Ask A Scientist Chemistry Archive

name         Amy G.
status       educator
age          30s

Question -   Are all strong acids liquids at room temperature and pressure?  I know that citric acid 
can be a solid under these conditions, but is there a stronger acid that is solid or is there a reason 
that strong acids are all liquid at room temperature and pressure?
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Amy,

The strength of common acids is related to the extent to which they dissociate (to yield H3O+ ions) 
when dissolved in a solvent -- almost always considered to be water. Using your example of citric 
acid, and the foregoing outlined here, citric acid is not acidic until its dissolved in water. This 
is also true of other common solid "acids."

Regards,
ProfHoff 787
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Your observation is correct MOST strong acids melt below room temperature, but exceptions are not too 
rare. For example HClO4-H2O the monohydrate of perchloric acid melts at about 50 C. and trichloroacetic 
acid has two crystal modification -- one melts at 58 C and the other at 49.6 C. The reason that strong 
acids "seem" to all be liquids is that they for the most part rapidly absorb water vapor to form 
solutions, so one seldom sees that anhydrous acid unless precautions are taken to exclude water.

Vince Calder
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