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When a carboxylic acid with low molecular weight is heated with $\ce{P2O5}$ we get the corresponding anhydride. My book says that this reaction happens with dehydrating agents such as $\ce{P2O5}$. I have also learned from my teacher that $\ce{H2SO4}$ is also a good dehydrating agent.

My question is what makes a molecule a good dehydrating agent? How can I identify one (or maybe what are the common dehydrating agents?) while solving questions?

Also, I have seen the use of $\ce{P4O10}$, but do $\ce{P2O5}$ and $\ce{P4O10}$ play the same role as a reagent?

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    $\begingroup$ Note that $\ce{P2O5}$ and $\ce{P4O10}$ are two ways to reference the identical chemical. The latter is a description of the actual molecule, which contains 4 phosphorous atoms and ten oxygen atoms (see the structure) while the former is a salt-like description only caring for the relative atom count. You should prefer the notation $\ce{P4O10}$ for being a better description of the molecules. $\endgroup$ – Jan Nov 3 '15 at 18:48
  • $\begingroup$ I think it is not possible to guess which substances are dehydrating agents. You have to learn this from the dehydrating property of the substance. Not all dehydrating reactions can be made with all dehydrating agents. $\endgroup$ – IV_ Apr 13 at 16:24

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