Reef Chemistry Question of the Day #74 Frozen Seawater

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #74

You are keeping a gallon container of seawater in your garage as a back up emergency supply.

Unfortunately, you forgot that it may freeze in your unheated garage. You recall that salt in water lowers the freezing point, but you have trouble remembering by how much?

At what temperature would the gallon of seawater first become more than 90% frozen?

A. 31°F
B. 28°F
C. 24°F
D. 15°F
E. 5°F
F. Below 0°F

By the way, Google is seriously in error on this question. If you Google "freezing point of seawater" they mistakenly pull out data for most dense seawater and post 39.2 degrees F in big letters, which is obviously incorrect.

Good luck!








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[Vanessa O.]

As a newbie, I have no clue.

 

[Randy Holmes-Farley]

As a newbie, I have no clue.

That's OK. It's probably not something you'd typicaly learn keeping a tank.

 

[beaslbob]

no clue here. A sounds as good as any.

 

[ajones2n]

Below zero

 

[tonizzy22]

Answer B at sea level

 

[Zero Nitrates]

Going with C

 

[bidds]

B: 24

 

[bidds]

B: 28

 

[Rybren]

The freezing point of seawater is 28.4 degrees F. I doubt that the gallon would be 90% frozen at that temp and have no idea how much colder it would have to be to get to that state (or how long it would have to remain at 28.4 to get to 90% frozen)

It's currently 4 degrees F outside, maybe I should grab a gallon and see how long it would take to get to ~90% frozen.

 

[seargentslice]

B. 28

 

[Rybren]

Just for grins, I placed 1G of 35ppt water outside on my back deck. The water was 68F and the outside temp was 4F. After 2 hours, there is no sign of freezing yet. The air temp has risen to 8F and the water temp has dropped to 32F

 

[beaslbob]

Below zero

in degrees C that would absolutely be true

 

[ajones2n]

I was going for the 90% frozen. At zero it would definitely be 90% frozen

 

[Rybren]

The water temp has dropped to 26F and a very thin skin of ice is starting to form on the surface around the edges of the container.

 

[cdness]

Well living in Fargo, ND and seeing some below zero temps right now, I know it would be 90% frozen at 0 degrees and lower... However where it would FIRST freeze to 90% is a little tougher. Given enough time, C or D should do it...

 

[Rybren]

Well, ~5 hours have gone by. The thermometer is still reading 26F and the container is starting to freeze solid. The outside air temp has risen to 15F.

The thermometer is just a cheapie plastic and glass wall unit that hasn't been calibrated, so there's a good chance that the water temp really is 28.4 degrees.

 

[Randy Holmes-Farley]

Here's a hint:

As the seawater begins to freeze, the salts are mostly excluded from the ice, becoming more concentrated in the remaining liquid.

 

[s2nhle]

Very interesting to know. The salt can melt the ice. So it must be pretty cold for the 90% of saltwater to freeze. so I guess it is somewhere between D and E.

 

[Rybren]

Here's a hint:

As the seawater begins to freeze, the salts are mostly excluded from the ice, becoming more concentrated in the remaining liquid.

That's what I would have thought - driving the temp even lower than 28.

My test gallon has been outside for almost 13 hours and the ambient temperature didn't make it above 15F. The liquid temp inside the container is still at 25-26 degrees. The ice is about 1/2" thick and slushy, so its nowhere near as solid as fresh water would be at this point. I would guess that the container is only ~10% frozen at the most.

Unfortunately, the temperature is supposed to warm up overnight and should be at 32 by morning.