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JEE Main levelSingle correct optionDifficulty 2 of 3pb027

A solution of $AlCl X_{3}$ in water turns blue litmus red mainly because

A $[Al (H X_{2} O) X_{6}] X^{3 +}$ is a strong enough polarising cation to release $H X^{+}$ from the coordinated water
B $Cl X^{-}$ hydrolyses to give $HCl$ and $OH X^{-}$
C $AlCl X_{3}$ dissolves with dissociation into $Al$ and $Cl X_{2}$
D $AlCl X_{3}$ is the salt of a strong base and a weak acid

Answer: A

The small, highly charged

Al X^{3 +}

polarises the O-H bonds of coordinated water:

[Al (H X_{2} O) X_{6}] X^{3 +} + H X_{2} O [Al (H X_{2} O) X_{5} (OH)] X^{2 +} + H X_{3} O X^{+}

. This produces

H X_{3} O X^{+}

and an acidic solution.

Cl X^{-}

is the conjugate base of a strong acid and does not hydrolyse.

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