that the water that does evaporate will remain for longer just above the surface of the water, and has more chances of returning to solution.
Secondly, the container itself will act as condenser. As the metal is typically cooler than the water vapour, the metal can cool down the vapour and turning it back into water, which conveniently flows back into the pot. The more exposed surface you have around the boiling water, the more important this effect will be.
You might also consider the power of the stove itself. This is hard to account for, but briefly, there might be some differences in the heat diffusion in the water.
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