🫴Physical Science Unit 7 – Solutions, Acids, and Bases

Key Concepts and Definitions

• Solutions homogeneous mixtures of two or more substances, consisting of a solute dissolved in a solvent
• Solute substance being dissolved, typically the smaller quantity in a solution (sugar, salt)
• Solvent substance doing the dissolving, usually present in a larger amount (water)
• Solubility maximum amount of solute that can dissolve in a given amount of solvent at a specific temperature
  • Affected by factors such as temperature, pressure, and the nature of the solute and solvent
• Concentration measure of the amount of solute present in a given amount of solution, expressed in various units (molarity, molality, mass percent)
• Acids substances that donate protons (H+ ions) in aqueous solutions, characterized by a sour taste and pH less than 7
• Bases substances that accept protons (H+ ions) in aqueous solutions, characterized by a bitter taste, slippery feel, and pH greater than 7
• pH scale logarithmic scale ranging from 0 to 14, used to measure the acidity or basicity of a solution

Types of Solutions

• Solid solutions solid solute dissolved in a solid solvent (alloys like brass, bronze)
• Liquid solutions liquid solute dissolved in a liquid solvent, most common type of solution (saltwater, sugar water)
  • Can be classified as unsaturated (more solute can be dissolved), saturated (no more solute can be dissolved), or supersaturated (more solute than normally possible, unstable)
• Gas solutions gas solute dissolved in a gas solvent (air, a mixture of gases)
• Aqueous solutions solute dissolved in water as the solvent, important in biological and chemical processes
• Non-aqueous solutions solute dissolved in a solvent other than water (iodine in alcohol)
• Colloids mixtures with particles larger than typical solutions but small enough to remain suspended (milk, fog)
  • Exhibit properties like the Tyndall effect (scattering of light) and Brownian motion (random motion of particles)

Solubility and Concentration

• Solubility rules general guidelines for predicting the solubility of ionic compounds in water
  • Compounds containing alkali metal ions, ammonium ions, and nitrate ions are generally soluble
  • Compounds containing chloride, bromide, and iodide ions are usually soluble, with some exceptions (silver, lead, mercury)
• Factors affecting solubility include temperature (higher temperature usually increases solubility for solids), pressure (higher pressure increases gas solubility), and polarity (like dissolves like)
• Concentration units include molarity (moles of solute per liter of solution), molality (moles of solute per kilogram of solvent), and mass percent (mass of solute divided by total mass of solution, expressed as a percentage)
• Dilution process of adding more solvent to a solution to decrease its concentration, can be calculated using the formula $M_1V_1 = M_2V_2$
• Colligative properties properties that depend on the concentration of solute particles, not their identity (vapor pressure lowering, boiling point elevation, freezing point depression, osmotic pressure)

Acids and Bases: Introduction

• Arrhenius definition acids produce H+ ions in aqueous solution, bases produce OH- ions
• Brønsted-Lowry definition acids are proton donors, bases are proton acceptors
  • Allows for the concept of conjugate acid-base pairs (HCl and Cl-, NH3 and NH4+)
• Lewis definition acids are electron pair acceptors, bases are electron pair donors
• Strong acids and bases completely dissociate in water (HCl, NaOH), while weak acids and bases only partially dissociate (CH3COOH, NH3)
• Monoprotic acids yield one H+ ion per molecule (HCl), while polyprotic acids yield multiple H+ ions per molecule (H2SO4)
• Amphoteric substances can act as both an acid and a base, depending on the environment (water, amino acids)

pH Scale and Indicators

• pH defined as the negative logarithm of the hydrogen ion concentration, $pH = -log[H+]$
  • Lower pH values (0-6) indicate acidic solutions, while higher pH values (8-14) indicate basic solutions
  • Neutral solutions have a pH of 7 at 25°C
• pOH analogous to pH, but for hydroxide ion concentration, $pOH = -log[OH-]$
  • Related to pH by the equation $pH + pOH = 14$ at 25°C
• Acid-base indicators compounds that change color depending on the pH of the solution
  • Examples include litmus (red in acid, blue in base), phenolphthalein (colorless in acid, pink in base), and universal indicator (multiple color changes across the pH scale)
• pH meters electronic devices that measure the pH of a solution more precisely than indicators
• Buffer solutions resist changes in pH when small amounts of acid or base are added, important in biological systems

Neutralization Reactions

• Neutralization reaction between an acid and a base produces water and a salt
  • Example: $HCl(aq) + NaOH(aq) -> H2O(l) + NaCl(aq)$
• Titration process of determining the concentration of an unknown solution by reacting it with a solution of known concentration
  • Involves using an indicator or pH meter to determine the endpoint (point at which neutralization is complete)
• Stoichiometry can be used to calculate the amounts of reactants and products in a neutralization reaction
• Heat of neutralization energy released when one mole of an acid and one mole of a base react to form water and a salt, typically around 55-57 kJ/mol

Applications in Daily Life

• Acid-base reactions play a crucial role in many everyday processes and products
• Digestion involves the use of hydrochloric acid in the stomach to break down food and the neutralization of this acid by bases in the small intestine
• Cleaning products often contain acids (toilet bowl cleaners) or bases (oven cleaners) to help remove stains and grease
• Soil pH affects plant growth, with most plants preferring slightly acidic to neutral soil (pH 6-7)
  • Soil pH can be adjusted using additives like lime (to raise pH) or sulfur (to lower pH)
• Pharmaceuticals many drugs are weak acids or bases, affecting their absorption and distribution in the body
• Environmental concerns acid rain (caused by sulfur and nitrogen oxides) can harm plants, animals, and infrastructure, while ocean acidification (caused by increased CO2 absorption) threatens marine life

Lab Techniques and Safety

• Proper personal protective equipment (PPE) should be worn when handling acids and bases, including gloves, lab coats, and safety goggles
• Acids and bases should be handled in a well-ventilated area, preferably a fume hood, to avoid inhaling fumes
• When diluting acids, always add the acid to water (AAA: Acid to Aqueous), never the reverse, to prevent splattering and heat generation
• Neutralize spills with the appropriate substance (sodium bicarbonate for acids, citric acid for bases) and clean up immediately
• Use precise measuring equipment like volumetric flasks, pipettes, and burets for accurate solution preparation and titrations
• Properly label all solutions with the substance name, concentration, date, and any necessary safety warnings
• Dispose of acids, bases, and their solutions in designated waste containers, never down the sink or in regular trash
• In case of contact with skin or eyes, flush the affected area with water for at least 15 minutes and seek medical attention if necessary