Solubility and Solutions: General Chemistry II Study Notes (CHM 1046, Chapter 11.3)

Solutions and Colloids

Introduction to Solutions

Solutions are homogeneous mixtures composed of two or more substances. The study of solutions is fundamental in chemistry, as it relates to how substances interact, dissolve, and form mixtures with unique properties. Colloids are mixtures where the particle size is intermediate between true solutions and suspensions.

• Solution: A homogeneous mixture of solute and solvent.

• Colloid: A mixture with dispersed particles larger than those in a solution but smaller than those in a suspension.

• Solute: The substance dissolved in a solvent.

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

Qualitative Descriptors of Solutions

Identifying Solutions

Qualitative analysis helps determine whether a mixture is a solution and distinguishes between solute and solvent. It also classifies the nature of the solution, such as whether it is electrolytic or non-electrolytic.

• Electrolytic Solution: Contains solutes that dissociate into ions and conduct electricity (e.g., NaCl in water).

• Non-electrolytic Solution: Contains solutes that do not dissociate into ions and do not conduct electricity (e.g., sugar in water).

Quantitative Descriptors of Solutions

Concentration Units

Quantitative descriptors measure how much solute is present in a solution. Several units are used to express concentration:

• Percent by Mass (% by mass): The mass of solute divided by the mass of solution, multiplied by 100%.

• Mole Fraction (X): The ratio of moles of a component to the total moles in the solution.

• Molarity (M): Moles of solute per liter of solution. Note: Volume changes with temperature.

• Molality (m): Moles of solute per kilogram of solvent.

Formulas:

• Percent by mass: $\text{Percent by mass} = \frac{\text{mass of solute (g)}}{\text{mass of solution (g)}} \times 100\%$

• Mole fraction: $X_A = \frac{\text{moles of A (mol)}}{\text{sum of moles of all components (mol)}}$

• Molarity: $M = \frac{\text{moles of solute (mol)}}{\text{liters of solution (L)}}$

• Molality: $m = \frac{\text{moles of solute (mol)}}{\text{mass of solvent (kg)}}$

Qualitative Descriptor of Solubility

"Like Dissolves Like" Principle

The solubility of substances depends on the nature of both solute and solvent. The general rule is "like dissolves like":

• Nonpolar molecules are soluble in nonpolar solvents.

• Ions and polar molecules are soluble in polar solvents.

• Hydrogen bonding solutes are more soluble in solvents capable of hydrogen bonding.

Example: Sodium chloride (NaCl) dissolves in water due to ion-dipole interactions; oil dissolves in hexane due to similar nonpolar characteristics.

Quantitative Descriptor of Solubility

Solubility Curves

A solubility curve graphically represents the amount of solute that can dissolve in a given amount of solvent at various temperatures. It is used to determine whether a solution is saturated, unsaturated, or supersaturated at a specific temperature.

• Saturated Solution: Contains the maximum amount of solute that can dissolve at a given temperature.

• Unsaturated Solution: Contains less solute than the maximum amount that can dissolve.

• Supersaturated Solution: Contains more solute than is normally possible at a given temperature; unstable and can precipitate excess solute.

Example: At 60°C, a solution containing 120 g of solute per 100 g water is saturated if the solubility curve shows 120 g at that temperature.

Factors Affecting Solubility

Temperature and Pressure Effects

Solubility is influenced by temperature and pressure, especially for gases and solids in liquids.

• Solids in Liquids: Increasing temperature usually increases solubility.

• Gases in Liquids: Increasing temperature decreases solubility; increasing pressure increases solubility.

Henry's Law: The solubility of a gas in a liquid is proportional to the pressure of the gas above the liquid.

$C = kP$

• $C$ = concentration of dissolved gas

• $k$ = Henry's law constant (specific to each gas)

• $P$ = partial pressure of the gas

Example: Carbonated beverages are bottled under high pressure to increase CO2 solubility; opening the bottle releases pressure, decreasing solubility and causing bubbles.

Applications and Real-World Examples

Environmental and Biological Implications

Solubility principles are important in environmental science and biology. For example, climate warming decreases oxygen solubility in oceans, affecting marine life. In diving, increased pressure causes more nitrogen to dissolve in blood, which can be dangerous if decompression is too rapid.

• Decompression Sickness: Rapid decrease in pressure causes dissolved gases to come out of solution, forming bubbles in blood.

• Hyperbaric Chamber: Used to treat decompression sickness by increasing pressure and allowing gases to re-dissolve.

Summary Table: Concentration Units

Unit	Definition	Formula	Notes
Percent by Mass	Mass of solute per mass of solution	$\frac{\text{mass of solute}}{\text{mass of solution}} \times 100\%$	Useful for solid-liquid solutions
Mole Fraction (X)	Ratio of moles of component to total moles	$\frac{\text{moles of A}}{\text{total moles}}$	Dimensionless
Molarity (M)	Moles of solute per liter of solution	$\frac{\text{moles of solute}}{\text{liters of solution}}$	Volume changes with temperature
Molality (m)	Moles of solute per kilogram of solvent	$\frac{\text{moles of solute}}{\text{kg of solvent}}$	Independent of temperature

Key Takeaways

• Solutions are homogeneous mixtures; colloids are intermediate between solutions and suspensions.

• Concentration can be expressed in several units: percent by mass, mole fraction, molarity, and molality.

• "Like dissolves like" is a guiding principle for solubility.

• Solubility curves help determine saturation status at different temperatures.

• Temperature and pressure affect solubility, especially for gases.

• Henry's Law quantifies the relationship between gas pressure and solubility.

Additional info: These notes expand on the provided lecture slides and images, adding definitions, formulas, and real-world context for clarity and completeness.