Can Adding Water to Molten Salt Increase the Boiling Point of Salt or Water?

[NormGirl]

If adding impurities, such as salt, to water will increase the boiling point of water, what if it's the other way around? Will adding water to molten salt increase the boiling point of salt (sodium chloride) or the water? Or water is not considered an impurities to molten salt?

 

[MrSid]

As you might guess, adding liquid water which has STP boiling point of 100C is a good way to get a lot of steam from contact with typical ionic melts which are in the 500-1500 C range. The experiment to determine mp depression for ionic melts with H2O as an impurity would require high pressure to keep the phases mingled. But since mp's are colligative properties, the same principles apply. For ionic melts transitioning to boiling points things become more complicated as H2O even under pressure can not be a liquid (above critical point).

However, this is exactly what is taking place deep beneath the Earth in subduction zones that take an oceanic plate and slide it under a continental plate. A moisture rich mineral phase begins to melt at some point below the lowest melting mineral, and the less dense hot fluidic magma rises through denser rock. As it nears the surface the hot vapor rich magma can depressurize and further fractionate depending on the cooling rate of the surroundings. If it rises to the surface it can release the water explosively in a volcanic eruption.