Properties of Solutions and Solution Chemistry (Chapter 13 Study Notes)

Properties of Solutions

Definitions: Solute, Solvent, Solution

Solutions are homogeneous mixtures composed of two or more substances. The solute is the substance dissolved, while the solvent is the substance in which the solute is dissolved. The resulting mixture is called a solution.

• Solute: Substance present in lesser amount, being dissolved.

• Solvent: Substance present in greater amount, doing the dissolving.

• Solution: Homogeneous mixture of solute and solvent.

• Example: Salt (solute) dissolved in water (solvent) forms a saltwater solution.

Dissolving Process

Dissolving is a physical process, but in some cases, it can be accompanied by chemical reactions. The process involves the interaction between solute and solvent particles.

• Physical Dissolution: No chemical change; solute particles disperse among solvent particles.

• Chemical Dissolution: Sometimes, dissolving can result in chemical changes (e.g., acid-base reactions).

Solubility and Saturation

Solubility is the maximum amount of solute that can dissolve in a given amount of solvent at a specific temperature and pressure. Solutions can be classified as:

• Saturated: Contains the maximum amount of solute that can dissolve.

• Unsaturated: Contains less solute than the maximum amount.

• Supersaturated: Contains more solute than is normally possible at that temperature; unstable.

Factors Affecting Solubility

Solubility depends on several factors:

• Temperature: Solubility of solids in liquids generally increases with temperature; for gases, solubility decreases as temperature increases.

• Pressure: Mainly affects gases; higher pressure increases gas solubility in liquids (Henry's Law).

• Nature of Solute and Solvent: "Like dissolves like"—polar solutes dissolve in polar solvents, nonpolar in nonpolar.

• Example: Carbonated drinks (CO2 dissolved in water under pressure).

Intermolecular Forces and Solubility

The type and strength of intermolecular forces between solute and solvent molecules affect solubility.

• Hydrogen Bonding: Strong hydrogen bonds increase solubility in water.

• Dipole-Dipole and London Dispersion: Affect solubility of nonpolar and polar substances.

• Example: Alcohols dissolve well in water due to hydrogen bonding.

Concentration Units

Common Units: Molarity (M), Molality (m), Mole Fraction (X), Percent by Mass (%), Parts per Million (ppm)

Concentration expresses the amount of solute in a given quantity of solvent or solution. Key units include:

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

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

• Mole Fraction (X): $X_A = \frac{\text{moles of component A}}{\text{total moles of all components}}$

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

• Parts per Million (ppm): $\text{ppm} = \frac{\text{mass of solute}}{\text{mass of solution}} \times 10^6$

It is important to know how to convert between these units and use them in calculations.

Colligative Properties

Definition and Types

Colligative properties depend on the number of solute particles in solution, not their identity. Major colligative properties include:

• Vapor Pressure Lowering

• Boiling Point Elevation

• Freezing Point Depression

• Osmotic Pressure

Vapor Pressure Lowering

Adding a nonvolatile solute to a solvent lowers the solvent's vapor pressure.

• Raoult's Law: $P_A = X_A P_A^0$

• $P_A$ = vapor pressure of solvent in solution

• $X_A$ = mole fraction of solvent

• $P_A^0$ = vapor pressure of pure solvent

Boiling Point Elevation and Freezing Point Depression

Solutions boil at higher temperatures and freeze at lower temperatures than pure solvents.

• Boiling Point Elevation: $\Delta T_b = K_b m$

• Freezing Point Depression: $\Delta T_f = K_f m$

• $K_b$ and $K_f$ are constants for the solvent; $m$ is molality.

Osmotic Pressure

Osmosis is the movement of solvent through a semipermeable membrane from low to high solute concentration.

• Osmotic Pressure Equation: $\Pi = MRT$

• $\Pi$ = osmotic pressure, $M$ = molarity, $R$ = gas constant, $T$ = temperature (K)

• Application: Cell membranes and medical solutions.

Electrolytes and van't Hoff Factor (i)

Electrolytes dissociate into ions, increasing the number of particles in solution. The van't Hoff factor ($i$) accounts for this effect.

• van't Hoff Factor: $i = \frac{\text{actual number of particles in solution}}{\text{number of formula units dissolved}}$

• For NaCl, $i \approx 2$ (Na+ and Cl-).

• Colligative property equations are multiplied by $i$ for electrolytes.

Osmosis and Tonicity

Hypotonic, Hypertonic, and Isotonic Solutions

The effect of solutions on cells depends on their relative concentrations:

• Hypotonic: Lower solute concentration than cell; water enters cell, may burst.

• Hypertonic: Higher solute concentration than cell; water leaves cell, may shrink.

• Isotonic: Equal solute concentration; no net water movement.

• Application: Medical IV solutions must be isotonic to prevent cell damage.

Colloids and Soap Action

Colloids

Colloids are mixtures where the dispersed particles are larger than in true solutions but small enough to remain suspended.

• Colloidal particles do not settle out and can scatter light (Tyndall effect).

• Examples: Milk, fog, gelatin.

Soap and Detergent Action

Soaps and detergents clean by forming micelles, which trap nonpolar dirt and grease inside their nonpolar centers, allowing them to be washed away by water.

• Micelle Formation: Soap molecules arrange with hydrophobic tails inward and hydrophilic heads outward.

• Application: Cleaning and emulsification of oils.

Summary Table: Colligative Properties

Property	Effect of Solute	Equation	Application
Vapor Pressure Lowering	Decreases	$P_A = X_A P_A^0$	Evaporation rates, humidity control
Boiling Point Elevation	Increases	$\Delta T_b = i K_b m$	Antifreeze in car radiators
Freezing Point Depression	Decreases	$\Delta T_f = i K_f m$	Salting icy roads
Osmotic Pressure	Increases	$\Pi = i M R T$	Medical IV solutions