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Gas liquefaction

Key idea: Cooling lowers molecular kinetic energy, and compression brings molecules closer. Below the critical temperature, intermolecular attractions can hold them together as a liquid.

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  • GCE A-Level H1 Chemistry 2027
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Gases such as nitrogen, oxygen and carbon dioxide can all be turned into liquids. Nothing happens to the molecules themselves: the same intermolecular attractions that act in the gas simply become able to hold the molecules together. This lesson explains how lowering the temperature and raising the pressure each make that possible.

Distinguish attractions between and within molecules

Check your understanding 1 Keep the molecules intact

When nitrogen gas becomes liquid nitrogen, which attraction holds neighbouring particles close together? Decide whether the N≡N bond inside each molecule changes.

Attraction between N₂ molecules
During liquefaction the N≡N bond
Show answer

London forces hold the N₂ molecules close in the liquid. The N≡N bonds stay intact: this is a change of state, not a chemical reaction.

Why a gas can become a liquid

The molecules of a gas attract one another, but under the starting conditions those attractions do not hold them close together. Two things decide whether they can:

  • how fast the molecules move. The average kinetic energy of the molecules depends on the temperature. Fast-moving molecules that come together have enough energy to pull apart again.
  • how far apart the molecules are. Intermolecular attractions act only over short distances. In a gas the molecules spend most of their time far apart, where the attractions are very weak.

A liquid forms when the attractions can keep the molecules close together, although they still move past one another. Liquefaction does not create a new kind of attraction, and it does not break or form any covalent bonds.

Guided practice 1

Cooling and compression

About 6 min

Problem

Explain how cooling and compression can help a gas liquefy. Compare compression at a constant temperature.

Try this before viewing the solution

Hints

Hint 1

effect of cooling

What happens to the kinetic energy of the molecules as the gas cools?

Hint 2

effect of pressure

What happens to the distance between molecules when the gas is squeezed into a smaller volume?

Show solution step by step

  1. Explain cooling

    Method

    State that the average kinetic energy of the molecules decreases.

    Reason

    Slower molecules are less able to escape the attractions of their neighbours.

    Working

    Answer

    Lower temperature: lower kinetic energy.

  2. Explain compression

    Method

    State that the molecules are brought closer together.

    Reason

    At smaller separations, the existing intermolecular attractions act more strongly and more often.

    Working

    Answer

    Higher pressure: smaller separation.

  3. Conclude liquefaction

    Method

    State that a liquid can form at a low enough temperature and a suitable pressure.

    Reason

    The attractions can then keep the molecules close while they continue moving.

    Working

    Answer

    Gas to liquid, with the molecules unchanged.

Check your understanding 2 Separate the two changes

Before comparing the panels below, predict what happens when the same gas is cooled at fixed volume, and when it is compressed at fixed temperature. Compare the cooled gas before condensation begins.

Cooling at fixed volume changes
Compression at fixed temperature changes
Show answer

The cooled-gas panel should have shorter motion arrows in the same volume. The compressed-gas panel should have closer particles but unchanged arrow lengths. Once a liquid forms, molecules occupy a much smaller volume; the cooled-gas panel represents the stage before that change.

Read the separate changes

Compare each gas panel with Starting gas. Count the circles and compare arrow lengths and container widths. The circles represent whole molecules; shorter arrows represent lower average speed, not shorter molecular bonds.

