There has obviously been a chemical
change because several chemical properties of the magnesium have been
modified: the color, the texture and the mass.
The increase in mass is due to the
fact that oxygen from the air has combined with the magnesium to make
magnesium oxide, MgO.
The chemical equation, Mg + O2
MgO shows this reaction
but it needs to be balanced to make 2Mg + O2
Using stoichiometry, we can convert
this eqation into an equation with moles:
2 mol Mg + 1 mol O2
2 mol MgO.
Next, we convert to grams using atomic
masses obtained from the periodic table:
48g Mg + 32g O2 80g
Lastly, we determine the same thing in
the proportions we used. In other words, we used only 0.15g of
Mg (not 48g) so everything needs to be divided by 320. So 80 /
320 = 0.25 g. If we burn 0.15 g of Mg, we obtain 0.25 g of MgO.