I've recently carried out an experiment trying to measure the boiling point elevation of water after adding 5M of NaCl and KCl. Although for some reason, KCl solution started boiling around 90 degree Celsius, theoretically, both KCl and NaCl should have the same effect on the boiling point of water right?


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    $\begingroup$ Well, yes, both should have the same (per mole, that is) and more importantly, positive effect. $\endgroup$ – Ivan Neretin Sep 16 at 11:13
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    $\begingroup$ Unless you are at a very high altitude, the boiling point of KCl solution should have been higher. Was the thermometer near the place that appeared to boil, or far away? Were you observing dissolved air bubbling out?? $\endgroup$ – DrMoishe Pippik Sep 17 at 0:51
  • $\begingroup$ @DrMoishePippik I was not at a very high altitude and the thermometer was dipped in the solution. Anyway, when can we say that the solution is "boiling" how many bubbles should appear? I repeated the experiment a few times. Does that affect the surrounding pressure in a way? $\endgroup$ – NoLand'sMan Sep 17 at 11:04
  • $\begingroup$ It may need to boil for a while to get dissolved air out. The temperature will not change appreciably unless a lot of water evaporates, increasing concentration. Measure the volume before and after, or boil in a graduated container. $\endgroup$ – DrMoishe Pippik Sep 17 at 21:04

Dissolving substances in water decreases water chemical potential

$$\mu=\frac {\mathrm{d}G} {\mathrm{d}n}$$

respectively activity $a$ :

$$\mu = \mu_9 + RT \ln {a}$$

As consequence, saturated water vapour pressure above the solution is lower than the saturate vapour pressure over pure water at the same temperature.

This leads to the higher boiling point of solution, compared to water, at the same atmospheric pressure.

See also ebullioscopy and ebullioscopic constant, useable to determine the molecular mass of the solute from the boiling temperature change and solute (w/w) concentration.

Similar principles, affecting freezing point, lead to cryoscopy


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