Oxygen

Discovery of Oxygen

The credit for discovering oxygen is shared by three scientists. A Swedish chemist, Carl Wilhelm Scheele, first isolated oxygen around 1772 by heating substances like potassium nitrate and mercuric oxide. He called the gas “fire air” because it made fires burn stronger.

An English chemist, Joseph Priestley, independently discovered oxygen in 1774. He heated mercuric oxide and noticed that candles burned more brightly in the gas. He also found that it made breathing easier. Priestley published his findings first, which is why he often gets credit for the discovery.

A French chemist, Antoine Lavoisier, studied the gas and named it “oxygen.” The name comes from Greek words meaning “acid-forming” because Lavoisier wrongly thought oxygen was present in all acids.

Occurrence of Oxygen on Earth

Oxygen is the most abundant element on Earth. It exists in two main forms:

Free state: Oxygen gas (O₂) makes up about 21% of the air we breathe. This percentage is perfect for life. Below 17%, breathing becomes difficult. Above 25%, many materials catch fire easily.

Combined state: Oxygen combines with other elements to form many compounds. Water (H₂O) is 89% oxygen by weight. The Earth’s crust is 46% oxygen by mass. About two-thirds of the human body is oxygen, mostly in water.

In the universe, oxygen is the third most common element after hydrogen and helium.

Laboratory Preparation of Oxygen

The most practical way to prepare oxygen in the laboratory is by heating potassium trioxochlorate(V), commonly called potassium chlorate.

Materials Needed

• Potassium trioxochlorate(V) – KClO₃ (about 10 grams)
• Manganese(IV) oxide – MnO₂ (about 2 grams) as catalyst
• Large test tube with delivery tube
• Bunsen burner and retort stand
• Pneumatic trough filled with water
• Gas collecting bottles
• Rubber stopper with glass tube

Chemical Equation

2KClO₃(s) → 2KCl(s) + 3O₂(g)

(Heat and MnO₂ catalyst shown above arrow)

This equation tells us that 2 moles of potassium chlorate produce 2 moles of potassium chloride (a salt) and 3 moles of oxygen gas.

Procedure

1. Mix about 10 grams of potassium chlorate with 2 grams of manganese dioxide in a test tube
2. Tilt the test tube at 30 degrees and shake it gently so the mixture spreads evenly in the bottom third
3. Connect the test tube to a delivery tube that leads into a pneumatic trough filled with water
4. Fill collecting bottles with water and invert them in the trough
5. Heat the mixture gently by moving the Bunsen flame back and forth along the test tube
6. Collect the oxygen gas by downward displacement of water in the inverted bottles
7. When a bottle is full of gas, cover it with a glass plate while still underwater, then remove it

Role of Manganese Dioxide

Manganese dioxide (MnO₂) acts as a catalyst. A catalyst speeds up a chemical reaction but is not used up in the reaction. Without MnO₂, potassium chlorate would need very high temperatures to decompose. The catalyst allows oxygen production at lower, safer temperatures.

Why Collection by Water Displacement Works

Oxygen is only slightly soluble in water, so very little dissolves. The gas pushes water out of the collecting bottle. Also, oxygen is denser than air, so it stays in the bottle when you turn it upright.

Industrial Preparation of Oxygen

Industries need much larger amounts of oxygen than laboratories. Two main methods are used:

Fractional distillation of liquid air: Air is cooled to very low temperatures until it becomes liquid. Oxygen has a higher boiling point (-183°C) than nitrogen (-196°C), so when liquid air is warmed slowly, nitrogen boils off first. The oxygen is left behind and collected separately.

Electrolysis of water: When electric current passes through water containing a small amount of acid or base, water breaks down into hydrogen and oxygen. Oxygen collects at the positive electrode (anode). This method is less common because it uses a lot of electricity.

