We can’t do without it
You breathe oxygen every moment. Hospitals give oxygen to patients, scuba divers carry tanks of compressed air, steel mills blow oxygen through molten iron, and rockets burn their fuel with liquid oxygen. What is oxygen like, and how do chemists make it?
This lesson has two parts:
- Properties of oxygen: its physical properties, how substances burn in it, and oxidation.
- Making oxygen in the lab: three reactions that make oxygen, catalysts, the apparatus, and how to test for and collect the gas.
A short practice set follows Part 1; do it before you start Part 2.
Part 1: Properties of oxygen
Physical properties: under ordinary conditions, oxygen is a colorless, odorless gas, slightly denser than air, and not very soluble in water. At −183 °C it condenses into a pale blue liquid, and at −218 °C it freezes into a pale blue solid.
Chemical properties: oxygen is fairly reactive and supports burning. A glowing wooden splint bursts back into flame in oxygen; that’s the standard test for oxygen.
Pick a substance and switch between “In air” and “In oxygen” to compare how it burns:
Glows red without smoke or flame.
Burns vigorously with a bright white light, releases heat, and produces a gas that turns limewater cloudy.
Burns with a faint, pale blue flame, releases heat, and produces a gas with a sharp, choking smell.
A little water in the bottom of the jar absorbs the sulfur dioxide so it doesn't pollute the room.
Burns with a bright blue-violet flame, releases heat, and produces a gas with a sharp, choking smell.
A little water in the bottom of the jar absorbs the sulfur dioxide so it doesn't pollute the room.
Burns, giving off thick white smoke and heat.
Burns even more vigorously with a white light, giving off thick white smoke and heat.
Only glows red-hot; it doesn't burn.
A match tied to the end of the wire starts it burning. A layer of water or sand in the bottom of the jar keeps hot, molten drops from cracking the glass.
Burns vigorously, throwing off sparks and lots of heat, and forms a black solid.
A match tied to the end of the wire starts it burning. A layer of water or sand in the bottom of the jar keeps hot, molten drops from cracking the glass.
Everything burns more vigorously in pure oxygen than in air, because the oxygen is more concentrated. A reaction of a substance with oxygen is an oxidation氧化反应. Burning is fast oxidation; breathing, food spoiling, and iron rusting are slow oxidation缓慢氧化, too slow to notice as they happen.
Part 1 practice
- A bright blue-violet flame
Sulfur burns with a faint pale blue flame in air and a bright blue-violet flame in oxygen.
- Burns vigorously, throws off sparks, and forms a black solid
The black solid is iron(II,III) oxide, .
- A white light, and a gas that turns limewater cloudy
Burning charcoal produces carbon dioxide.
- Thick white smoke
The white smoke is tiny particles of solid phosphorus pentoxide.
- A gas with a sharp, choking smell
Burning sulfur produces sulfur dioxide.
- Needs water or sand in the bottom of the jar to keep it from cracking
Molten drops from the burning iron would crack the bottom of the jar.
- In air, it burns with a faint pale blue flame
That's sulfur burning in air.
Part 2: Making oxygen in the lab
In the last two reactions, manganese dioxide is a catalyst催化剂: a substance that changes the rate of a reaction while its own mass and chemical properties are the same at the end as at the start.
Industry makes oxygen on a large scale by distilling liquid air. Liquid nitrogen boils at a lower temperature than liquid oxygen, so the nitrogen boils off first. This is a physical change.
A lab setup for making a gas has two parts. The generator depends on the states of the reactants and whether they need heating; the collection method depends on how soluble the gas is and how its density compares with air’s.
Generators: chosen by the reactants’ states and whether heating is needed
Example: heating KMnO₄ to make O₂. Tilt the mouth of the tube slightly down so condensed water can’t run back onto the hot glass and crack it; a plug of cotton keeps KMnO₄ powder out of the delivery tube.
Example: H₂O₂ solution with MnO₂ to make O₂. Adding the liquid from a separatory funnel lets you control how fast the reaction goes.
Collection: chosen by the gas’s solubility and density
For gases that don’t dissolve in or react with water, such as O₂ and H₂. Fill the jar with water and turn it upside down in the trough first. The gas collected this way is quite pure.
Jar mouth up; for gases denser than air, such as O₂ and CO₂. The gas sinks to the bottom and pushes the air out of the top.
Jar mouth down; for gases less dense than air, such as H₂. The gas collects at the top and pushes the air out through the mouth below.
Collecting oxygen: oxygen isn’t very soluble in water, so it can be collected by water displacement. It’s slightly denser than air, so it can also be collected by upward displacement of air (with the jar mouth up).
