Why does adding salt to boiling water cause it to flare up suddenly?

I was expecting that the water would become less "boily" since the ions in $\ce{NaCl}$ would require energy to disassociate. Instead it turned whitish and the water level suddenly rose and then subsided. I think the white color can be attributed to the fact that table salt is white but why did the water suddenly flare up ?

• – Mithoron May 5 '18 at 16:18

The salt does not instantly dissolve so the surface of the crystals suddenly provides a lot of nucleation sites for the water to form vapour - hence the surge as it boils from these surfaces. The white is due to turbulence from the boiling just like waterfalls look white.

It seems that water does not begin boiling at 100.001 degrees C. A pot of water keeps getting hotter and hotter until it produces $$H_2O$$ vapor at a rate sufficient to produce a visible bubble in one location, and then turbulence shakes some more vapor free. The extra nucleation sites proposed by Waylander suggests that most pots don't have many nucleation sites. Of course! Who wants a rough pot that is hard to clean? Much preferred is a smooth pot. Other nucleation sites (or turbulence) can cause the same boil-up: pasta added to a slightly boiling pot will flare up too. And this suggests that the hot water is actually hotter than equilibrium, either because of a hotter temperature than 100 degrees, and perhaps the vapor bubbles in the water are actually at a little more than atmospheric pressure, or that the vapor space just above the hot water is not 100% water vapor (i.e., not 1 atmosphere of $$H_2O$$), so more water vapor produced by more nucleation sites can be instantly accommodated if a boil-up occurs - this is not an adjustment of an equilibrium, but a collapse of a non-equilibrium situation.

The same thing can be done at room temperature with Diet Coke. I opened a cold fresh bottle very carefully and heard a little hiss - I know that the pressure in a bottle of Diet Coke is about 45 psi, because I have hooked up an automobile tire gauge to a bottle cap and screwed it on, then shook the bottle to get an equilibrium. But total equilibrium was not achieved, because the Coke in the bottle did not fizz. I poured some into a glass carefully - small amount of fizz, which stopped. Then I salted the liquid - viola! FIZZ! just because I added some nucleation sites. That reminded me of the Mentos addition videos.