Question

Why magnecium float on water​

Answer 1

Answer:

The hydrogen gas evolved due to this reaction sticks on the surface of magnesium ribbon making it float in hot water.

Answer 2

Explanation:

Note that magnesium metal has a density of [math]1.74g/cm^3[/math], so at first glance it shouldn’t be able to float on water. Indeed, if you take a block of magnesium and drop it in water, it most definitely sinks.

However, a common high school chemistry demo involves placing clean shavings/ribbons of magnesium metal in water. (I haven’t seen it in forever, but I seem to recall you need to clean the surface of the magnesium metal really well, i.e. with steel wool or something.)

There will be a very slow reaction between the metal and water that produces tiny bubbles of hydrogen gas. These gas bubbles cling to the small metal ribbons and can cause them to float to the surface of the water. [1]

[math]Mg(s) + 2H_2O(l) \longrightarrow Mg(OH)_2(s) + H_2(g)[/math]

Note that the magnesium hydroxide that is produced is insoluble in water. It forms a solid layer on the magnesium ribbon, inhibiting the reaction. Thus the demo only works with tiny pieces of metal, where the small amount of hydrogen gas produced becomes significant when compared with the weight of the metal