It is not surprising, then, that nonpolar gases are most soluble in nonpolar solvents. Consequently, the only way they can interact with a solvent is by means of London dispersion forces, which may be weaker than the solvent–solvent interactions in a polar solvent. Nonpolar gases such as N 2, O 2, and Ar have no dipole moment and cannot engage in dipole–dipole interactions or hydrogen bonding. Thus we need to consider only the energy required to separate the solvent molecules and the energy released by new solute–solvent interactions. Even so, energy is required to disrupt these interactions.įor solutions of gases in liquids, we can safely ignore the energy required to separate the solute molecules because the molecules in the gas phase are already separated. The London dispersion forces, dipole–dipole interactions, and hydrogen bonds that hold molecules to other molecules are generally weak. Solutions of Molecular Substances in Liquids In contrast, adding a seed crystal to a saturated solution reestablishes the dynamic equilibrium, and the net quantity of dissolved solute no longer changes. If the rate of crystallization in Equation 9.2.1 is greater than the rate of dissolution, crystals or a precipitate will form (part (d) in Figure 9.2.1). In this video from Mike Shin a small seed crystal is introduced into the middle of the solution Consequently, adding a small particle of the solute, a seed crystal A solid sample of a substance that can be added to a supercooled liquid or a supersaturated solution to help induce crystallization., will usually cause the excess solute to rapidly precipitate or crystallize, sometimes with spectacular results.įigure 9.2.2 Crystallization of Sodium Acetate out of a Supersaturated Solution: The crystallization of sodium acetate out of a supersaturated solution can be spectacular. Like a supercooled or superheated liquid, a supersaturated solution is unstable. When the solution is cooled, it can therefore become supersaturated An unstable solution with more dissolved solute than it would normally contain under the given set of conditions. The solubility of most solids increases with increasing temperature.īecause the solubility of most solids increases with increasing temperature, a saturated solution that was prepared at a higher temperature usually contains more dissolved solute than it would contain at a lower temperature. We can prepare a homogeneous saturated solution by adding excess solute (in this case, greater than 35.9 g of NaCl) to the solvent (water), stirring until the maximum possible amount of solute has dissolved, and then removing undissolved solute by filtration. Using the value just stated, a saturated aqueous solution of NaCl, for example, contains 35.9 g of NaCl per 100 mL of water at 20☌. When a solution is saturated and excess solute is present, the rate of dissolution is exactly equal to the rate of crystallization (part (b) in Figure 9.1.1). If a solution contains less than the maximum amount of solute, it is unsaturated. As you learned in Chapter 4, the lattice energies of the sodium halides increase from NaI to NaF.Ī solution with the maximum possible amount of solute is saturated A solution with the maximum possible amount of a solute under a given set of conditions. At 70☌, however, the solubilities increase to 295 g of NaI, 119 g of NaBr, 37.5 g of NaCl, and 4.8 g of NaF. At 20☌, for example, 177 g of NaI, 91.2 g of NaBr, 35.9 g of NaCl, and only 4.1 g of NaF dissolve in 100 g of water. In general, the solubility of a substance depends on not only the energetic factors we have discussed but also the temperature and, for gases, the pressure. Even for very soluble substances, however, there is usually a limit to how much solute can dissolve in a given quantity of solvent. Solubility is often expressed as the mass of solute per volume (g/L) or mass of solute per mass of solvent (g/g), or as the moles of solute per volume (mol/L). The maximum amount of a solute that can dissolve in a solvent at a specified temperature and pressure is its solubility A measure of the how much of a solid substance remains dissolved in a given amount of a specified liquid at a specified temperature and pressure.
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