[GrumpyShedMonster (OP)]

One for a cold morning: I'd run out of de-icer, and the frost was really tough to scrape off.  When melting the ice off my car I thought if I just add a couple of tablespoons of salt to a quart of warm water - that should both melt the ice and prevent refreezing - as I think sea water freezes at -21°C, and its hardley ever that cold.

No - that wasn't enough salt - It soon refroze. 

What would the correct proportions of salt to water be to get that seawater -21°C freezing temperature?

[LeeE]

This wikipedia subsection:

http://en.wikipedia.org/wiki/Sea_water#Salinity

starts by saying:

Although the vast majority of seawater has a salinity of between 3.1% and 3.8%, seawater is not uniformly saline throughout the world

Although it doesn't say what that percentage actually means i.e. by volume or mass, or relative salinity.

Digging a bit deeper from there should clear it up though.

[Bored chemist]

"as I think sea water freezes at -21°C"
I don't, sea water freezes at about -2C rather than -20C

[GrumpyShedMonster (OP)]

I have been confusing two facts - admittedly I was told them at school over 20 years ago.
1) Seawater is indeed about the same salinity as 2 tablespoons of salt per quart. However it freezes around -2°C as stated above.
2) A saturated solution of saltwater freezes at -21°C (and would require over 10 times as much salt - nearly half a pound)

For you metric dudes  Seawater equivalent salinity is 35g Salt per litre
Saturated salt solution around 380g per litre.

So to answer my own question:
Assuming a straight line relationship between freezing point and amount of salt dissolved (and I am not certain of this)  I would need around 5 tablespoons of salt per quart to get a mixture that wouldn't freeze on my car windows until -10°C.

[chris]

This wikipedia subsection:

http://en.wikipedia.org/wiki/Sea_water#Salinity

starts by saying:

Although the vast majority of seawater has a salinity of between 3.1% and 3.8%, seawater is not uniformly saline throughout the world

Although it doesn't say what that percentage actually means i.e. by volume or mass, or relative salinity.

In the context of solutions like this, the percentage means grams per hundred ml of water.

So a 5% glucose solution would be 5g glucose dissolved in 100 ml water (i.e. final volume of solvent + solute is 100ml)

Chris

[LeeE]

Ta for that Chris, but if you dissolved 5g of something, such as glucose, in 100ml of water, wouldn't the final volume be a tiny bit greater than 100ml?

[chris]

Ta for that Chris, but if you dissolved 5g of something, such as glucose, in 100ml of water, wouldn't the final volume be a tiny bit greater than 100ml?

Indeed it would, LeeE, which is why I wrote "final volume of solvent + solute is 100ml" - in other words you need to make up the volume to 100ml post-dissolving; chemists have fancy volumetric flasks for doing this accurately. Biologists tend to chuck the salt in and then "top it up a bit" to what looks right

Chris

[Geezer]

Biologists tend to chuck the salt in and then "top it up a bit" to what looks right

Chris

Lee, please disregard the last post from the so called "Chris". I'm pretty sure we've got an impersonator on the forum who is just trying to stir things up - no pun intended. We will try to find a solution soon - no pun intended.

[LeeE]

Well. what with dealing with living organisms, and all that, I've always thought that the 'science' of biology was bound to be a bit hit and miss at times, so that comment seemed quite reasonable to me  []

[chris]

I told 20 crap jokes yesterday to make people laugh, but no pun in ten did...

[Bored chemist]

If the chemists are being "fancy" about this sort of thing they do it by weight (I bet you will never see that on a bumper sticker).
However, I have heard that the more adventurous biologists have learned to use such complex equipment as a long necked flask with a mark on it.
This puts them one-up on physicists.