Dec 27, 2006
How Does Salt Affect the Boiling Point of Water
The question how does salt affect the boiling point of water is a common one Sciencebase readers ask. It’s presumably a course question and those visitors are after a quick answer for their science essay.
The fact that dissolving a salt in liquid, such as water, affects its boiling point comes under the general heading of colligative properties in chemistry. These are effects that determine how a liquid will behave once it becomes a solution, as it were. The degree of change depends on the amount of solute dissolved in the bulk liquid. Adding salt to water raises its boiling point, slightly.
If a substance has a lower vapour pressure than the liquid then dissolving that substance in the liquid, common salt (NaCl) in water (H2O), for instance, will lower the overall vapour pressure of the resulting solution compared with the pure liquid. A lower vapour pressure means that the solution has to be heated more than the pure liquid to make its molecules vapourise. In other words, the boiling point is raised.
Conversely, the addition of common salt to water will lower the solution’s freezing point. This effect is exploited in cold weather when adding grit (rock salt) to the roads. The salt dissolves in the water condensing on the road surface and lowers its freezing point so that the temperature has to fall that bit further before ice will form on the roads.
A much more fun use for freezing point depression is to add salt to ice to make icecream. The About site has some instructions on how to do this, although it’s probably not too tasty.
Curiously, at least one Sciencebase reader was searching for the phrase “how does sugar affect the boiling point of water?” and landed on this page. This is essentially the same question as does salt affect the boiling point of water. The nature of the solute, the material being dissolved in the solvent, is pretty much irrelevant at a first estimate. Rather it is the amount of material that is dissolved (which depends on the materials solubility) that influences the boiling and freezing points as described above.
David Bradley
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Mick S said,
January 24, 2007 at 3:41 pm
I think the only thing you haven’t mentioned is Raoult’s law. Raoult’s law states that: the vapor pressure of each chemical component in an ideal solution is dependent on the vapor pressure of the individual component and the mole fraction of the component present in the solution
The last sentence in the first paragraph of your post doesn’t mention that there is a maximum, it is not a linear scale, it tends towards a maximum value, in other words it plateaus. Solubility curves also complicate the picture. Remember the solubility curve for NaCl is unusual, as it’s solubility hardly varies with temperature whereas it does for other salts.
SATHYABHAMA said,
February 10, 2007 at 7:59 am
i would like to know what is the effect of salt on boiling point of mixture of solvents (ammonia+water)
David Bradley said,
February 10, 2007 at 9:44 am
Adding one solvent to another, providing they mix (are miscible) is equivalent to adding any solute so the boiling point of the other is raised and the freezing point lowered. Adding a third factor, a salt, for example, simply make working out that the actual raisings and lowerings a little more complicated, but possible.
jessica said,
March 21, 2007 at 8:47 pm
I think that if you put so much salt in water it will affect the boiling point of water.
David Bradley said,
March 21, 2007 at 9:05 pm
Jessica, dissolving a salt in water will indeed affect the boiling point of the resulting solution, it will raise the boiling point relative to the boiling point of water. Fundamentally, this is because it will require more energy to release the water molecules from the solution into the vapor phase.
Fayyaz said,
October 4, 2007 at 12:50 pm
The boiling point is 100 degrees but when you add the salt how many degrees does it go up.
what degrees does the salt water stop at on the thermometer…
David Bradley said,
October 4, 2007 at 1:39 pm
Fayyaz, thanks for your query. You could do the calculations yourself, it depends on how much salt you add and follows a formula - http://www.nyu.edu/classes/tuckerman/honors.chem/lectures/lecture_13/node7.html
But, as an example, as “salty” as seawater is, its boiling point is just 100.6 degrees Celsius, 0.6 degrees above the standard boiling point of pure water at atmospheric pressure.
db
Fayyaz said,
October 5, 2007 at 12:13 pm
what if i only put 3 tbs of salt.
plz give me a degreeze.
im beggin u…….
J Dizzle said,
October 7, 2007 at 12:03 am
Would thid be a good science project for 7th grade?
