Reef Chemistry Question of the Day #150 Evaporation

[Randy Holmes-Farley]

Reef Chemistry Question of the Day [HASHTAG]#150[/HASHTAG]

Water will evaporate slowest from which of the following solutions, assuming they are all equally exposed to the same air at the same temperature?

A. Pure fresh RO/DI water at 0 ppm TDS.
B. Tap water at 145 ppm TDS
C. Hyposaline seawater used to treat fish, with a specific gravity of 1.009
D. Seawater at 35 ppt salinity
E. The calcium part of a two part system that has a calcium concentration of 37,000 ppm calcium.

Bonus: explain why.

Good luck!




































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[beaslbob]

E
the more junk in the water the slower it evaporates

 

[Cory]

I really dont know but will guess e also.

 

[Mrezy33]

A watched pot never boils.

 

[KingBlingTX]

The Calcium concentrate due to colligative properties. The higher concentration of ions elevates the boiling point of the solution. The type of ions doesn't matter; only the concentration.

 

[zzzdiver]

E

 

[Ontheway]

It might be related to surface tension. May the liquids with weaker surface tension tend to evaporate easier? This specific liquid should also be able to create a good foam. My vote to 35 ppt water, D.

 

[Ontheway]

(Seems I answered for the fastest evoporating )

 

[Randy Holmes-Farley]

And the answer is...E. The calcium part of a two part system that has a calcium concentration of 37,000 ppm calcium.

The more ions or molecules in a system other than water, the more the vapor pressure is reduced. Raoult's Law indicates that the vapor pressure lowering is directly related to the number of ions and molecules in the water aside from the water. While it is just an approximation for any real solution, it is good enough for this purpose since E has far more ions in it than any other solution in the question.

Interestingly, when people try to test this at home or in school chemistry labs, they often find that salt solutions evaporate a bit faster than pure water. The explanation is likely that, as we all see for ourselves, salt solutions can form salt creep along the sides of a container, and that wet salt provides additional surface area for water to evaporate from, which tends to increase the rate of evaporation.

Happy Reefing.