Does salt added to water decrease the time needed to reach boiling point?

[PeterHarrison84]

I'm not sure if this would be better in another thread of not but here I go...

I have heard that if you put salt into water it will make it boil faster. Several people have also told me they read this same statement in cooking magazines and what not. Not wanting to trust a cooking magazine as a reputable source on thermodynamics I figured this was the place to ask.

Is this true? If so, what is the reason behind it?

Thanks

 

[russ_watters]

I'm not much into cooking, but salt increases the boiling point of water, which would make it come to a boil slower, unless it had a significant impact on the heat transfer itself, which I doubt.

 

[daveb]

Yes, and that's the reason it's used in cooking. Cooking food at a hotter temperature decreaeses the time it takes to cook the food. I don't know if the total time (time to boil water + time to cook food) is increased or decreased, since you not only have to take into account the heat capacity of the water and food but also the rate of heat transfer for both as well.

 

[sdemjanenko]

boiling is relative to the mixture. if it boils faster than normal, which salt does not do but a higher altitude will do that does not mean that it is safe at that point since the boiling temperature changes, in the altitude case it lowers. You need to get the water to 212 F or 100 C which is the normal boiling temperature to make sure that whatever you are cooking, if its cooking is safe to eat and is cooked the right length of time. In normal environments boiling is only an indication of the temperature, nothing more.

 

[Gokul43201]

I have heard that if you put salt into water it will make it boil faster. Several people have also told me they read this same statement in cooking magazines and what not.

The above statement is false.

What is true, however, is that you can cook foods a little bit faster in salt water. The reason this is true, is pointed out above - the boiling point of water is raised slightly by dissolving salt in it. Hence, it is possible to maintain the water in the liquid state at a higher temperature than if it were pure. But how much higher is this temperature, really?

The answer lies in what is known as the ebullioscopic constant (Kb) of water. This constant relates the elevation of the boiling point ($\Delta T$) to the amount of salt in the water, through the relation:

$$\Delta T = m \cdot K_b$$

where m is the number of moles of salt in about a liter of water.

A mole of common salt (mostly NaCl) weighs about 60g. But a liter of water will not dissolve more than about 30-35g of NaCl (and the solubility is fairly constant in temperature), so the largest amount of useful salt is about 1/2 a mole per liter (this is a lot more than just a pinch of salt). Now, the value of Kb for water is only 0.5 K/mol/L. So, by adding 0.5 moles (per liter) of salt to water, you can raise its boiling point by only about 0.25K (=0.25C = 0.5F).

That's hardly a noticeable effect. Yikes!

(Looks like this just debunked the idea that adding a little salt makes stuff cook "much faster" by raising the boiling point of water. The difference is barely observable. I'd always imagined it was an increase of at least a few degrees... until now!)

 

[Bystander]

(snip)A mole of common salt (mostly NaCl) weighs about 60g. But a liter of water will not dissolve more than about 30-35g of NaCl (and the solubility is fairly constant in temperature), (snip)

Solubility limit and temperature dependence is incorrect (4 1/2 m at 273 K to 6 m at 373 K) --- 12m x 0.5 K is 6 K elevation at saturation and 1 atm.

Boiling faster? Slower. Run the Langmuir evaporation rate expression --- vapor pressure hasn't changed, MW hasn't changed, and difference in inverse of square root of T slows it down.

 

[turbo]

The water will not boil faster (sooner) with salt added. It will start to boil at a later time, but at a higher temperature, which is of interest to some cooks, who wish to cook some foods for a shorter period of time at a higher temperature - pasta comes to mind.

 

[DaveC426913]

Sounds like fertile ground for some empirical evidence. Who's up for a kitchen experiment?

 

[Gokul43201]

Solubility limit and temperature dependence is incorrect (4 1/2 m at 273 K to 6 m at 373 K) --- 12m x 0.5 K is 6 K elevation at saturation and 1 atm.

Oops, missed a factor of 10 in the solubility, didn't I? It's 30g/100cc not 30g/L. Surely a 30% variation with temperature satisfies the "fairly constant" criterion for my envelope back.

Nevertheless, for a liter of water, this says you need nearly a couple pounds of salt to make a paltry 5K difference to the boiling point. That still reduces the cookbook tip of "adding a little bit of salt to raise the boiling point" to rubbish, no?

Boiling faster? Slower. Run the Langmuir evaporation rate expression --- vapor pressure hasn't changed, MW hasn't changed, and difference in inverse of square root of T slows it down.

This is a different question than what I think the cookbooks are talking about - which is the time it takes to cook a thing (not the time it takes to boil the water).

 

[Bystander]

Salting cooking water is strictly flavoring --- how much time do you gain salting water for pasta? Nothing measurable.

 

[Gokul43201]

As for the other question of the boiling time, do you think Langmuir gives a good approximation to the boiling rate?

I'm skeptical. The vapor is hardly thermalized with the liquid, and the process (boiling) is not at all like evaporation.

