Two aluminium pieces are taken. One is dipped in HCl solution and the other piece in $H N O_{3}$ solution. In the first case the metal piece disappeared after some time whereas in the second case, the piece is not disappeared.

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Solution

On reaction with $H C l$ following reaction occurs:
$3 A l + 3 H C l ⟶ {A l C l}_{3} + \frac{3}{2} H_{2}$
Hence, $A l$ keeps on reacting and eventually the metal disappears.
On reaction with ${H N O}_{3}$ following reaction occurs:
$4 A l + 2 {H N O}_{3} ⟶ 2 {A l}_{2} O_{3} + H_{2} + N_{2}$
Hence, $A l$ keeps on reacting until a passive layer of aluminium oxideis formed and eventually the reaction stops and metal does not disappear.

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Two aluminium pieces are taken. One is dipped in HCl solution and the other piece in HNO3 solution. In the first case the metal piece disappeared after some time whereas in the second case, the piece is not disappeared.

Two aluminium pieces are taken. One is dipped in HCl solution and the other piece in $H N O_{3}$ solution. In the first case the metal piece disappeared after some time whereas in the second case, the piece is not disappeared.