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Well, the question is short. Can lewis acids and bases change the colour of a litmus paper? I can't seem to find the answer online (at least according to my high school level chemistry search history).

The intuitive answer I feel is that they shouldn't be able to, as lewis acids and bases are defined only by their ability to donate or accept electrons. On the other hand, Arrenhius and Broensted-Lowry acids seem to be able to change the colour of litmus, possibly due to the presence of H+ or OH-

As a bonus question, is there any indicators that can be used to test the presence of lewis acids that change their colour after donating a lone pair of electrons?

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    $\begingroup$ Let's see. Will litmus react to AlCl3, a Lewis acid? Without water, probably no, but then again, without water many ordinary Broensted-Lowry acids and bases won't react either. With water, it will, because AlCl3 will partially turn into a BL acid due to hydrolysis. $\endgroup$ Feb 5, 2021 at 16:27
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    $\begingroup$ @IvanNeretin, yes you are right to point that out. I think dry HCl is a good example. But when it dissolves in water, i learnt it forms H3O+ and OH- which is are required for the litmus paper to change colour, without which HCl wouldnt change colour. Would AlCl3 also form a similiar ions? $\endgroup$ Feb 5, 2021 at 16:34
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    $\begingroup$ Yes, Anhydrous $\ce{AlCl3}$ reacts violently with water, producing insoluble $\ce{Al(OH)3}$ and gaseous $\ce{HCl}$. If enough water is present $\ce{HCl}$ will soon get dissolved producing $\ce{H3O+}$ which will dissolve $\ce{Al(OH)3}$. $\endgroup$
    – Maurice
    Feb 5, 2021 at 17:08

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You should keep in mind that the concept of acid-bases is not restricted to water. Lewis acid definition is applicable to gaseous, aqueous and non-aqueous systems. Consequently, all Bronsted-Lowry acids are Lewis acids. The idea applies to bases. Note that the converse is not true just like an apple is fruit but not all fruits are apples. The idea of litmus test is rather restricted to aqueous systems. So in short, the all relatively strong Lewis acids and bases in water can affect the litmus.

If you put a blue litmus is very dry HCl (gas), it would still turn it red because paper is never dry. It always has moisture absorbed in it.

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