The aqueous solution of potash alum is acidic due to hydrolysis of

${\mathrm{Al}}^{3+}$ having high charge density will associate strongly with water through its fractional negatively charged oxygen pole and in such interaction there will be hydrolysis to some extent :

${\mathrm{Al}}^{3+} + {\mathrm{H}}_2\mathrm{O} \rightleftharpoons {\left[\mathrm{Al}\left(\mathrm{OH}\right)\right]}^{2+} + {\mathrm{H}}^{+}$

The product cation may hydrolyze further.