Salt and the Boiling Point of Water

Boiling point elevationColligative properties determine how a solvent will behave once it becomes a solution. The degree of change depends on the amount of solute dissolved in the bulk liquid, not the type of solute. So, without my doing your homework for you…how does adding salt to water affect its boiling point? You will find several clues and several keywords above and below.

The fact that dissolving a salt in a liquid, such as water, affects its boiling point comes under the general heading of colligative properties in chemistry. In fact, it’s a generic phenomenon dissolve one substance (the solute) in another (the solvent) and you will raise its boiling point.

So, here’s a rough explanation of what’s going on. If a substance has a lower vapour pressure than the liquid (it’s relatively non-volatile in other words) 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 vaporise. It is an effect of the dilution of the solvent in the presence of a solute. If you want to know about tungsten and why it is used in incandescent light bulbs please check out the Wikipedia entry.

Put another way, if a solute is dissolved in a solvent, then the number of solvent molecules at the surface of the solution is less than for pure solvent. The surface molecules can thus be considered “diluted” by the less volatile particles of solute. The rate of exchange between solvent in the solution and in the air above the solution is lower (vapour pressure of the solvent is reduced). A lower vapour pressure means that a higher temperature is necessary to boil the water in the solution, hence boiling-point elevation.

Conversely, adding common salt to water will lower its 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 more before ice will form on the roads.

A much more fun use for freezing point depression is to add salt to ice to make ice cream. 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.

149 thoughts on “Salt and the Boiling Point of Water”

  1. i would like to know what is the effect of salt on boiling point of mixture of solvents (ammonia+water).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.4 cups of wate and 3 tbs of salt

    plzzzzzz!!!.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.

  2. 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.

  3. i want to know the synopsis and the apparatus and the procedure to measure the elevation in boiling point.

  4. this is a really helful site…the information which i was not able to find anywhere i found it here.

  5. Read the post Kyongera. I don’t believe colligative boiling point depression is that significant. Reducing the air pressure can lower the boiling point considerably, however.

  6. Thank you for making this incredible story you’re a lifesaver, without this article I would have failed my report thanks. You’re the best, now can you make one about matter gas, liquid and solid thank you.

  7. @Bob I’m not here as a teacher, I can chastise whom I like. My article was not full of grammatical errors, it may be written in British English but I’ve read it again and find only one error where I’d added some additional material and overlooked a necessary question mark. As to adding salt to ice cream, I didn’t say it was “yucky”, that’s not a word I would use. Moreover, salt can be used as an ingredient, but that was not what the correspondent was referring to. The reason I did not give actual figures is that this page is commonly found by students hoping someone will do their homework for them. The lowest temperature possible for liquid salt solution is -21.1. Celsius. At that point the salt begins to crystallise.

    This page has some additional information as well as a nice app showing you how freezing point depression occurs when a solute is added to a solvent – http://antoine.frostburg.edu/chem/senese/101/solutions/faq/why-salt-melts-ice.shtml

  8. I was intrigued by the question about the opposite effect being observed and very surprised at your answer. And is there somewhere in the article where the amount of ratio of solute and solvent are discussed? Now I’m curious how much salt has how much effect on boiling point.

    And with meat thermometers, probed used to test far greater temperatures I deal with in my cars and motorcycles, and a pyrometer I know to be right on the money, you can bet I’m gonna test this one. I’ll write back later.

  9. Thank you so much for this article. It helped me so much with my science fair. I did it on how the amount of salt dissolved on water affects how much the boiling point of water changes and this article helped me a great deal. You taught me more than my science teacher. I never heard of colligative properties until I read this article and I’m in the 8th grade! Thanks again!

  10. But we found the opposite–salt water began a rolling boil under 100 degrees Celsius. We’re only a few hundred feet above sea level. Why did this happen.

  11. @Lottie Define salt water. If you’d actually bothered to read the post properly, you’d have learned that it depends on how much salt is added. That’s the whole point!

  12. i love this web site!!! i had to do my science fair which was really challenging because i didnt understand half of what the other articles said but with this one article i was able to write my paper and understand everything!! thank you soo much!! and please keep writing!!

