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I am confused with the meaning of "(aq)" in chemical equations, and the meaning of "dissolved". What is the actual meaning it is supposed to convey?

For instance, in the document "Preparing Bromine Water", bromine molecules in bromine water are labelled $\ce{Br2_{(aq)}}$.

  1. What does dissolving in water mean? Must it be that a hydration shell is formed around the molecule? See:Dissolving of non polar gases in water (liquid)

  2. Even if no hydration shell is formed, small molecules just insert themselves into the space between solvent molecules, is it still called "dissolve"? It does seem more like being "dispersed", as in a colloid.

  3. Must "(aq)" mean that there is a hydration shell surrounding the molecule/ion?

  4. For non-polar molecules dissolved in water, do we label it as "(aq)" in chemical equations? Must they form hydration shells?

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The abbreviation (aq) in chemical equations or symbols simply means that this material is present in water as a homogeneous phase. The state symbol has nothing to do with the actual and true "structure" in solution. In short forget about hydration shell.

From IUPAC's recommendations "Notation for states and processes, significance of the word standard in chemical thermodynamics, and remarks on commonly tabulated forms of thermodynamic functions" Pure & Appl.Chem., Vol.54, No.6, pp. 1239—1250, 1982.

the symbol for a solution in which water is the solvent (an aqueous solution) is aq; in the past this symbol has sometimes been used to denote an infinitely dilute aqueous solution, but infinite dilution should henceforward be denoted by the extra symbol $\infty$.

$\ce{CO2 (aq)}$ would imply dissolved carbon dioxide in water (i.e.,to our eyes it is a single phase, just like an unopened bottle of coke).

$\ce{Br2 (aq)}$ would imply homogeneous solution of liquid bromine in water.

$\ce{NaCl (aq)}$: Salt dissolved in water.

A colloid is a heterogeneous phase.

But above all, do not worry too much about these minor symbolism issues. A chemical equation is not meant to convey everything. It is just a shorthand notation to describe chemical reactions. A real experiment or a research paper should describe the experimental condition with sufficient details in such a way that an experienced person should be able to repeat it.

You can also use (sln) as the state symbol, when the substance is dissolved but not present in water. From the same document, I quote an example.

$$\ce{Na (s) + 5C2H5OH (l) = 0.5 H2 (g) + \{C2H5ONa + 4 C2H5OH\} (sln)}$$

Try to interpret it and see if it makes sense.

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  • $\begingroup$ I disagree that a colloid solution is a heterogeneous phase, unless detergent or protein solutions should be considered heterogeneous. $\endgroup$
    – Buck Thorn
    Jun 18 at 6:11
  • $\begingroup$ See also the IUPAC Green Book, printed page 54. media.iupac.org/publications/books/gbook/… $\endgroup$
    – mhchem
    Jun 18 at 6:31
  • $\begingroup$ @Buckthorn, you are stepping in a grey area, as per the IUPAC, "The term (colloidal) refers to a state of subdivision, implying that the molecules or polymolecular particles dispersed in a medium have at least in one direction a dimension roughly between 1 nm and 1 μm, or that in a system discontinuities are found at distances of that order." $\endgroup$
    – M. Farooq
    Jun 18 at 13:18
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    $\begingroup$ Detergent solution is also tricky, under what concentration, above CMC or below CMC, should we consider them as a colloid. At a molecular level every solution is heterogenous, right (if only we could see molecules). $\endgroup$
    – M. Farooq
    Jun 18 at 13:21

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