Question

why does magnesium ribbon start floating on hot water

Answer 1

HEY MATE . HERE IS YOUR ANSWER .

Magnesium ribbon starts floating in hot water as it is lighter than water. It reacts slowly with boiling water to form MgO and H2.

HOPE IT HELPS YOU .

Answer 2

when mg reacts with hot water it forms Mg(OH)2
and hydrogen is liberated
this hydrogen stick to magnesium and hence it floats