Comparison Table: Laboratory vs Industrial Preparation

Common Exam Mistakes (WAEC Examiner Reports)

WAEC examiners have noted these frequent errors in questions about oxygen:

• Confusing “preparation” with “test”: Students write about glowing splint test when asked to describe preparation. Remember: preparation means making the gas, testing means identifying it.
• Omitting the catalyst: Many students forget to mention manganese dioxide or don’t explain that it’s a catalyst. Always state its role clearly.
• Wrong collection method: Some students say “upward displacement” when oxygen is collected by “downward displacement of water.” Know why: oxygen is denser than air but only slightly soluble in water.
• Unbalanced equations: Check that the number of atoms on both sides of the equation are equal. For KClO₃ → KCl + O₂, you need 2KClO₃ to produce 3O₂.
• Poor diagram labels: When drawing the apparatus, label all parts: delivery tube, test tube, pneumatic trough, collecting bottle, water, and the mixture being heated.
• Not stating “heat” or “catalyst”: Write these above or below the arrow in chemical equations. Don’t leave them out.

Practice Questions

Multiple Choice Questions

1. Which catalyst is used in the laboratory preparation of oxygen from potassium chlorate?
   a) Iron(III) oxide
   b) Manganese(IV) oxide ✓
   c) Copper(II) oxide
   d) Zinc oxide

2. What percentage of the atmosphere is oxygen?
   a) 78%
   b) 50%
   c) 21% ✓
   d) 15%

3. The most abundant element in the Earth’s crust is:
   a) Silicon
   b) Nitrogen
   c) Oxygen ✓
   d) Carbon

4. In the laboratory preparation of oxygen, the gas is collected by:
   a) Upward displacement of air
   b) Downward displacement of air
   c) Downward displacement of water ✓
   d) Upward displacement of water

Essay/Theory Questions

1. (a) Draw a labelled diagram of the apparatus used to prepare oxygen in the laboratory. (6 marks)
   (b) Write a balanced equation for the reaction. (2 marks)
   (c) State two precautions to observe during the preparation. (2 marks)

   Examiner’s tip: Your diagram must show the test tube tilted at an angle, the pneumatic trough, delivery tube, and labels for all parts. For precautions, mention: ensure no mixture touches the rubber stopper; heat gently to avoid explosion; don’t collect the first few bubbles as they contain air from the apparatus.

2. (a) State three ways in which oxygen exists on Earth. (3 marks)
   (b) Explain why manganese dioxide is added when preparing oxygen from potassium chlorate. (3 marks)
   (c) Give two reasons why oxygen is collected over water. (4 marks)

   Examiner’s tip: For part (a), mention free state in air, combined in water, combined in minerals/rocks. For part (b), explain what a catalyst does – speeds up reaction without being used up, allows lower temperature. For part (c), say oxygen is only slightly soluble in water AND it is denser than air.

3. (a) Name two scientists who discovered oxygen and state the year. (4 marks)
   (b) Compare the laboratory and industrial methods of preparing oxygen under these headings: (i) raw material (ii) cost per litre (iii) quantity produced. (6 marks)

   Examiner’s tip: For part (a), Carl Scheele (1772) and Joseph Priestley (1774) are the most important. For part (b), make clear comparisons: laboratory uses potassium chlorate, industrial uses air; laboratory is expensive, industrial is cheap; laboratory produces small amounts, industrial produces large amounts.

Memory Aids

For the balanced equation: Remember “2-2-3” – 2 KClO₃ gives 2 KCl plus 3 O₂

For discoverers: “SchemePriest” – Scheele (Swedish) and Priestley (English)

For occurrence: “FREE combines EARTH” – oxygen is FREE in air, COMBINES in water and EARTH (rocks)

For catalyst: “MnO₂ makes oxygen faster” – Manganese dioxide speeds up oxygen release

For collection method: “Down in Water” – oxygen collected by Downward displacement of Water

Related Topics

To understand oxygen better, study these related chemistry topics:

• Physical Properties of Oxygen – Learn about oxygen’s colour, smell, density, and physical state
• Chemical Properties of Oxygen – Understand how oxygen reacts with metals, non-metals, and compounds
• Test for Oxygen – Know how to confirm the presence of oxygen gas using a glowing splint
• Uses of Oxygen – Discover oxygen’s applications in medicine, industry, and daily life
• Oxides and their Classification – Study acidic, basic, amphoteric, and neutral oxides