What burns: metals, non-metals and their oxides
Most metals burn in oxygen to form basic oxides. For example 2Mg + O₂ → 2MgO and 4Na + O₂ → 2Na₂O. A few of these oxides dissolve, and the solution is alkaline: it turns red litmus paper blue. Most non-metals burn to form acidic oxides: C + O₂ → CO₂ and S + O₂ → SO₂. When these dissolve in water they give acids, which turn blue litmus red. So the kind of element decides the kind of oxide: metal gives a basic oxide, non-metal gives an acidic oxide. Many common fuels are hydrocarbons, compounds of hydrogen and carbon: methane is natural gas, and propane and butane are in the cooking gas that comes in cylinders. When a hydrocarbon burns, the carbon and the hydrogen both combine with oxygen. Careful: rusting is a slow reaction of iron with oxygen and water, not combustion, because it does not give heat and light quickly.
| Element burned | Oxide formed | In water | Litmus |
|---|---|---|---|
| metal | basic | alkaline solution | red → blue |
| non-metal | acidic | acid | blue → red |
| Acidic oxides | Basic oxides |
|---|---|
| CO₂ | Na₂O |
| SO₂ | CaO |
Oxide cards. Write ten oxide formulas on cards (for example CO₂, SO₂, MgO, CaO, Na₂O, K₂O) and a second set with the name of the element that burned. Match each oxide with its element and sort the pairs into acidic and basic oxides by asking: is the element a metal or a non-metal? Add a note for each card: what would the solution in water do to blue litmus? Do not burn anything: the cards are the experiment.