Why are some salts insoluble?

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Saturated solutions

If more and more of a substance is added to a solvent, it initially dissolves until a certain concentration is reached that does not change with further substance addition. The excess substance remains as a solid (or, in the case of liquids that are not completely miscible, as a liquid) phase. Such a solution is called a saturated solution. The amount of a substance that dissolves in a certain amount of a solvent is a characteristic property of the substance in question. It is called its solubility. Depending on the type and age of the tables, the solubility is given in / 100 solvent or / 1000 solvent or solution. It is always important to specify the temperature, because solubilities are usually clearly temperature dependent.

In a saturated solution there is a dynamic equilibrium between dissolved and undissolved substance, i.e., on average over time, as many particles are deposited on the crystal surface as are detached in other places, for example:

These processes lead to a dynamic equilibrium, which is formulated in the following form as a solution equilibrium:

How high the solubility is depends not only on the enthalpy of solution, but also on the entropy of solution. The energetic conditions alone say nothing about the solubility. Potassium nitrate thus dissolves endothermically, but is very soluble in water.

The temperature dependence of the solubility is explained by the energetic conditions during the dissolution process. Substances that dissolve endothermically are more soluble at higher temperatures, i.e. their saturation concentration is higher. Exothermically soluble substances show reduced solubility at elevated temperatures. This behavior follows the principle of "escape from compulsion" which was formulated qualitatively by Le Chatelier (1888) and applies to all dynamic equilibria. If one exerts a "compulsion" on such a system by supplying heat, the system reacts to this in such a way that it tries to use up the supplied heat, whereby a new equilibrium is established. The solubility of most salts increases with increasing temperature, i.e. they dissolve endothermically. What is striking is the behavior of sodium chloride, the solubility of which hardly increases with increasing temperature. According to the laws discussed, this must be due to the fact that hydration enthalpy and lattice energy have approximately the same amounts, i.e. the total enthalpy of solution is approximately zero.

In most cases, the solubility of gases decreases with increasing temperature, because the dissolution process is exothermic. The gas molecules, which exert negligible forces on one another, are more strongly attracted by the liquid molecules when they dissolve, which corresponds to energy gain and thus an exothermic reaction. You can therefore remove dissolved gases from liquids by heating.