Testing: a glowing splint lowered into the jar relights if the gas is oxygen. When collecting by air displacement, hold the glowing splint at the mouth of the jar; if it relights there, the jar is full.
Making oxygen by heating potassium permanganate and collecting it over water takes seven steps. (Chinese students remember them with seven one-syllable words: check, load, clamp, light, collect, remove, extinguish.) Put the steps in order:
Steps to place (click to add)
Your order (click a step to send it back)
- Check that the apparatus is airtightIf the apparatus leaks, you won't collect any oxygen, so check before loading the chemical.
- Add potassium permanganate, put a loose plug of cotton in the mouth of the test tube, and insert the stopper and delivery tubeThe cotton keeps powdered potassium permanganate from being blown into the delivery tube.
- Clamp the test tube to the stand, tilted slightly downwardTilting the mouth down keeps condensed water from running back onto the hot glass and cracking it.
- Light the burner, warm the whole tube first, then heat under the solidWarming the whole tube first keeps it from cracking due to uneven heating.
- Start collecting when bubbles come out of the tube steadily and evenlyThe first bubbles are just the air from the tube; waiting for steady bubbles gives purer oxygen.
- When you're done, take the delivery tube out of the water firstIf the burner goes off first, the gas in the tube cools and contracts, and water is sucked back into the hot tube, which can crack it.
- Then turn off the burnerRemove the tube first, then the flame, so this step comes last.
Example 1: Tube out first, or burner off first?
After collecting oxygen over water, why must you take the delivery tube out of the water before turning off the burner?
- Suppose the burner goes off first. The test tube stops being heated, the gas inside cools, and the pressure drops.
- What happens next? The delivery tube is still in the water, so the air pressure outside pushes water up the tube and back into the hot test tube.
- The result. Cold water hitting hot glass can crack the test tube.
- Conclusion. Take the tube out of the water first, then turn off the burner.
Example 2: Is manganese dioxide a catalyst?
Adding a pinch of manganese dioxide to hydrogen peroxide solution produces lots of bubbles right away. Afterward, the manganese dioxide is filtered out, washed, dried, and weighed: its mass hasn’t changed. Added to fresh hydrogen peroxide, it makes it bubble rapidly again. What can you conclude?
- It changed the rate. With manganese dioxide, hydrogen peroxide breaks down much faster.
- Its mass didn’t change. It weighs the same after the reaction.
- Its chemical properties didn’t change. It still speeds up fresh hydrogen peroxide.
- Conclusion. Manganese dioxide is a catalyst for this reaction. All three pieces of evidence are needed; showing that it speeds things up isn’t enough on its own.
Common mistakes
Mistake 1: “Oxygen is flammable.” Oxygen supports burning, but it doesn’t burn itself. When a glowing splint relights in oxygen, it’s the wood that burns.
Mistake 2: “A catalyst always speeds up a reaction and makes more product.” A catalyst changes the rate (some even slow reactions down), but it doesn’t change how much product forms.
Mistake 3: “Start collecting as soon as bubbles appear.” The first bubbles are the air that was already in the test tube. Wait until the bubbles come out steadily and evenly.
Quick check
What is the correct way to test whether a jar of gas is oxygen?
Oxygen supports burning, so a glowing splint relights. A splint going out (B) only shows the gas doesn't support burning, and D is the test for carbon dioxide.
When making oxygen from potassium permanganate, why is a plug of cotton placed in the mouth of the test tube?
When heated, fine potassium permanganate powder can be carried along by the gas and block the delivery tube or get into the water, so a plug of cotton stops it.
Summary
- Oxygen is colorless and odorless, slightly denser than air, and not very soluble in water. It is fairly reactive and supports burning; a glowing splint relights in it.
- Charcoal, sulfur, phosphorus, and iron all burn more vigorously in oxygen than in air: sulfur burns blue-violet, and iron throws sparks and forms a black solid.
- Lab methods: heating potassium permanganate, decomposing hydrogen peroxide with manganese dioxide as a catalyst, or heating potassium chlorate with manganese dioxide.
- When heating potassium permanganate, take the delivery tube out of the water before turning off the burner. Collect oxygen over water or by upward air displacement.
On to AP (for later)
Reactions with oxygen are classic redox reactions (topic 4.9). Topic 5.11 explains how a catalyst speeds up a reaction: it provides a pathway with a lower activation energy. And a gas collected over water contains water vapor, so in AP gas calculations (topic 3.4) you subtract the vapor pressure of water using Dalton’s law of partial pressures to get the pressure of the gas itself.