A magnesium ribbon burns with a dazzling flame in air (oxygen) and changes into a white substance 'X'. The X is:

When a magnesium ribbon burns in the air, it reacts with oxygen to form a white substance called magnesium oxide (MgO). The chemical reaction is: 2Mg + O2 → 2MgO. Magnesium oxide is a white powder that is formed after the combustion of magnesium, which burns with a bright, dazzling flame.