system
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human
What happens when you boil an acidic solution?

I was making tomato paste the other day and I noticed that the recipe didn't suggest I add lemon juice or citric acid.  This got me thinking.  When you boil down a liquid that is slightly acidic (boil off the water in the case of tomato paste), how is the PH affected?  Does it become more acidic?  Less?  Stay the same?
gpt
That depends on the relative vapor pressure of the acid in question, compared to water. 

If it has a higher "relative volatility" than water, the acid will evaporate and boil much faster than the water will, thus reducing the concentration of the acid. However the acid fumes can be captured and condensed using a distilling apparatus designed to handle acids, thus producing a more concentrated acid. 

If the acid has a lower vapor pressure than water, the water will tend to evaporate and boil much faster than the acid, thus concentrating it. this can be done with sulfuric acid. Though for technical reasons it's difficult to concentrate H2SO4 over 90% this way.