Oxygen

Oxygen (O2) is the second most abundant gas in air (21%) and one of the most heavily reused facts in 0620: it drives combustion, rusting, oxide classification and the most famous gas test in the course.

Where oxygen appears in 0620

• Gas tests: oxygen relights a glowing splint — a required test in analysis.
• Chemistry of the environment: the 21% composition of clean dry air.
• Combustion and fuels: complete vs incomplete combustion of hydrocarbons.
• Oxides: metals form basic oxides, non-metals form acidic oxides.
• Rusting: oxygen is one of the two things iron needs to rust.

Position in the periodic table

• Symbol: O
• Atomic number: 8
• Group 16 (VI), Period 2
• Electron configuration: 2, 6
• Forms O2- ions in ionic compounds
• Diatomic molecule: O2 (double bond, O=O)

Physical properties

Property	Detail
State at room temperature	Gas
Colour	Colourless
Smell	Odourless
Boiling point	-183 C
Solubility	Slightly soluble in water (supports aquatic life)
Density	Slightly denser than air

Test for oxygen

A glowing splint relights in oxygen. This is the standard test, and it is different from the hydrogen test (a lighted splint gives a squeaky pop).

Composition of air

Oxygen makes up about 21% of clean, dry air. This is shown experimentally by passing air over heated copper:

2Cu(s) + O2(g) -> 2CuO(s)

The copper removes the oxygen and the gas volume falls by about one-fifth, confirming the ~21% figure.

Combustion

Oxygen is required for combustion, and all combustion reactions are exothermic.

Complete combustion (excess oxygen)

CH4(g) + 2O2(g) -> CO2(g) + 2H2O(l)

Products: carbon dioxide and water. See complete combustion.

Incomplete combustion (limited oxygen)

2CH4(g) + 3O2(g) -> 2CO(g) + 4H2O(l) (carbon monoxide formed)

CH4(g) + O2(g) -> C(s) + 2H2O(l) (soot/carbon formed)

CO is toxic: it binds to haemoglobin and reduces oxygen transport in the blood. See incomplete combustion.

Reactions forming oxides

Reaction	Equation	Oxide type
Magnesium burning	2Mg(s) + O2(g) -> 2MgO(s)	Basic
Iron burning	3Fe(s) + 2O2(g) -> Fe3O4(s)	Basic
Sulfur burning	S(s) + O2(g) -> SO2(g)	Acidic
Carbon burning	C(s) + O2(g) -> CO2(g)	Acidic

Metal oxides are generally basic; non-metal oxides are generally acidic. See oxides.

Rusting

Iron rusts only when both oxygen and water are present:

4Fe(s) + 3O2(g) + 6H2O(l) -> 4Fe(OH)3(s)

Remove either the oxygen or the water and rusting stops — the basis of every prevention method. See rusting and prevention.

Making and separating oxygen

In the lab, oxygen is made by decomposing hydrogen peroxide over a manganese(IV) oxide catalyst:

2H2O2(aq) -> 2H2O(l) + O2(g)

Industrially it comes from fractional distillation of liquid air: nitrogen boils off first at -196 C, leaving liquid oxygen at -183 C.

Key facts

• Symbol: O — proton number: 8
• Position: Group VI, Period 2
• Electron configuration: 2, 6 (forms O2-); diatomic O2 with an O=O double bond
• Key property: supports combustion (relights a glowing splint)
• Abundance: ~21% of clean dry air
• Main roles in 0620: combustion, oxide formation, rusting

Common exam mistakes

• Swapping the splint tests. Oxygen relights a glowing splint; hydrogen gives a squeaky pop with a lighted splint. Do not mix them.
• Saying oxygen “burns” or is “flammable”. Oxygen supports combustion but does not itself burn — the fuel burns in it.
• Forgetting rusting needs both oxygen and water. Either one alone will not rust iron.
• Thinking air is mostly oxygen. Air is about 78% nitrogen and only 21% oxygen.

Worked exam questions

Describe an experiment to show that air contains approximately 21% oxygen. (4 marks)

Mark scheme

• Pass a fixed volume of air repeatedly over heated copper between two gas syringes [1]
• The copper reacts with the oxygen: 2Cu + O2 -> 2CuO / the copper turns black [1]
• Record the gas volume before and after [1]
• The volume falls by about one-fifth (~20-21%), showing that fraction was oxygen [1]

Examiner note: the copper must be in excess and the air passed over it repeatedly, so that all the oxygen is removed. State that the reading is taken once the volume stops changing.

Explain why incomplete combustion of methane in a faulty gas heater is dangerous. (2 marks)

Mark scheme

• Incomplete combustion produces carbon monoxide (CO) [1]
• CO is toxic / colourless and odourless so it is not detected / it binds to haemoglobin and stops oxygen being carried in the blood [1]

Examiner note: name the gas as carbon monoxide, not “carbon” or “CO2”. The danger mark needs the toxic/haemoglobin idea, not just “it is poisonous”.