The Heavy Ash: Burning Magnesium
Observe mass gain and new substance formation in combustion
What this lesson covers
Why it matters
A ribbon of magnesium vanishes in a flash of blinding white light, leaving behind a pile of white ash. Surprisingly, the ash weighs more than the original metal.
The idea in plain words
Balance the atoms: 2 Mg atoms and 2 O atoms on both sides.
Predict first
Why does the mass of the solid product increase after burning?
Mass is conserved, but the solid gains the mass of the oxygen atoms it grabbed from the air.
- Magnesium absorbs heat energy which has mass
- Oxygen from the air combines with magnesium to form a new substance — correct
- The balance is broken by the intense heat
- Magnesium turns into a heavier isotope
What you do
Ignite the magnesium ribbon. Observe the dazzling white light and the formation of white powder.
Check yourself
What is the chemical equation for this reaction?
Magnesium oxide is MgO. The balanced equation requires 2 Mg and 1 O₂ molecule to produce 2 MgO units.
What type of chemical change is this?
Two substances (Mg and O₂) combine to form one new substance (MgO).
Why is this change considered irreversible?
Chemical changes form new substances with different properties. You cannot simply 'un-burn' MgO to get Mg metal back by physical means.
- Mg + O₂ → MgO₂
- 2Mg + O₂ → 2MgO — correct
- Mg + O → MgO
- Decomposition
- Combination — correct
- Physical change
- You cannot turn the white ash back into magnesium ribbon by cooling it — correct
- The magnesium disappears completely
- It requires too much energy to reverse