Supersaturated solutions of most solids in water are...

Name: Problem 17, Chapter 11: Chemistry
Uploaded: 2019-09-20T03:11:51-08:00
Duration: 58 s
Channel: Aadit Sharma
Description: Supersaturated solutions of most solids in water are prepared by cooling saturated solutions. Supersaturated solutions of most gases in water are prepared by heating saturated solutions. Explain the reasons for the difference in the two procedures.

Supersaturated solutions of most solids in water are prepared by cooling saturated solutions. Supersaturated solutions of most gases in water are prepared by heating saturated solutions. Explain the reasons for the difference in the two procedures.

Key Concepts

Supersaturation

Supersaturation refers to a state in which a solution contains more solute than it would normally hold at equilibrium under a given set of conditions. This metastable condition can occur when the solution is prepared at one temperature where solubility is higher and then moved to another temperature where the solubility should be lower, yet the excess solute remains dissolved temporarily.

Temperature Dependence of Solubility

The solubility of substances in a solvent often depends on temperature, but the direction of this dependence can differ between types of solutes. For many solids, solubility increases with temperature, so cooling a saturated solution results in a supersaturated condition. Conversely, for gases dissolved in water, solubility typically decreases as temperature increases, which means that heating a saturated solution can produce a supersaturated state.

Dissolution Thermodynamics

The temperature dependence of solubility is closely related to the thermodynamic nature of the dissolution process. For many solids, dissolution is generally endothermic, meaning that heat is absorbed and solubility increases with temperature. In contrast, the dissolution of gases in water is usually exothermic, so an increase in temperature shifts the equilibrium to favor the release of dissolved gas. This difference in enthalpy changes underlies the contrasting methods used to prepare supersaturated solutions for solids versus gases.

Kinetic Stability of Supersaturated Solutions

Even when a solution is supersaturated, the excess solute does not immediately precipitate out due to kinetic factors. The nucleation process required for crystallization can be hindered, allowing the supersaturated state to persist temporarily. This kinetic inhibition is a critical factor that enables practical preparation and temporary stability of supersaturated solutions before crystallization occurs.

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