Quick Summary
- Oxygen makes up 21% of air and is essential for life and combustion
- Laboratory test: glowing splinter rekindles in pure oxygen
- Oxygen is colorless, odorless, and slightly soluble in water
- Prepared in lab by decomposing hydrogen peroxide or heating potassium chlorate
- Used in hospitals, welding, and rocket fuel
What is Oxygen?
Oxygen is a very important gas we need to stay alive. Every time you breathe in, you take oxygen from the air into your lungs. Your body uses this oxygen to turn food into energy. Without oxygen, we cannot survive for more than a few minutes.
In chemistry, oxygen has the symbol O and atomic number 8. It exists as a molecule with two oxygen atoms joined together (O₂). This is called diatomic oxygen. At room temperature, oxygen is a colorless gas with no smell or taste.
Scientists discovered oxygen in the 1770s. A Swedish chemist named Carl Wilhelm Scheele found it first, but an English scientist called Joseph Priestley published his findings earlier. Both men heated different substances and collected the gas that came out.
Properties of Oxygen
| Property | Description |
|---|---|
| Physical State | Gas at room temperature |
| Color | Colorless (pale blue as liquid) |
| Smell | Odorless |
| Taste | Tasteless |
| Solubility in Water | Slightly soluble (dissolves a little) |
| Density | Heavier than air |
| Support for Combustion | Supports burning (but does not burn itself) |
| Reactivity | Very reactive, forms oxides with most elements |
How to Test for Oxygen in the Laboratory
When you want to know if a gas is oxygen, you use a simple test that works every time. Here is how you do it:
Step 1: Light a wooden splinter (a thin piece of wood like a matchstick) and let it burn for a few seconds.
Step 2: Blow out the flame so the splinter is no longer burning brightly. It should be glowing red at the tip with some smoke coming from it.
Step 3: Quickly put the glowing splinter into the gas you want to test.
Step 4: Watch what happens. If the gas is oxygen, the splinter will burst into flames again. We say it “rekindles.” This means oxygen is present.
This test works because oxygen helps things burn. When you blow out the splinter, there is not enough oxygen around it to keep the flame going. But when you put it in pure oxygen or oxygen-rich gas, combustion starts again immediately.
You must use a glowing splinter, not a burning one. If the splinter is still burning with a flame when you put it in, you will not see a clear result. The test only works when the wood is hot and glowing but not flaming.
Why Does This Test Work?
Oxygen is needed for combustion. When something burns, it combines with oxygen in a chemical reaction that gives out heat and light. This process is called oxidation.
Wood contains carbon and hydrogen. When wood burns, these elements react with oxygen to form carbon dioxide and water vapor. The reaction releases energy as heat and light.
In normal air, oxygen makes up only 21%. This is enough to keep a flame burning, but not enough to relight a glowing splinter. However, in pure oxygen or oxygen-rich environments, there is so much oxygen available that even a glowing ember can burst into flames.
Think about fanning a dying fire at home. When you blow air on hot coals, the extra oxygen makes them glow brighter and sometimes burst into flames. The glowing splinter test works on the same principle.
Laboratory Preparation of Oxygen
In Nigerian secondary school chemistry labs, you can prepare oxygen using several methods. The most common ones for WAEC practicals are:
Method 1: Decomposition of Hydrogen Peroxide
Hydrogen peroxide (H₂O₂) breaks down into water and oxygen when you add a catalyst like manganese(IV) oxide:
2H₂O₂(aq) → 2H₂O(l) + O₂(g)
This method is safe and produces oxygen quickly. The manganese(IV) oxide speeds up the reaction but does not get used up.
Method 2: Heating Potassium Chlorate
When you heat potassium chlorate with manganese(IV) oxide as a catalyst, it breaks down to give oxygen:
2KClO₃(s) → 2KCl(s) + 3O₂(g)
This method needs careful heating. You must use a Bunsen burner and test tube.
Method 3: Heating Potassium Permanganate
Purple potassium permanganate crystals decompose when heated strongly:
2KMnO₄(s) → K₂MnO₄(s) + MnO₂(s) + O₂(g)
After preparing oxygen by any method, you collect it over water or by downward displacement of air (because oxygen is heavier than air). Then you test it with the glowing splinter.
Difference Between Oxygen and Ozone
Oxygen exists in two forms called allotropes. The normal form we breathe is O₂. Another form called ozone has the formula O₃.
| Property | Oxygen (O₂) | Ozone (O₃) |
|---|---|---|
| Formula | O₂ | O₃ |
| Smell | Odorless | Sharp smell (like chlorine) |
| Color | Colorless | Pale blue |
| Stability | Very stable | Unstable, breaks down to O₂ |
| Where Found | Air (21%) | Upper atmosphere (ozone layer) |
| Effect on Health | Essential for life | Toxic in large amounts |
The ozone layer in the sky protects us from harmful sun rays. But breathing ozone at ground level is dangerous to your lungs.
