Back in the days when the principal reason for teaching about solubility equilibria was to prepare chemists to separate ions in quantitative analysis procedures, these problems could be mostly ignored. All solids that dissociate into ions exhibit some limit to their solubilities, but those whose saturated solutions exceed about 0.01 mol L–1 cannot be treated by simple equilibrium constants owing to ion-pair formation that greatly complicates their behavior. For example, the solubilities of the [sparingly soluble] oxide and hydroxide of magnesium are represented by, \[Mg(OH)_{2(s)} → Mg^{2+} + 2 OH^– \label{10}\], \[MgO_{(S)} + H_2O → Mg^{2+} + 2 OH^– \label{11}\]. The nucleation problem: precipitation is [theoretically] impossible! Suppose that a dilute solution of AgNO3 is added dropwise to a solution containing 0.001M Cl– and 0.01M CrO42–. The LibreTexts libraries are Powered by MindTouch® and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. Calcium stearate is less dense than water, so it forms a scum that floats on top of the water surface; anyone who lives in a hard-water area is likely familiar with the unsightly "bathtub rings" it leaves around the high-water mark or the shower-wall stains. Volume of treated water: 1000 L + 10 L = 1010 L. Concentration of OH– on addition to 1000 L of pure water: Initial concentration of Cd2+ in 1010 L of water: The easiest way to tackle this is to start by assuming that a stoichiometric quantity of Cd(OH)2 is formed — that is, all of the Cd2+ gets precipitated. What is the equilibrium state of this solution with respect to gypsum? If this succeeds in removing the "hardness cations", the water has been "softened". To keep things as simple as possible, we will not distinguish between them in what follows, and just use the formula H2CO3 to represent the two species collectively. An old chemist's trick is to use the tip of a glass stirring rod to scrape the inner surface of a container holding a supersaturated solution; the minute particles of glass that are released presumably serve as precipitation nuclei. University-level students should be able to derive these relations for ion-derived solids of any stoichiometry. Such a solution is said to be undersaturated. Much more seriously from an economic standpoint, evaporation of water in boilers used for the production of industrial steam leaves coatings on the heat exchanger surfaces that impede the transfer of heat from the combustion chamber, reducing the thermal transfer efficiency. In any ionic solution, small clumps of oppositely-charged ions are continually forming by ordinary collisional processes. Many insoluble salts can exist in more than one crystalline form (, Other ions present in the solution can often get incorporated into the crystalline solid, usually replacing an ion of similar size (, What you were likely taught about the dissociation of salts in water is wrong! As a consequence, the concentration of "free" Cd2+(aq) in an aqueous cadmium iodide solution is only about 2% of the value you would calculate by taking K1 as the solubility product. We can express this quantitatively by noting that the solubility product expression, \[[Ca^{2+}][F^–]^2 = 1.7 \times 10^{–10} \label{8}\], must always hold, even if some of the ionic species involved come from sources other than CaF2(s). For river and lake waters, 10–5 M would be more typical; this would simply shift the curves downward without affecting their shapes. And of course, there are a number of general solubility rules — for example, that all nitrates are soluble, while most sulfides are insoluble. The point of showing this pair of plots is to illustrate the great utility of log-concentration plots in equilibrium calculations in which simple approximations (such as that made in Equation \(\ref{9b}\) can yield straight-lines within the range of values for which the approximation is valid.


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