💏Intro to Chemistry Unit 3 – Substances and Solutions Composition

Key Concepts

• Matter can be classified as pure substances or mixtures based on their composition and properties
• Pure substances have a fixed composition and distinct properties (melting point, boiling point) while mixtures vary in composition and properties
• Solutions are homogeneous mixtures composed of a solute dissolved in a solvent
  • Solute is the substance being dissolved (sugar)
  • Solvent is the substance doing the dissolving (water)
• Concentration is the amount of solute present in a given amount of solution
  • Can be expressed in various units (molarity, molality, mass percent)
• Solubility refers to the maximum amount of solute that can dissolve in a solvent at a given temperature
  • Affected by factors such as temperature, pressure, and the nature of solute and solvent
• Colligative properties are properties of solutions that depend on the concentration of solute particles, not their identity
  • Include vapor pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure

Types of Substances

• Elements are pure substances that cannot be broken down into simpler substances by chemical means (oxygen, iron)
• Compounds are pure substances composed of two or more elements chemically combined in a fixed ratio (water, salt)
• Homogeneous mixtures have a uniform composition throughout (air, brass)
  • Also called solutions when a solute is dissolved in a solvent
• Heterogeneous mixtures have a non-uniform composition and distinct phases (oil and water, sand and gravel)
• Alloys are homogeneous mixtures of two or more metals or a metal and a nonmetal (steel, bronze)
• Colloids are mixtures with particles larger than those in solutions but small enough to remain suspended (milk, fog)
  • Exhibit properties such as the Tyndall effect and Brownian motion

Solutions and Their Properties

• Solutions can exist in any physical state (solid, liquid, or gas)
  • Gaseous solutions (air)
  • Liquid solutions (salt water)
  • Solid solutions (alloys)
• The solubility of a solute in a solvent depends on the intermolecular forces between the solute and solvent particles
  • "Like dissolves like" principle: polar solvents dissolve polar solutes, nonpolar solvents dissolve nonpolar solutes
• Saturated solutions contain the maximum amount of solute that can dissolve at a given temperature
  • Adding more solute results in undissolved solute settling at the bottom
• Unsaturated solutions contain less than the maximum amount of solute that can dissolve at a given temperature
  • More solute can be added until saturation is reached
• Supersaturated solutions contain more solute than can normally dissolve at a given temperature
  • Prepared by cooling a saturated solution without allowing solute to crystallize

Concentration Measurements

• Molarity (M) is the number of moles of solute per liter of solution
  • $Molarity = \frac{moles\ of\ solute}{liters\ of\ solution}$
• Molality (m) is the number of moles of solute per kilogram of solvent
  • $Molality = \frac{moles\ of\ solute}{kilograms\ of\ solvent}$
• Mass percent is the mass of solute divided by the total mass of the solution, expressed as a percentage
  • $Mass\ percent = \frac{mass\ of\ solute}{total\ mass\ of\ solution} \times 100\%$
• Parts per million (ppm) and parts per billion (ppb) are used for very dilute solutions
  • ppm represents milligrams of solute per liter of solution or milligrams of solute per kilogram of solvent
  • ppb represents micrograms of solute per liter of solution or micrograms of solute per kilogram of solvent
• Mole fraction is the ratio of the number of moles of one component to the total number of moles in the solution
  • $Mole\ fraction = \frac{moles\ of\ component}{total\ moles\ in\ solution}$

Solubility and Factors Affecting It

• Temperature affects solubility differently for solid and gas solutes
  • Solubility of solid solutes generally increases with increasing temperature
  • Solubility of gas solutes decreases with increasing temperature
• Pressure affects the solubility of gases in liquids
  • Henry's law states that the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid
• The presence of a common ion decreases the solubility of a slightly soluble salt
  • Common ion effect: the solubility of a slightly soluble salt decreases when a soluble compound containing one of the ions is added to the solution
• The pH of the solution can affect the solubility of substances
  • Acidic solutions can increase the solubility of basic compounds, while basic solutions can increase the solubility of acidic compounds
• The polarity of the solute and solvent determines solubility
  • Polar solutes dissolve in polar solvents, nonpolar solutes dissolve in nonpolar solvents

Colligative Properties

• Vapor pressure lowering occurs when a nonvolatile solute is added to a solvent
  • The solute particles reduce the surface area available for solvent molecules to escape, lowering the vapor pressure
• Boiling point elevation is the increase in the boiling point of a solution compared to the pure solvent
  • $\Delta T_b = K_b \times m$, where $K_b$ is the molal boiling point elevation constant and $m$ is molality
• Freezing point depression is the decrease in the freezing point of a solution compared to the pure solvent
  • $\Delta T_f = K_f \times m$, where $K_f$ is the molal freezing point depression constant and $m$ is molality
• Osmotic pressure is the pressure that must be applied to a solution to prevent the flow of solvent molecules across a semipermeable membrane
  • $\Pi = MRT$, where $M$ is molarity, $R$ is the gas constant, and $T$ is the absolute temperature
• Colligative properties are used in applications such as antifreeze, desalination, and osmosis

Practical Applications

• Freeze distillation is a process that uses freezing point depression to concentrate solutions (maple syrup production)
• Dialysis is a medical treatment that uses osmosis and a semipermeable membrane to remove waste products from blood
• Reverse osmosis is a water purification method that uses osmotic pressure to remove dissolved solids from water
• Fractional crystallization is a separation technique that uses differences in solubility to isolate components of a mixture (purifying salt from seawater)
• Antifreeze solutions use colligative properties to lower the freezing point of water and prevent engine damage in cold temperatures
• Salting roads in winter lowers the freezing point of water, preventing ice formation and improving road safety
• Osmotic drug delivery systems use osmotic pressure to control the release of medication over time

Common Mistakes and Tips

• Remember that molarity is moles of solute per liter of solution, not per liter of solvent
• When calculating molality, use the mass of the solvent, not the mass of the solution
• Be careful with units when converting between concentration measurements
• Remember that colligative properties depend on the number of solute particles, not their identity
  • Electrolytes dissociate into multiple particles, increasing the effect on colligative properties
• When using the vapor pressure lowering, boiling point elevation, or freezing point depression equations, make sure the units match the constants ($K_b$ and $K_f$ are molal constants)
• In the osmotic pressure equation, use molarity, not molality
• When solving problems involving colligative properties, identify the solute and solvent, and determine the concentration in the appropriate units