Because not all of the oxygen was reacted, there appears to be a mass loss.
Solid MgO is produced from the simultaneous combustion of solid magnesium metal and oxygen gas, which burns with a flash of light and heat. The initial masses of magnesium and oxygen added together are less than MgO.
Magnesium oxide is created when oxygen and magnesium interact. After burning, it transforms into a white magnesium oxide powder. Magnesium and oxygen react to produce a light powerful enough to momentarily impair your vision. Magnesium burns so brilliantly because it produces a lot of heat while doing so. This powder is produced by magnesium transferring two electrons to oxygen atoms. It is an exothermic process.
2Mg + O2 → 2MgO is a prime illustration of an oxidation and combination process.
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