Phase ChangesPhysical Science

Mr. MossRHS

When 2 states are present at the same time, we describe each as a phase.

Here, we see 2 phases of water:◦ Solid Phase◦ Liquid Phase

Phases

A Phase Change is the reversible physical change that occurs when a substance changes from one state of matter to another.

There are 6 common phase changes◦ Melting◦ Freezing◦ Vaporization◦ Condensation◦ Sublimation◦ Deposition

Phase Change Definition

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Dep

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Melting

Freezing

Condensation

Vaporization

Phase Change NOTE:

◦ The temperature of the substance DOES NOT change during a phase change!!!!

All phase changes are related to energy and temperature.

Law of Conservation of Energy◦ Neither created or destroyed.◦ Transferred or Transformed

In phase changes, energy is either absorbed or released.◦ Endothermic – energy is absorbed.

Ice melting◦ Exothermic – energy is released.

Water freezing

Energy during Phase Changes

1g of ice absorbs 334 joules (J) of energy as it melts.

This amount of energy is the Heat of Fusion for water.

This is another term for the melting process.

Heat of Fusion

Phase Diagrams

Phase Diagrams

The Triple Point is where the substance exists in all 3 states.

The Critical Point is where, under extreme high temps and pressure, the liquid and gaseous states are indistinguishable.

Triple and Critical Point

Water is a molecule made up of 2 atoms of Hydrogen and 1 atom of Oxygen.

The arrangement of molecules becomes less orderly as water melts.

The arrangement of molecules becomes more orderly as water freezes.

Melting & Freezing

Endothermic process Heat energy is transformed into increased

kinetic energy. Causes molecules to vibrate more rapidly.

◦ Some gain enough to overcome the attractive forces and move from the fixed locations.

◦ This is the melting point. Any energy gained after the phase change

increases the average kinetic energy and the temperature rises.

Melting

Exothermic Average kinetic energy (Ke) decreases

causing molecules to slow down. At the freezing point, the attractive forces

begin to draw the molecules into an orderly arrangement.◦ Continues until all have been fixed.

Any energy removed after the phase change is complete decreased the average kinetic energy of the molecules and the temperature decreases.

Freezing

Vaporization happens when a substance changes from a liquid into a gas.◦ Endothermic◦ Heat of vaporization

Water gains 2258 J of energy when it vaporizes at 100° C.

◦ 2 vaporization processes Boiling Evaporation

Takes place at the surface of a liquid Occurs at temps below the boiling point

Vaporization

Process that changes a substance from a liquid to a gas at temps below the boiling point.

In a closed container:◦ Water vapor collects above the liquid◦ Pressure caused by molecules colliding with

container is called vapor pressure. Pressure increases as temp increases.

Evaporation

As you apply heat energy, temperature and vapor pressure increase.

When vapor pressure = atmospheric pressure, the water boils.

Called the boiling point. Depends on Atmospheric Pressure

◦ Higher elevations have less atmospheric pressure Takes longer to cook food.

Boiling

The phase change in which a substance changes from a gas to a liquid.

The water vapor transferred heat to the glass and condensed into a liquid.

Condensation

Sublimation is the phase change from a solid to a gas without going through the liquid phase.◦ Endothermic◦ Dry Ice

Deposition is the phase change from a gas to a solid without going through the liquid phase.◦ Exothermic◦ Frost on windows

Sublimation & Deposition