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When magnesium is burned in air, it chemically combines with oxygen gas to form a compound called magnesium oxide which is heavier than the metal i.e. the white ash is heavier than the original piece of magnesium.
Thus when an element is heated or burnt in air, it combines with oxygen from air and gains in weight.
Kavungya answered the question on March 25, 2019 at 09:23
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