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:


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


Where $Q$ is the actual saturation of the solution. And the answer is that no precipitation occurs since $Q &lt; K_\mathrm{s}$ 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.
The curly brackets denote "activity of" the species therein. 

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