Gases in the Atmosphere
Inorganic Chemistry · 1 question type
What combustion is
- Combustion is the reaction of a substance with oxygen, producing one or more oxides
- Every combustion reaction is exothermic — heat (and usually light) is released
- A combustion reaction is also classed as an oxidation reaction in the "addition of oxygen" sense
Three required combustion reactions
| Element burnt | Observations | Balanced equation |
|---|---|---|
| Magnesium | Intense white flame; fine white powder collects | 2 Mg(s) + O2(g) → 2 MgO(s) |
| Hydrogen | Burns quietly with a pale blue flame; a colourless liquid (water) condenses on a cool surface | 2 H2(g) + O2(g) → 2 H2O(l) |
| Sulfur | Burns with a pale blue flame; produces a colourless, choking gas | S(s) + O2(g) → SO2(g) |
- The magnesium product is an alkaline metal oxide
- The hydrogen product is water itself, the simplest non-metal oxide
- The sulfur product is sulfur dioxide, an acidic non-metal oxide and a major air pollutant linked to acid rain
Observations when magnesium or sulfur burns in oxygen
Question: State one or two observations when magnesium burns in oxygen or air, or give the appearance of the magnesium oxide formed (1–2 marks).
Asked in 4 of the 23 papers, and a fifth asks which metal burns with a bright white flame. The two observations are a bright white flame (or light) and a white solid, powder or ash left behind; white smoke or a grey powder is also allowed. "White precipitate" is rejected, because nothing is in solution. When only the appearance of the oxide is wanted, the flame earns nothing, so describe the solid.
Balance the equation as 2Mg + O₂ → 2MgO; one paper gave a second mark for the state symbols (s), (g) and (s). Sulfur was set once: it burns with a blue flame and gives a choking gas, and universal indicator added to that gas turns red, orange or yellow because sulfur dioxide forms an acidic solution.