Burning of magnesium ribbon
Aim: to observe what happens when magnesium ribbon burns in air, and to find out whether the product is acidic or basic.
2Mg(s) + O2(g) → 2MgO(s) + heat + lightmagnesium + oxygen → magnesium oxide
What is happening
When magnesium is heated strongly it combines with oxygen from the air. It burns with a dazzling white light and leaves behind a white powder, magnesium oxide.
- Two substances join to make one product, so it is a combination reaction.
- Magnesium gains oxygen, so magnesium is oxidised.
- Heat and light are given out, so the reaction is exothermic.
Why the ribbon is cleaned first
Magnesium kept in air slowly forms a thin, dull coating of magnesium oxide (with some magnesium carbonate). The coating keeps oxygen away from the metal underneath, so the ribbon does not catch fire easily. Rubbing it with sandpaper removes the coating and exposes shiny, fresh metal.
Observations
- After rubbing, the ribbon changes from dull grey to shiny silver.
- In the flame it burns with a brilliant, dazzling white light.
- White smoke rises and a white, powdery ash is left behind.
- A lot of heat is given out.
Testing the ash
Magnesium oxide dissolves slightly in water to form magnesium hydroxide. The solution turns red litmus paper blue, so magnesium oxide is a basic oxide. Oxides of metals are generally basic.
MgO(s) + H2O(l) → Mg(OH)2(aq)
Mass of the product
The magnesium oxide weighs more than the ribbon did, because oxygen from the air is now joined to the magnesium. No mass is created: it comes from the air.
Precautions
Never look straight at burning magnesium. The light is intense enough to hurt your eyes.
- Hold the ribbon with tongs, at arm's length, away from your face and clothes.
- Burn it over a watch glass so the ash is collected and the bench is protected.
- Never use water on burning magnesium; smother it with dry sand.
Conclusion
Magnesium burns in air to form magnesium oxide, a white, basic oxide. The reaction is a combination reaction in which magnesium is oxidised and energy is released as heat and light.