Forces Between Molecules · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

What boiling really breaks

Science · Matter & Materials · ages 15-16
Name ______________________   Date ____________

  1. For small molecules of similar size and shape, which of these three attractions is usually strongest?

    • Dispersion forces
    • Ordinary dipole-dipole attraction
    • Hydrogen bonding

  2. When liquid water boils into water vapour, which attractions are overcome?

    • The covalent O-H bonds inside each water molecule
    • The attractions between separate water molecules
    • The forces holding each oxygen atom's nucleus together

  3. In the hydrogen bonding studied here, hydrogen is covalently bonded to an atom from which set?

    • Nitrogen, oxygen, or fluorine
    • Carbon, sulfur, or phosphorus
    • Sodium, chlorine, or magnesium

  4. At standard atmospheric pressure, water boils at 100°C, unusually high for such a small molecule. What explains this?

    • Covalent O-H bonds must be broken whenever water boils
    • Hydrogen bonds add attractions between water molecules
    • Small molecules always boil higher than large molecules

  5. Two similarly shaped nonpolar molecular substances A and B are compared at the same pressure. Treat dispersion as their only relevant attraction. B has a larger, more easily distorted electron cloud than A. Which is expected to boil higher?

    • B, because its dispersion attractions are stronger
    • A, because boiling breaks its covalent bonds more easily
    • Both equally, because nonpolar molecules cannot attract

  6. Hydrogen bonding guarantees a higher boiling point than any substance with only dispersion forces, whatever the molecular sizes.

    Circle one:   True   False

  7. A brimful glass holds a small dome of water above its rim. Which effect helps the surface resist spreading and spilling?

    • Pressure from air trapped inside each individual water molecule
    • Elastic stretching of the covalent bonds inside each molecule
    • Surface tension from attractions between water molecules

  8. At room temperature and atmospheric pressure, F2 and Cl2 are gases, Br2 is liquid and I2 is solid. Which trend helps explain these states?

    • Only bromine and iodine can form hydrogen bonds
    • Larger electron clouds give stronger dispersion forces
    • Covalent bonds split more easily in the larger molecules
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Answer key

For grown-ups. Fold this page away before handing over the rest.

What boiling really breaks W1-mt_0uBZWotvZB-s1

  1. Hydrogen bonding · Hydrogen bonding is usually strongest in this restricted comparison. Molecular size and shape prevent a universal ranking.
  2. The attractions between separate water molecules · Boiling separates H2O molecules. Their covalent O-H bonds stay intact.
  3. Nitrogen, oxygen, or fluorine · The taught donor bonds are N-H, O-H and F-H. The attraction also needs a nearby suitable lone pair.
  4. Hydrogen bonds add attractions between water molecules · Hydrogen bonding adds strong attractions between water molecules. Boiling overcomes these attractions without splitting H2O.
  5. B, because its dispersion attractions are stronger · Under the stated dispersion-only model, B is expected to boil higher because its more easily distorted electron cloud gives stronger dispersion attractions.
  6. False · The combined attractions matter. Large nonpolar molecules can have strong dispersion attractions, so this guarantee is false.
  7. Surface tension from attractions between water molecules · Cohesion between water molecules produces surface tension. Hydrogen bonding contributes to this cohesion.
  8. Larger electron clouds give stronger dispersion forces · Increasing electron-cloud size strengthens dispersion attractions. This helps hold Br2 and I2 in condensed states.
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