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When a molecule of water ($\ce{H2O}$) and a molecule of hydrochloric acid ($\ce{HCl}$) react together, would they become a molecule made of molecules? What's the name for such thing? How would it be represented - just as $\ce{H2OHCl}$?

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  • $\begingroup$ It's quite nice question, but I heavily edited it. You can also edit if you disagree with some of changes. $\endgroup$ – Mithoron Aug 26 '15 at 23:44
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The key to your question lies in the word "bound". Without mentioning the state of your species we can say in general, species which are joined by covalent bonds are single molecules, there are countless examples of such reactions in organic chemistry where two small molecules condense into a single molecule.

Species with intramolecular forces (hydrogen bonds, van der waals etc) can be called bound or perhaps "clusters", which is quite a broad term. Real bonding however is not so easily defined as the molecular orbitals may generate a wide spectrum of charge distributions etc. Quantum mechanics gives a much clearer description of the bonding of a particular molecule or extended system.

The terminology you write your $\ce{H2OHCl}$ is similar to how we write water of crystallisation in a solid. If this is where your idea came from, it should be remarked that this is a distinct concept.

To conclude; species that are covalently bound (that could have strong polarisation even to bordering ionic bonding) are molecules. Intermolecular forces are precisely that, between distinct molecules. We should remark that we can also have extended solids with covalent bonding (e.g. group 14). In addition some regard metals as molecules too, molecules whose molecular orbitals are distributed over the entire structure etc (research tight binding model for more information). But I feel that wasn't the focus of your question?

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  • $\begingroup$ In fact, such clusters are sometimes referred to as "van der Waals molecules". $\endgroup$ – Ivan Neretin Aug 27 '15 at 8:02
  • $\begingroup$ @IvanNeretin T be honest I think there is a bit of a grey area in terms of the nomenclature. It is best to think of interactions in terms of MO diagrams in my opinion. When one draws the cut off between a molecule or not is vague. $\endgroup$ – AngusTheMan Aug 27 '15 at 9:34

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