Properties of Solutions: Composition, Types, and Concentration Units

Properties of Solutions

Introduction

Solutions are fundamental mixtures in chemistry, consisting of two or more substances physically combined but not chemically bonded. Understanding their composition, types, and concentration units is essential for analyzing chemical processes and reactions.

Classification and Separation of Mixtures

Types of Mixtures

• Mixture: A combination of two or more substances physically mixed, not chemically combined.

• Defining Characteristics of Mixtures:

  1. Variable composition

  2. Retention of some properties of components

  3. Components can be separated by physical means (e.g., filtration, crystallization, extraction, chromatography, distillation)

Classification of Mixtures

• Heterogeneous Mixture: Composition is not uniform; visible boundaries exist between components. Examples: Champagne, smoke, milk, concrete

• Homogeneous Mixture (Solution): No visible boundaries; components are mixed as individual atoms, ions, or molecules. Composition is uniform throughout. Examples: Saline solution, 14-karat gold, malt vinegar

Complexity of Mixtures

• Synthetic mixtures: Glass, soap, brass, steel, gasoline

• Natural mixtures: Seawater, lakes, soil, rocks, Earth's atmosphere

• Living mixtures: Trees, insects, dogs, cats, students, etc.

Common Types of Solutions

Definition

Solution: A solute dispersed homogeneously in a solvent, which generally determines the physical state of the solution.

Table: Common Types of Solutions

Solute	Solvent	Solution Phase	Examples
Gas	Gas	Gas	Air (O2, Ar, CO2 in N2)
Gas	Liquid	Liquid	CO2 in water (carbonated drinks)
Liquid	Liquid	Liquid	Alcohol in water (vodka, wine)
Solid	Liquid	Liquid	NaCl in water (saline solution)
Solid	Solid	Solid	14-karat gold (Au in Ag)

Solution Composition

Components of Solutions

• Solute: Substance being dissolved (lesser amount)

• Solvent: Substance doing the dissolving (greater amount)

Concentration of Solutions

• Concentration: Quantity of solute dissolved in a given quantity of solution (or solvent)

• Concentrated solution: Contains a relatively large number of solute particles per unit volume

• Dilute solution: Contains a relatively small number of solute particles per unit volume

Methods for Describing Solution Composition

Concentration Units

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

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

• Percent by mass: $\frac{\text{grams of solute}}{\text{grams of solution}} \times 100\%$

• Percent by volume: $\frac{\text{milliliters of solute}}{\text{milliliters of solution}} \times 100\%$

• Mass/volume percent: $\frac{\text{grams of solute}}{\text{milliliters of solution}} \times 100\%$

• Mole fraction (χ): $\chi = \frac{\text{moles of component}}{\text{total moles in solution}}$

Key Terms

• Solute: The substance present in the lesser amount, being dissolved.

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

• Mole: $6.022 \times 10^{23}$ (Avogadro's number) of entities (atoms, molecules, ions).

• Molar mass (MW): The mass in grams of one mole of a substance.

Example: Calculating Molarity

• To prepare a solution with a desired molarity, calculate the required moles of solute and dissolve in the appropriate volume of solvent.

• Example: To make 0.10 M NaCl solution, dissolve 0.10 moles of NaCl in enough water to make 1.0 L of solution.

Example: Calculating Molality

• Molality is calculated using the mass of the solvent, not the total solution.

• Example: Dissolve 10.0 g of KCl (molar mass = 74.55 g/mol) in 100.0 g of water. Moles of KCl = $\frac{10.0\ g}{74.55\ g/mol} = 0.134\ mol$ Molality = $\frac{0.134\ mol}{0.100\ kg} = 1.34\ m$

Summary Table: Solution Concentration Units

Unit	Definition	Formula
Molarity (M)	Moles of solute per liter of solution	$M = \frac{\text{moles of solute}}{\text{liters of solution}}$
Molality (m)	Moles of solute per kilogram of solvent	$m = \frac{\text{moles of solute}}{\text{kilograms of solvent}}$
Percent by mass	Grams of solute per grams of solution × 100%	$\frac{\text{grams of solute}}{\text{grams of solution}} \times 100\%$
Mole fraction (χ)	Moles of component per total moles in solution	$\chi = \frac{\text{moles of component}}{\text{total moles}}$

Applications and Importance

• Understanding solution composition is essential for laboratory preparations, chemical reactions, and industrial processes.

• Concentration units allow chemists to quantify and compare solutions accurately.

• Separation techniques are vital for purifying substances and analyzing mixtures in research and industry.

Additional info:

• Further topics in solution chemistry include colligative properties, vapor pressures, and the energetics of solution formation, which are covered in subsequent sections of the textbook.