Reef Chemistry Question of the Day #256 Evaporation Rate Fresh vs Salt

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #256

If all other things are identical (air flow, water flow, temperatures, organic surface films, etc.) except the salinity, how do the water evaporation rates from freshwater and seawater aquariums compare?

A. Salt water evaporates faster
B. Fresh water evaporates faster
C. They will be the same

Good luck!








































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[dragon99]

B. Freshwater wins

 

[Maggie321]

I have both salt and fresh... the saltwater evaporates way faster... I don't know why, but I bet you do!

 

[timnem70]

Yep. Fresh. Its the chemical makeup of salt that takes more energy for It to evaporate. Although, seems as if I am filling or topping off my reef much more than I did 30 years ago with fresh (so thos may be a trick questiin)You can get really low in tank water with Fresh before filling it back up as other than quicker temperature fluctuations, the water that evaporates in our reefs could be deadly since the water is what actually evaporates and will leave you with really high SG. as the salt volume stays the same but the water is gone. Hence why we don't fill the ATO with saltwater, just RODI.

 

[timnem70]

So, my answer was fresh but I believe water will evaporate at the same rate? So...D?

 

[Klwheat]

Hmm...let’s guess. If I add salt to water I’m trying to boil, it decreases the boiling point. So, maybe saltwater will evaporate faster.

 

[JimWelsh]

"Paging Monsieur Raoult. François-Marie Raoult, please pick up the White Courtesy Phone...."

 

[Randy Holmes-Farley]

Hmm...let’s guess. If I add salt to water I’m trying to boil, it decreases the boiling point..

Does it?

 

[Maggie321]

My theory... same volume of saltwater and freshwater... the salt takes up space, therefore less h2o in the same volume. The same amount of evaporation requires more top offs in salt because there is less actual water in the same space.

 

[siggy]

it decreases the boiling point

I believe it conducts heat efficiently due to more free electrons(energy) fresh is faster so B SW requires more energy to release h2o.

 

[jlfalin]

Coligative properties. Adding salt lowers the freezing point and increases the boiling point. For freezing, it does this by inhibiting crystalization of the water by preventing contact of the molecules. For boiling, the salt ions block the surface and prevent water molecules from escaping to the gas state. Using this idea salt should prevent water from escaping the salt water and therefore slow evaporation. Therefore, I say freshwater evaporates more quickly.

 

[Js.Aqua.Project]

Freshwater would evaporate faster per research comparing Fresh water and salt water solutions:

"Under conditions of constant water temperature, the effects exhibited by each of the variables were found to be related to vapor pressure. For each of the variables the following was noted:
A. Air temperature - as air temperature increased, evaporation decreased.
B. Relative humidity - as relative humidity increased, evaporation decreased.
C. Salt concentration - as salt concentration increased, evaporation decreased.
D. Wind speed - as wind speed increased, evaporation increased."

https://www.twdb.texas.gov/publications/reports/numbered_reports/doc/R77/R77.pdf Page 64

 

[GSPClown94]

Freshwater should evaporate faster, I vaguely remember one of my professors mentioning something about the bonds between the salt and water molecules being stronger than the bonds between just water molecules.

 

[FailingUpwards]

Freshwater would evaporate faster per research comparing Fresh water and salt water solutions:

"Under conditions of constant water temperature, the effects exhibited by each of the variables were found to be related to vapor pressure. For each of the variables the following was noted:
A. Air temperature - as air temperature increased, evaporation decreased.
B. Relative humidity - as relative humidity increased, evaporation decreased.
C. Salt concentration - as salt concentration increased, evaporation decreased.
D. Wind speed - as wind speed increased, evaporation increased."

https://www.twdb.texas.gov/publications/reports/numbered_reports/doc/R77/R77.pdf Page 64

But then paragraphs 2&3 of page 25 contradict that as they say "The data presented in Tables 1 and 2 do not readily allow direct comparison of evaporation rates as solution temperatures were not constant; however, certain trends can be noted".
Also, "The effect of salt concentration is not clearly evident and appears erratic."

If you dig into the tables you'll note that the measured evaporation rates for the same day, you have a higher evaporation for 50ppm than H2O for the 90 degree bath, but at 76 degrees, higher evaporation rates on some with the NaCl solutions.

The data isn't really conclusive and I would disagree with the conclusion listed above.

My gut feeling with all variables removed (same temp, same air flow, same container, etc.) the evaporation rate will be the same as it's only H2O that is evaporating

 

[Greenstreet.1]

I say B cause if I remember correctly as the salinity rise it reduces water vapor pressure at the surface.
B[emoji848][emoji848]final answer.

 

[Frizzles7]

My thought is that it takes more energy to separate the salt from the water than to just separate water molecules. If more energy is needed, then all other things equal, the saltwater will evaporate more slowly. Therefore B is my answer.

 

[Chuk]

Salt decreases the boiling pt/ vapor pressure of water so I vote saltwater

 

[etmanning1]

My stance on this:

FW evaporates more quickly due to it being purer BUT saltwater tanks seem to evaporate more because of increased surface area agitation.

 

[Alfrareef]

A. More evaporation area...

 

[Lousybreed]

Reef Chemistry Question of the Day #256

If all other things are identical (air flow, water flow, temperatures, organic surface films, etc.) except the salinity, how do the water evaporation rates from freshwater and seawater aquariums compare?

A. Salt water evaporates faster
B. Fresh water evaporates faster
C. They will be the same

Good luck!








































.

Freshwater evaporates faster. If that said freshwater lake was at 5,000 ft it would evaporate even faster. The ionic interactions with salt water reduce evap rate. Great question Randy! Also the viscosity of salt water is slightly higher than freshwater for the same reasons. Disclaimer: I was an engineer at a solar salt plant in California. Having a thriving population of brine shrimp also increases evap in salt water due to the food chain they create