  13. This page has a lot of info about salt and the boiling point of water but not what I was looking for.

  14. i love this web site!!! i had to do my science fair which was really challenging because i didnt understand half of what the other articles said but with this one article i was able to write my paper and understand everything!! thank you soo much!! and please keep writing!!

  15. really because english isnt my first language i didnt get everything, but i think thats the easiest way to explain that… :D

  16. I like this a lot. Maybe you guys can have this site Pointing to other sites?
    I like this and I second the thought on a printer friendly version :D

  17. Yes, thanks for the info. However, do you have a printer friendly version of this info, because I uusually print my research notes from the internet?

  18. i’m doing a University lab on this and we have to find the use of data for why salty water boils faster.

  19. good logical explination of the expernment of the boiling point .it helped me in my science fair

  20. Good logical explinations of the elavation of the boiling point. It helped me in my science fair project reasherch.

  21. This helped me a lot! The explanation was a little confusing but it was much better than any other sites I went to.

  22. my son is in 4th grade and doing a science project on this subject do you suggest we do the project with and without salt and the times it will take both to boil

  23. Great info on this site, it really helped with my 5th grade science fair project. I dissolved 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, & 11 Tablespoons of salt into 2c distilled water and measured the boiling temp. The results were interesting. At some point, when all of the salt wouldn’t dissolve into the water, the boiling temperature stayed the same.

  24. Amanda it doesn’t really have any relevance to cooking. The boiling point elevation due to adding salt to a pan of boiling vegetables is very small. However, colligative properties like bp elevation and freezing point depression don’t just apply to adding “salt” to water, they apply to dissolving any solute in any solvent and so have implications for understanding a huge range of natural phenomena as well industrial processes.

  25. ok so i just turned in my project yesterday and there this one question I was asked i was just like “ummmm” and was totally clueless so i was wondering ….what are th practical applications for this project..i mean ya sure we can use like while cookin and stuff but i mean like IMPORTANT stuff..not saying food is not important or somthing..but u kno wat i mean..;)

  26. Really…? It didn’t help you? It gives you all the key terms, background information, offers suggestions and tries to get you think and yet Kelly, you still cannot work it out…? No gold star for you, then!

  27. umm thanx..but this website didnt rly help me a lot im have to do a science fair project on how table salt effect the boiliong point of tap water so this didnt help me

  28. All liquids and solids have a tendency to evaporate to a gaseous form, and all gases have a tendency to condense back into their original form (either liquid or solid). At any given temperature, for a particular substance, there is a pressure at which the gas of that substance is in dynamic equilibrium with its liquid or solid forms. This is the vapor pressure of that substance at that temperature.

  29. Thanks so much! This really helped because im doing something related to this for my 8th grade science fair project. But could you define vapor pressure

    Thanks again.

  30. This was really helpful for my science project. thanks! My project is Which Solute Affects The Boiling Point Or Melting Point Of Water The Most. This is just what I needed. Do you have any ideas on how to measure when exactly the ice reaches its freezing point?

  31. Harman, Amanda – in the first place, the boiling point of the solution increases only slightly when adding salt. There is a limit, of course, to how much salt might be dissolved in any given volume of water beyond which you would have a saturated solution containing undissolved particles of salt. Add more and more salt beyond this and you are talking about a pile of wet salt rather than a solution of salt in water.

  32. Amanda,
    The boiling temperature would increase “forever” only if you could add an infinite amount of salt to the water. Do you think that this is possible? You can do an experiment to find out!

  33. but how much can the boiling point be changed? if I continue adding salt to the water will the boiling point continue to increase forever or is there a certain temperature where the boiling point can no longer increase? if this is true then what is the temperature and why does is stop as this particular temperature? plz answer ASAP
    thanks so much

  34. This was very helpful in someways, especially with the question “how does sugar affect the boiling point of water?” Anyways I am conducting an experiment and its problem is “how do adding matter to water affect it’s boiling point?” I was wondering if you could give me some suggestions on matter to use in conducting my experiment. and i was also wondering if this would be a closely accurate way of measuring boiling point: Putting salt into a pot along with water, put on stove, wait for it to boil, stick a liquid thermometer in the pot and the temperature of the solution will be the boiling point? i really appreciate your help. and i thought that this article was perfectly understandable if you read it through and I am not in high school. :D

  35. Well, Erudite, the clue is in the original post and the typo in your original comment, which I fixed

    Adding “a salt” or any other soluble substance to the liquid will affects its physical properties.

