Solutions: Saturation, Solubility, and Temperature Effects

Solutions and Solubility

Types of Solutions Based on Saturation

When an ionic solid dissolves in a solvent, its ions disperse throughout the solution. The amount of solute that can dissolve in a solvent depends on the solution's saturation level. The following are key types of solutions based on the amount of dissolved solute:

• Saturated Solution: A solution in which the maximum amount of dissolved solute is present at a given temperature. No more solute can dissolve under these conditions.

• Unsaturated Solution: A solution that contains less solute than the maximum amount that can dissolve at a given temperature. Additional solute can still be dissolved.

• Supersaturated Solution: A solution that contains more dissolved solute than is normally possible at equilibrium. This is typically achieved by dissolving solute at a higher temperature and then cooling the solution carefully.

Example: If 100 g of water contains the maximum amount of dissolved solute, it is saturated. If less than the maximum is present, it is unsaturated. If more than the equilibrium amount is dissolved, it is supersaturated.

Definitions and Key Terms

• Solute: The substance that is dissolved in a solvent to form a solution.

• Solvent: The substance in which the solute is dissolved, typically present in greater amount.

• Saturation Point: The concentration at which no more solute can dissolve in the solvent at a specific temperature.

Application Example

Example Problem: Caffeine is about 10 times as soluble in warm water as in cold water. A student puts a hot-water extract of caffeine into an ice bath, and some caffeine crystallizes. What is the identity of the solution before it is placed in an ice bath?

• a) Saturated

• b) Super saturated (Correct answer)

• c) Unsaturated

• d) Not enough information to answer the question.

Explanation: The solution was supersaturated before cooling, as it contained more dissolved caffeine than is possible at the lower temperature, causing crystallization when cooled.

Temperature Effects on Solubility

General Trends

The solubility of substances in solvents is affected by temperature. The following trends are commonly observed:

• Solubility of Gases: As temperature increases, the solubility of gases in liquids generally decreases.

• Solubility of Most Solids: As temperature increases, the solubility of most solid solutes in liquids generally increases.

Practice Question

In general, as the temperature increases, the solubility of gas in a given liquid decreases, and the solubility of most solids in a given liquid increases.

• a) Increases, increases

• b) Increases, decreases

• c) Decreases, increases (Correct answer)

• d) Decreases, decreases

Summary Table: Effects of Temperature on Solubility

Type of Solute	Effect of Increasing Temperature
Gas	Solubility decreases
Most Solids	Solubility increases

Key Equations

• Solubility (S): The maximum amount of solute that can dissolve in a given amount of solvent at a specific temperature. $S = \frac{\text{mass of solute}}{\text{mass of solvent}}$