Water is a substance characterised by properties such as high boiling point and its ability to act as a universal solvent. So why do we say that “the smallest unit of water is a water molecule”? Single molecules cannot possess such properties. Am I missing something here?

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    $\begingroup$ You need a lot more water molecules until you have water. A drop of 10,000 will show a huge difference e.g. in vapour pressure. Even a billion would still be quite strongly influenced by it's surface, and not show the ideal bulk water properties. $\endgroup$ – Karl Jan 31 at 9:09
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    $\begingroup$ Not all water is defined the same. "Water" defines a molecule - $H_2O$ - but in our everyday parlance also a bulk substance which is a collection of such molecules. Perhaps it would help to say "the smallest unit of a volume of water molecules is a single water molecule". Not very enlightening perhaps. $\endgroup$ – Buck Thorn Jan 31 at 10:57
  • $\begingroup$ Water is a substance. Would it perhaps be more correct to say that a water molecule is the smallest unit that defines the properties of the substance water? $\endgroup$ – user70417 Jan 31 at 11:12
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    $\begingroup$ @J.Smith I don't think that's going to solve your question, because you still need to define "properties". The properties in question can depend on how many water molecules you have. $\endgroup$ – Buck Thorn Jan 31 at 11:36
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    $\begingroup$ The problem here is that conventional usage of "the smallest unit of water is a water molecule" leaves out a lot of qualifications. The sentence ignores the qualification that some properties are intrinsic to the molecule but others only appear when there are a lot of molecules together. Every molecule in bulk water has the same mass, geometry and electric dipole but many of the bulk properties only appear when there are millions of the molecules (since they depend on how the molecules interact). $\endgroup$ – matt_black Jan 31 at 13:37

No, you aren't missing anything. Indeed, one molecule doesn't have any boiling point, neither high nor low. In fact, you've just stumbled upon one of the holes deliberately left in the early definitions. It is a very common thing in the textbooks on natural sciences: first we give the kids a simplified definition, so they don't have to read a ton of prerequisites before they start reading the book, and then they kinda already know what a molecule is, so why bother returning to it.

If we were to invent a more-or-less realistic definition of a molecule in simple terms, it would go like this: "the smallest unit of matter such that the matter retains its properties when split to these units and then allowed to settle down".

Indeed, say we have a glass of water, and we magically split it to individual molecules and then step aside. The molecules will assemble back into liquid water; probably it will be all over the place, but who cares, it is liquid water all the same, with unchanged boiling point and stuff.

Now what if we magically split a sample of some matter to smaller units, like atoms? Well, as soon as we step aside, those will assemble back to molecules, but not necessarily the same molecules. They may well be different, and so will be the physical properties of the sample.

This definition is not without its flaws either, but that's another story.

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    $\begingroup$ Many molecules won't survive being taken into vacuum. ;-) Imo trying to define real world stuff like molecules in terms of abstract entities like "smallest unit of matter" is generally a bit of a philosophical exercise in futility. You get to statements which are often flawed, true in best case, but all pretty useless in gaining further understanding. $\endgroup$ – Karl Jan 31 at 9:22

The word "water" can be used to mean different things. In everyday language we usually use "water" to mean a large collection of water molecules, which we associate with such phenomena as boiling, freezing, solubilization of salts, etc. We can conjure a different definition of "water", namely, a single water molecule, with molecular formula $\ce{H2O}$. These are not the same thing. In a way, one is a subset of the other.

The solvation capabilities and boiling point of water are thermodynamic properties, usually (but not only) defined for macroscopic ("bulk") quantities of water under equilibrium conditions. The boiling point defines a temperature at which a transition occurs between the liquid and the gas states of a substance. The boiling point is a property of the ensemble, the entire collection of molecules. It is important to emphasize that when chemists discuss properties such as boiling points they are pretty strict. Such properties depend on the pressure, amongst other things. Even the geometry of the water volume can influence the experimentally observed boiling point. Chemical thermodynamic properties belong by definition to a collection of molecules under well defined equilibrium (constant) conditions. Thermodynamicists observed that properties like solubility and phase transition temperatures reach limiting values provided you have sufficient molecules, that is, above a limit dealing with bulk matter. In fact, a thermodynamic limit can be defined by observing how properties change with the amount of a substance.

It doesn't make much sense to attribute a bulk property such as a boiling point to a single water molecule, but you can say that a single molecule of water has intrinsic molecular properties that lead reproducibly, when the molecule aggregates with others of its kind, to specific macroscopic properties.

On a historical note, properties such as the boiling point were defined quantitatively as the foundations of thermodynamics were layed down and a formal molecular theory of matter began to be developed, the latter involving amongst other things statement of Avogadro's hypothesis and, later, advances in spectroscopy.

  • $\begingroup$ Thanks for the response Try Hard. Does that mean that we cannot define a single molecule of water to be the substance “water”? Since water is characterised by the aforementioned thermodynamic properties. $\endgroup$ – user70417 Jan 31 at 13:10
  • $\begingroup$ Water is divisible down to the molecular level. It exhibits properties (thermodynamic or possibly other) that may depend on the number of such molecules and their interactions. $\endgroup$ – Buck Thorn Jan 31 at 13:36
  • $\begingroup$ Then let me ask this question: What properties characterise a substance? Are they thermodynamic properties or something else? What am I missing here? $\endgroup$ – user70417 Jan 31 at 22:49
  • $\begingroup$ The word "substance" typically implies a macroscopic ensemble, a large collection of molecules. A thermodynamic description might be adequate, assuming the substance is in equilibrium, which for a pure homogeneous liquid such as water would mean stating what temperature and pressure it is at. Then you can proceed to list all of the properties you might be interested, extensive (such as volume, enthalpy of formation, etc) or intensive (molar properties such as molar heat capacities, thermal expansion coefficient, etc). $\endgroup$ – Buck Thorn Jan 31 at 22:59
  • $\begingroup$ With thermodynamic properties you would not be speaking of one water molecule, but of the properties of the ensemble of molecules. But of course properties can also include spectroscopic properties that are impossible to explain without an atomic/molecular/QM description of matter. When you start dealing with certain spectroscopic properties, you move away from the aggregate properties and shift into the realm of the molecule. The aggregate properties become "perturbations" on molecular properties that are similar in an ensemble and in a single molecule. Often, but not always. $\endgroup$ – Buck Thorn Feb 1 at 8:58

the smallest unit of water is a water molecule

To say that a molecule is the smallest unit of a substance that shares its properties is not a good definition, and has the same problems as defining an atom as the smallest unit that has all the properties of a given element.

For the definition of an atom, you can salvage that definition by specifying exactly what properties are retained. For example, the electronic transitions involving inner electrons will be the same for an isolate atom and for different allotropes of the element (and you can use that to identify atoms from their spectrum even if they are extraterrestrial).

Similarly, you can specify a small set of properties that a bulk sample of a pure molecular substance has in common with a single one of the molecules it contains: Composition, bond lengths and angles (with some minor variations), electronic spectra (more or less).

Of course, if your bulk sample is in the gas phase, there will be more properties that are the same, so pure water in the gas phase or water in air will share most properties with a single isolated water molecule.


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