How is ammonia liquefied

Compressing ammonia gas under high pressure forces the molecules into close proximity. In a normal gas, the separation between each molecule is generally large - approximately 1000 molecular diameters is a good generalization. By contrast, the separation between the molecules in a condensed phase (solid or liquid) is more likely to be one to two molecular diameters, thereby explaining why the molar volume of a solid or liquid is so much smaller than the molar volume of a gas. 

As a direct consequence of the large intermolecular separations, we can safely say no interactions form between the molecules in ammonia gas. The molecules are simply too far apart. We saw in the previous chapter how the property known as pressure is a macroscopic manifestation of the microscopic collisions occurring between gas particles and, say, a solid object such as a container s walls. But the gas particles can also strike each other on the same microscopic scale we say the resultant interactions between molecules are intermolecular. 

Intermolecular interactions only operate over relatively short distances, so we assume that, under normal conditions, each molecule in a gas is wholly unaffected by all the others. By contrast, when the gas is compressed and the particles come to within two or three molecular diameters of each other, they start to notice each other. We say the outer-shell electrons on an atom are perturbed by the charges of the electrons on adjacent atoms, causing an interaction. We call these interactions bonds, even though they may be too weak to be formal bonds such as those permanently connecting the atoms or ions in a molecule. 

The intermolecular interactions between molecules of gas are generally attractive, so, by way of response, we find that, once atoms are close enough to interact, they prefer to remain close - indeed, once a tentative interaction forms, the atoms or 

A formal bond involves the permanent involvement of electrons in covalent or ionic bonds see p. 64. Interactions between molecules in a compressed gas are temporary. 

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