# What do curly brackets {} mean?

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.

• 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. Commented May 30, 2016 at 17:57
• 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. Commented May 30, 2016 at 18:51
• 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 $a_{\ce{Ca^2+}}a_{\ce{OH-}}^2$ Commented May 30, 2016 at 18:59
• 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 Commented May 30, 2016 at 19:01