# What is the standard way to denote physical states in a chemical reaction?

A lot of older posts that I come across have their reactions written like:

$$\ce{Cu(NO3)2_{(aq)} + 2NaHCO3_{(s)} -> CuCO3_{(s)} + 2NaNO3_{(aq)} + H2O_{(l)} + CO2_{(g)}}$$

($$\ce{Cu(NO3)2_{(aq)} + 2NaHCO3_{(s)} -> CuCO3_{(s)} + 2NaNO3_{(aq)} + H2O_{(l)} + CO2_{(g)}}$$)

I just go ahead and remove the subscript to convert it into:

$$\ce{Cu(NO3)2 (aq) + 2NaHCO3 (s) -> CuCO3 (s) + 2NaNO3 (aq) + H2O (l) + CO2 (g)}$$

($$\ce{Cu(NO3)2 (aq) + 2NaHCO3 (s) -> CuCO3 (s) + 2NaNO3 (aq) + H2O (l) + CO2 (g)}$$)

i.e. convert the phases of matter from subscript to inline text.

I didn't find any meta question related to this already, and I am not sure which book to look into either. Is the subscript notation discouraged? Or is it actually the standard notation instead? I don't know, but I've almost always seen the latter being used in both my books or on the Internet.

States of aggregation should not be subscripted, it is not wrong, but the recommendations (Sec. 2.1.) are different.

The document recommends that:

The symbols should be printed in roman type without full stops (periods) and should be placed in parentheses after the symbol for a physicochemical quantity or chemical substance to which they relate.
(....)
Extra information on the state of a substance can be imparted either by use of simple subscripts and superscripts

Examples given are of standard entropy and molar heat capacity.

For completeness, here is a list of the different "states of aggregation" symbols that the IUPAC recommends:

$$\begin{array}{c|c} \text{State of aggregation}&\text{Symbol}\\\hline \text{gas or vapor}&\text{g}\\ \text{liquid}&\text{l}\\ \text{solid}&\text{s}\\ \text{condensed phase}^1&\text{cd}\\ \text{fluid}^2&\text{fl}\\ \text{liquid crystal}^3&\text{lc}\\ \text{crystalline solid}^4&\text{cr}\\ \text{amorphous solid}&\text{am}\\ \text{vitreous substance}^5&\text{vit}\\ \text{adsorbate}^6&\text{ads}\\ \text{monomeric form}&\text{mon}\\ \text{polymeric form}^7&\text{pol}\\ \text{solution}^8&\text{sln}\\ \text{aqueous solution}^9&\text{aq} \end{array}$$

Notes:

1. either the solid or liquid state
2. either the gaseous or the liquid state
3. crystalline liquid
4. where polymorphism occurs, it may be necessary to augment the symbol cr with a descriptor for the crystal modification under discussion; the preferred descriptors are Roman numerals, with textual definition of the crystallographic significance of the numerals used (see examples given later in the Appendix)
5. a glass
6. a species adsorbed on a substrate
7. in many cases the monomeric or polymeric character of the entity will be clear from the context without the symbol, and the symbol should be used only in cases where ambiguity might result
8. see Appendix I, Section A.I.9 in reference 1; in many contexts it will be clear whether a liquid solution or a solid solution is meant, but where this is unclear it must be made clear by supplemental notation (see e.g. paragraph 2.3 below)
9. the symbol for a solution in which water is the solvent; 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 (see paragraph 2.2 and Note b).