Unit 10: Liquids and Solids
The forces between whole molecules, not within them, and how those forces decide whether a substance flows, holds a fixed shape, or conducts electricity.
10 topics · 40 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
10.1 London Dispersion Forces
- How it worksHow does a temporary, randomly shifting electron imbalance in one atom induce a matching dipole in its neighbor?
- Worked exampleFind which noble gas has the higher boiling point, helium or xenon, and explain why using London dispersion forces.
- Common mistakeWhy isn't London dispersion force something that only exists between nonpolar molecules?
- Going furtherWhat happens to the strength of London dispersion forces as a molecule's electron cloud gets larger and more polarizable?
Your turn1 of 2
Xenon boils at a much higher temperature than helium, even though neither noble gas has any permanent dipole. What explains xenon's higher boiling point?10.1 Dipole-Dipole Forces
- How it worksHow does the positive end of one polar molecule line up with the negative end of its neighbor to create an attractive force?
- Common mistakeWhy isn't a dipole-dipole force anywhere near as strong as an actual ionic or covalent bond?
- Going furtherWhat happens to a substance's boiling point when you compare molecules of similar mass that differ only in polarity?
Your turn1 of 2
Dipole-dipole forces hold polar molecules together in a liquid or solid. Why isn't a dipole-dipole force anywhere near as strong as an actual ionic or covalent bond?10.1 Hydrogen Bonding
- How it worksHow does a hydrogen atom bonded to a small, highly electronegative atom create an unusually strong dipole-dipole attraction?
- How it worksWalk me through why only bonds to nitrogen, oxygen, or fluorine count as hydrogen bonds strong enough to matter.
- Common mistakeWhy isn't a hydrogen bond a real covalent bond even though it's drawn as a connection between two molecules?
- Going furtherWhat happens to DNA's double helix once the hydrogen bonds holding its two strands together are broken by heating?
Your turn1 of 2
Water boils at 100 degrees C, but H2S, a heavier molecule with a similar bent shape, boils at about negative 60 degrees C. What explains this huge difference?10.1 Ranking Intermolecular Forces and Boiling Point Trends
- How it worksHow do you rank London dispersion, dipole-dipole, and hydrogen bonding by strength to predict relative boiling points?
- Worked exampleFind which of butane, acetone, and ethanol has the highest boiling point by identifying each one's dominant intermolecular force.
- Common mistakeWhy doesn't a molecule's molar mass alone determine its boiling point?
- Ties togetherHow does this ranking of intermolecular forces connect back to the electronegativity differences that make a bond polar in the first place?
Your turn1 of 2
Butane, acetone, and ethanol have fairly similar molar masses. Which one has the highest boiling point, based on each one's dominant intermolecular force?10.2 Vapor Pressure and Volatility
- Common mistakeWhy isn't a liquid's vapor pressure the same thing as the atmospheric pressure pushing down on it?
- How it worksHow does the balance between molecules escaping into vapor and vapor molecules recondensing establish a liquid's equilibrium vapor pressure?
- How it worksWalk me through how raising a liquid's temperature increases its vapor pressure by giving more molecules enough energy to escape.
- Going furtherWhat happens to a liquid's vapor pressure at the exact temperature where it starts to boil?
Your turn1 of 2
A liquid has a vapor pressure of 145 torr at 289 kelvin. Its molar heat of vaporization is 27 kilojoules per mole. Use the Clausius-Clapeyron equation to find its vapor pressure in torr at a temperature 29 kelvin higher.10.3 Phase Changes and Heating Curves
- How it worksHow does a heating curve show energy going into breaking intermolecular forces instead of raising temperature during a phase change?
- Worked exampleFind the total energy needed to convert 50.0 g of ice at 0 degrees C completely into steam at 100 degrees C.
- Common mistakeWhy doesn't all the heat added during melting or boiling show up as a temperature increase?
- Going furtherWhat happens to a heating curve's flat plateaus once the substance is swapped for one with a much smaller heat of vaporization?
Your turn1 of 2
Find the total energy needed to convert 142 g of ice at 0 degrees C completely into steam at 100 degrees C (heat of fusion 334 J/g, specific heat of liquid water 4.184 J per gram per degree C, heat of vaporization 2260 J/g).10.4 Phase Diagrams
- How it worksHow do the three curves on a phase diagram mark the exact pressure-temperature combinations where two phases coexist?
- How it worksWalk me through tracing a path across water's phase diagram from solid ice to liquid to steam by changing pressure and temperature.
- Worked exampleFind the phase of water at 0.5 atm and negative 5 degrees C using a phase diagram.
- Common mistakeWhy isn't the triple point on a phase diagram just an arbitrary reference point instead of a real physical condition?
- Going furtherWhat happens to water's melting point curve, uniquely among most substances, when pressure is increased instead of decreased?
Your turn1 of 2
Water's melting point drops by about 0.0073 degrees C for every 1 atm increase in pressure above 1 atm (its melting point is 0 degrees C at 1 atm, unlike almost every other substance). If the pressure on a block of ice is raised to 363 atm, find its new melting point in degrees C.10.5 Crystalline Solids and Unit Cells
- How it worksHow does a tiny repeating unit cell, stacked in three dimensions, build an entire visible crystal?
- Worked exampleFind the number of atoms actually contained in one face-centered cubic unit cell.
- Common mistakeWhy isn't a unit cell a physical piece you could cut out of a crystal with a knife?
- Going furtherWhat happens to a unit cell's packing efficiency as you move from simple cubic to body-centered cubic to face-centered cubic?
Your turn1 of 2
A cubic unit cell has atoms at 7 equivalent corner positions (each shared among 8 unit cells) and 4 equivalent face-center positions (each shared between 2 unit cells), with no atoms anywhere else. Find the total number of atoms per unit cell.10.5 Ionic, Molecular, Covalent-Network, and Metallic Solids
- How it worksHow do ionic, molecular, covalent-network, and metallic solids each hold their particles together with a completely different kind of force?
- Worked exampleFind which type of crystalline solid, ionic, molecular, or covalent-network, best describes quartz, SiO2, and explain why.
- Common mistakeWhy isn't a metal's ability to bend and stretch explained by strong, fixed bonds the way a covalent network solid's hardness is?
- Ties togetherHow does a metallic solid's sea of delocalized electrons connect back to the molecular orbital idea of electrons spread across many atoms?
Your turn1 of 2
Quartz, SiO2, is extremely hard and has a very high melting point. Which type of crystalline solid best describes it, and why?10.5 Band Theory: Conductors, Insulators, and Semiconductors
- How it worksHow do molecular orbitals from millions of atoms merge into continuous bands of allowed energy inside a solid?
- How it worksWalk me through the difference between a conductor, an insulator, and a semiconductor in terms of the gap between valence and conduction bands.
- Common mistakeWhy isn't a semiconductor just a weak conductor sitting partway between a real conductor and a real insulator?
- Going furtherWhat happens to a semiconductor's conductivity when it's heated or doped with atoms carrying one extra valence electron?
Your turn1 of 2
Copper and diamond are both made of atoms bonded to atoms of the same element, yet copper conducts electricity effortlessly while diamond does not conduct at all. What explains this difference?