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Question
When magnesium is burnt in air, there is an increase in mass after the reaction.
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Solution
When magnesium ribbon is burnt in air, a white solid, magnesium oxide, is formed. The mass of magnesium oxide is more than the mass of magnesium. This is because the mass of oxygen used is not taken into account. If that is taken into account, the total mass of the reactants and products is found to be nearly equal.
\[\ce{\underset{magnesium}{2Mg} + \underset{air}{O2} -> \underset{magnesium oxide}{2MgO}}\]
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