Why is a magnesium ribbon cleaned with sandpaper before burning?

Correct answer: To remove the magnesium oxide layer on the surface, which hinders burning.

A thin layer of magnesium oxide
\(\mathrm{MgO}\)
forms on the surface of magnesium ribbon when it is exposed to air. This layer prevents the magnesium underneath from making direct contact with oxygen, so the ribbon may not burn readily or the reaction may not be clear. Sandpaper removes this layer, allowing fresh magnesium to burn with a dazzling white flame and form
\(\mathrm{MgO}\)
; cleaning does not increase the melting point.