Liquids, Solids, and Intermolecular Forces (Chapter 12) – Study Notes

Liquids, Solids, and Intermolecular Forces

12.1 Water, No Gravity

Water behaves uniquely in microgravity environments, such as on the space station, due to intermolecular forces. These forces cause water molecules to stick together, forming spherical blobs in the absence of gravity. The existence of condensed states (solid, liquid, gas) depends on the balance between intermolecular forces and thermal energy.

• Intermolecular forces are attractive forces between particles (molecules, atoms, or ions) that hold substances together.

• Condensed states (liquid and solid) exist when intermolecular forces are strong relative to thermal energy.

12.2 Three States of Water

Water can exist as a solid (ice), liquid, or gas (steam), each with distinct properties. The densities and molar volumes of these states differ significantly.

• Ice and liquid water are much denser than steam.

• The density of ice is less than that of liquid water, which is unusual for solids and essential for life (ice floats).

Phase	Temperature (°C)	Density (g/cm³, 1 atm)	Molar Volume	Molecular View
Gas (steam)	100	5.90 × 10-4	30.6 L	Widely spaced molecules
Liquid (water)	20	0.998	18.0 mL	Close-packed, disordered
Solid (ice)	0	0.917	19.6 mL	Close-packed, ordered

Properties of the Three States of Matter

The main difference between solids, liquids, and gases is the freedom of movement of their constituent particles.

State	Density	Shape	Volume	Strength of Intermolecular Forces
Gas	Low	Indefinite	Indefinite	Weak
Liquid	High	Indefinite	Definite	Moderate
Solid	High	Definite	Definite	Strong

Liquids

• Particles are closely packed but can move around each other, making liquids incompressible.

• This mobility allows liquids to flow and take the shape of their container, but not expand to fill it.

Gases

• Particles have complete freedom of motion and are far apart, making gases highly compressible.

• Gases expand to fill and take the shape of their container.

Solids

• Particles are closely packed and fixed in position, making solids incompressible and rigid.

• Solids retain their shape and volume.

• Crystalline solids have ordered structures (e.g., salt, diamond).

• Amorphous solids lack long-range order (e.g., glass, plastic).

Changes Between States

Changing the state of matter involves altering the kinetic energy or freedom of the particles, typically by changing temperature or pressure.

• Heating causes solids to melt and liquids to boil.

• Pressure changes can induce transitions between states (e.g., gas to liquid by increasing pressure).

12.3 Intermolecular Forces

The type and strength of intermolecular forces determine the physical properties of substances, such as melting and boiling points.

• All particles are attracted by electrostatic forces.

• Stronger intermolecular forces result in higher melting and boiling points.

Types of Intermolecular Forces

• Dispersion forces (London forces): Present in all atoms and molecules due to temporary dipoles from electron movement. Strength increases with molar mass and surface area.

• Dipole-dipole forces: Occur in polar molecules with permanent dipoles. The positive end of one molecule is attracted to the negative end of another.

• Hydrogen bonding: A special, strong dipole-dipole interaction when H is bonded to N, O, or F. Responsible for water's high boiling point.

• Ion-dipole forces: Occur between ions and polar molecules, important in solutions of ionic compounds.

Effect of Molecular Size and Shape

• Greater molar mass means more electrons, leading to stronger dispersion forces and higher boiling points.

• More surface area (less branching) increases dispersion forces.

Noble Gas	Molar Mass (g/mol)	Boiling Point (K)
He	4.00	4.2
Ne	20.18	27
Ar	39.95	87
Kr	83.80	120
Xe	131.30	165

Examples

• n-Pentane (straight chain) has a higher boiling point than neopentane (branched) due to more surface contact.

• Formaldehyde (polar, with C=O bond) has higher boiling/melting points than ethane (nonpolar).

Summary Table: Types and Strengths of Intermolecular Forces

Type	Relative Strength	Occurs Between
Dispersion	Weakest	All molecules/atoms
Dipole-dipole	Intermediate	Polar molecules
Hydrogen bond	Strong (but < covalent)	H with N, O, or F
Ion-dipole	Strongest	Ions and polar molecules

Key Equations

• Heat required for phase change: $q = n \Delta H$

• Heat for temperature change: $q = mC\Delta T$

Conceptual Questions

• Which state of matter is compressible? Gas

• Which halogen has the highest boiling point? I2 (due to highest molar mass)

Additional info: These notes are based on textbook slides and include expanded explanations, definitions, and tables for clarity and completeness.