Question

Why do magnesium ribbon floats on the surface of dilHCL

Answer 1

Because the hydrogen gas formed in the reaction sticks to the surface of magnesium ribbon and makes the ribbon float on the surface.
Hope u got it...

Answer 2

When Mg reacts with hcl it forms magnesium chloride and h2 gas is released
This hydrogen gas sticks to the surface of magnesium and this it appears to be floating because of resuction in weight..

Hope this helps!