 

[Bystander]

Langmuir's about all we got --- boiling vs. evaporation? Not a whole lot of difference beyond the temperature range of the superheat necessary for boiling. Throw in surfactants and you've got an interesting problem --- to which I've no quick answer at the moment.

 

[NoTime]

If you want to cook things faster...
That's what they have pressure cookers for.

I was always under the impression of what Bystander said.
It's for flavoring.

 

[DaveC426913]

It's for flavoring.

Also prevents pasta from sticking together.

But neither of these rule out the 'faster cooking' theory.

 

[PeterHarrison84]

Thanks for the info everybody. Although I will still continue putting an excessive amount of salt in everything regardless of the boiling time simply because it's delicious!

 

[cracker]

Yeah what they said ^ ^ ^ I learned this in Chemistry

The more poluted the water is the higher its boiling point and the more pure the water is then the lower the boiling point is

Also on days when the pressure is low then the boiling point is lower because the water is able to escape easier because there is less pressure pusshing down on it. Also the higher your elevation the lower the pressure. and just because water is boiling don't mean its hot but most of the time it is so be carefull lol

 

[Gokul43201]

Langmuir's about all we got --- boiling vs. evaporation?

I can't say I recall very much of how Langmuir's derived, or where exactly it applies (I think it's used for determining deposition rates in processes like CVD, and also maybe adsorption rates), but I find it wholly unpalatable (for the moment) that a puddle of water on the floor disappears faster if the water's colder!

Here's a preliminary alternative thought: At a constant heat transfer rate into the liquid (changes in heat transfer properties over ~10K being neglected), the boiling rate is nothing but the ratio of this heating rate to the enthalpy of vaporization. From Hess, it seems that the enthalpy of vaporization should be suppressed for salts whose dissolution is endothermic (assuming I'm not making a sign error), which gives me a bigger boiling rate with increasing boiling point.

(However, Clausius-Clapeyron, which I'm not sure if I can use here, suggests differently...gah!)

 

[Bystander]

I can't say I recall very much of how Langmuir's derived, or where exactly it applies (I think it's used for determining deposition rates in processes like CVD, and also maybe adsorption rates), but I find it wholly unpalatable (for the moment) that a puddle of water on the floor disappears faster if the water's colder!

Here's a preliminary alternative thought: At a constant heat transfer rate into the liquid (changes in heat transfer properties over ~10K being neglected), the boiling rate is nothing but the ratio of this heating rate to the enthalpy of vaporization. From Hess, it seems that the enthalpy of vaporization should be suppressed for salts whose dissolution is endothermic (assuming I'm not making a sign error), which gives me a bigger boiling rate with increasing boiling point.

(However, Clausius-Clapeyron, which I'm not sure if I can use here, suggests differently...gah!)

J, "Mdot," or whatever for mass flux = p_sat/(sqrt(2piMRT)); calling p_sat constant for boiling at one atmosphere is what slows the rate --- mixture of non-volatile (salt) and volatile (water) might be better handled as xp_sat --- ain't really looked at it. For the puddle on the floor, I'm sure you remember that p_sat is a function of T --- I just pulled a fast one for the constant pressure boiling of a mixture. Remember the other things going on in evaporation vs. boiling --- heat transfer rate is low, enthalpy of vaporization is high --- in non-equilibrium situations, you're looking at a steady-state balance of heat loss through evaporation, or boiling, with heat transfer to the liquid.

 

[NoTime]

Also prevents pasta from sticking together.

I find a little olive oil does a lot better job with the sticky issue.

But neither of these rule out the 'faster cooking' theory.

Yea! But I think Gokul did.

 

[Chronos]

Turbo is correct, salt water not only boils at a higher temperature, it heats more rapidly than unsalted water.

 

[profg]

i just did an experiment on this today, and the more salt in the water, the faster it boiled

 

[Loren Booda]

Did no one mention the heat of dissolution? When I add salt to water near boiling, that water seems to boil briefly.

The National Research Council of the National Academy of Sciences in Washington, D.C., has determined that the recommended safe minimum daily amount is about 500 milligrams of sodium.

 

[Noog_Z]

Um.. my class just did a experiment on how adding the mass of salt to boiling water increases its temperature and now we have to write a thing on it, and we have to research it...

even thought i wasn't there for two days I'm sort of getting it.

this is what i got out of the prac.

if u put in 2g of salt to tap water with a temperature of...26°C, in the 1st minute the temperature is raised to 42°, whereas if u put in 4g, in the first minute, its raised to 46° and if u ut no salt in its raised to just 30°, by the results in my table, it seems to me that although salt helps it boil faster, it doesn't get to boiling point, because, at 10 minutes, no salt went up to 100°, in 9 minutes, with 2g, 96°, and with 4g in 7 minutes, 94°. My teacher said that as son as u record two temperatures the same in 2 minutes, to stop. so, even though the salt boiled faster, it didn't get as high as no salt.(: (: (: wow, i think i have my conclusion for my writing done... yay!

