Question

Explain dynamic equilibrium with respect to solution formation. What is a saturated solution? An unsaturated solution? A supersaturated solution?

Short answer

Dynamic equilibrium is reached when the solute's dissolution and crystallization rates are equal. A saturated solution contains the maximum solute at equilibrium, an unsaturated solution contains less solute than the maximum, and a supersaturated solution contains more solute than the maximum amount in a stable state.

Step-by-step solution

Understanding Dynamic Equilibrium

Dynamic equilibrium in solution formation occurs when the rate of dissolution (the process of a solute dissolving in a solvent) is equal to the rate of crystallization (the process of solute coming out of solution and forming crystals). At this point, the amount of dissolved solute remains constant over time, although particles are still being exchanged between the dissolved and solid states.

Defining Saturated Solution

A saturated solution is a solution in which the maximum amount of solute has been dissolved at a given temperature. Any additional solute will not dissolve because the solution equilibrium has been reached; the dissolution and crystallization processes are occurring at the same rate.

Explaining Unsaturated Solution

An unsaturated solution is a solution that contains less solute than the maximum amount that can be dissolved at a given temperature. In other words, there is still capacity for more solute to dissolve without reaching the equilibrium point.

Describing Supersaturated Solution

A supersaturated solution is a solution that contains more solute than what would normally be possible at a given temperature. This occurs when a solution is saturated at an elevated temperature and then carefully cooled without disturbing the solution. If successful, it remains clear and contains more solute in the dissolved state than would be expected. Disturbing a supersaturated solution can cause rapid crystallization.

Key concepts

Saturated Solution

Imagine trying to dissolve sugar in your tea. At first, the sugar dissolves quite easily, but after adding a few spoonfuls, you'll notice that no more sugar dissolves, no matter how much you stir. What you have now is a saturated solution. This means that you have added as much sugar (solute) as the tea (solvent) can hold at that temperature. At this point, the solution has reached a balance. Any additional sugar will simply settle at the bottom because the solution can't accommodate more. This capacity is influenced by temperature; warming up the tea would allow more sugar to dissolve, while cooling it down might cause some sugar to crystallize out.

Unsaturated Solution

Now let's go back to when you first started stirring sugar into the tea. Initially, the sugar disappeared as it dissolved, and you could have kept adding and dissolving more. This is what's known as an unsaturated solution. There is less solute—sugar, in this case—than the solvent—tea—can dissolve at the given temperature. The unsaturated solution is like a sponge that is not yet fully soaked; it still has room to absorb more. In unsaturated solutions, the rate of dissolution is faster than the rate of crystallization because there are fewer solute molecules to re-form into solid structure.

Supersaturated Solution

But what if we heat the tea, dissolve more sugar, and then gently cool it down without stirring? This scenario could create a supersaturated solution, a tricky and unstable situation where more sugar is dissolved in the tea than typically possible at that cooler temperature. Supersaturation is an overfilled state—like overstressing that sponge beyond its limits. It's a delicate balance; even a small disturbance, such as a bump or introducing a single sugar crystal, can trigger rapid crystallization, where the excess sugar swiftly turns into solid particles and falls out of solution.

Rate of Dissolution

The rate of dissolution refers to how quickly a solute dissolves in a solvent. It's affected by various factors such as temperature, agitation (stirring), and the surface area of the solute. For instance, granulated sugar dissolves quicker than a sugar cube because its larger surface area comes into contact with more solvent. Temperature plays a significant role, too: Higher temperatures generally increase the rate of dissolution because the solvent's molecules move faster and interact more frequently and vigorously with the solute.

Rate of Crystallization

Opposite the process of dissolving is crystallization. The rate of crystallization indicates how quickly the solute particles come out of the solution and form crystals. This can occur when a saturated solution starts losing its solvent (through evaporation, for example) or if a supersaturated solution is disturbed. The rate of crystallization matches the rate of dissolution at equilibrium in a saturated solution. At this stable state, the number of solute particles leaving the solution is equal to the number entering it, so there is no net change in concentration.