The Three States of Matter

  • Gas: molecules/atoms have enough energy to move freely

○ Particles so far apart from each other that intermolecular forces not considered

○ Indefinite shape and volume

  • Liquid: strong intermolecular forces and molecular motions

○ Particles are always in contact but have enough energy to slide past each other

○ Indefinite shape & definite volume

  • Solid: strongest intermolecular forces, but the molecular motions are minimal

○ Particles don’t have enough energy to move → always in contact and in fixed position

○ Definite shape and definite volume


  • Particles retain their chemical identity in all 3 states, but the volume, density, and interparticle distances are different

○ Ex: water is still water, but can be solid, liquid, or gas

  • Liquids & solids are incompressible (condensed state) & their density does not change with temperature

○ These similarities are due to the molecules being close together in solids and liquids; far apart in gases

The Liquid State

  • Liquids have low compressibility, lack of rigidity, and high density compared to gasses
  • Surface tension: tendency of molecules to be pulled from the surface to the interior of a liquid; resistance of a liquid to an increase in its surface area

○ Stronger InterMF = stronger surface tension b/c molecules resist being stretched/broken

  • Viscosity: a measure of a liquid’s resistance to flow
  • Capillary action: spontaneous rising of a liquid in a narrow tube

○ Capillary action depends on the cohesive and adhesive forces present → during capillary action, the liquid molecules simultaneously adhere to the tubing while pulling each other up

Cohesive forces: intermolecular forces among the molecules of the liquid

  • Molecules attracted to same molecules

Adhesive forces: forces between the liquid molecules and their container

  • Molecules attracted to another type of molecule

Concave vs Convex Meniscus

Water has both strong adhesive and cohesive forces, but bcuz the adhesive forces are stronger water will have a concave meniscus (water is attracted to the glass)

Nonpolar liquids have stronger cohesive than adhesive forces (not attracted to glass) → convex meniscus

Changes of State

Phase Changes: when a substance changes from solid to liquid to gas

Changes of State Melting or Fusion
Solid → Liquid Melting (endo)
Liquid → Solid Freezing (exo)
Liquid → Gas Vaporization (endo)
Gas → Liquid Condensation (exo)
Gas → Solid Deposition (exo)
Solid → Gas Sublimation (endo)

Melting point: temp at which the substance goes from a solid to a liquid (or from a liquid to a solid)

      ○ The strength of the InterMF determines the temp at which these phase changes will occur

Boiling point: temp at which a substance goes from a liquid to a gas (or from a gas to a liquid)

Key Points

1. At a substance’s MP or BP, it is at equilibrium & the two phases can exist simultaneously

2. Phase changes involve changes in interMFs

3. Energy of phase changes: melting = freezing < vaporization = condensation

4. The temp of a substance does not change as the substance goes from one phase to another

       ● Only after all of the substance has changed phases does adding heat change the temp of the substance

Heat of fusion: The enthalpy change that occurs at the melting point when a solid melts
Heat of vaporization: The energy required to vaporize 1 mole of a liquid at a pressure of 1 atm

    ○ Water has high HoV so can absorb lots of heat and resist chemical change; needs lots of energy to freeze → cools air when it is warm and releases heat in the winter, stabilizes ocean temperature, climate

When curve quickly changes slope to 0, all energy is used to overcome intermolecular forces holding the substance’s molecules together Evaporation 


  • Evaporation: liquid becomes gas below a substances BP (occurs only for particles at the surface of a liquid)
  • The particles with the highest KE can overcome the InterMF forces within the liquid and evaporate as a gas
  • Is a cooling process because the particles with the highest KE diffuse away from the liquid so the average KE of remaining particles decrease


  • Boiling: process by which a liquid becomes a vapor when it is heated to its boiling point
  • As temp of liquid increases, vapor pressure increases until the vapor pressure of the liquid become equal to the surrounding atmospheric pressure → At this temp the liquid will boil (BP)

         ○ As atmospheric pressure increase, BP of liquid increases

         ○ The normal BP is the temperature at which the liquid boils at standard pressure

Evaporation vs Boiling

  • Vaporization occurs in two ways: boiling and evaporation
  • Evaporation is slower, occurs only from the surface of the liquid, does not produce bubbles, and leads to cooling.
  • Boiling is faster, can occur throughout the liquid, produces lots of bubbles, and does not result in cooling.

Vapor Pressure and Changes of State

  • Vapor Pressure: Liquid molecules at the surface escape into the gas phase → gas particles create pressure above the liquid in a closed container
  • Weaker IMF → Lower BP → will have higher vapor pressure before reaching boiling point ○ Liquids are said to be volatile—they evaporate rapidly from an open dish
  • Stronger IMF → higher BP/fewer molecules break away → will have lower vapor pressure before reaching boiling point.
  • Generally, the vapor pressure of a liquid is related to temperature and intermolecular forces ○ Vapor pressure increases significantly with temperature.

                  ■ Temperature of the liquid increases = more molecules will have the minimum energy needed to overcome InterMFs and escape into the vapor phase

  • Gases are often collected over water so the vapor pressure of water must be subtracted from the total pressure in calculations to find pressure of the gas

Phase Diagrams

  • Phase diagram: way of representing the phases of a substance as a function of temperature and pressure. (in a closed system)

           ○ Lines represent phase changes

  • Triple point: condition of temp and pressure where all three phases are present
  • Critical temperature: temperature above which the vapor cannot be liquefied no matter what pressure is applied
  • Critical pressure: pressure required to produce liquefaction at the critical temperature
  • Critical point: critical temperature + critical pressure

Phase Diagram for Water

  • Density and Phase Diagrams: the slope of the line between the solid and liquid region indicates which of these 2 phases is denser

               ● MP Curve has positive slope (/) → solid is denser

         ○ Increasing pressure = increases melting point

  • MP curve has negative slope (\)→ liquid is denser

         ○ Increasing pressure = decreases melting point

              ■ Water has a negative slope, but most other substances have a positive slope