Question

Give two reaction where water acts as catalyst.

Answer 1

1. Magnesium + silver nitrate: $Mg\left(s\right)+2AgN{O}_3\left(s\right)\stackrel{water}{\to }Mg\left(N{O}_3{)}_2\right(s)+2Ag(s)$

The reaction between magnesium and silver nitrate is triggered by water. When the first drop of water falls into the mixture a vigorous reaction ensues which starts to spit and bubble.

2. Aluminium + iodine: $2Al\left(s\right)+3I_2\left(s\right)\stackrel{water}{\to }A{l}_2{I}_6\left(s\right)$

The reaction between aluminium and iodine is catalyzed by water.

This is a spectacular demonstration as clouds of purple iodine vapour are produced.