Boiling water & salt

  1. Hi all,
    there's so many forums here, I really don't know if this is the right one. However...

    Today, I cooked some noodles.
    The water was boiling low, just simmering.
    I had a taste, and decided the noodles needed more salt.
    So I added some salt.
    And, can you believe, where I put in the salt, the water started to boil & bubble very intensely for a second or so, then went back to normal.
    I became curious, and repeated this several times with the same result.
    Later (eating some very salty noodles), I wondered what's behind this.
    Any ideas? Thanks!
  2. jcsd
  3. Before I explain let me tell you this. If you see a ||||| do not be allarmed. It is to prevent my diagram from being squashed into meaningless rubish by the server as ot will automatically remove mutliple spaces from this post reply.

    Understanding whats going on here will require you know about
    1)The polar nature of water and salt
    2)What actually happens when something changes state(state in terms of liquid, gas or solid)

    I will explain theses two concepts to you and then combine them to give a conclusion (this will be the answer to your question)

    A water molecule consists of 2hydrogen atoms and 1 oxygen atom. The hydrogen atoms each contain 1 proton and 1 electron. The oxyen atom contains 8 protons and 8 elctrons. Since the electrons effectively (and i use effectively in the broadest sense) fill up the shells in the fashion 2,8,8,8,8,etc... The first 2 electons fill up the first shell and the other 6 almost fill the second shell. The oxygen steals the electron from each of the hydrogen atoms in order to fill its second shell. This creates positively chared hydorgen atoms and a negatively charged oxygen atom. Since opposites attract they do so froming a water molecule.

    A water molecule is called a polar molecule because it has a positive end and a negative end as depicted by the diagram bellow where the + is the oxygen and the 2 -'s are the hydrogens.

    Water is a polar molecule||_|/\ _

    Now if we look at salt which is also polar


    you can see how the hydrogens on the water would be attraced to the clorine in the salt and would orient itself accordingly and the same with the oxygen and the Sodium(Na).

    Here is another concept. The heat of a particle is litteraly how much it is vibrating. If a group of water molecules has hardly any heat energy then they assume the solid state as their particles have not got enough engergy to vibrate and escape the strong attractive forces between the moleucles. So they stick together to form a solid. If they have more heat energy they now have enough heat energy to vibrate a little away from each other. They can now slide over eachother (a liquid). Heat them some more and they have completely escaped the attractive force binding them and therefore move to someextent, indepenedant of eachother ( a gas).

    Combining these two concepts we can see how adding salt to water would distort the system of acctraction and repulsion forces between the molecules. We can see that since the relationship of these forces has been affected, the heat energy required to break free from these them (turn into gas or evaporate) would be different. So in an area of the water where the water has not got enough energy to turn into gas, you are changing the amount of energy required to change the water into steam so that the required energy level is below the actual energy level of the water in the saucepan. (when i say enegy level i am refering to the amount of heat energy in the saucepan)

    Its been an honour answering your question. Was my answer usefull to you? please tell me by emailing me
  4. Bystander

    Bystander 4,077
    Science Advisor
    Homework Helper
    Gold Member

    And, for a more pedestrian explanation, the salt furnishes nucleation sites for boiling of superheated water --- try it with sugar, sand, baking soda, ground glass --- you'll get the general picture.
  5. Thanks Bystander and especially mceddy2001!
    That was very helpful.

    I still have some questions:

    Let's assume it's the attraction between H+ and (another molecule's) O--, which is mainly responsible for the high boiling point of water. (This sounds reasonable, since e.g. pure oxygen has a much lower boiling point.)
    Now, if I add the salt, there is also attraction between Na+ and O--, and also between Cl- and H+. I can see that this changes the energy needed for a water molecule to escape, but I can't see why this necessarily decreases that energy.
    After all, you could argue that both Na and Cl are heavy atoms compared to H2O, so a water molecule should be held down by their attraction, just as if a heavy weight was being attached to it...

    You seem to suggest that his works with the other substances, too. Well, one should expect that, with sand, the effect will NOT vanish after a second or so. Because the sand will not dissolve, as the salt does. I'll try when I find the time. Or maybe someone else?
  6. Integral

    Integral 7,288
    Staff Emeritus
    Science Advisor
    Gold Member

    This post is mostly hot air, with little that applies to this problem. Read Bystanders post, it says all that needs to be said.
  7. well if you take a look at the structure of how water groups, the molecules cluster very close together forming a sperical blob. If you look at you can see how 280 water molecules arrange in a blob. By looking at this diagram you can get an idea of how disruptive adding ions into the blob arrangement would be.

