Explaining the Properties of Solids & Liquids (College Board AP® Chemistry): Revision Note

Vapor Pressure & Boiling Point

• When a substance changes state, the intermolecular forces between its particles are overcome or re-formed; the covalent bonds inside the molecules do not break

• The type and strength of the intermolecular forces can affect the observable properties of solids and liquids, including:

  • Melting point

  • Boiling point

  • Vapor pressure

  • Solubility

  • Viscosity

  • Surface tension

• Vaporization is when a liquid changes its physical state into a gas

  • The reverse of this process, converting a gas back to a liquid, is known as condensation

What is vapor pressure?

• Liquids, and even some solids, are continuously vaporizing

  • Ethanol (CH₃CH₂OH) placed in an evacuated, closed container quickly begins to vaporize

  • This results in the development of an increasing pressure exerted by the vapor in the space above the liquid

  • After a short time, the pressure of the vapor reaches a constant value

  • This constant value is known as vapor pressure

Vapor pressure in a closed flask

The vapor pressure of liquids

• The vapor pressure of a liquid is the pressure exerted by its vapor when the liquid and vapor are in dynamic equilibrium

  • A dynamic equilibrium is a state in which the rates of vaporization and condensation are equal

• Volatile substances have high vapor pressures

  • This means that, at a given temperature, more of the substance is present as vapor at equilibrium than for a substance with a low vapor pressure

• Generally, vapor pressure increases with temperature

  • This is because more molecules of the liquid have enough kinetic energy to overcome the intermolecular forces and escape into the vapor

• The vapor pressure of a liquid depends on intermolecular forces

  • A liquid with stronger intermolecular forces has a lower vapor pressure because there is a greater attraction between the molecules

What is the boiling point?

• The temperature at which the vapor pressure of a liquid equals the pressure exerted on the liquid (atmospheric pressure, unless the vessel containing the liquid is closed) is called the boiling point of the liquid

  • The temperature at which a given liquid boils increases with increasing external pressure

  • When the pressure is 1 atm, the boiling point is known as the normal boiling point

• At a given external pressure, liquids with low vapor pressures have high boiling points, while volatile liquids have low boiling points

  • So, stronger intermolecular forces mean a lower vapor pressure and a higher boiling point

Vapor pressure vs temperature

Melting Point & Intermolecular Interactions

• The conversion of liquid to solid is called freezing

  • The reverse process is called melting or fusion

• The temperature at which a pure liquid freezes is called the freezing point

  • The temperature at which a solid melts is called the melting point, which is identical to the freezing point

• Melting or freezing occurs at the temperature where the liquid and solid are in dynamic equilibrium

• Unlike boiling points, melting points are affected significantly only by large pressure changes

  • Melting points also tend to increase with the strength of the intermolecular forces, but the link is less simple than for boiling points: when a solid melts, the interactions are only rearranged, not overcome completely

• Melting and boiling points are both indicators of the strength of intermolecular forces

  • Stronger intermolecular forces between the particles in the solid mean that more energy is needed to overcome these forces

  • This, in turn, leads to a higher melting point

• For example, the melting and boiling points of the halogens (group 17) increase in the order:

F₂ < Cl₂ < Br₂ < I₂ < At₂

• Down the group the molecules have more electrons and larger, more polarizable electron clouds, so their London dispersion forces are stronger

Melting and boiling points of the halogens

Particulate-Level Representations

• The properties of solids and liquids can be better understood by showing the arrangement of particles in each state

• The particles in a solid experience intermolecular forces that are strong enough to restrict their movement

  • This means that particles can only vibrate about their fixed positions

  • Therefore, solids are rigid with well-defined structures that cannot be easily compressed

• The particles in a liquid are attracted by the same intermolecular forces, but they have enough kinetic energy to keep breaking and re-forming these attractions as they move past one another

  • The arrangement and movement of particles in a liquid are influenced by the nature and strength of the intermolecular forces

  • Hence, the particles in a liquid can move relative to one another, which allows liquids to flow

  • Like solids, liquids are not easily compressed because there is little empty space between the particles

• Particulate diagrams are most useful when they show the interactions between particles, for example dashed lines for the hydrogen bonds between water molecules, and link them to a property such as a high boiling point

• The solid and liquid phases of a particular substance typically have similar volumes

  • This is because, in both phases, the particles are in close contact at all times

Particulate representation of a solid and a liquid