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Offline jace

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h2o
« on: 23/04/2006 13:47:08 »
why does the boiling point of water go down if u add salt or sugar;):)


jack


 

Offline ukmicky

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Re: h2o
« Reply #1 on: 23/04/2006 14:37:53 »
HMMM , It doesn't i believe if you add salt to water the dissolved salt particles increase the density of the fluid increasing its boiling point and lowering it freezing point.

I could be wrong but i think that salt raises the boiling point and lowers the freezing point of water more than sugar as salt particles are smaller than sugar particles  and so more can be dissolved into water increasing the density more before the point of supersaturation.

Michael
« Last Edit: 23/04/2006 15:54:21 by ukmicky »
 

Offline jace

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Re: h2o
« Reply #2 on: 23/04/2006 16:59:39 »
i think this is wrong as i diid an experiment on it and the boiling point went down

jack
 

Offline rosy

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Re: h2o
« Reply #3 on: 23/04/2006 17:44:46 »
Ur, the change in boiling and freezing point observed should be as Micky says but it's not actually to do with the density... it's thermodynamics (what isn't!)

The boiling point is expected by theory to increase, because the disorder (entropy) of the solution is increased by the presence of solutes and so the dilute solution is favoured over the more concentrated solution and the gas at a slightly higher temperature than the pure solvent.
Adding solutes will also depress freezing point since the solution is favoured over the more concentrated solution and the solid solvent.

I don't know the details of your experiments but as it's been tested quite often in the past my instinct is to suggest there's something wrong with your data rather than the theory...

The reason more salt dissolves in water than sugar isn't down to the particle size its to do with the different interactions between the particles, with each other in the solid and with the water molecules and between the water molecules.
 

another_someone

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Re: h2o
« Reply #4 on: 23/04/2006 18:00:46 »
quote:
Originally posted by jace

i think this is wrong as i diid an experiment on it and the boiling point went down

jack



What were you measuring?

This is way outside of my field (well, most things are), but I will make a guess.

I am guessing that since you are talking about adding salt and sugar, which are cooking ingredients, that you may simply be talking about your experiences at the kitchen stove, rather than a laboratory (my apologies if I am misjudging you).

When people here are talking about liquids boiling, they are talking about changing from liquid to gas.  What is often observed when cooking something on a stove is the formation of bubbles when boiling.  It may be that adding impurities into water will increase the amount by which bubbles are formed, but that is not the same as increasing the amount of vapour that is formed.



George
« Last Edit: 23/04/2006 18:02:34 by another_someone »
 

Offline elegantlywasted

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Re: h2o
« Reply #5 on: 23/04/2006 18:12:25 »
Jace, I did the same experiment in chem last week, the NaCl solution boiled at 98 degrees Celcius, and the sugar soultion at 105. I still dont really understand why the salt was at 98, I thought it was a mistake on my part, but if you had similar results...

-Meg
 

Offline ukmicky

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Re: h2o
« Reply #6 on: 23/04/2006 18:26:23 »
quote:
Originally posted by rosy

Ur, the change in boiling and freezing point observed should be as Micky says but it's not actually to do with the density... it's thermodynamics (what isn't!)


The reason more salt dissolves in water than sugar isn't down to the particle size its to do with the different interactions between the particles, with each other in the solid and with the water molecules and between the water molecules.

Its so good to be right for once :), its just a  shame it was for the wrong reasons :(

Michael
 

Offline rosy

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Re: h2o
« Reply #7 on: 23/04/2006 18:29:07 »
What was the boiling point of water with no solute (Jack or Meg)?
 

another_someone

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Re: h2o
« Reply #8 on: 23/04/2006 19:02:58 »
quote:
Originally posted by rosy
The boiling point is expected by theory to increase, because the disorder (entropy) of the solution is increased by the presence of solutes and so the dilute solution is favoured over the more concentrated solution and the gas at a slightly higher temperature than the pure solvent.



What about when one talks, not of a dilute solution of salt, but a saturated solution of salt?

