2
$\begingroup$

There is a so common understanding (IMHO misunderstanding) that you don't add salt to water on the beginning but later to make cooking faster. Ignoring the fact that the difference of boiling points and time needed to reach that higher is very minimal I would like to approach this problem theoretically. So there are two cases - first one when salt is added before boiling - then the boiling point increases but with same ammount of energy it should't affect final time of boiling.

Regarding second case and following this example https://chem.libretexts.org/Courses/Valley_City_State_University/Chem_122/Chapter_3%3A_Solutions_and_Solubility/3.5%3A_Freezing_Point_Depression_and_Boiling_Point_Elevation

boiling point of water+salt is elevated from 100 to 101.78 by adding 10g NaCl to water, how about adding same amount of NaCl to already boiling water but when temperature is under 101.78 ?

$\endgroup$
1
  • 2
    $\begingroup$ Why not do the experiment? $\endgroup$
    – John Doty
    Commented Dec 16, 2022 at 21:43

1 Answer 1

2
$\begingroup$

Dissolving a solute in water raises its equilibrium boiling point.

(The reason is that impure water is less than 100% water, but the vapor phase above it is still 100% water. Since mass tends to move to areas of lower concentration—strictly, lower chemical potential, but this doesn't affect the reasoning here—the liquid phase is favored, so a higher temperature is required to reattain equilibrium with the gas phase.)

As you note, the boiling point elevation for a palatable amount of salt is minimal for cooking purposes but still detectable by a suitably sensitive thermometer.

One way to demonstrate the effect is to place the water (with inert boiling chips to ensure plenty of heterogeneous nucleation sites) in a container of controllable temperature; the water and surrounding air should be well stirred to ensure temperature uniformity. Upon boiling at slight overheating above 100°C, one adds salt that has been stored in the same container to avoid temperature differences. The boiling is then expected to stop, both because the dissolution of sodium chloride in water is endothermic and because the equilibrium boiling temperature is now higher. (A momentary froth of bubbles might be seen as the undissolved salt provides nucleation sites for pure-water boiling and as any attached air bubbles detach and float up.) The endothermic effect can be decoupled by bringing the solution to the original boiling temperature, where no boiling is expected. Boiling is expected to resume only upon additional heating to achieve slight overheating over the new equilibrium boiling temperature.

Does this get at what you're asking about?

$\endgroup$
1
  • $\begingroup$ Yes, thank you, that was my concern $\endgroup$
    – nusch
    Commented Dec 18, 2022 at 16:44

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.