 

[DaveC426913]

if u put in 2g of salt to tap water with a temperature of...26°C, in the 1st minute the temperature is raised to 42°, whereas if u put in 4g, in the first minute, its raised to 46° and if u ut no salt in its raised to just 30°, by the results in my table, it seems to me that although salt helps it boil faster, it doesn't get to boiling point, because, at 10 minutes, no salt went up to 100°, in 9 minutes, with 2g, 96°, and with 4g in 7 minutes, 94°. My teacher said that as son as u record two temperatures the same in 2 minutes, to stop. so, even though the salt boiled faster, it didn't get as high as no salt.

Wow. I was just about to comment on how hard this was to read when I noticed this status message:

Last edited by Noog_Z; Y at 11:53 PM.. Reason: spelling mistakes (:

This is after editing?

 

[Noog_Z]

and all those weird equation thingies made no sense to me, its not that hard to figure out what happens to boiling water when salt is added.

 

[Noog_Z]

Wow. I was just about to comment on how hard this was to read when I noticed this status message:

Last edited by Noog_Z; Y at 11:53 PM.. Reason: spelling mistakes (:

This is after editing?

hey, I am a teenager! I am more focused on what is actaully in the paragraph more than my spelling. (:

how could u not understand that? what made it hard to read? I am confused?

 

[blubricks]

I apologize for reviving a dead thread tat surely none of the OP's are around to defend, but like a book it doesn't lose relevance by being old. I was searching the physics of salt water for cooling purposes, not heat, and so I will continue searching, though...my answer is in here, I am just not smart enough to see it. The reason I felt compelled to post is that everyone lost the "implied" goal of the OP. The OP mentioned cooking, I myself am a chef and remember my basic science classes, and for what I can extrapolate (big word, so I sound intelligent) from such a "smart" group of people is yes salt has an effect on the boiling point of water. What everyone missed, was the effect on the food being boiled. I suppose I best not guess on the physics, but I am curious as to how an increase of salinity increases absorption and thereby more efficiently spreading heat through the mass of the food. I will mention that, as a chef, only foods high in carbohydrates are boiled. corn, potatoes, etc are boiled. Ribs, pork and beef, come to mind, but I suspect this is a crude way of steaming (and a horrible way to treat meat) and "tenderizing" tough proteins. I won't bother with my perspctive on coagulation and such. I merely am searching for an answer and saw some misguided, yet highly "intelligent" people along the way.

 

[blubricks]

Knowledge is worthless without purpose.

 

[coelho]

Wow that's a quite old thread... anyway, now that it was summoned from the depths of time, i think it would be useful to make some comments.

Reading the posts, i did see lots and lots of theory, but almost nobody (except one person) actually did an experiment to verify if it was actually true or not.

Put a pan with pure water on the stove and let it heat until it almost start boiling. When the inner walls of the pan were covered with little bubbles, throw a handful of salt in the water. If it were done at the right time, a LOT of bubbles will be produced when the salt were added, creating the impression that the water started to boil when one added the salt.

The same effect can be achieved throwing a spoonful of sugar (or salt, or any powder) in a cup of coke (or any other carbonated drink). "Mentos" (that mint) works as well as sugar.

This happens because the bubbles need a surface to grow on. Thats why there are bubbles on the inner walls of a cup of carbonated drinks, or of a pan of boiling water. The added salt (or sugar or mentos or whatever), being a powder, has a very large surface, and so allows the creation of a lot of bubbles, giving the impression the water (or coke) started to boil.

 

[kwinb]

just helped my niece test her hypothesis that salt added to water would decrease the time needed to reach boiling (100 C) using a candy thermometer

she used the 2 cups of water and did 3 tests each with 0, 1, 2, 3, and 4 tablespoons of salt

pot was identical, and was cooled to same starting temperature each time

she let the salt dissolve first, before initiating boiling. She found that the time to boiling point was nearly identical for 0 and 1 tablespoons, but the time to boiling decreased for each of the next sets of tests by about 1 to 1.5 minutes.

She is full of questions, and possible next steps

It is so nice to see her excited about it

 

[turbo]

just helped my niece test her hypothesis that salt added to water would decrease the time needed to reach boiling (100 C) using a candy thermometer

she used the 2 cups of water and did 3 tests each with 0, 1, 2, 3, and 4 tablespoons of salt

pot was identical, and was cooled to same starting temperature each time

she let the salt dissolve first, before initiating boiling. She found that the time to boiling point was nearly identical for 0 and 1 tablespoons, but the time to boiling decreased for each of the next sets of tests by about 1 to 1.5 minutes.

She is full of questions, and possible next steps

It is so nice to see her excited about it

It's nice to see that you are willing to explore the scientific method with her. Many adults are not so willing and go down the "everybody knows" route, which is counter-productive with kids. Kudos!