As the water boils off, the salt will precipitate out of solution, and will thus crystallise.  In crystallising, will not the salt generate heat through the latent heat of crystallisation?  If this is so, how would it effect the amount of heat (and thus energy) required to biol off the water from the saturated solution?



George
 

another_someone

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Re: h2o
« Reply #9 on: 23/04/2006 19:04:45 »
quote:
Originally posted by rosy

What was the boiling point of water with no solute (Jack or Meg)?



Other question what was the purity of the water without the salt/sugar was it distilled water?



George
 

Offline elegantlywasted

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Re: h2o
« Reply #10 on: 23/04/2006 19:20:42 »
it was distilled water, boiled at 99 the first go, and 100 the second. We used 100 in our equations... my prof said that elevation has alot to do with it, as well as the air pressure in the lab. The lab I worked it is sealed off from the rest of the school with its own air system. That could have had an effect on the results.

-Meg
 

Offline jace

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Re: h2o
« Reply #11 on: 23/04/2006 19:53:01 »
my results say the more salt you put in the less the boiling point as my results went down

jack
 

Offline parsley

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Re: h2o
« Reply #12 on: 23/04/2006 21:24:16 »
We were always taught in food tech that the reason you add salt to water when you boil pasta is because it raises the boiling point so the pasta cooks more quickly, however food tech lessons are a distant memory for me now. I remember my chemistry teacher saying something similar more recently though. This sounds alot like a physics experiment - what is in theory supposed to happen, and what actually happens are opposites!

"I just set fire to the table!"
Bring on the chemicals!

 

Offline ukmicky

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Re: h2o
« Reply #13 on: 23/04/2006 21:29:28 »
Hi rosy :)
As salt desolves in water some of its molecules break up and some of the hydrogen bonds break apart and become sodium hydroxide and hydrogen chloride, this requires energy which i assume is taken from the water in the form of heat . Now as water boils away and salt crystal are formed is this process reversed and if so is energy once again taken from the water to fuel this process or is energy released back into the water as everything reforms.

Or have i got it wrong again, go easy on me

Michael
« Last Edit: 23/04/2006 21:32:29 by ukmicky »
 

another_someone

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Re: h2o
« Reply #14 on: 23/04/2006 21:34:03 »
Out of curiosity, I thought I'd hunt around for some answers on this.

Not a lot found, but one sight asked a similar, though not exactly the same, question:

http://www.newton.dep.anl.gov/askasci/gen01/gen01846.htm
quote:

Question -   When the water is close to the boiling point happen
something strange to me: if I put salt in the water it suddenly boils (on
the contrary if put the salt before it takes longer to reach the boiling
point). Why?
---------------------------------------
Davide,

In order to facilitate boiling, a nucleation point, a point where bubbles
can begin to form, is helpful. Typically, a nucleation point is something
that has a large surface to volume ratio, like a porous stick, or grains
of sand, rice grains, bits of rocks, or broken pieces of porcelain - and
as you found out, salt. You can try this same experiment with such
materials - but be VERY careful that you do not scald yourself!

As to why adding salt before the water comes to a boil seems to prolong
the advent of boiling - there is a colligative property known as "boiling
point elevation". It is also possible that the addition of salt can change
the specific heat of the system so that the saline solution has a higher
heat capacity then an equivalent amount of water ---- but I really think
these effect are too small for the amount of salt that you are mixing in
the water to be noticeable. More then likely, this is more a perception
issue then a real objective observation.

Greg (Roberto Gregorius)
====================================================================
All powders carry some trapped air between grains.
This air acts as nucleation points for liquid converting to vapor.
If the liquid is superheated (hotter than boiling,
but not hot enough to start it's own microscopic bubbles a few molecules
wide),
then suddenly providing new nucleation sites can cause extremely sudden
boiling.
Heating clean water in a clean container,
or water heated in its volume rather than at a surface,
(microwave ovens do that),
often develops some degree of superheating before steady boiling begins.
That makes heated water dangerous at that stage.

