Change of Phase and Thermodynamics

M. Rocha   

Physics 1  - Chapters 17 and 18

Phase transition diagram illustrating state changes between solid, liquid, and gas including melting, freezing, vaporization, condensation, sublimation, and deposition

Phase Transitions

Inter Molecular Forces in Play

Van der Waals (Atraction)

Illustration of neutral atoms with electron clouds inducing temporary dipoles
Illustration of neutral atoms showing London dispersion forces

Electron Overlap (Repulsion)

Diagram illustrating balance between attractive Van der Waals forces and repulsive electron overlap forces between molecules
Potential energy vs intermolecular distance curve showing regions of electron repulsion, optimal attraction well, and separated molecules

Inter Molecular Forces in Play

Temperature and pressure make molecules be either too far, at the optimal distance for attraction, or too close

Potential Energy

Phase transitions depend on the molecular properties of the material, temperature and pressure

Pressure vs temperature phase diagram showing solid, liquid, and gas regions separated by phase boundaries and the triple point

Evaporation

Evaporation is the change of phase from liquid to gas

Diagram of water molecules evaporating from the surface of a liquid into gas
Phase change diagram highlighting evaporation from liquid to gas

Evaporation

Evaporation is the change of phase from liquid to gas

Animation of boiling liquid with vapor bubbles forming below the surface and rising

Evaporation can happen below the surface when boiling

Evaporative Cooling

Because only the most energetic molecules can escape the surface, evaporation removes internal energy from the liquid, thus evaporation cools

Illustration of hot sun sweating during a heat wave
Illustration of sweating person using evaporative cooling
Diagram of dog panting for evaporative cooling
Diagram showing energetic molecules escaping a liquid surface during evaporation

Condesation

Condensation is the reverse of evaporation, a change of phase from gas to liquid

Phase change diagram highlighting condensation from gas to liquid
Water droplets condensing on a cold glass surface
Water vapor condensing on a mirror in a steamy bathroom

Condesation

Condensation is the reverse of evaporation, a change of phase from gas to liquid

Diagram comparing high-speed vapor molecules bouncing apart versus low-speed molecules coalescing into liquid water

a)  At high speeds, molecules of water vapor bounce apart and remain a gas.

b) At lower speeds, molecules of water vapor are more likely to stick together and form a liquid

Checkpoint 

Why is it that a 90 degree day in a dry climate feels more comfortable than a 90 degree day in a humid place?

In a dry climate you’re cooled by evaporation, in a wet climate you’re heated by condensation

Melting

Phase change diagram highlighting melting from solid to liquid

Melting is the change of phase from solid to liquid.

While melting, the solid absorbs heat from the environment 

Ice cubes absorbing heat and melting into water
Molecular representation of ordered crystal lattice in ice breaking down into disordered liquid state during melting

Freezing

Phase change diagram highlighting freezing from liquid to solid

Freezing is the change of phase from liquid to solid.

While freezing, the solid releases heat into the environment 

Water releasing heat to the environment as it freezes into ice crystals

Sublimation

Phase change diagram highlighting sublimation from solid directly to gas

Sublimation is the change of phase from solid to gas without passing through the liquid phase.

 

Solid dry ice sublimating into visible carbon dioxide vapor

Solid carbon dioxide (dry ice) sublimates at -109 °F.

Illustration of sublimation process in freeze-drying and dry ice conditions

Energy and Changes of Phase

Temperature versus heat graph illustrating heating curves and latent heat plateaus for melting and vaporization of water

Energy and Change of Phase

Heating curve diagram of water showing energy required for phase transitions and temperature increases

1 Calorie = 4.2 Joules

Phase transitions require energy

Heating 1 gram of water

Graph section detailing specific heat capacity and energy transfer for warming liquid water

Water Heat Capacity in calories = 1 cal/g °C

\Delta T = \frac{\mathrm{Heat \ Transferred}}{\mathrm{Heat \ Capacity} \times \mathrm{mass}} = 1 \degree C / cal

Checkpoint 

It takes 80 Calories for 1 gram of water to change phase from solid to liquid, and the specific heat capacity of  water is 1 Calorie/(g °C). How much energy do you need to melt 1 gram of ice and end up with water at 20 °C ?

100 Calories

\Delta T = \frac{\mathrm{Heat \ Transferred}}{\mathrm{Heat \ Capacity} \times \mathrm{mass}}

Thermodynamics

Connecting heat to mechanical energy

Remember

Heat (Q) : Energy transfer due to temperature differences

Work (W): Energy transfer due to acting forces

\Delta T = \frac{Q}{c \ m} \Rightarrow Q = \Delta T \ c \ m
W = F \ d

First Law of Thermodynamics

Whenever heat is added to a system, it transforms to an equal amount of some other form of energy

Energy is conserved!

Diagram illustrating the First Law of Thermodynamics: Heat added equals increase in internal energy plus work done by the system

First Law of Thermodynamics

Whenever heat is added to a system, it transforms to an equal amount of some other form of energy

Energy is conserved!

Diagram of a piston-cylinder apparatus where added heat causes gas expansion to do mechanical work
Diagram of gas pressure and temperature relationship in a closed thermodynamic system

Adiabatic Process

Compressing or expanding a gas while no heat enters or leaves the system is said to be an adiabatic process

Diagram of adiabatic compression: compressing gas without heat loss raises its temperature

Adiabatic Process

When a gas adiabatically expands, it does work on its surroundings and gives up internal energy, and thus becomes cooler.

Illustration of adiabatic expansion where gas does work on its surroundings, loses internal energy, and cools down

Second Law of Thermodynamics

Heat of itself never flows from a cold object to a hot object

The second law of thermodynamics describes the direction of heat flow in natural processes

Heat Engine and Second Law

A heat engine is any device that changes internal energy into mechanical work

Heat engine schematic diagram showing heat transfer from a high-temperature reservoir to produce work with exhaust heat expelled to a low-temperature reservoir
Heat engine efficiency diagram comparing heat input, useful work output, and waste heat according to the second law of thermodynamics
Diagram of refrigerator cooling cycle showing compressor, condenser coils, expansion valve, and evaporator coils transferring heat outside

How a Refrigerator Works?

The End

Adiabatic Process

Diagram demonstrating adiabatic expansion of rising warm air cooling and condensing into clouds

Copy of Change of Phase and Thermodynamics

By Miguel Rocha

Copy of Change of Phase and Thermodynamics

Physics 1 - Week 7 - Chapters 17-18

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