Uses of Oxygen
Oxygen has many important uses in everyday life and industry:
In Hospitals: Doctors give oxygen to patients who have trouble breathing. You see oxygen cylinders in hospitals across Lagos, Abuja, and other Nigerian cities. COVID-19 patients needed oxygen support to help them breathe.
In Welding: Welders use oxy-acetylene torches that mix oxygen with acetylene gas. This produces a very hot flame (up to 3,500°C) to cut and join metals. You see this at mechanic workshops in Nigeria.
In Steel Making: Factories blow oxygen into molten iron to remove impurities and make steel. Ajaokuta Steel Company in Kogi State would use this process.
In Rocket Fuel: Rockets carry liquid oxygen to burn their fuel in space where there is no air.
In Water Treatment: Water companies bubble oxygen through sewage to help bacteria break down waste.
At High Altitudes: Pilots and mountain climbers carry oxygen because air gets thinner as you go higher.
Common Exam Mistakes
WAEC examiners report that students often make these mistakes when answering questions about oxygen tests:
Mistake 1: Writing “the splinter burns” instead of “the glowing splinter rekindles.” The word “rekindle” is important. It means the splinter lights up again after being blown out.
Mistake 2: Saying oxygen burns. Oxygen does NOT burn. It supports combustion (helps other things burn), but oxygen itself is not flammable.
Mistake 3: Confusing the oxygen test with the hydrogen test. Hydrogen burns with a “pop” sound. Oxygen rekindles a glowing splinter. These are different tests.
Mistake 4: Not explaining WHY the test works. Always explain that oxygen supports burning, so it provides enough oxygen for the hot wood to burst into flames again.
Mistake 5: Writing about ozone when the question asks about oxygen. Read questions carefully. Ozone (O₃) and oxygen (O₂) are different substances.
Mistake 6: Poor drawing of apparatus for oxygen preparation. Label all parts clearly: delivery tube, conical flask, beehive shelf, trough, gas jar.
Practice Questions
Multiple Choice Questions
1. What is the correct test for oxygen gas?
a) It turns lime water milky
b) It produces a ‘pop’ sound with a lighted splinter
c) It rekindles a glowing splinter ✓
d) It bleaches moist litmus paper
2. Which catalyst is commonly used in the laboratory preparation of oxygen from hydrogen peroxide?
a) Platinum
b) Manganese(IV) oxide ✓
c) Iron filings
d) Copper turnings
3. What percentage of air is oxygen?
a) 78%
b) 50%
c) 21% ✓
d) 1%
4. Which of these statements about oxygen is FALSE?
a) Oxygen is colorless and odorless
b) Oxygen supports combustion
c) Oxygen is highly flammable ✓
d) Oxygen is slightly soluble in water
Essay/Theory Questions
Question 1 (6 marks): Describe how you would prepare and collect oxygen gas in the laboratory using hydrogen peroxide.
Answer Guide:
- Pour hydrogen peroxide solution into a conical flask (1 mark)
- Add manganese(IV) oxide as a catalyst (1 mark)
- Fit the flask with a delivery tube leading into a trough of water (1 mark)
- Collect the oxygen gas over water in an inverted gas jar (1 mark)
- Test the gas with a glowing splinter which rekindles (1 mark)
- Correct diagram with labels (1 mark)
Tip: Always draw and label your diagram. Examiners give marks for clear drawings.
Question 2 (4 marks): State four uses of oxygen.
Answer Guide:
- In hospitals for patients with breathing difficulties (1 mark)
- In oxy-acetylene welding and cutting of metals (1 mark)
- In steel manufacture to remove impurities from iron (1 mark)
- As rocket fuel oxidizer in space exploration (1 mark)
Tip: Give specific uses, not vague answers like “for breathing” or “for burning.”
Question 3 (5 marks): Explain why a glowing splinter rekindles in oxygen.
Answer Guide:
- Oxygen supports combustion/burning (1 mark)
- The glowing splinter is hot wood that is not burning with a flame (1 mark)
- When placed in oxygen, there is plenty of oxygen available (1 mark)
- The hot wood reacts rapidly with oxygen to produce heat and light (1 mark)
- This causes the splinter to burst into flames again (rekindle) (1 mark)
Tip: “Explain” means you must give reasons, not just state facts. Show cause and effect.
Memory Aids
Use this mnemonic to remember the properties of oxygen:
CODS HELP
- C – Colorless
- O – Odorless
- D – Denser than air
- S – Supports combustion
- H – Heavier than air
- E – Essential for life
- L – Liquid oxygen is pale blue
- P – Prepared from H₂O₂ or KClO₃
To remember the oxygen test: “Glowing Goes Bright” – A glowing splinter goes bright (rekindles) in oxygen.
Oxygen formula: O-TWO (O₂) – Oxygen travels with two atoms together.
Related Topics
- Test for hydrogen gas
- Test for carbon dioxide
- Combustion and oxidation reactions
- Allotropes of elements
- Preparation of gases in the laboratory