AI-generated practice question — model-generated for extra practice, not a previous-year CBSE board question.
Magnesium ribbon is cleaned with sandpaper before burning because its surface is covered with a thin layer of magnesium oxide (formed due to reaction with atmospheric oxygen). This oxide layer prevents the ribbon from burning properly. Removing it with sandpaper exposes the pure shiny metal, allowing it to burn readily in air.
Source: Chapter 3, Section 3.2.1
The key point examiners look for is: (1) identifying that a pre-existing oxide layer forms on the surface, and (2) stating that this layer must be removed to allow proper burning/reaction. Many students only mention "to clean it" — you must explain why cleaning is necessary (the oxide layer blocks the reaction). The source explicitly states that "surfaces of metals such as magnesium… are covered with a thin layer of oxide" which acts as a protective coat.