Preparation of Oxygen and Its Properties
Senior 2 · Chemistry
Preparation of Oxygen and Its Properties
Lesson 3 of 4
25 min
A gas jar full of pure oxygen looks exactly like a jar of air, or even an empty one. So how can you tell that it is there, and what can it do? The answers are oxygen's properties. Physical properties describe the substance itself: its colour, smell, density and solubility. Chemical properties describe how it reacts with other substances.
Oxygen's chemical properties explain the fire in your kitchen, the rust on a mabati roof and the energy your cells release from food.
| Property | Oxygen |
|---|---|
| State at room temperature | gas, made of diatomic molecules, O2 (O=O) |
| Colour, smell, taste | colourless, odourless, tasteless |
| Density at STP | 1.43 g/dm³, slightly denser than air (1.29 g/dm³) |
| Solubility in water | slightly soluble: about 49 cm³ dissolves in 1 dm³ of water at 0 °C. This small amount keeps fish and other water life alive. |
| Boiling point | −183 °C; liquid oxygen is pale blue |
| Effect on litmus | none; oxygen is neutral |
Push a glowing splint into the gas (a splint that has been lit and blown out, so that it still glows red). In oxygen it relights and bursts into flame. This is the standard test for oxygen.
Worked example. Four unlabelled jars contain air, oxygen, carbon dioxide and hydrogen. A glowing splint relights only in the oxygen jar. In air it just keeps glowing, because air is only about 21% oxygen. In carbon dioxide it goes out. A burning splint held at the mouth of the hydrogen jar gives a "squeaky pop". Use a glowing splint for oxygen, not a burning one: a burning splint burns in air too, so it would prove very little.
Oxygen does not burn itself; it supports combustion. Combustion is a rapid reaction with oxygen that gives out heat and light. Substances burn far more fiercely in pure oxygen than in air, because air is almost four-fifths unreactive nitrogen. Any reaction in which a substance gains oxygen is called oxidation, and its product is an oxide, a compound of oxygen with one other element.
| Metal | What you see in oxygen | Balanced equation |
|---|---|---|
| Magnesium | dazzling white flame; white powder (magnesium oxide) | 2Mg(s) + O2(g) → 2MgO(s) |
| Calcium | brick-red flame; white solid (calcium oxide) | 2Ca(s) + O2(g) → 2CaO(s) |
| Iron (steel wool) | burns with bright yellow sparks; black solid, iron(II,III) oxide | 3Fe(s) + 2O2(g) → Fe3O4(s) |
| Copper | does not burn; turns black on the surface when heated (copper(II) oxide) | 2Cu(s) + O2(g) → 2CuO(s) |
Sodium (teacher demonstration only) burns with a yellow-orange flame to give a white solid. It is written as sodium oxide, 4Na + O2 → 2Na2O, although in plenty of oxygen some sodium peroxide, Na2O2, also forms.
| Non-metal | What you see in oxygen | Balanced equation |
|---|---|---|
| Carbon (charcoal) | glows bright red and then white; colourless gas that turns limewater milky | C(s) + O2(g) → CO2(g) |
| Sulfur | pale blue flame in air, brilliant blue in oxygen; choking, poisonous gas (fume cupboard) | S(s) + O2(g) → SO2(g) |
| Phosphorus (teacher only) | dazzling white flame and dense white smoke | P4(s) + 5O2(g) → P4O10(s) |
| Hydrogen | burns with a pale blue flame; a mixture of hydrogen and oxygen explodes | 2H2(g) + O2(g) → 2H2O(l) |
When carbon burns in a limited supply of oxygen, poisonous carbon monoxide forms instead: 2C + O2 → 2CO.
Mugisha burns magnesium ribbon in one jar of oxygen and sulfur in another. He adds a little water to each jar, shakes it, and adds universal indicator.
Conclusion: metals tend to form basic oxides and non-metals tend to form acidic oxides. You will build on this rule in the next unit.
Open the Balancing Chemical Equations simulation. On the first screen, choose "Combust Methane" and change the numbers in front of each formula until the balance scales level. Then try the Game, level 1, to practise balancing more equations.
Tip
Three gas tests to know by heart
Oxygen relights a glowing splint. Hydrogen gives a squeaky pop with a burning splint. Carbon dioxide turns limewater milky. When balancing a combustion equation, balance the element that is not oxygen first, and adjust the number of O2 molecules last.
Describe the test for oxygen and the positive result.
Solution: Put a glowing splint into the gas. It relights (bursts into flame) if the gas is oxygen.
Balance: __Fe + __O2 → Fe3O4.
Solution: There are 3 Fe on the right, so write 3Fe. There are 4 O on the right, which is 2 molecules of O2. The equation is 3Fe + 2O2 → Fe3O4.
Why does a glowing splint relight in oxygen but not in air?
Solution: Air contains only about 21% oxygen, diluted by nitrogen. Pure oxygen supplies oxygen to the hot wood so fast that the burning speeds up enough to produce a flame.
Sulfur is burned in oxygen and the product is shaken with water containing blue litmus. Write the equation for the burning and state the colour change.
Solution: S + O2 → SO2. The litmus turns red, because sulfur dioxide is an acidic oxide and forms an acidic solution.
What mass of magnesium oxide forms when 4.8 g of magnesium burns completely? (Mg = 24, O = 16)
Solution: Moles of Mg = 4.8 ÷ 24 = 0.20 mol. From 2Mg + O2 → 2MgO, 2 mol Mg give 2 mol MgO, so 0.20 mol MgO forms. M(MgO) = 24 + 16 = 40 g/mol, so the mass = 0.20 × 40 = 8.0 g.
Summary
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