To bypass the superheated-liquid stage, it is customary to, before very
hot, put in a few
dry "boiling-stones" of some inert and porous substance (carbon, stone, or
DuPont's Teflon (TM)).
Their job is to keep some trapped air bubbles available to the liquid at
all times.
If you do this to both solutions I think the difference in their behavior
will decrease.

If you boil the water with boiling stones in it, then cool the water, then
heat again,
the boiling stones will not help the second boil as they do the first.
During the first boil, the air in the bubbles are replaced by steam.
When the liquid is cooled, these steam-bubbles can collapse out of existence,
and all parts of the stone's surface are wetted completely.
So, for the second boil they provide no air-bubbles, no nucleation-points.
These stones have no effect.
At these times you may throw in a few new (dry) stones, and they will serve.
Later, after they are removed and dried, they can all be used again.



http://www.answers.com/topic/solution
quote:

The addition of solute affects the boiling point, freezing point, and vapor pressure of the solution, in general raising the boiling point, depressing the freezing point, and lowering the vapor pressure (see Raoult's law). A number of substances (acids, bases, and salts) exhibit characteristic behavior in aqueous solution. These substances dissociate in water to form positive and negative ions that enable the solution to conduct electricity. Such solutions are called electrolytic (see electrolyte).


The addition of some solutes to a solvent will raise the temperature of the solution, while others may lower the temperature and still others will have no noticeable effect. This behavior depends on the heat of solution of the solute in the given solvent. The heat of solution, i.e., the amount of heat given off or absorbed during the process of solution, is equal to the difference between the energy that must be supplied to break up the crystals of the solute and the energy that is released when the solute particles are taken into solution by the solvent (see enthalpy). If the heat of solution is negative (i.e., more energy is required to break up the crystal than is released in forming the solution), then the temperature will decrease; if the heat of solution is positive, the temperature will increase.



And here is an answer that looks closer to your question:

http://antoine.frostburg.edu/chem/senese/101/solutions/faq/boiling-pt-elevation-experiment.shtml
quote:

What's wrong with this experiment for measuring boiling point elevation?


Hi, I'm doing a lab on the effect of salt on the boiling point of water... The procedure is to fill a metal pot 3/4 full with water, add salt, place a thermometer in for 5 minutes, then place the pot on the stove and begin heating the pot. I was hoping you could give me some information on what happens.
Gianna Massouras
quote:

Gianna,
There are some flaws in this experiment that you'll have to correct if you want to use the results to show how salt affects the boiling point of water.
The molecules in the liquid are in constant motion. They constantly collide with each other and with the walls of their container; every now and then, a collision will cause one of the molecules to be ejected from the liquid into the air above it, creating a vapor pressure above the liquid. Heating the liquid increases the average speed of the molecules, increases the number of molecules ejected, and increases the vapor pressure.
A liquid boils when its vapor pressure becomes equal to atmospheric pressure. Low atmospheric pressure causes the boiling point to go down; high pressure drives it up. Atmospheric pressure varies a bit from day to day, depending on the weather, and it varies from place to place, depending on the altitude. So it's quite possible that you'll get a boiling point for the salt water that's less than 100, which seems to contradict what your textbook predicts (namely, that salt increases the boiling point temperature).
You can look at the effect of salt without the pressure effect if you measure the boiling point of the water before adding the salt. That way you can compare the boiling point of the water and the solution at the same pressure.
Another thing that affects the vapor pressure of the water is the relative number of water molecules in the solution. The higher the percentage of water, the more molecules will escape into the vapor, and the higher the vapor pressure will be. A salty solution has a lower percentage of water molecules than pure water does. So dissolving salt in water decreases the vapor pressure of the water. The more salt you dissolve, the lower the vapor pressure of the water becomes. You'll have to heat the solution to a higher temperature than before to get its vapor pressure equal to atmospheric pressure
You must carefully measure the amount of salt you use to be sure that it's salt and not something else that is affecting the boiling point. The boiling point goes up by roughly 1C for every mole of NaCl per liter- it's a very small effect, and one you're likely to miss completely if you're sloppy or your thermometer isn't very precise.
Finally, make sure that the temperature you're measuring is really the boiling point. The first bubbles that appear aren't steam- they're dissolved air, which comes out of solution as the temperature rises. You won't be at the boiling point until the bubbles forming at the bottom rise all the way to the top and burst on the surface. Don't wait too long; as the liquid boils, some will evaporate, increasing the salt concentration. The boiling point temperature will slowly climb as the experiment proceeds!
Author: Fred Senese senese@antoine.frostburg.edu








