Properties of Solutions

Introduction to Solutions

Solutions are homogeneous mixtures composed of two or more substances. Understanding their properties is essential in chemistry, as solutions are common in both natural and laboratory settings.

• Solution: A homogeneous mixture of two or more substances in a single phase.

• Solute: The component present in a lesser amount; it is dissolved in the solvent.

• Solvent: The component present in a greater amount; it dissolves the solute and determines the phase of the solution.

• Both solute and solvent can be solid, liquid, or gas.

Example: Dissolving potassium dichromate (K2Cr2O7) in water forms a solution of 2K+(aq) + Cr2O72−(aq).

Types of Solutions

• Solid in Liquid: e.g., salt in water

• Liquid in Liquid: e.g., alcohol in water

• Gas in Liquid: e.g., carbon dioxide in soda

Formation of Solutions

Whether a mixture forms a solution depends on the properties of the solute and solvent, particularly their intermolecular forces (IMFs).

• "Like dissolves like": Polar solvents dissolve polar solutes; nonpolar solvents dissolve nonpolar solutes.

• Solution formation can be exothermic (releases heat) or endothermic (absorbs heat).

Solubility

Solubility of Gases in Liquids

The solubility of gases in liquids depends on temperature and pressure.

• As temperature increases, gas solubility decreases (gases escape more easily).

• As pressure increases, gas solubility increases (more gas is forced into solution).

Henry's Law: The solubility of a gas is directly proportional to its partial pressure above the solution.

• = concentration of dissolved gas

• = Henry's law constant (depends on gas and solvent)

• = partial pressure of the gas

Solubility of Liquids in Liquids

• Miscible: Liquids that mix in all proportions (e.g., ethanol and water).

• Immiscible: Liquids that do not mix significantly (e.g., oil and water).

Solubility of Solids in Liquids

• Generally, solubility of solids increases with temperature.

• Supersaturated solutions can be formed by dissolving more solute at high temperature and then cooling.

Concentration Units

Common Concentration Units

• Molarity (M): Moles of solute per liter of solution.

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

• Mole Fraction (): Ratio of moles of a component to total moles in solution.

• Mass Percent: Mass of solute divided by total mass of solution, multiplied by 100%.

• Parts per million (ppm):

• Parts per billion (ppb):

Note: Molarity depends on temperature (volume changes), while molality and mole fraction do not.

Colligative Properties

Definition and Overview

Colligative properties are physical properties of solutions that depend only on the number of solute particles, not their identity. These include vapor pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure.

Vapor Pressure Lowering (Raoult's Law)

• Adding a nonvolatile solute to a solvent lowers the vapor pressure of the solution.

• Raoult's Law: The vapor pressure of a solution is proportional to the mole fraction of the solvent.

• Vapor pressure of solution is always less than that of the pure solvent.

Boiling Point Elevation

• The boiling point of a solution is higher than that of the pure solvent.

• The increase in boiling point () is proportional to the molality of the solute.

• = ebullioscopic constant (depends on solvent)

Freezing Point Depression

• The freezing point of a solution is lower than that of the pure solvent.

• The decrease in freezing point () is proportional to the molality of the solute.

• = cryoscopic constant (depends on solvent)

Osmotic Pressure

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

• Osmotic pressure () is the pressure required to stop osmosis.

• = molarity, = gas constant, = temperature in Kelvin

Electrolytes and Colligative Properties

Electrolytes vs. Nonelectrolytes

• Electrolytes: Substances that dissociate into ions in solution and conduct electricity (e.g., NaCl, strong acids/bases).

• Nonelectrolytes: Substances that do not form ions in solution and do not conduct electricity (e.g., sugar, ethanol).

Types of Electrolytes

• Strong Electrolytes: Completely dissociate in solution (e.g., NaCl, HCl).

• Weak Electrolytes: Partially dissociate in solution (e.g., acetic acid, ammonia).

Van't Hoff Factor ()

• The van't Hoff factor () represents the number of particles into which a solute dissociates in solution.

• For nonelectrolytes, ; for electrolytes, equals the total number of ions produced per formula unit.

• Colligative property equations for electrolytes are multiplied by to account for the increased number of particles.

Summary Table: Colligative Properties

Key Takeaways

• Solutions are homogeneous mixtures with properties dependent on the amount and type of solute and solvent.

• Colligative properties depend only on the number of solute particles, not their identity.

• Electrolytes affect colligative properties more strongly due to dissociation into multiple particles.