What method is suitable for the separation of gases present in air?

1. Analyze the question Air is a mixture of gases (mainly nitrogen about 78%, oxygen about 21%, argon, CO₂ and traces of others). We need the industrial method used to separate these gases.

2. Explain the concept Because air is a mixture, its components can be separated by physical means. Each gas has a different boiling point, so we use fractional distillation of liquid air:

| Gas | Boiling point | |---|---| | Nitrogen | −196 °C | | Argon | −186 °C | | Oxygen | −183 °C |

3. Step-by-step solution

1. Remove dust, water vapour and CO₂ from the air (CO₂ is removed with NaOH, and water vapour is frozen out or absorbed), so they do not freeze and block the equipment.
2. Compress the air and cool it. Then let it expand suddenly (the Joule-Thomson effect) to cool it to about −200 °C, which turns it into liquid air.
3. Let the liquid air warm slowly in a fractionating column.
4. Nitrogen has the lowest boiling point, so it boils off first and is collected at the top. Argon comes next, and oxygen, with the highest boiling point, stays as liquid longer and is collected last.

4. Check the options

• A. Catalytic cracking is used to break large hydrocarbons into smaller ones in petroleum refining. It has nothing to do with air.
• B. Fractional distillation of liquid air separates the gases by their boiling points. This is correct.
• C. Thermal decomposition breaks a compound down by heat. Air is a mixture, not a compound, so this is wrong.
• D. Catalytic decomposition also applies to compounds (for example H₂O₂ with MnO₂), so it is wrong.

Answer: B. Fractional distillation of liquid air

Exam tip: Remember "mixture means physical separation". Crude oil and liquid air both use fractional distillation, and the component with the lowest boiling point comes off first. Cracking is only for hydrocarbons.