  36. My project is “Do substances affect the boiling point of water?”, and salt is one of the substances, along with sugar, hot chocolate mix, Kool-Aid, and honey.

  37. im doing a science fair project about lowering the freezing point of water but i couldnt understand this article.could you state it smpler and add pictures so i understand. i have to write a conclusion 2 pages long and i need as much help as i can get!

  38. Cassie, I’m not sure what you’re talking about. Which temperature are you talking about? The temperature at which the water boils? Yes, that’s affected by various factors including substances dissolved in the water.

  39. Erudite, it raises the boiling point by such a small amount at the quantities of salt you would add to food without making it inedible that it’s irrelevant to the cooking time. Adding salt to cooking food is about taste, increasing “more-ishness” and reducing the bitter sensation some foods produce.

  40. hi i am in the 6 th grade and as im am writitng this i am in language a class .i am trying to finish my science and the research i have done was no help until i found this wwebsite

  41. Ogisal, not quite sure what you’re asking. Salt lowers the freezing point of water from O Celsius to a little below 0, it doesn’t lower the boiling point of ice from 0 to 5…that makes no sense.

  42. Helped with my research towards how adding salt affects the bp of water in my year 8 course. Doesn’t salt also lower the bp of ice from 0’c to 5’c?

    Thank you again!

  43. So… pepper (since it doesn’t dissolve in water) would not have an effect on the boiling point or freezing point of water, but would sugar have the same effect as salt?

  44. Hannah – Does pepper dissolve in water? What about sugar? If it is soluble in the solvent then it can affect the freezing point and the boiling point. I think I said all this earlier in the comment thread.

  45. How does pepper affect the boling point or freezing point of water…. and what about other things like sugar or flour or anything else

  46. Wow this is a great solution to this question. I finally figured it out! i have been workin for that question for a long time, and now its becoming easy. Thanks to this site=)

  47. Maramii – thanks for posting. I’m not sure why you posted your write-up subsequent to this comment, but the word is colligative, as opposed to colligate, but otherwise it looks okay as a write-up. I’d be more specific with your research citations. http://www.google.com is a bit broad, isn’t it?

  48. My experiment is about will amount of salt affect the boiling point of water? The boiling point of a substance is the temperature at which the vapor pressure of the substance is equal to the atmospheric pressure. Dissolving salt in water will lower its vapor pressure because the solute particles limit the ability of water particles to escape the liquid’s surface (evaporation). As a result, a higher temperature is required to reach
    the necessary vapor pressure and so the boiling point is increased. The greater the amount of salt (or any non-volatile solute) added, the greater the increase to the boiling point will be.

    Thank you for the site and in my opinion its terrific.
    Maryam:p

  49. This website is awesome it gives you a lot of astounding information and its great this helps me with my science fair so thanks

  50. Aaliyah, yes it does, as discussed here. But, the important point is that the substance has to dissolve in the water for it to have an effect on lowering freezing point and raising boiling point. Adding an insoluble material will have no effect on the temperature at which any solvent freezes or boils.

    For those interested in taking this further consider solids with something dissolved in them, alloys are a good example in which a metal is dissolved in another metal. Would you expect an alloy to have a higher melting point than the pure metal?

  51. A Sciencebase reader visited this page having searched Google with the question:

    “what is water that has little salt dissolved in it (two words)?”

    I cannot know for sure what they are looking for but perhaps it was brackish water, briny water (brine), gripe water possibly…anyone else got any ideas…?

