 Okay, so let's work through some examples. Firstly, we'll practice writing equilibrium expressions for reactions. So let's take the first one here, the reaction between hydrogen and carbon monoxide to give methanol. Remember that the expression involves products overreactions, reactants, sorry, and that the concentrations must be raised to the power of the stoichiometric coefficient. So this is going to look like KEQ equals... Our only product is methanol, so the concentration of methanol over the concentration of hydrogen times the concentration of carbon monoxide. And then you can see that hydrogen has a stoichiometric coefficient of 2, so we raise its concentration to the power of 2, it's squared, and the others have stoichiometric coefficients of 1, so they stay the same. So that is the equilibrium expression for that particular reaction. Note that because all the species in this particular reaction are gases, we could also write an expression for Kp, in which case we would use the pressures of each gas. Alright, now let's take the next one, sodium chloride dissolving. Now, equilibria can be physical as well as chemical, so in this case we've got something dissolving rather than an actual chemical reaction taking place. We're going to be looking more closely at dissolution equilibria in lessons to come. For now though, there's something important that you need to know. Because in equilibria what we're interested in is changes in concentration or pressure, we can ignore reactants or products that are in the solid phase or the pure liquid phase. So they do not appear in the equilibrium expression. The reason is that while the amount of a solid might change, its concentration, the number of molecules in a given volume remains constant. It's only when a substance is dissolved in a solvent, or when its particles are far apart like in a gas, that it's possible to change its concentration. When it's in a solid or a liquid form, the particles are essentially as close together as they can get. So as long as that reactant or product stays pure, its concentration doesn't change. So when sodium chloride dissolves, we ignore the solid reactant here, and we write the expression like this. It's going to be products over reactants. So our products are sodium ions, so we want the concentration of the sodium ion and the concentration of the chloride ion. Both of those are aqueous remember, that means they're dissolved in water, so they do have concentrations. And normally we would then put this over the concentration of the reactant, but in this case our reactant is solid, so we just ignore it. Actually what happens is that the unchanging concentration of that solid reactant is sort of subsumed into the value of keq, and it's taken into account in that way. Similarly in this reaction here where we have calcium carbonate solid decomposing into calcium oxide and carbon dioxide gas, two of our species here are solids. So our equilibrium expression is going to be quite simple. We ignore the calcium carbonate, we ignore the calcium oxide. The carbon dioxide is our only product, so that's our equilibrium expression for this particular reaction. And it even has a stoichiometric coefficient of 1, so we don't even have to raise it to any power. So for this particular reaction we have the unusual situation where the equilibrium constant is exactly equal to the concentration of one of the products.