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I learned that square brackets ex: $[\ce{H2O}]$ can be used to denote the concentration of water molecules in a fluid. The notation used much in equilibrium problems and such.

However, sometimes I have seen curly brackets {} being used in similar scenarios, but I never really understood the difference. When are curly brackets used and what do they mean?


I found this example from an exam where the problem is to see if a solution of $\ce{Ca(OH)_2}$ in water is concentrated enough to result in precipitation.

Following is what how they solve it in the example:

  1. Solubility constant for $\ce{Ca(OH)_2}$ is given as $K_\mathrm{s} = \{\ce{Ca^{2+}}\}\{\ce{OH^-}\}^2$ (100% saturation point I think)
  2. $Q = \{\ce{Ca^{2+}}\}\{\ce{OH^-}\}^2 \approx [\ce{Ca^{2+}}][\ce{OH^-}]^2$ (actual saturation I think)

Where $Q$ is the actual saturation of the solution. And the answer is that no precipitation occurs since $Q < K_\mathrm{s}$ in this specific example.

So they seem to suggest that {} is something different than the concentration [] and that it can be approximated to the concentration.

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  • $\begingroup$ Hmm, the place I've seen curly brackets as you describe is for inorganic crystalline substances - see this link for an explanation. That said, they are used to denote the constitution of structural units and not used to enclose chemical species, so the use case I present might not be relevant to yours. $\endgroup$ – Todd Minehardt May 30 '16 at 17:57
  • $\begingroup$ It doesn't seem to have anything with crystalline substances to do. I found an example where they use the denotation which I edited in. $\endgroup$ – dekuShrub May 30 '16 at 18:51
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    $\begingroup$ In that context I would guess it is referring to the activity, although I don't think that is a standard notation for it, usually one would write $a_{\ce{Ca^2+}}a_{\ce{OH-}}^2$ $\endgroup$ – orthocresol May 30 '16 at 18:59
  • $\begingroup$ Curly braces are often also used to denote encounter pairs, or molecules held together in solvent cages, but that's obviously not what you're talking about $\endgroup$ – orthocresol May 30 '16 at 19:01
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The curly brackets denote "activity of" the species therein.

See the Wikipedia section: Basic definitions and properties of Equilibrium constant

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It could be a matter of clearness on the overall expression, for example, if you are dealing with complex compounds like $\ce{Mn[H_{2}O]_{6}}$ it could be confusing to express it's concentration using [], so you can use {}

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    $\begingroup$ Are you trying to write $\ce{[Mn(H2O)6]^2+}$? $\endgroup$ – Jan May 30 '16 at 17:46

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