Liquids, Solids, and Intermolecular Forces

Vocabulary terms covering the states of matter, various types of intermolecular forces, phase changes, and the properties associated with them such as boiling point and miscibility.

Intermolecular Forces

Forces (energy) between molecules that determine the state of a substance—solid, liquid, or gas—at room temperature.

Dispersion Forces (London Forces)

Intermolecular forces present in all molecules and atoms resulting from the instantaneous uneven distribution of electrons; their strength increases with increasing molar mass.

Dipole-Dipole Force

The force that exists in all polar molecules possessing permanent dipoles that interact with the permanent dipoles of neighboring molecules.

Hydrogen Bonding

A particularly strong type of dipole–dipole attraction occurring in polar molecules containing hydrogen atoms bonded directly to fluorine, oxygen, or nitrogen ($F$, $O$, or $N$).

Ion-Dipole Forces

Very strong intermolecular forces that exist between an ion and a polar molecule, such as during the dissolution process of $NaCl$ in water.

Miscibility

A liquid’s ability to mix with another liquid without separating into two phases; generally, polar liquids are miscible with other polar liquids.

Evaporation (Vaporization)

A physical change in which a substance is converted from its liquid state into its gaseous state, which increases with surface area, temperature, and decreasing strength of intermolecular forces.

Condensation

A physical change in which a substance is converted from its gaseous state to its liquid state.

Volatile

A term describing liquids that evaporate easily.

Nonvolatile

A term describing liquids that do not vaporize easily.

Dynamic Equilibrium

The point at which the rates of condensation and evaporation become equal, resulting in a constant number of gaseous molecules above a liquid.

Vapor Pressure

The partial pressure of a liquid’s vapor when it is in dynamic equilibrium with its liquid.

Plateau (on a heating curve)

A horizontal line on a heating curve that represents a change of state occurring at a constant temperature.

Sloped line (on a heating curve)

A line on a heating curve that represents a temperature change.

Thermal Energy

Energy whose strength relative to intermolecular forces determines the state of matter; weaker intermolecular forces relative to this energy result in gaseous samples.

Methanol ($CH_3OH$)

A compound with a molar mass of $32.0\,g/mol$ that exhibits hydrogen bonding and has a boiling point of $64.7^{\circ}C$.

Formaldehyde ($CH_2O$)

A polar molecule with a molar mass of $30.0\,g/mol$ and a boiling point of $-19.5^{\circ}C$.

Ethane ($C_2H_6$)

A nonpolar gas molecule with a molar mass of approximately $30.1\,g/mol$ and a boiling point of approx $-88$ to $-89^{\circ}C$, depending on comparison data.