    By looking at the blob you can see why water has got such a high boiling point, because they are tightly packed and therefore the residual electro static forces attracting the paticles together needs to be overcome by being heated to 100 deg C or 373K to be politically correct.

    It is my belief that adding salt weakens this K'nex Ferrice wheel, making it easier to change the state.

  8. mceddy: adding impurities like salt to water *raises* the latter's boiling point. no real controversy there. but what the original poster is referring to here is the phenomenon of vigorous localized bubbling, not a general change in the melting point. bystander has the short, sweet, and correct answer.

    [post corrected; apparently i don't know my chemistry]
    Last edited: Nov 3, 2003
  9. Njorl

    Njorl 818
    Science Advisor

    Adding salt raises the boiling point of water. It does not lower it. The sudden boiling is a short-term effect. Once the salt dissolves, it winds up raising the boiling point.

  10. !!! sorry 'bout that. i'll correct my post.
  11. Im sorry im a physician not a chemist, I oviously stand corrected.
  12. Thanks for the response everybody.
    Njorl, are you sure? Yes I know salt water is denser than pure water which backs up your statement, but please consider this:

    When it's sub zero in winter, and the sidewalk has frozen over, you can scatter some salt upon it, and the ice will eventually melt. Because adding the salt lowers the melting point, right? Couldn't it be that my little effect comes from a similar mechanism - salt lowering the boiling point? (AFAIK, a substance with lower melting point also has a lower boiling point, or am I naive here...?)

    In another experiment, you use a mixture of icicles & water, and add a LOT of salt - temperature will go down to -20°C or so without the mixture freezing solid.

    Both experiments will obviously not work with sand, etc. So maybe Bystander's 'nucleation sites' theory is not the whole story... well I'm not sure at all...
  13. Humm, I have literally dropped hundreds of tons of salt, onto highways covered with snow, or ice. When salt and water mix, the salt dissolves into the water, releasing heat. (exothermic reactions)

    As for the responce to the initial question, AFAIK (from chemistry) Bystander has it right.
  14. very fine sand, or ground glass, would probably give a similar effect. although salt has admittedly a bit more going for it, since dissolution is net exothermic.

    impurities generally *raise* boiling points and *lower* melting points.
  15. The boiling point of the salt water is higher because part of the pressure of the liquid comes from the impurities added to the water (the total pressure at the boundary of water and air is the same, only now some of that boundary is occupied by the solutes) and so it reduces the vapor pressure by that amount.

    Water boils when the vapor pressure exceeds the atmospheric pressure. With the pressure from vapor reduced because of the impurities added to the water, the vapor pressure has to rise farther before the water can boil.
  16. Good points just that the impurities are not already in the water, they are being added to it while it is right near boiling temp, "just simmering" was what the poser of the question (arcnets) had stated.

    Perhaps it is the exothermic reation that excites the water to boil instantly and/but only slightly, upon introduction, coupled with nucleation? perahaps?
  17. Could be. Also the surface tension is being broken at the point where the salt is being added. The combination of nucleation, exothermic reaction (which should cause an associated rise in the vapor pressure), occuring at the very point where the surface tension has been broken, probably culminates in boiling the water instantly.
  18. Thanks for all the input. I'll do more experiments when I find the time.
  19. Bystander

    Bystander 4,077
    Science Advisor
    Homework Helper
    Gold Member

    "... net exothermic ...," and "... surface tension being broken ..." are incorrect. Thanks for playing "Guess That Thermodynamic Property," and come back to play again some time. There are libraries full of tabulated measurements of these properties. In the future, please consult the tables for quantitative information rather than assuming qualitative values to support arm-waving rationalizations.
  20. Breaking the surface tension into smaller areas is exactly what is happening, bystander. The salt is providing nucleation points, without which the water would continue to heat into one large bubble with a single surface (as can happen with a smooth vessel heated in a microwave or a container of water boiled in space where gavity does not force the bubbles to separate).
  21. sorry for the misinformation. would 'thermodynamically favourable' be correct?
Know someone interested in this topic? Share this thead via email, Google+, Twitter, or Facebook

Have something to add?