The Liquid Phase (College Board AP® Chemistry): Revision Note

The Liquid Phase

• The particles in a liquid are in close contact with each other, and they are continually moving and colliding

• Unlike solids, the particles of liquids are able to move relative to one another, which means that liquids can flow

Particles in a liquid

Diagram showing the arrangement and movement of particles that make up the liquid phase

• Because there is little empty space between the particles, liquids are nearly incompressible

• Unlike gases, the size of liquid and solid particles compared to the distance between them cannot be ignored

  • Hence, they have fixed volume and there are no equations to describe the properties of liquid or solid state as they exist for gases

• The properties of liquids such as surface tension, viscosity, boiling point and vapor pressure are dependent on two main factors:

  • The nature and strength of the intermolecular forces between the particles of the liquid

  • These forces also affect how the particles are arranged and how freely they move, e.g. polarity and hydrogen bonding hold particles together more strongly

  • Temperature

• Generally:

  • Surface tension — the energy required to increase the surface area of a liquid by a unit amount — increases with increasing strength of intermolecular force

  • Viscosity, which measures the resistance of a liquid to flow, also increases with increasing intermolecular attraction but decreases with increasing temperature

  • Vapor pressure and boiling point of a liquid are dependent on the strength of the intermolecular force between the particles of the liquid

    • The stronger the intermolecular force, the lower the vapor pressure and the higher the boiling point of the liquid

Molar Volume in the Solid and Liquid Phase

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

  • This is because the distance between the particles of the substance in the solid phase is approximately the same as the distance between the particles in the liquid phase

  • Molar volume is the volume occupied by one mole of a substance at a given temperature and pressure

Molar volumes of metals in the solid and liquid states

The table shows that the molar volumes of metals are similar in both solid and liquid states. The slight differences in value are due to the small space between the particles of the metals in the liquid state, allowing them to flow.