  52. This helped me out so much with my science fair project and my research paper… It was exactly what I was looking for. Thanks. :)

  53. We were wondering about it. And there wasn’t really a premise accompanying this lab, nor did it really fit in with the sequence of our class outline. Apparently it was out of order, which we found out a couple days later. On all our previous labs they provided extremely detailed instructions. On this one, they left out a critical detail, hence our failure. And yes, NaCl…I knew that but am a fast typer! Thanks!

  54. Yes, very funny comments. But was wondering if you are suggesting that a 10 M solution of salt will increase the temp 6 degrees celcius?

  55. Jen – First, it’s NaCl (symbol for chlorine/chloride has a lower case l not L). It’s important to get that kind of thing right from the start, it is a lower case second letter for all symbols for the chemical elements with two letters.

    As to your problem. Did you understand the basic premise of the experiment? Did you not wonder that failing to mix the materials would not likely have the desired effect. I try to teach my kids to think rationally and critically, I’d have expected them to be wondering from the start whether mixing was required. Go back and try it again, but mix the solution thoroughly so that all the sodium chloride is dissolved before carrying out the measurements.

  56. Hello! I think I did something wrong! My home lab said to use distilled water and measure the boiling point at 4 intervals, each with zero, one, two and three tablespoons of NaCL respectively. We did this with a pan on the stove and a glass thermometer which we suspended above the bottom of the pan. We got the exact opposite effect..the temperature slightly decreased. Any suggestions on what we may have done wrong? I know we didn’t thoroughly mix the NaCL and water together, but our lab did not specify this. Was this our glitch?

  57. Yamum, I’m not into providing a plain and simple answer, I’d rather guide you as to how you might figure it out for yourself, that way you will have a deeper understanding of the principles involved rather than simply being able to fill in the blank space on your homework or test sheet.

    If adding a solute to a solvent (i.e. adding salt to water) raises its boiling point, do you think that implies that will require more or less energy to cause it to boil? If it requires more energy then it will take longer to boil than if it requires less. So, what’s your answer?

  58. ummm ok lets take my hypothesis to the lab.. two beakers one with pure water and the other with NaCl solution.. which one will evaporate first.. the pure water of course :)

  59. The picture was chosen simply as a dramatic depiction of water under pressure, not exactly related to the question in hand. As to your hypothesis, there is no way to make all other factors constant between a land-locked region with a (freshwater) river and a (salty) coast. What might be worth comparing would be the climate/rainfall between somewhere on Lake Baikal (Russia, large freshwater lake) and a region on a similar latitude and altitude with a saltwater lake…

  60. another thing .. i wondered about the picture here.. is it related to the effect of the salt on the waterbioing point?? and if it is not what are the uses of this fact (for example the depression in the freezing point fact is used in making ice cream )

  61. suppose that all other factors are constant or the same in the two areas, will the level of the rainfall be increased in the river area where the salt concentration is low??

  62. Why do you think that would be, explain your reasoning. Rainfall patterns are dependent on many more factors than local vapor pressure, but I can see where you’re coming from. Please expand on your hypothesis.

  63. i have a question regarding this topic
    if there is two towns one has a river and the other has a sea, will the river town be more rainy than the sea town (suppose that the weather in the two areas is the same)

  64. 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!

  65. 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 use salt in cooking.

  66. 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?

  67. 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.

  68. 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

  69. 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

  70. 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!

  71. could we plez use smaller words dude so i can pwn my science experiment and accually know what im talking about

  72. 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

  73. 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

  74. This actually did help. Most websites don’t actually help and/or answer your questions.

    so THANK YOU :)

    julia

  75. 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

  76. what conclusions can be drawn from this experiment? aiso what has ebullioscopic constant to do with this experiment

  77. 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.

  78. 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.

  79. 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

  80. 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”.

  81. 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!

  82. 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.

  83. 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.

  84. 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.

  85. Anything that dissolves in the solvent will affect its colligative properties. I don’t believe pepper dissolved in water, but sugar certainly does.

  86. 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!!!

  87. 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…

  88. 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………

  89. With appropriate background reading and supervision, I’m sure it would make an interesting science project, yes.

  90. 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…

  91. 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.

  92. 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.

  93. i would like to know what is the effect of salt on boiling point of mixture of solvents (ammonia+water)

  94. 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.

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