Properties and uses of Oxygen gas
Properties of oxygen gas
- slightly soluble in water
- neutral to litmus
- 1.1 times denser than air
- does not burn
- supports combustion
Test for oxygen gas
To test whether a gas is oxygen or not, dip a glowing splint in the gas. Oxygen will relight a glowing splint.
Combustion of non-metals in oxygen gas
Combustion of sulphur in oxygen gas
- Sulphur burns in oxygen with a bright blue flame, forming cloudy fumes sulphur dioxide. The fumes of sulphur dioxide have a choking smell. Sulphur dioxide dissolves in water to form sulphurous acid (H2SO3(aq)).
- S(s) + O2(g) → SO2(g)
- SO2(g) + H2O(l) → H2SO3(aq)
Combustion of carbon in oxygen gas
- Carbon burns in oxygen with an orange flame forming a colourless gas, carbon dioxide. Carbon dioxide dissolves in water to form carbonic acid.
- C(s) + O2(g) → CO2(g)
- CO2(g) + H2O(l) → H2CO3(aq)
Combustion of phosphorus in oxygen gas
- Phosphorus burns in oxygen gas with a very bright, yellow gas to form a dense white smoke of phosphorus(V) oxide mixed with a little phosphorus(III) oxide.
- 4P(s) + 5O2(g) → 2P2O5(s) or P4O10(s) (phosphorus(V) oxide)
- 4P(s) + 3O2(g) → 2P2O3(s) or P4O6(s) (phosphorus(III) oxide)
- The above mentioned oxides of phosphorus dissolve in water to form phosphoric acid and phosphorus acid respectively
- P2O5(s) + 3H2O(l) → 2H3PO4(aq) (phosphoric acid)
- P2O3(s) + 3H2O(l) → 2H3PO3(aq) (phosphorous acid)
Combustion of metals in oxygen gas
Combustion of calcium in oxygen gas
- Calcium burns in oxygen with a bright yellow flame and forms a white solid of calcium oxide. Calcium oxide dissolves in water to form an alkaline solution of calcium hydroxide (also known as lime water)
- 2Ca(s) + O2(g) → 2CaO(s)
- CaO(s) + H2O(l) → Ca(OH)2(aq)
Combustion of magnesium in oxygen gas
- Magnesium burns in oxygen with a very bright light and forms a white ash of magnesium hydroxide. Magnesium hydroxide dissolves in water to form an alkaline solution of magnesium hydroxide.
- 2Mg(s) + O2(g) → 2MgO(s)
- MgO(s) + H2(l) → Mg(OH)2(aq)
Uses of oxygen
- Medical purposes – in hospitals, people with respiratory illnesses and people under anaesthetics are given pure oxygen as they cannot take enough oxygen efficiently from the air.
- In sea diving – sea divers carry tanks of oxygen mixed with helium to breathe under water.
- By astronauts – astronauts carry oxygen in their suit backpacks to breathe in space where there is no oxygen.
- Oxyacetylene welding – acetylene gas when mixed with oxygen burns with flame of temperatures of about 3000°C. This is used in cutting and welding metals.
- In steel manufacture – oxygen is blown into molten pig iron at high pressure. It oxidises impurities such as carbon, sulphur and phosphorus to oxides which escape as gases.
- As a fuel oxidant – rockets carry oxygen to burn their fuels in outer space where there is no oxygen.
Laboratory preparation of oxygen
In the laboratory oxygen is prepared from hydroxide using the apparatus shown below:
- Setup the apparatus as shown above.
- Use manganese(IV) oxide as a catalyst.
- Tip the conical flask enough for the catalyst to come into contact with hydrogen peroxide.
- Do not heat. The catalyst speeds the reaction enough.
- The hydrogen peroxide breaks down as follows:
- 2H2O2(aq) → O2(g) + 2H2(l)
- The oxygen is collected above water in the measuring cylinder.