Phase Changes and Phase Diagrams

​10.2 Properties of Liquids

Learning Objectives

• Distinguish between adhesive and cohesive forces

• Define viscosity, surface tension, and capillary rise

• Describe the roles of intermolecular attractive forces in each of these properties/phenomena

​Liquid to Gas Phase Transition

Vaporization of a sample of liquid is a phase transition from the liquid phase to the gas phase.

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Phase Transition: Liquid to Gas

Vaporization of a sample of liquid is a phase transition from the liquid phase to the gas phase. There are two types of vaporization: evaporation and boiling.

• Evaporation occurs at temperatures below the boiling point, and occurs on the liquid’s surface. For molecules of a liquid to evaporate, they must be located near the surface, be moving in the proper direction, and have sufficient kinetic energy to overcome intermolecular forces present in the liquid phase.

• Boiling, by contrast, is rapid vaporization that occurs at or above the boiling temperature and at or below the liquid’s surface.

​Dew Point: the atmospheric temperature (varying according to pressure and humidity) below which water droplets begin to condense and dew can form.

​A supercritical fluid is a substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist.

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Freezing is a phase transition in which a liquid turns into a solid when its temperature is lowered to its freezing point.

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Sublimation is the phase transition from the solid to the gaseous phase, without passing through an intermediate liquid phase.

Water transitions from ice to liquid to water vapor as heat is added to it.

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Major Features of a Phase Diagram

Phase diagrams are useful because they allow us to understand in what state matter exists under certain conditions.