When magnesium burns, it reacts with oxygen in the air to form magnesium oxide, which appears as a white powder. The fact that the mass of the white powder (magnesium oxide) produced is greater than the mass of the magnesium burned indicates that the process involves a chemical reaction where additional mass is gained from the surrounding environment.
This observation is consistent with the law of conservation of mass, which states that mass cannot be created or destroyed in a closed system. In this case, the increase in mass is due to the magnesium combining with oxygen from the air to form magnesium oxide.
Specifically, during the combustion of magnesium:
\[ 2 \text{Mg} + \text{O}_2 \rightarrow 2 \text{MgO} \]
Here, two moles of magnesium (Mg) react with one mole of oxygen gas (O₂) to produce two moles of magnesium oxide (MgO). The mass of the oxygen that combines with the magnesium contributes to the total mass of the magnesium oxide produced.
Overall, the increased mass in the end product shows that the magnesium has reacted with oxygen, making it a clear example of a chemical change where reactants have transformed into products, resulting in a net gain in mass due to the incorporation of oxygen.