Saturated Solutions and Dynamic Equilibrium

When a solid is added to a liquid, a competition begins between two opposing rates of change. On the one hand, particles leave the surface of the solid and move into solution, and on the other hand, particles leave the solution and return to the solid form. 

Solute Concentration and Rate of Return to Solid Form 

More collisions per second between solute and solid 

Tllc cunccnrratitm ol solute in the solution is low, io ihc rate of return is low. 

Ihc concentration of L lme in ihc soliuion is higticr, so the rate of return is higher 

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In a saturated solution, a dynamic equilibrium is established between the dissolved solute in the solution and the undissolved solute. At equilibrium, the rate of the forward reaction (dissolving) equals the rate of the reverse reaction (crystallization or precipitation). The solubility of a substance is usually expressed as the mass or amount of solute present in 1 dm of solution. 

When a solution is saturated, there is dynamic equilibrium. Solute molecules move back and forth between the solute phase and the solution phase at equal rates. 

Solubility Equilibrium. The misconceptions regarding the amount of solid materials in equilibrium and the dynamic aspect are equally important in the discussion. If one observes a saturated sodium chloride solution together with solid sodium chloride, and adds an additional portion of solid sodium chloride to it, this portion sinks down without dissolving (see E6.2). If one measures the density of the saturated solution before and after the addition of salt portions, one gets the same measurements (see E6.2). The concentration of the saturated solution does not depend on how much solid residue is present equilibrium sets in between the saturated solution and arbitrary amounts of solid residue (see Fig. 6.2) ... 

SECTION 13w2 The equilibrium between a saturated solution and undissolved solute is dynamic the process of solution and the reverse process, crystalizadon, occur simultaneously. In a solution in equilih-rium with undissohred solute, the two processes occur at equal rates, ing a saturated solution. If there is less solute present than is needed to saturate the solution, the solution is unsatairated. When solute concentration is greater than the equilihrium concentration ralue, the solution is supersaturated. This is an unstable condition, and separation of some solute from the solution will occur if the process is initiated with a solute seed crystaL The amount of solute needed to form a saturated solution at any particular temperature is the solubility of that solute at that temperature. 

A saturated solution of sucrose (CJ2H22O11) is made by dissolving excess table sugar in a flask of water. There are 50 g of undissolved sucrose crystals at the bottom of the flask in contact with the saturated solution. The flask is stoppered and set aside. A year later a single large crystal of mass 50 g is at the bottom of the flask. Explain how this experiment provides evidence for a dynamic equilibrium between the saturated solution and the undissolved solute. 

We achieved, that by contact of polyurethane foam with water solution of molybdophosphate, contain by pH 1-2,5 mixture of saturated (5 NMR P=-3.20 p.p.m. apply to 85 % H PO ) and unsaturated monovacant (x=0-t-4) (5 NMR P = -0,96 p.p.m.) heteropolycomplexes Keggin staicture, equilibrium discharge in the direction produced of saturated heteropolycomplex of Dowson stmcture and on the surface of polyurethan foam formed 18-molybdo-2-phosphate acid ( P = -2,40 p.p.m. in ether extract). The formed surfaces heteropolycomplex is stable for action 1 M solution of strong acids and basics and have ion exchanged properties in static and dynamic conditions to relation to macro and micro amount of M(I) ... 

In a saturated solution, any solid solute present still continues to dissolve, but the rate at which it dissolves exactly matches the rate at which the solute returns to the solid (Fig. 8.17). In a saturated solution, the dissolved and undissolved solute are in dynamic equilibrium with each other. 

FIGURE 8.17 The solute in a saturated solution is in dynamic equilibrium with the undissolved solute If we could follow the solute particles (yellow spheres), we would sometimes find them in solution and sometimes back within the solid. Red, green, and blue lines represent the paths of individual solute particles. The solvent molecules are not shown. 

Several observations show that saturated solutions are at dynamic equilibrium. For example, if O2 gas enriched in the oxygen-18 isotope is introduced into the gas phase above water that is saturated with oxygen gas, the gas in the solution eventually also becomes enriched in the heavier isotope. As another example, if finely divided ciystalline salt is in contact with a saturated solution of the salt, the small crystals slowly disappear and are replaced by larger crystals. Each of these observations shows that molecules are moving between the two phases, yet the concentrations of the saturated solutions remain constant. 

