The reaction 2Mg + O$_2$ $\longrightarrow$ 2MgO, in which magnesium burns in air to form a white powder, is an example of a:

1. Identify the reactants and products: Mg and O$_2$ combine to give a single product MgO. 2. Two reactants forming one product is the signature of a combination reaction. 3. There is no single reactant splitting up, so it is not decomposition. 4. No ions are exchanged between two compounds, so displacement and double displacement do not apply. _Source: Balbharati (Maharashtra Board) Class 10 Science & Technology, Ch 3 "Chemical Reactions and Equations", p.45_