Science Notes — MIXTURES, ELEMENTS AND COMPOUNDS

Subtopic: Oxygen (Age 13 — Kenya)

What is oxygen?
Oxygen is a chemical element found on the Periodic Table. Its symbol is O and each oxygen atom has 8 protons in its nucleus (atomic number 8). In nature oxygen usually exists as a molecule of two atoms, written O₂.

Physical properties

• Colourless, odourless gas at room temperature
• Does not burn, but supports burning (helps things burn faster)
• Boiling point: −183°C ; melting point: −219°C (so it is a gas at normal temperatures)
• Slightly soluble in water — some oxygen dissolves in lakes and rivers (important for fish)

Where is oxygen found?

Oxygen is very common. You find it in:

• The air (about 21%) — we breathe it.
• Water (H₂O) — two hydrogen atoms and one oxygen atom.
• Rocks and soil as oxides (e.g., iron oxide = rust).
• Living things — plants, animals and people are made of oxygen-containing compounds.

Important chemical reactions with oxygen

Some simple reactions (showing how oxygen is involved):

• Combustion (burning): Fuel + O₂ → heat, light, products (e.g., wood + O₂ → CO₂ + H₂O + ash).
• Rusting (iron oxide formation): 4Fe + 3O₂ → 2Fe₂O₃ (iron reacts slowly with oxygen and water).
• Photosynthesis (plants): 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (plants release oxygen).
• Respiration (animals): C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (animals use oxygen to get energy).

How to test for oxygen (class demonstration)

Glowing splint test — safe and simple (teacher supervision needed):

1. Light a splint (thin stick) and blow it out so it is glowing (not flaming).
2. Place the glowing splint into a jar that may contain oxygen.
3. If the splint re-ignites (glows into flame), oxygen is present because it supports burning.

Lab preparation (for teacher use): Oxygen may be prepared by decomposing hydrogen peroxide using a catalyst (manganese(IV) oxide). Equation: 2H₂O₂ → 2H₂O + O₂. This must be done in a lab under supervision — follow safety rules.

Uses of oxygen

• Breathing — essential for animals and humans.
• Medical oxygen in hospitals for sick patients.
• Industrial uses — steel making, welding (oxygen helps reach high temperatures).
• Water treatment and life support in aquariums to keep fish alive.

Safety notes

• Oxygen itself does not burn, but it makes fires burn stronger and faster.
• Keep flammable materials and open flames away from oxygen cylinders and oxygen-rich areas.
• Only use compressed oxygen cylinders with trained adults and equipment.

Oxygen and the environment

Plants release oxygen during photosynthesis. Protecting forests and plants helps keep the air healthy. Ozone (O₃) is another form of oxygen high in the atmosphere that protects us from harmful sunlight — but at ground level ozone can be harmful.

Summary (key points)

• Oxygen is an element (O), normally found as O₂ gas.
• It makes up about 21% of the air and is needed for respiration and many chemical reactions.
• It supports burning — be careful around oxygen sources.

Quick quiz (try these)

1. What is the chemical symbol for oxygen?
2. What percentage of air is oxygen?
3. Give one test used to show oxygen is present.
4. Write the formula for water and show how many oxygen atoms it has.

Use these notes to review in class or at home. For practical activities, always work with a teacher or adult and follow safety rules.