Boiling water & salt

  • Thread starter arcnets
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  • #26
Humm enthalpy of NaCl, Humm doesn't work on ice at -18° C as it no longer generates enough heat, to melt, well, ya it does, but exeedingly slowly. Raise the temp to ~ -5° C and the salt will melt into the ice with much greater rapidity, as a matter of fact the dissolution rate at, or near, those temperatures, increases with the temperature increase.

Perhaps in the Boiling water the dissolution rate is quick enough to generate many nucleation sites, hence the apperance of "accelerated boiling"
(Rapid Vapor Generation)
 
  • #27
What about duterium and tritium? If memory serves me correctly, they are acids.
 
  • #28
NateTG
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Originally posted by S = k log w
What about duterium and tritium? If memory serves me correctly, they are acids.
duterium and tritum are isotopes of Hydrogen.

Duterium and tritium are not acids in the same way that steel is not a hovercraft.
 
  • #29
Originally posted by NateTG
duterium and tritum are isotopes of Hydrogen.

Duterium and tritium are not acids in the same way that steel is not a hovercraft.
Would they make H20 an acid, or am I wrong?
 
  • #30
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Wrong --- what effect are you proposing for boiling water and salt?
 
  • #31
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I have done some more experiments.
First, I used pure water (just simmering) and tried to reproduce the effect with salt, then with sand.
Observation: No effect(!)
Then I remembered, in the original experiment I had some molten butter in the water.
I tried to reproduce that.
Observation: With salt only a small effect (if any), with sand no effect.
Next, I thought, what else was there in the original experiment? The noodles, of course. But also 2 spoons of 'Instant Fleischbrühe' (German for 'meat extract powder').
Sorry to bother you with these 'unphysical' ingrediences, but what influences may they have?

Still puzzled...
 
  • #32
Originally posted by Bystander
Wrong --- what effect are you proposing for boiling water and salt?
If it were so that duteurium and tritium were acids, depending on their chemical reactions, it might change the soln., and perhaps, it's boiling point. Do light water v heavy water NaCl solutions have different boiling points?
 
  • #33
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Originally posted by Bystander
Could you rephrase "Breaking the surface tension into smaller areas..." for us? It's a bit unclear what you're saying --- I doubt very much that you're asserting that NaCl emulsifies water. Please keep in mind that the intuitive dependence of evaporation rate on surface tension has been examined for liquids, isotropic solids, and anisotropic solids, and demonstrated to be non-existant --- Langmuir's model for rate of mass transfer to the vapor phase still holds.

By the by, surface tensions of "most" electrolyte solutions are GREATER than that of pure water; non-electrolytes generally reduce surface tension in aqueous solution.
No, nothing that exotic. I was just suggesting that the salt aides the formation of bubbles, the formation of bubbles relies on surface tension to hold the bubble together and the bouyancy of the bubbles breaks the surface tension at the surface of the water.
 
  • #34
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Originally posted by S = k log w
If it were so that duteurium and tritium were acids, depending on their chemical reactions, it might change the soln., and perhaps, it's boiling point. Do light water v heavy water NaCl solutions have different boiling points?
Yes, higher --- and, I'm not going to look up exact numbers for you --- "deuteroim oxide" Googles 4.5k sites, "tritium oxide" 450 --- have at 'em.
 
  • #35
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Originally posted by Artman
No, nothing that exotic. I was just suggesting that the salt aides the formation of bubbles, the formation of bubbles relies on surface tension to hold the bubble together and the bouyancy of the bubbles breaks the surface tension at the surface of the water.
Surface tension is NOT a "breakable" quality --- surface tension is an intensive thermodynamic variable used to describe systems. The product of surface tension and area is surface free energy which is significant only in cases where a phase is in very finely divided condition, aerosols, mists, Pt on a platinized electrode --- aggregate dimensions measured in 10s of microns or less. Increases in surface area, bubble formation, require an INCREASE in free energy over that of a "bulk" material.

Phase separations, bubble formation, droplet formation, crystallization, depend upon some nucleation process --- in pure phases, nucleation is limited to the "fluctuations" or fluctuation phenomena of statistical mechanical arguments (as far as current understanding goes). In the everyday world, dust motes, scratches, divots, and other irregularities furnish nucleation sites for phase transitions.

