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Factors Affecting Solubility

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[AUDIO LOGO] INSTRUCTOR: We know that the solubility of compounds is impacted by temperature, but the solubility can also be greatly impacted by the presence of other solutes. Common ions in solution can have the impact of decreasing the solubility. In this beaker, I have a saturated solution of sodium chloride. You can see there's just a little bit of solid here at the bottom. When I add concentrated hydrochloric acid, what will happen to the solution in this beaker? So here I have concentrated hydrochloric acid. When I add this to our beaker of saturated sodium chloride, you can see the presence of a white precipitate. The sodium chloride solid is crashing out of the solution. This is because of the presence of a common ion. So if we look at our equation, sodium chloride solid is in equilibrium with sodium plus ions and CL minus ions. When I add HCl remember that this is a strong acid. So it completely dissociates into H plus and Cl minus ions. So the effect is we are increasing the concentration of the Cl minus ions. So when we increase the concentration in this equilibrium, the equilibrium shifts back to the left, and the net effect is to increase the amount of solid that we have, noticed by the white precipitate in the beaker. In addition to the presence of common ions, the solubility can also be greatly impacted by the pH. In this beaker, I have milk of magnesia. Milk of magnesia is a saturated solution of magnesium hydroxide or MgOH2. So we have solid MgOH2 in equilibrium with Mg2 plus ions and OH minus ions. What will happen when I reduce the pH of this solution by adding HCl? So when I add HCl to this solution of milk of magnesia, I'm lowering the pH. And you can see that as I add the HCl, the solution is turning clear. The solid is going away. So the solubility of the solid is increasing as I lower the pH. We can explain that again by looking at the equilibrium expression. So the milk of magnesia is MgOH2 solid, and this is in equilibrium with Mg2 plus ions and hydroxide ions. So again, as I'm adding the acid, I am adding protons or H plus ions. These H plus ions will neutralize these pH ions in solution to form water. So as I am forming this water in the neutralization, the concentration of these OH ions here are going down. As these are getting pulled out of solution, the equilibrium is going to shift to the right. And the net effect is that we're reducing the amount of solid. We're forming more ions in our beaker. Lastly, the solubility can be impacted by the formation of complex ions. So remember that a complex ion has a metal center that is coordinated to ligands. Here I have nickel with 6 NH3 ligands coordinated to it. We can look at the reaction for this. And you'll notice that the Kf, the formation constant for this reaction is very, very large, 10 to the positive 9. What that means is if we have nickel present and we have NH3 present, this is going to form. And the formation of this complex ion can greatly impact the solubility of salts. So in this beaker, I have a saturated solution of nickel carbonate. So I have nickel ions and carbonate ions in equilibrium with solid nickel carbonate. When I add concentrated ammonia or NH3, I'm going to form that nickel complex ion. So what will happen to the solution when I add this concentrated NH3 to my saturated solution? So when I add concentrated ammonia to the saturated nickel carbonate solution, so you can see that the solution turned blue. The blue is from the formation of the nickel complex with the NH3 ligands. And you can also notice that the solid nickel carbonate is no longer present in the beaker. So we can think about this by looking at the solubility expression for the nickel carbonate. So I have NiCO3 solid in equilibrium with Nicole 2 plus ions plus carbonate ions. So when I'm forming this complex when I add the NH3, I'm reducing the concentration of the free nickel ions. So as this concentration decreases, my equilibrium here shifts to the right, and my solid now goes to form ions and solution. So again, the effect is forming this complex will reduce the concentration of the free nickel ions, and therefore, it increases the solubility of my nickel carbonate.