 So in this video, we're going to look at a very particular kind of equilibrium, which is the equilibrium that's set up when something dissolves in water, specifically when ionic compounds or salts dissolve in water. So up until now we've been talking about equilibria and how they are defined by the equilibrium constant, KEQ. When we specifically talk about things dissolving, we give the equilibrium constant a special name. It's called KSP and the SP stands for solubility product. So this is specifically for situations where you've got something dissolving and precipitating in solution. So here's an example. We've got potassium carbonate and we've put some of that into water and the forward reaction in this equilibrium is that the solid is dissolving into aqueous ions, which are free in solution. So forward reaction is dissolving and the reverse reaction is that the free ions in solution are re-precipitating as the solid. So the reverse reaction is precipitating. Bear in mind that this is what we would tend to call a physical equilibrium. It's not actually a chemical reaction. We're not producing any new compounds. We're just moving between the solid state and the aqueous state. But it is nevertheless a true equilibrium. Okay, to write the equilibrium expression for a solubility equilibrium, you do it exactly the same way as you do for any other equilibrium. But the thing to remember is that in this kind of equilibrium, the reactant is always solid. It's the salt that's dissolving. And because of this rule that we have, that we ignore the solids and pure liquids, it doesn't appear in the equilibrium expression. So for this particular one we've got Ksp, solubility product instead of Keq. It's exactly the same thing. We're just giving it a different name to indicate that it's this particular kind of equilibrium. So we've got Ksp equals and it's products over reactant. So our products are the potassium ion here and it has a stoichiometric coefficient of 2, so we raise it to the power of 2. And the carbonate ion, concentration of the carbonate ion, our reactants of course are the solid and so they get ignored. So that's our equilibrium expression.