Question

why AlCl3 is an acid?

Answer 1

No, acids have at least Hydrogen (H) in their molecular formula.

Answer 2

\(AlCl_3\) is a \(\sf lewis~ acid\) because the aluminium can accept electrons (into it's empty p orbital), it's not a Bronsted-Lowry acid though because there aren't any protons it can give.

Answer 3

The answer depends on the level you are doing chemistry.
aaronq's answer is the right one.

Though at a more basic level, you can say that it will react to form H3O+ ions in the presence of water:

\(AlCl_3+6H_2O\rightarrow Al(OH)_3+3H_3O^++3Cl^-\)

or

\(AlCl_3+3H_2O\rightarrow Al(OH)_3+3HCl\) (AlCl3 forms hydrochloric acid in water)