Freezing Point Sea water

dratsea

dratsea

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We put salt on the roads to melt the ice so I know that the salt dissolved in the sea water will depress the freezing point and, at least down here in the South, I can leave the salt water circuits full over winter. Well I have done that for the last 15 years and not had a problem. What do you do? And if, like me, you have relied on the effect of the salt on the freezing point do you know the freezing point of seawater, the temperature seawater will begin to freeze?






-5C






-7C





I guessed it was somewhere thereabouts but in truth it is actually -1.91C for North Atlantic water (and in North Pacific, which is a bit less salty, is -1.90C). We have forcasts of -5C tonight so just been down and drained last dregs from the raw water pumps, heat exchangers and raw water strainers. Hope to sleep a bit better tonight.

John
 
I can't answer your question but can remember from my school physics that 0 degrees F was set as the lowest temperature reached when ice and salt are mixed.

100 degrees was supposed to be the temperature of human blood - Snr Fahrenheit got that slightly wrong!
 
I received the following reply from VicS,following a query I had, which I found to be very helpful.

"Don't worry unduly. Sea water does not freeze in the say way that fresh water does ie solid at 0C
Sea water starts to freeze at about -2C but the ice that forms is pure ice, ie no salt, so the salt concentration in the water that's left is increased. As it increases so the freezing temperature falls and the water does not finally freeze solid until considerably colder.
I don't think I have sen a figure for seawater but it is about -21C for a salt solution."
 
A lot will depend on whether the boat is in or out of the water. The air temp outside may get down to -5 but if the boat is sitting in relatively warm seawater then inside the hull should stay above outside air temp.
 
Even if the boat is out of the water the mass of the engine itself and the boat and air within it will often see you through a major temperature dip before the water in the engine gets near to freezing.
I got caught out - hadn't winterised my boat on the hardstanding before the freeze set in.
The plastic pipe leading up to the engine was to all intents and purposes frozen solid and I had to take the pipe off and beat the ice out of it, but the engine itself was still ice free (Phew!)
Engine now sitting with any remaining pockets of water well diluted with antifreeze (ran it for 30 Mins collecting exhaust water, treating it, and passing it back to the pump suction)
Also undisturbed water often supercools - particularly if it's reasonably clean.
I had a bottle of 100% liquid fresh water sitting in the cabin on the same day - when I gave it a nudge I could watch the ice crystals forming in seconds in the bottle until it was 75/25% ice/water. - So don't shake your engine!
 
Frozen Over

Ipswich Haven was frozen the other week .. It was well past lunch time before you could see the water moving again ..

Woodbridge was frozen last year so we could be in for the start on an ICE AGE :eek:
 
The freezing point of seawater varies with salinity which varies considerably in different seas. It also varies with pressure and so changes a little with depth, although the question relates to surface freezing. The extremely low temperatures for salt water freezing mentioned above may apply for saturated salt solutions but certainly not for seawater which is relatively dilute. A typical sea surface freezing point would be in the region of -2C. At a pressure of 10 atmospheres, the same salt solution would freeze at around -2.6C.
 
There was a time when Dover Council used to hose the roads down with seawater until one year it froze -late fifties or early sixties if I remember.
 
The freezing point of seawater varies with salinity which varies considerably in different seas.
Click to expand...
That is why in my original post on the subject, which Puddock so conveniently quotes above, I said about -2C rather than any more precise figure.

However it is true that salt water, even a solution as dilute as seawater, does not completely freeze until it reaches a very much lower temperature than the temperature at which ice starts to form. For a pure salt solution -21.1C at standard pressure. The reason being that ice and salt behave as simple eutectic system

Saltandwater.jpg

The ice forming when a dilute solution (anything less than 23.3%) is cooled concentrates the salt in the remaining solution, the process of ice formation and salt concentration continuing until the last remaining solution contains 23.3% of salt finally freezes at -21.1C.

A detailed and easy to follow explanation and intereptation of the phase diagram can be found at
http://www.chemguide.co.uk/physical/phaseeqia/saltsoln.html

I do realise that the above refers to pure salt solutions rather than to seawater and would be interested if anyone can come up with a link to the same information specifically for seawater.
 
Sea water ice.

Ipswich Haven Marina has a freshwater spring just off the harbour office so the freshwater will show some freezing but other areas will remain ice free ( near the lock ), until such time that the temperature of the surface water reaches - 20s.
As posted above, if your pride and joy is in the water, there is a degree or two of warming from that sea water. This is ok for us darn sarf but it must be getting close to critical temperature up norf.
 
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