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General Chemistry Principles, Patterns, and Applications, 2011

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Figure 13.4 Dissolution <strong>and</strong> Precipitation<br />

(a) When a solid is added to a solvent in which it is soluble, solute particles leave the surface of the solid <strong>and</strong> become<br />

solvated by the solvent, initially forming an unsaturated solution. (b) When the maximum possible amount of solute<br />

has dissolved, the solution becomes saturated. If excess solute is present, the rate at which solute particles leave the<br />

surface of the solid equals the rate at which they return to the surface of the solid. (c) A supersaturated solution can<br />

usually be formed from a saturated solution by filtering off the excess solute <strong>and</strong> lowering the temperature. (d)<br />

When a seed crystal of the solute is added to a supersaturated solution, solute particles leave the solution <strong>and</strong> form<br />

a crystalline precipitate.<br />

Factors Affecting Solubility<br />

The maximum amount of a solute that can dissolve in a solvent at a specified temperature <strong>and</strong> pressure is<br />

its solubility. Solubility is often expressed as the mass of solute per volume (g/L) or mass of solute per<br />

mass of solvent (g/g), or as the moles of solute per volume (mol/L). Even for very soluble substances,<br />

however, there is usually a limit to how much solute can dissolve in a given quantity of solvent. In general,<br />

the solubility of a substance depends on not only the energetic factors we have discussed but also the<br />

temperature <strong>and</strong>, for gases, the pressure. At 20°C, for example, 177 g of NaI, 91.2 g of NaBr, 35.9 g of<br />

NaCl, <strong>and</strong> only 4.1 g of NaF dissolve in 100 g of water. At 70°C, however, the solubilities increase to 295 g<br />

of NaI, 119 g of NaBr, 37.5 g of NaCl, <strong>and</strong> 4.8 g of NaF. As you learned in , the lattice energies of the<br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

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