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

Vapor Pressure & Boiling Point

• Intermolecular forces are broken or formed when solids and liquids change state

• 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.

  • If we place a quantity of ethanol (CH₃CH₂OH) in an evacuated, closed container, it 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 attains a constant value.

  • This constant value is known as vapor pressure

Vapor Pressure

A vapor pressure develops in the evacuated closed container as an equilibrium is established between the rate of vaporization of liquid ethanol and the rate of condensation of gaseous ethanol.

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 describes a state in which the rate of vaporization and condensation of a liquid are equal

• Volatile substances have high vapor pressures

  • This means that they evaporate quicker than substances with low vapor pressure

• Generally, vapor pressure increases with temperature

  • This is because more molecules of the liquid have sufficient energy to break the intermolecular forces and change to 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 normal boiling point

• At a given external pressure, liquids with low vapor pressures have a higher boiling point 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 Graph

A graph showing the relationship between vapor pressure of several covalent compounds and their boiling points

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

  • Hence, the melting points of crystalline solids cannot be categorized in as simple a fashion as boiling points

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

  • Stronger intermolecular forces of attraction between the solid particles means that more energy is required to overcome these forces

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

• For example, the melting and boiling point of group 7A elements increases from
              F₂ < Cl₂ < Br₂ < I₂ < At₂

Melting & boiling points of the Group 7a elements

The melting & boiling points of the Group 7a elements increase going down the group as the strength of the intermolecular forces increase

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 or oscillate about their fixed positions

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

• The particles in a liquid experience some intermolecular forces but these are not as strong as those in a solid

  • 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 for liquids to flow

  • Similar to solids, liquids are not easily compressed due to a lack of empty space between the particles

• The solid and liquid phases for a particular substance typically have similar volume

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

Particulate Representation of Solid and Liquid

In a solid, particles are closely packed and can only exhibit vibrational motion about their fixed positions. However, in a liquid, particles are not as closely packed as solids and are capable of both vibrational and translational motion meaning the liquid substance can flow.