David Bradley said,
October 7, 2007 at 2:09 pm
With appropriate background reading and supervision, I’m sure it would make an interesting science project, yes.
g-DUB said,
October 8, 2007 at 12:45 pm
ok
4 cups of water and 3 tbs of salt.
what douse the boiling point of 3tbs of saltwater go up to on the thermomiter.
thank u
u r genuros………
David Bradley said,
October 8, 2007 at 2:33 pm
Okay. Okay. Okay. I’m not doing anyone’s homework, but here’s a clue:
The boiling point of water will rise one half degree Celsius for every 58 grams of salt dissolved per kilogram of water.
Now go and do the calculations…
fuzzy said,
October 9, 2007 at 10:15 am
my thermomiter is skrooed up becayse i put it on the fire.
please just tell me the boiling point of saltywater and i will get off ur back. u wont hear from me ever again. I SWER….plzzzzzz.
4 cups of wate and 3 tbs of salt
plzzzzzz!!!
Erik Martinez said,
October 11, 2007 at 9:55 pm
Will Pepper, Sugar, or other type of solutes affect the boiling point of water also?
David Bradley said,
October 11, 2007 at 10:57 pm
Anything that dissolves in the solvent will affect its colligative properties. I don’t believe pepper dissolved in water, but sugar certainly does.
David Bradley said,
November 16, 2007 at 10:20 pm
In answer to the many visitors who have asked questions along the lines of “is the boiling point of water affected by adding water?” No, it isn’t unless some component in the pepper is dissolved in which it case the rise in boiling point would be proportional to the change in concentration. But, nothing in pepper dissolves as well as salt or sugar in water.
Somebody said,
November 20, 2007 at 11:35 pm
Will you please change the title of your article so the judges AND MY SCIENCE TEACHER DON’T GET SUSPICIOUS. Sorry I wrote in caps lock. Maybe you could change it to some thing like salt and boiling water. Thanks.
David Bradley said,
November 21, 2007 at 8:50 am
Hey, “Somebody”, of what would your science teachers and judges be suspicious? The fact that you were planning to crib the entire article and claim it as your own (in which case you’ll not get a great grade anyway because the article isn’t a project write-up)? Or, the fact that you’d had the initiative to search for useful background information on the internet? The article exists on this site because lots of people want to know how salt affects the boiling point of water, I have no intention of changing.
Anonymous said,
November 29, 2007 at 12:57 am
what are you talking about.
all i am trying to do is find the answer but you are putting way more mubo-jumbo in here than i need!
David Bradley said,
November 29, 2007 at 7:41 am
Hello Anonymous (mmsput), it always surprises me how ignorant some people can be. You criticise my post on the fascinating subject of colligative properties and obviously have no interest in actually finding out about the topic before you rubber stamp your homework. Why not try and learn something about the world instead of just completing the assignment blind. There is, as I’ve said repeatedly, no single answer to this problem. YOU HAVE TO WORK IT OUT!
Incidentally, the phrase you are looking for is not “mubo-jumbo”, but “mumbo jumbo”.
David Bradley said,
December 17, 2007 at 11:27 am
The title of the post is one of the most popular search phrases with which visitors hit the Sciencebase site. I’m wondering whether to expand the post to explain why adding salt to the roads in winter is a good idea? Anyone care to write a guest post for me?
db
Harmon said,
January 2, 2008 at 4:04 pm
David,
In some climates, adding salt doesn’t do much more than adding sand. That is true of the coldest days here in ND; when it gets below 0 (F), ice doesn’t turn liquid, even in the presence of a lot of salt! Remember that the effect of the solute depend on the number of particles (molecules or ions) dissolving, not directly on the mass.
David Bradley said,
January 2, 2008 at 5:35 pm
Harmon, you’re right of course regarding the fact that some climates are just too cold for improving road surface grip with salt in icy conditions, although I hate to see that 0F you mention as opposed to the SI unit of temperature, but the point is well taken nevertheless.
By the way, I also fixed the typo you spotted that had me listing hydrogen instead of dihydrogen monoxide in this modified version of the original item.
nikhita said,
January 18, 2008 at 3:43 pm
what conclusions can be drawn from this experiment? aiso what has ebullioscopic constant to do with this experiment
David Bradley said,
January 18, 2008 at 4:46 pm
Nikhita, you tell me! What conclusions do you draw from this “experiment” regarding the term “ebullioscopic constant”. I am sure you can find a definition of that on the web that will assist you in undertaking your school project.
db
G-gee said,
February 5, 2008 at 3:36 am
This was very helpful in aiding me with my project. Thanks and good luck with everything.