Starting gasTwelve molecules are spread through a large container. Motion arrows represent relatively high average speed. Particle positions and sizes are schematic, not to scale. All panels show twelve unchanged molecules. Gas arrows represent motion; liquid motion is omitted. Compare cooling and compression separately with the starting gas. Arrow lengths and container dimensions are schematic. T means temperature.Starting gasLarge separationHigher average speedCool at the same volumeThe same twelve molecules occupy the same volume. Shorter motion arrows represent lower average speed and kinetic energy. This panel shows a cooled gas before any condensation. Particle positions and sizes are schematic, not to scale. All panels show twelve unchanged molecules. Gas arrows represent motion; liquid motion is omitted. Compare cooling and compression separately with the starting gas. Arrow lengths and container dimensions are schematic. T means temperature.Cool at the same volumeSame separation shownLower average speedCompress at the same TThe same twelve molecules occupy a smaller volume, reducing their average separation. Motion arrows have the same length as in the starting gas because temperature is held constant. T means temperature. Particle positions and sizes are schematic, not to scale. All panels show twelve unchanged molecules. Gas arrows represent motion; liquid motion is omitted. Compare cooling and compression separately with the starting gas. Arrow lengths and container dimensions are schematic. T means temperature.Compress at the same TSmaller separationSame average speedLiquid can formAt a sufficiently low temperature and suitable pressure, the same twelve molecules can form a liquid. Molecules are close together in an irregular arrangement near the bottom of the container. Their continuing motion is not drawn. Particle positions and sizes are schematic, not to scale. All panels show twelve unchanged molecules. Gas arrows represent motion; liquid motion is omitted. Compare cooling and compression separately with the starting gas. Arrow lengths and container dimensions are schematic. T means temperature.Liquid can formMolecules remain closeThey still move
Cooling slows the molecules; compressing at constant temperature brings them closer together. Under the right conditions, either change can turn the gas into a liquid.

Each panel changes one condition compared with the starting gas. They are not two steps that must both happen: cooling, compression or both together may cause liquefaction, depending on the substance and the starting conditions.

Check your understanding 3 Explain the compressed panel

The compression panel has closer circles but the same motion-arrow lengths as the starting gas. Explain how it can favour liquefaction without lowering the average kinetic energy. Use the displayed particle count to check that no molecules were added. Without the diagram, both panels show twelve molecules and equal-length arrows, but the compressed container is narrower.

Show answer

Twelve circles in each panel show that the number of molecules is unchanged. Equal arrow lengths represent the same average speed at constant temperature, so average kinetic energy has not decreased. The smaller volume reduces separation, bringing the same intermolecular attractions into play more often and more effectively. A liquid can form if the temperature is low enough for those attractions to keep molecules close.

Check your understanding 4 When compression is not enough

A gas is compressed to a very high pressure but stays a gas, because its temperature is high. Suggest why cooling it may then cause it to liquefy.

Show answer

At a high temperature, the molecules move so fast that even when they are pushed close together the attractions cannot hold them: they keep escaping. Cooling lowers their average kinetic energy. Once it is low enough, the attractions can keep the compressed molecules close together, and a liquid forms. This is why some gases cannot be liquefied by pressure alone until they are cooled below a certain temperature.

Comparing gases

Gases differ in how strongly their molecules attract one another, depending on the intermolecular forces present and the size of the molecules. That decides how far each gas must be cooled before it liquefies.

Check your understanding 5 Hold pressure fixed

Two gases of similar molecular size are cooled at the same pressure, at which both can condense. A has stronger intermolecular attractions than B. Which is expected to condense at the higher temperature?

Higher condensation temperature
Show answer

Gas A is expected to condense at the higher temperature. Its stronger attractions can hold molecules close even with greater average kinetic energy. Gas B must be cooled further at the same pressure.

Try it yourself 2

Which gas liquefies first?

Minimal support

Problem

Two gases of similar molecular size are compared at a pressure at which both can condense. Gas A has stronger intermolecular attractions than gas B. Explain the higher-condensation-temperature prediction using attraction strength and molecular kinetic energy.

Try this before viewing the solution

Hints

Hint 1

hold pressure constant

What differs between the gases while the pressure is held fixed?

Hint 2

allow greater motion

Which attractions can hold molecules together even when they move faster?

Show solution step by step

  1. Make the prediction

    Method

    Select gas A.

    Reason

    Its stronger attractions can hold its molecules together in a liquid even when they have more kinetic energy.

    Working

    Answer

    Gas A liquefies at the higher temperature.

  2. Connect temperature and attraction

    Method

    State that gas B must be cooled further at the same pressure.

    Reason

    Its molecules must slow down more before its weaker attractions can hold them together.

    Working

    Answer

    At equal pressure, A liquefies at a higher temperature than B.

Common mistakes

  • Saying high pressure makes the intermolecular forces stronger. It brings the molecules closer, where the same attractions act more effectively.
  • Saying liquefaction breaks or forms covalent bonds. The molecules are unchanged.
  • Assuming any gas will liquefy if the pressure is high enough. Above a certain temperature it must be cooled as well.
  • Treating cooling and compression as two steps that must both happen. Either, or both, can cause liquefaction.

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