Physical Properties Of Oxygen

Understanding Physical Properties

A physical property is any characteristic you can observe or measure without changing what the substance is made of. For example, when you measure oxygen’s boiling point, it’s still oxygen afterward. This is different from chemical properties, which involve oxygen turning into something else (like when oxygen reacts with iron to form rust).

Understanding oxygen’s physical properties helps you recognize the gas, handle it safely in the laboratory, and explain why certain methods work for collecting and storing it.

Detailed Physical Properties of Oxygen

1. Colour

Oxygen gas (O₂) at room temperature and normal pressure is colourless. You cannot see it. This is why you can’t see the oxygen in the air around you, even though it makes up 21% of the atmosphere.

However, when oxygen is cooled to very low temperatures and becomes a liquid (below -183°C), it turns pale blue. Solid oxygen is also pale blue. In Nigeria’s hot climate, you will only encounter oxygen as a colourless gas.

2. Odour and Taste

Oxygen is completely odourless – it has no smell at all. It is also tasteless – you cannot taste it. This is important for safety. Unlike gases like chlorine (which smells sharp) or hydrogen sulfide (which smells like rotten eggs), you cannot detect oxygen by smell. This means you need proper tests, not your nose, to identify oxygen.

3. Physical State

At normal room temperature (about 25°C) and atmospheric pressure, oxygen exists as a gas. More specifically, oxygen is diatomic, meaning its molecules consist of two oxygen atoms bonded together (O₂).

Oxygen remains a gas until temperatures drop far below anything experienced naturally on Earth. Even in the coldest parts of Nigeria during harmattan, oxygen stays gaseous.

4. Density

Oxygen has a density of about 1.43 grams per litre (g/L) at room temperature and normal pressure. Air has a density of about 1.29 g/L. This means oxygen is denser (heavier) than air – about 1.1 times heavier.

Because oxygen is denser than air, it sinks. If you release oxygen gas from a cylinder at ground level in a room, it will stay low rather than rising to the ceiling. This property is useful when collecting oxygen in the laboratory.

5. Solubility in Water

Oxygen is slightly soluble in water. This means a small amount of oxygen dissolves in water, but most of it remains as gas bubbles.

At 20°C, about 0.04 grams of oxygen dissolve in 1 litre of water. This might seem tiny, but it’s crucial for life. Fish and other water animals depend on this dissolved oxygen to breathe. The cooler the water, the more oxygen it can hold. This is why fish in hot ponds in Lagos during hot season sometimes struggle to breathe – warm water holds less oxygen.

In the laboratory, oxygen’s low solubility in water allows us to collect it by displacing water in a pneumatic trough. If oxygen were very soluble, it would dissolve into the water instead of pushing it out.

6. Effect on Litmus Paper

Oxygen has no effect on litmus paper. It does not turn red litmus blue or blue litmus red. This tells us oxygen is a neutral gas – it is neither acidic nor basic (alkaline).

This property helps distinguish oxygen from gases like ammonia (which turns red litmus blue because it’s basic) or chlorine (which bleaches litmus paper).

7. Melting and Boiling Points

Oxygen has very low melting and boiling points compared to most substances:

• Boiling point: -183°C (-297°F). This is the temperature at which liquid oxygen turns into gas. Below this temperature, oxygen is liquid.
• Melting point: -218°C (-361°F). This is the temperature at which solid oxygen turns into liquid. Below this temperature, oxygen is solid.

These extremely low temperatures explain why we only see oxygen as a gas in everyday life. Even in a freezer at -20°C, oxygen would still be a gas. You would need special equipment using liquid nitrogen to cool oxygen enough to make it liquid.

8. Molecular Structure

Each oxygen molecule (O₂) consists of two oxygen atoms joined by a strong double covalent bond. The bond length is 121 picometres (extremely tiny – about one ten-billionth of a metre). This double bond gives oxygen its stability as a diatomic gas.

Comparison with Air and Other Gases

Why These Properties Matter in the Laboratory

Collection method: Because oxygen is only slightly soluble in water and denser than air, we can collect it over water in inverted bottles. The gas displaces the water downward and fills the bottle from top to bottom.

Storage: Being denser than air means oxygen can be stored in open containers for short periods without immediately escaping (though it will eventually mix with air). Gas cylinders are better for long-term storage.

Safety: Since oxygen is colourless and odourless, you cannot detect leaks by sight or smell. Laboratories must use proper ventilation and oxygen detectors. Never rely on your senses to detect oxygen.

Industrial use: The different boiling points of nitrogen (-196°C) and oxygen (-183°C) allow industries to separate them through fractional distillation of liquid air. Nitrogen boils off first, leaving purer oxygen behind.