George
 

another_someone

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Re: h2o
« Reply #15 on: 23/04/2006 21:48:09 »
quote:
Originally posted by ukmicky

Hi rosy :)
As salt desolves in water some of its molecules break up and some of the hydrogen bonds break apart and become sodium hydroxide and hydrogen chloride, this requires energy which i assume is taken from the water in the form of heat . Now as water boils away and salt crystal are formed is this process reversed and if so is energy once again taken from the water to fuel this process or is energy released back into the water as everything reforms.

Or have i got it wrong again, go easy on me

Michael



I don't believe this is right (but then, I ,may be wrong).

Sodium chloride is not a molecule, it a a collection of ions.  There are no molecular bonds in the sense that you would have in organic chemistry.  When you dissolve NaCl in water, all you get is a collection of free floating Na and Cl ions, all totally separated from each other.

Although metallic sodium can break apart water molecules, I am not aware that sodium ions (as would be found in NaCl) would have this effect (but what do I know?).



George
 

Offline daveshorts

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Re: h2o
« Reply #16 on: 23/04/2006 22:40:24 »
An issue that could be clouding the results is that if you were using a very clean vessel to boil the water in, there could be nowhere for bubbles to form (nucleation points) so the water will become super heated (above its boiling point). The salt if it wasn't quite dissolved may have been giving the steam bubbles some nucleation points.

Any arguements based on salts giving or removing energy from the water don't work as this will just increase or decrease the overall temperature a bit, not affect when it boils.
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Offline parsley

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Re: h2o
« Reply #17 on: 24/04/2006 16:40:40 »
If we are talking about salt precipitating from a saturated solution of salt water increasing the temperature, then could it have something to do with an energy like lattice enthalpy?
Obviously a lattice enthalpy being forming the ionic solid from its gaseous ions, this isn't quite that, as the ions are in solution, but surely there is a similar principle at work: that forming an ionic solid is exothermic, and this energy would be lost as heat to the surroundings which would cause the measured temperature to go up, if not the actual temperature at which the water boils.
Also, could it be something to do with adding an ionic force to the already existent hydrogen bonds that increases the intermolecular forces slightly, causing the boiling point to rise?
I am curious now.

"I just set fire to the table!"
Bring on the chemicals!

 

Offline Cut Chemist

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Re: h2o
« Reply #18 on: 09/05/2006 03:26:29 »
The pressence of a solute decreases the entropy of water because the water molecules line up around the solute (the oxygens are attracted to the cations, and the hydrogens are attracted to the anions)increasing the order of the system.  This increases the boiling point because the gas state has a much higher entropy than the liquid state, and this makes the difference in entropy greater.  Therefore more heat is required to change from liquid to gas.
 

Offline Cut Chemist

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Re: h2o
« Reply #19 on: 09/05/2006 03:47:56 »
Change in Gibbs free energy (G)  = change in enthalpy (H) - (Temerature)x change in Entropy (S)

For the reaction to be spontaneous delta G must be negative
so delta H must be less than T x delta S
at constant delta H (like on your stove),
but lowered delta S - the temperature must be increased in order for the water to boil.

so if entropy (disorder) decreases
then temperature must rise.

Does that make sense??? Its physical chemistry.
 

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Re: h2o
« Reply #19 on: 09/05/2006 03:47:56 »

 

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