PDA

View Full Version : boiling salt vs fresh water

captnjj
06-08-2007, 10:34 AM
Which will boil faster, salt or fresh water, assuming the same heat source and similar containers?

Belrix
06-08-2007, 11:04 AM
Which will boil faster, salt or fresh water, assuming the same heat source and similar containers?
In general, adding stuff, like salt, to water lowers the freezing point and raises the boiling point.

Sea water boils at 103 degrees C.

Balthisar
06-08-2007, 11:06 AM
Salt raises the boiling point, and I assume salt water has the same (or close to the same) specific heat as non-salt water, ergo non-salt water will boil faster.

If there's some weird physics reason this isn't so, I'd kind of like to hear about it myself.

David Simmons
06-08-2007, 12:40 PM
Sea water boils at 103 degrees C. There seems to be a difference of opinion as to the boiling point of seawater. This site (http://dbhs.wvusd.k12.ca.us/webdocs/Solutions/BP-Elev-Probs.html) gives a value of 100.64 C.

In any case we can find out thusly. The specific heat of pure water is about 1. I'ts actually 1 only at a specifi temperature, which I can't remember, but it's close to 1 over a wide range of temperatures. The specific heat of seawater is about .93 according to this site. (http://www.chrismanual.com/Intro/prop.htm)

If we start with one gram of pure water at 0 C we must add 100 calories to reach 100 C. If we start with 1 gram of sea water we need 100.64*.93 =93.6 calories to reach 100.64 C. If the boiling point is 103 C then it takes 103*.93 = 95.8 calories. I believe the heat of vaporization is the same in both cases since it is only the water that vaporizes.

So the sea water should boil first in either case.

Great Dave
06-08-2007, 01:32 PM
I thought that adding salt to water gives more nucleation points for vapor to form bubbles, and hence, boil faster.

Squink
06-08-2007, 01:59 PM
Colligative properties (http://en.wikipedia.org/wiki/Colligative_properties)

GorillaMan
06-08-2007, 02:18 PM
There seems to be a difference of opinion as to the boiling point of seawater.
I presume it depends which sea you take it from - after all, water from the Dead Sea would not be representative.

Quercus
06-08-2007, 02:32 PM
There seems to be a difference of opinion as to the boiling point of seawater. This site (http://dbhs.wvusd.k12.ca.us/webdocs/Solutions/BP-Elev-Probs.html) gives a value of 100.64 C.

In any case we can find out thusly. The specific heat of pure water is about 1. I'ts actually 1 only at a specifi temperature, which I can't remember, but it's close to 1 over a wide range of temperatures. The specific heat of seawater is about .93 according to this site. (http://www.chrismanual.com/Intro/prop.htm)

If we start with one gram of pure water at 0 C we must add 100 calories to reach 100 C. If we start with 1 gram of sea water we need 100.64*.93 =93.6 calories to reach 100.64 C. If the boiling point is 103 C then it takes 103*.93 = 95.8 calories. I believe the heat of vaporization is the same in both cases since it is only the water that vaporizes.

So the sea water should boil first in either case.Excellent analysis.

I'll just add that if we're instead of equal masses, we're using equal volumes of fresh- and sea- waters, the difference narrows, as the specific gravity of seawater is about 1.025. This doesn't make enough of a difference to change the answer, though. Raising 1 cc of seawater (which is 1.025 g) from 0 C to 103 C still ends up needing only 98.2 calories.

Belrix
06-08-2007, 03:29 PM
Frankly I just googled for the temperature and came up with this site: Properties of Sea Water (http://ijolite.geology.uiuc.edu/02SprgClass/geo117/lectures/Lect15.html).

I don't have the chemistry to figure further so I'll just step to one side.

Balthisar
06-08-2007, 04:40 PM
So I was totally off. Not being a chemist, would anyone care to explain why the specific heat decreases with the addition of moderate amounts of salts? Since we're speaking "stove top" here, examples using volume instead of mass might be more meaningful.

Is it because the specific heat of the salts is lower, and they displace that much water to really lower it?

essell
06-08-2007, 04:51 PM
I know that adding a solute into a liquid, like water, will raise the boiling point, because I was taught that at A-Level chemistry. But I can't remember why. No wonder I got an E.

I always instinctively felt that the molecules of solute added were 'holding' onto the heat energy preventing the liquid molecules from using the energy. (I think the specific heat capacity of solids is generally higher than liquids. Right? Else they'd melt. I'm wondering now.)

Paging Stranger.

Squink
06-08-2007, 05:20 PM
Colligative Properties of Solutions (http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/collig.html) at HyperPhysics.

David Simmons
06-08-2007, 09:17 PM
(I think the specific heat capacity of solids is generally higher than liquids. Right? Else they'd melt. I'm wondering now.)

Paging Stranger.Wrong. Water has one of the highest heat capacities of any material, 1 cal/gm. Iron, for example, has a heat capacity of 0.11 cal/gm; copper 0.093 cal/gm; lead 0.032 cal/gm, and on and on.

David Simmons
06-08-2007, 09:26 PM
As of the time this web page was created, the only natural material known with a higher heat capacity that water was liquid ammonia. (http://hypertextbook.com/physics/thermal/heat-sensible/)

MikeS
06-08-2007, 10:43 PM
I thought that adding salt to water gives more nucleation points for vapor to form bubbles, and hence, boil faster.Someone correct me if I'm wrong, but I'm pretty sure that nucleation sites only occur at the interface between two substances — between the surface of the glass and the water in the case of a glass of soda, for example, or between a speck of dust and humid air in the case of a rain cloud. If the salt is completely dissolved in the water, then these interfaces don't exist.

It's true that you can cause superheated water to boil by adding salt to it, thereby giving it nucleation sites. But these nucleation sites are extant only because the salt has not yet dissolved when you first add it — you could get the same result by adding something non-soluble like silica (sand).

David Simmons
06-09-2007, 12:43 AM
Someone correct me if I'm wrong, but I'm pretty sure that nucleation sites only occur at the interface between two substances — between the surface of the glass and the water in the case of a glass of soda, for example, or between a speck of dust and humid air in the case of a rain cloud. If the salt is completely dissolved in the water, then these interfaces don't exist.It's my understanding that this is correct. Googling nucleation sites got this Wiki segment from the article on such sites.... nucleation occurs on the heating surface, at nucleation sites. Typically, nucleation sites are tiny crevices where free gas-liquid surface is maintained or spots on the heating surface with lower wetting properties. Substantial superheating of a liquid can be achieved after the liquid is de-gassed and if the heating surfaces are clean, smooth and made of materials well wetted by the liquid.That statement that "Substantial superheating of a liquid can be achieved after the liquid is de-gassed" reminds me of something I read. The bubbles that appear first when water is heated are air bubbles. Cool water can hold more dissoved oxygen that warmer water. So when the water is heated the dissolved oxygen begins to bubble out of solution and the bubble provide lots of nucleation sites. This and imperfections in the surface of the containter means that water will boil at its correct temperature unless it is degassed and in a clean, smooth walled container.

Dissolved salt is just dissociated Cl- and Na+ ions with no physical interface with the H2O.