Is there a difference between two molecules, of the same substance, one in liquid phase and the other in gaseous phase?

Question

What are the main differences? Volume, shape, or what?

Answer 1

While in a dilute gas the intermolecular distances are about one order of magnitude larger than nearest-neighbors distances in a molecule, this is not the same in liquid and solid phases. This is the reason one could expect variations of physical molecular properties with the environment. The site you cited in a comment does not directly address the issue of the modifications of one molecule as a function of different condensed phases. Unfortunately, it is not easy to find a comprehensive discussion of this point in textbooks.

Here, I can list and briefly discuss a few issues.

First of all, condensed phases include phases under very high pressure, then, from dilute gases to a very dense phase, there is a continuum of average configurations. But, while in a dilute phase molecular identities are quite obvious, in a dense phase, identification of individual molecules may become impossible. In the so-called molecular liquids or solids, there is a visible separation between intra- and inter-molecular distances, allowing to speak about molecules. However, even in such cases, experiments under pressure may eventually blur the individual molecules.

Even without the effect of pressure, the chemical environment, changing the interactions, may modify molecular properties as measured in vacuum. This is true for bond lengths and angles, but also for electronic properties like molecular polarizabilities. In general it is an interesting and non simple exercise to try to connect condensed phase properties with their isolated molecule counterparts. Since thee are many properties which could be used to discuss quantitatively this issue, I would leave a discussion to more specific questions.