CHAPTER 9
CONTACT EQUILIBRIUM PROCESSES - THEORY (cont'd)

SOLID/LIQUID EQUILIBRIA

Liquids have a capacity to dissolve solids up to an extent, which is determined by the solubility of the particular solid material in that liquid.

Solubility is a function of temperature and, in most cases, solubility increases with rising temperature. A solubility curve can be drawn to show this relationship, based on the equilibrium concentration in solution measured in convenient units, for example gkg^-1, as a function of temperature. Such a curve is illustrated in Fig. 9.1 for sodium nitrite in water.

Figure 9.1 Solubility of sodium nitrite in water

There are some relatively rare systems in which solubility decreases with temperature, and they provide what is termed a reversed solubility curve. The equilibrium solution, which is ultimately reached between solute and solvent, is called a saturated solution, implying that no further solute can be taken into solution at that particular temperature.

An unsaturated solution plus solid solute is not in equilibrium, as the solvent can dissolve more of the solid. When a saturated solution is heated, if it has a normal solubility curve the solution then has a capacity to take up further solute material, and so it becomes unsaturated. Conversely, when a saturated solution is cooled it becomes super saturated, and at equilibrium that solute which is in excess of the solubility level at the particular temperature will come out of solution, normally as crystals. However, this may not occur quickly and in the interim the solution is spoken of as super saturated and it is said to be in a metastable state.


Contact-Equilibrium Processes - THEORY > EQUILIBRIUM-CONCENTRATION RELATIONSHIPS

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