Phase Changes and Energy

Phase Changes

• Phase change is a transformation from one state of matter to another
• Temperature remains constant during a phase change.
• Added energy breaks attractive forces holding particles together instead of increasing kinetic energy

Energy and Phase Change

• As energy is added to a material, the temperature changes
• At certain points, the temperature stops increasing
• Added energy converts the material from one state of matter to another

Increasing Kinetic Energy of Liquid Water Molecules

• Added energy is absorbed by water molecules and turned into increased molecular motion
• Increasing the temperature

Breaking Intermolecular Forces

• Added energy breaks hydrogen bonds between water molecules
• It does not increase molecular motion
• Temperature remains constant

Increasing Kinetic Energy of Gaseous Water Molecules

• Once all hydrogen bonds are broken, added energy increases molecular motion
• Increasing the temperature of the gas molecules

Types of Phase Changes

• Melting: solid to liquid

• Freezing: liquid to solid (opposite of melting), example = freezing water

• Vaporization: liquid to gas

• Condensation: gas to liquid (opposite of vaporization), example = water vapor

• Sublimation: solid to gas directly (opposite of deposition), example = dry ice

• Deposition: gas to solid, example = frost and ice formation

• During phase change, the actual particles remain the same

• Most of water on Earth is in liquid form

• A small fraction is solid or gas

Heating a Liquid

• As a liquid is heated, the particles gain vibrational and rotational energy
• When enough energy is gained, the remaining attractive forces between particles are broken, and the liquid becomes gas
• The temperature at which the attractive forces break is the boiling point
• Strength of intermolecular forces relates to the type of force and the size and weight of the molecules.

Boiling Point

• The liquid becomes gas as intermolecular forces between the molecules are broken
• The temperature at which the attractive forces break

Intermolecular forces

• Depend on the type of forces between molecules, the size and the weight the molecules
• Stronger forces result in a higher boiling point

Evaporation vs Vaporization

• With evaporation, liquid changes to vapor below the boiling point
• With vaporization, liquid changes to vapor at its boiling point

Condensation

• Liquid changes to vapor, with particles of the gas having low energy
• Occurs when particles get closer and become a liquid
• Happens on the surface of cold glass, where water vapor is cooled down, is lowered to change the gas into the liquid.

Evaporation and Condensation

• A liquid doesn't have to be boiling for some vaporization to occur.
• Below the boiling point, most of the particles don't have enough energy to overcome the attractive forces and escape into the gaseous state.
• Some particles may have enough energy to break free and become gas in a process called evaporation.
• Individual kinetic energies of particles fall within a distribution

Evaporation vs Condensation

• Evaporation takes places at all temperatures at the surface of the liquid due to the individual kinetic energy of surface particles
• Condensation takes place at a certain temperature, at a curtain surface

Particles and Evaporation

• At the process of evaporation, increasing temperature results in more particles having enough energy to evaporate
• Particles having enough energy can escape the liquid and enter the gas phase
• By increasing temperature, the fraction of molecules with enough energy to evaporate will increase