💏Intro to Chemistry Unit 10 – Liquids and Solids

Key Concepts

• Matter exists in three main states: solid, liquid, and gas, each with distinct properties and behaviors
• Phase transitions occur when a substance changes from one state to another (melting, freezing, vaporization, condensation, sublimation)
• Intermolecular forces, including dipole-dipole interactions, hydrogen bonding, and London dispersion forces, influence the properties and behaviors of liquids and solids
  • Strength of intermolecular forces determines physical properties such as melting point, boiling point, and viscosity
• Crystalline solids have a highly ordered, repeating arrangement of particles, while amorphous solids lack long-range order
• Surface tension, capillary action, and viscosity are unique properties of liquids that arise from intermolecular forces
• Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid phase at a given temperature
• Heating curves illustrate the relationship between temperature and heat energy added to a substance during phase transitions

Properties of Liquids

• Liquids have a definite volume but no fixed shape, conforming to the shape of their container
• Fluidity allows liquids to flow and take the shape of their container due to weak intermolecular forces
• Surface tension is the tendency of liquid surfaces to minimize their surface area, caused by cohesive forces between molecules
  • Responsible for the formation of droplets, meniscus, and the ability of some insects to walk on water (water striders)
• Capillary action is the ability of liquids to flow through narrow spaces without the assistance of external forces, caused by adhesive and cohesive forces
  • Enables plants to transport water from roots to leaves and paper towels to absorb liquid spills
• Viscosity is a measure of a liquid's resistance to flow, determined by the strength of intermolecular forces and temperature
  • Honey has a higher viscosity than water due to stronger intermolecular forces
• Liquids are nearly incompressible because molecules are already close together, with little empty space between them

Properties of Solids

• Solids maintain a fixed shape and volume due to strong intermolecular forces that hold particles in a rigid, ordered arrangement
• Crystalline solids have a regular, repeating pattern of particles (crystal lattice) with long-range order
  • Examples include diamonds, salt (NaCl), and quartz (SiO2)
• Amorphous solids lack a regular, repeating pattern of particles and have no long-range order
  • Examples include glass, rubber, and plastics
• Solids have a higher density than liquids and gases because particles are tightly packed together
• Elastic properties allow some solids to deform under stress and return to their original shape when the stress is removed (rubber bands)
• Plastic deformation occurs when a solid permanently changes shape under applied stress (bending a metal spoon)
• Thermal expansion causes solids to expand when heated and contract when cooled due to changes in the average kinetic energy of particles

Phase Transitions

• Melting is the transition from solid to liquid, occurring when the thermal energy overcomes the intermolecular forces holding the solid together
  • Melting point is the temperature at which a substance transitions from solid to liquid at a given pressure
• Freezing is the transition from liquid to solid, occurring when thermal energy is removed and intermolecular forces cause particles to arrange in a fixed, ordered pattern
• Vaporization is the transition from liquid to gas, occurring when thermal energy overcomes the intermolecular forces in the liquid
  • Boiling is vaporization that occurs throughout the entire liquid at a specific temperature (boiling point)
  • Evaporation is vaporization that occurs only at the surface of the liquid at any temperature
• Condensation is the transition from gas to liquid, occurring when thermal energy is removed and intermolecular forces cause particles to come together
• Sublimation is the direct transition from solid to gas, skipping the liquid phase (dry ice, CO2)
• Deposition is the direct transition from gas to solid, skipping the liquid phase (formation of frost)

Intermolecular Forces

• Dipole-dipole interactions occur between polar molecules, where the positive end of one molecule attracts the negative end of another
  • Polar molecules have an uneven distribution of charge due to differences in electronegativity between atoms (HCl, H2O)
• Hydrogen bonding is a strong type of dipole-dipole interaction that occurs when hydrogen is bonded to a highly electronegative atom (N, O, F)
  • Responsible for the unique properties of water, such as high boiling point and surface tension
• London dispersion forces are weak intermolecular forces that arise from temporary fluctuations in the distribution of electrons, creating instantaneous dipoles
  • Present in all molecules, but strongest in larger, more polarizable molecules (I2, hydrocarbons)
• Ion-dipole interactions occur between ions and polar molecules, such as the attraction between Na+ and the negative end of a water molecule
• Strength of intermolecular forces determines physical properties such as melting point, boiling point, and viscosity
  • Stronger intermolecular forces lead to higher melting and boiling points and greater viscosity

Crystal Structures

• Unit cell is the smallest repeating unit that makes up a crystal lattice, containing atoms, ions, or molecules
• Packing efficiency refers to the percentage of space occupied by particles in a unit cell
  • Close-packed structures (hexagonal close-packed, HCP; cubic close-packed, CCP) have the highest packing efficiency (74%)
• Cubic crystal systems include simple cubic (SC), body-centered cubic (BCC), and face-centered cubic (FCC) arrangements
  • SC has particles at each corner of the cube
  • BCC has particles at each corner and one in the center of the cube
  • FCC has particles at each corner and one at the center of each face of the cube
• Hexagonal crystal system consists of layers of close-packed particles stacked in an ABABAB pattern (HCP)
• Coordination number is the number of nearest neighbors a particle has in a crystal structure
  • SC: 6, BCC: 8, FCC: 12, HCP: 12
• Polymorphism occurs when a substance can exist in more than one crystal structure (carbon: diamond, graphite)

Applications in Everyday Life

• Liquid crystals, used in LCD displays, have properties of both liquids and solids, allowing them to flow like liquids while maintaining some ordered arrangement of molecules
• Surfactants lower the surface tension of liquids, making them useful in detergents, soaps, and emulsifiers (lecithin in food products)
• Antifreeze, such as ethylene glycol, lowers the freezing point of water, preventing damage to car engines and pipes during cold weather
• Phase change materials (PCMs) absorb and release heat during phase transitions, making them useful for temperature regulation in buildings and textiles
• Supercooled liquids, such as glass, remain liquid below their normal freezing point due to slow crystallization kinetics
• Superabsorbent polymers, used in diapers and water retention gels for plants, can absorb and retain large amounts of liquid due to cross-linked polymer networks
• Solid-state electronics, such as transistors and integrated circuits, rely on the properties of semiconductors (silicon, germanium) in their crystalline form

Lab Experiments and Demonstrations

• Measuring the surface tension of liquids using a tensiometer or capillary rise method
  • Comparing surface tension of water, ethanol, and soapy water
• Demonstrating capillary action using glass capillary tubes or paper chromatography
  • Observing the rise of colored water in paper towels or the separation of ink pigments on chromatography paper
• Investigating the relationship between viscosity and temperature using a falling sphere viscometer
  • Measuring the time taken for a metal sphere to fall through a liquid at different temperatures (glycerol, honey)
• Growing crystals from supersaturated solutions and observing their crystal structures
  • Copper sulfate, alum, or borax crystals grown by slow evaporation or cooling of saturated solutions
• Demonstrating the effect of intermolecular forces on boiling point using a series of alcohols (methanol, ethanol, propanol)
  • Observing the trend in boiling point as the size and strength of intermolecular forces increase
• Investigating the heat of fusion and heat of vaporization using calorimetry
  • Measuring the energy required to melt ice or vaporize water using a calorimeter and heating device
• Demonstrating sublimation and deposition using dry ice (solid CO2)
  • Observing the direct transition from solid to gas (sublimation) and gas to solid (deposition) as dry ice sublimates and forms frost