Actual formation/growth of bubbles/droplets/crystals is driven by the difference in chemical potential of the system undergoing the phase change and the chemical potential it would have at equilibrium at the same conditions. Boiling ALWAYS indicates superheating of a liquid phase; addition of nucleation sites always results in an increase in the rate of boiling; if the nucleation site population is "permanent," as in the case of boiling stones, the degree of superheat is reduced, that is, the temperature measured in a boiling liquid for a given heat transfer rate is less than that measured for the same liquid without the stones.

The actual mechanisms for nucleation are NOT terribly well understood; Adamson is a great place to start for anyone who is interested in pursuing an introduction to surface chemistry/physics.
 
  • #36
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Originally posted by arcnets
I have done some more experiments.
First, I used pure water (just simmering) and tried to reproduce the effect with salt, then with sand.
Observation: No effect(!)
Then I remembered, in the original experiment I had some molten butter in the water.
I tried to reproduce that.
Observation: With salt only a small effect (if any), with sand no effect.
Next, I thought, what else was there in the original experiment? The noodles, of course. But also 2 spoons of 'Instant Fleischbrühe' (German for 'meat extract powder').
Sorry to bother you with these 'unphysical' ingrediences, but what influences may they have?

Still puzzled...
Several points (of many possible): 1) you are dealing with a heat transfer process, and the degree of superheat you can establish in the pan/pot/cauldron is going to be proportional to the magnitude of the enhanced boiling effect you observe --- translated to English, turn the burner up to full blast; 2) the butter and noodles may have contributed to the superheat at simmer by quenching the pot's built in nucleation sites with butter, and by reduced thermal conductivity of the water-butter-noodle system; 3) butter on the surface is probably going to quench nucleation sites on sand as it enters the mix; 4) "3)" suggests a couple other experiments you can try (I hope you're treating this with the respect due something that can scald you rather severely) --- at full heat (max superheat), try the salt, the sand, and wetted salt and wetted sand (I'd say a slurry in a teaspoon with enough water that you can dump the slurry rather than having to snap it into the water); 5) once you get the sand to work, if you've got a thermometer that lets you read to tenths of a degree at 100 C, try measuring temperature at the bottom of the pan for a) water, b) water plus salt, c) water plus salt plus sand, and d) for the last mess after you've let it cool, and brought it back to a boil.

Don't run your utility bill up too much, but have fun.
 
  • #37
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Bystander,
thanks indeed for your very detailed analysis on my little 'crazy cooking scientist' experiment. Don't worry, I won't burn myself - I have enough experience with making e.g. 'fish in beer batter' in a pan full of oil (boy, can that stuff explode... ).

OK. If I read you correctly, then the thermometer should show well above 100°C because of the fat, and the salt/sand is there to provide nucleation sites to make the water evaporate, right? Let's see...
 
  • #38
What noodles are you using? What is the gluten content? Are you using rice noodles? Have the noodles been artificially colored? Do they contain eggs? Have you considered the fact that they may contain insect parts? Are you sea level? (See Boyle's law).
 
  • #39
markci
The nucleation theory IS the whole story.

Adding salt to water RAISES the boiling point. Period. In fact, adding _anything_ that dissolves will raise the boiling point of any solvent.
 
  • #40
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"The thread that wouldn't die." Absolute statements have the absolutely nasty habit of turning around and biting people who make them --- depending upon the solute, the boiling temperature of an aqueous solution at 1 atm can be increased, decreased, or totally unaffected; the partial pressure of water measured over such a solution may be greater than, equal to, or less than the product of water mole fraction and the vapor pressure of pure water at the temperature of the boiling solution. Interested parties may consult King, Rowlinson, Lewis & Randall, Klotz, Glasstone, or PM me for a longer bibliography of less commonly available sources on the topic (dead serious interest in the topic for the latter case --- I ain't digging into that mess for free).
 
  • #41
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I feel I should give an exact recipe that guarantees to reproduce this effect. I'll try...
 
  • #42
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"Nucleate(d) boiling" is a heat transfer problem, and been the subject of much study for the past century and a half or so, remains the subject of much study, and will remain such; don't kick yourself too hard if you can't come up with a precise recipe --- if you do, don't report it here, patent it and sell it to the chemical process industries. You were more than specific enough in your description of the phenomenon of interest --- anyone who's ever been near a kitchen with eyes and ears open has seen and heard the water in a clean pot "bump."
 

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