If you take solid NaCl and add it to water, dissolution occurs rapidly at first but then slows down as more and more NaCl is added. Eventually the dissolution stops because a dynamic equilibrium is reached where the number of Na+ and Cl- ions leaving a crystal to go into solution is equal to the number of ions returning from the solution to the crystal. At this point, the solution is said to be saturated in that solute. 

In a saturated solution, an equilibrium exists between the rate of precipitation of solute particles and the rate of dissolution of solute particles. The rate of precipitation equals the rate of dissolution. The shape of a crystal of solute added to a saturated solution will change after a period of time at a constant temperature and pressure, but its mass will remain the same. The equilibrium between dissolving and precipitation is dynamic, a continuous process. 

In a saturated solution, solute particles are dissolving and recrystallizing at the same rate. It is a state of dynamic equilibrium. There is constant exchange, yet there is no net change. 

Depending on the amount of solute present, the rates of solvation and crystallization may eventually equalize no more solute appears to dissolve and a state of dynamic equilibrium exists between crystallization and solvation (as long as the temperature remains constant). Although solute particles continue to dissolve and crystallize in solutions that reach equilibrium, the overall amount of dissolved solute in the solution remains constant. Such a solution is said to be a saturated solution it contains the maximum amount of dissolved solute for a given amount of solvent at a specific temperature and pressure. An unsaturated solution is one that contains less dissolved solute for a given temperature and pressure than a saturated solution. In other words, more solute can be dissolved in an unsaturated solution. 

Solubility equilibria resemble the equilibria between volatile liquids (or solids) and their vapors in a closed container. In both cases, particles from a condensed phase tend to escape and spread through a larger, but limited, volume. In both cases, equilibrium is a dynamic compromise in which the rate of escape of particles from the condensed phase is equal to their rate of return. In a vaporization-condensation equilibrium, we assumed that the vapor above the condensed phase was an ideal gas. The analogous starting assumption for a dissolution-precipitation reaction is that the solution above the undissolved solid is an ideal solution. A solution in which sufficient solute has been dissolved to establish a dissolution-precipitation equilibrium between the solid substance and its dissolved form is called a saturated solution. 

Solubility is a solute s tendency to dissolve in a solvent. On the MCAT, the solute will usually be a salt, and the solvent will most often be water. Dissolution of a salt is reversible on a molecular scale dissolved molecules of the salt reattach to the surface of the salt crystal. For a dissolving salt, the reverse reaction, called precipitation, takes place initially at a slower rate than dissolution. As the salt dissolves and the concentration of dissolved salt builds, the rate of dissolution and precipitation equilibrate. When the rate of dissolution and precipitation are equal, the solution is said to be saturated the concentration of dissolved salt has reached a maximum in a saturated solution. Just like any other reaction, the equilibrium established at the saturation point is dynamic the concentrations of products and reactants remain constant, but the forward and reverse reactions continue at the same rate. 

The dynamic equilibrium between a mineral and its saturated solution (i.e. the point at which no more mineral will dissolve), for example ... 

Such a solution is said to be saturated. Saturation occurs at very low concentrations of dissolved species for slightly soluble substances and at high concentrations for very soluble substances. When imperfect crystals are placed in saturated solutions of their ions, surface defects on the crystals are slowly patched with no net increase in mass of the solid. Often, after some time has passed, we see fewer but larger crystals. These observations provide evidence of the dynamic nature of the solubility equilibrium. After equilibrium is established, no more solid dissolves without the simultaneous crystallization of an equal mass of dissolved ions. 

Solubility Product. If calcium sulfate powder (gypsum) is mixed well with water and the suspension is left to stand, a white solid sinks down to the bottom (see E6.5). The question arising from the amount of solid substance is whether a part of the calcium sulfate dissolves or if the substance is insoluble. Testing the electrical conductivity (see E6.5), however, shows a much higher value than with distilled water calcium sulfate dissolves in very minute amounts a dynamic equilibrium is formed between the solid residue and the saturated solution ... 

A saturated solution need not be accompanied by excess solid in its container, although if it does, there will be a dynamic equilibrium between the rate of solution and the rate of return. A saturated solution without excess solid can be made by first adding solid to a solvent until no more will dissolve, and then filtering out the excess solid. 

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