Common Exam Mistakes (WAEC Examiner Reports)

WAEC examiners have identified these frequent errors regarding oxygen’s physical properties:

• Confusing physical and chemical properties: Students write “oxygen supports burning” when asked for physical properties. Supporting combustion is a chemical property because it involves oxygen reacting with other substances. Stick to observable features like colour, state, density.
• Saying “very soluble in water”: Oxygen is SLIGHTLY soluble, not very soluble. This distinction matters when explaining why collection over water works.
• Wrong density comparison: Some say oxygen is “lighter than air.” No – oxygen is heavier (denser) than air. This is why we collect it by downward displacement of water, not upward.
• Vague temperature values: Don’t write “very cold” or “low temperature.” Give the actual values: boiling point -183°C, melting point -218°C (or solidification point -225°C in older texts).
• Not stating “diatomic”: When asked about oxygen’s molecular form, say it exists as O₂ (diatomic molecules), not as single O atoms.
• Mixing up “no effect” with “neutral”: When describing litmus test, say “no effect on litmus paper” rather than just “neutral,” which can be unclear.

Practice Questions

Multiple Choice Questions

1. Which of the following is NOT a physical property of oxygen?
   a) It is colourless
   b) It is denser than air
   c) It supports combustion ✓
   d) It is slightly soluble in water

2. The boiling point of oxygen is:
   a) 0°C
   b) -100°C
   c) -183°C ✓
   d) -273°C

3. Oxygen can be collected over water because it is:
   a) Very soluble in water
   b) Slightly soluble in water ✓
   c) Insoluble in water
   d) Denser than water

4. At room temperature, oxygen exists as:
   a) Solid
   b) Liquid
   c) Gas ✓
   d) Plasma

Essay/Theory Questions

1. (a) State five physical properties of oxygen. (5 marks)
   (b) Explain why oxygen is collected over water and not over air. (3 marks)
   (c) What is meant by saying oxygen is a “diatomic gas”? (2 marks)

   Examiner’s tip: For part (a), list: colourless, odourless, tasteless, slightly soluble in water, denser than air, no effect on litmus paper, boiling point -183°C (any five). For part (b), explain that oxygen is only slightly soluble in water so it doesn’t dissolve much, and it displaces water easily. For part (c), state that diatomic means each molecule has two atoms (O₂).

2. (a) Distinguish between physical and chemical properties of a substance, giving one example of each for oxygen. (4 marks)
   (b) State three physical properties that make oxygen different from nitrogen. (6 marks)

   Examiner’s tip: For part (a), physical properties can be observed without changing the substance (example: oxygen is colourless); chemical properties involve reactions that change the substance (example: oxygen supports burning). For part (b), mention: oxygen has higher density than nitrogen (1.43 vs 1.25 g/L), oxygen has higher boiling point (-183°C vs -196°C), oxygen is slightly more soluble in water than nitrogen.

3. (a) Copy and complete the table below for oxygen:

   Property	Value or Description
   Colour	__________
   Smell	__________
   Boiling point	__________
   Effect on litmus	__________
   Solubility in water	__________

   (5 marks)

   (b) Explain why fish can survive in rivers and lakes. (3 marks)
   (c) Why is it dangerous to assume a room is safe just because you cannot smell any gas? (2 marks)

   Examiner’s tip: For table – colourless, odourless, -183°C, no effect, slightly soluble. For part (b), explain that oxygen from air dissolves slightly in water, and fish use gills to extract this dissolved oxygen for respiration. For part (c), many gases including oxygen are odourless, so you cannot detect them by smell; dangerous gases or gas leaks might be present without any warning smell.

Memory Aids

For state and senses: “COT Gas” – Colourless, Odourless, Tasteless Gas

For temperature values: “One Eight Three DOWN” – boiling point is 183 degrees below zero (DOWN = minus)

For density: “O is heavier” – Oxygen is heavier/denser than air

For solubility: “SLIGHTLY saves fish” – Oxygen is SLIGHTLY soluble in water, which is enough to save fish

For litmus test: “NO LIT” – oxygen has NO effect on LITmus paper

For collection: “DOWNER water” – collected by DOWNward displacement ovER water

Related Topics

To build complete understanding of oxygen, study these related topics:

• Oxygen (general) – Discovery, occurrence, and laboratory preparation of oxygen
• Chemical Properties of Oxygen – How oxygen reacts with metals, non-metals, and compounds
• Test for Oxygen – The glowing splint test to identify oxygen gas
• Uses of Oxygen – Applications in medicine, welding, and life support systems
• Oxides – Compounds formed when oxygen combines with other elements