David Bradley said,
February 5, 2008 at 7:28 am
Glad to be of service G-Gee
db
A Rod said,
February 18, 2008 at 1:16 pm
Great great great this article has helped me so much!
Thank you
Julia said,
March 2, 2008 at 1:02 am
This actually did help. Most websites don’t actually help and/or answer your questions.
so THANK YOU :)
julia
julia said,
March 2, 2008 at 8:41 pm
David,
this has absolutely nothing to do with salt or the boiling point of water but as i was reading through your posts i laughed… you are certainly an entertaining person…very funny guy!
julia
David Bradley said,
March 3, 2008 at 8:11 am
Thanks Julia, glad you’re enjoying my stuff
db
julia said,
March 4, 2008 at 11:31 pm
you are so welcome.
I have a question for you…
What is the link between water’s boiling point and vapor pressure?
If you could email me that would be fab!
missylynn182@gmail.com
David Bradley said,
March 5, 2008 at 7:39 am
Well Julia, that’s another question I’d prefer to defer to your brain. With a little background reading you could work it out. But, to put it briefly, the boiling point of a liquid is the temperature at which its vapor pressure equals the pressure being exerted on it (usually by the atmosphere).
db
julia said,
March 7, 2008 at 2:14 am
i did thanks!
your mom said,
March 16, 2008 at 8:23 pm
could we plez use smaller words dude so i can pwn my science experiment and accually know what im talking about
David Bradley said,
March 17, 2008 at 9:03 am
Find yourself a dictionary, why don’t you? I just checked Sciencebase.com on the blog readability index and it comes out as “High Shool”. So unless you’re ten and under, you should be able to cope with this site in all its glory!
mkabayi tshabalala said,
April 23, 2008 at 4:25 pm
sumbody plzzz help me out on this im clueless……how is the boiling pint of water affected by adding different quantities of salt???????????aaaaaaaaaaaaaaaaah iesh i think i got a clue but, i wud lyk a little more ground info
David Bradley said,
April 23, 2008 at 4:39 pm
Common salt dissolves readily in water, forming positive sodium ions and negative chloride ions, which are surrounded by an ever changing flux of water molecules, which themselves are linked by a fluxional network of hydrogen bonds between the H atoms and the O atoms in the water molecules themselves. It is the interaction of all these bonds and ions and molecules that leads to a rise in the energy required to release water molecules from the surface of the liquid into the vapor phase. More energy means a higher temperature. Put simply the salt ions make the liquid bind together more tightly than it does in the pure liquid.
(This is very much a glossy answer, but hopefully you will be able to figure it out on this basis)
db
Sofia said,
July 3, 2008 at 5:12 am
Does salt affect the boiling point of alcohol in the same way as water?
David Bradley said,
July 3, 2008 at 8:16 am
Sofia, rather than my answering that question directly ask yourself whether or not and to what degree salt dissolves in alcohol and whether the concept of colligative properties as discussed above applies to all solvents and solutes or whether it is restricted to water.
Derek said,
July 4, 2008 at 2:37 am
Since it’s clear from your post that salt only has a limited affect on the boiling point of water, why is it that some cooks salt water before cooking pasta? I had assumed it was because it would boil quicker but that doesn’t seem to be the case. Could it really be just taste?
David Bradley said,
July 4, 2008 at 7:26 am
Derek, that’s right, salt has minimal effect on cooking times and is added for the simple fact that it makes food taste better, not only by adding a stimulating saltiness, but by blocking bitterness, to a degree. I’ve written a short follow-up post on this, especially for readers curious about why we usesalt in cooking.
milo said,
July 8, 2008 at 12:03 am
DB, I cannot believe you don’t like doing peoples homework! :-)
For me, the coolest thing about colligative properties is the fact that all of them are really entropic effects.
There is an interesting analysis of freezing point depression by Bob Wolke (http://pubs.acs.org/isubscribe/journals/cen/86/i13/html/8613newscripts.html) where he contends that melting ice with salt is nit really a colligative effect, but rather a perturbation of the ice water equillibrium.
Always a fun topic!