Changes of phase
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Changes of Phase. Phase. “Phase” refers to the state of matter a material is in: solid, liquid or gas. (we will ignore other states like plasma for now since they make life a bit complicated)

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Changes of Phase

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Changes of phase

Changes of Phase


Phase

Phase

  • “Phase” refers to the state of matter a material is in: solid, liquid or gas.

    • (we will ignore other states like plasma for now since they make life a bit complicated)

  • A change in phase means that the material is breaking or forming bonds (so there is a change in energy.

  • It also means that the physical and sometimes chemical properties have changed.


A touch of background

A Touch of Background

  • Matter is held together by bonds

  • These bonds occur due to electrons from one atom or molecule being attracted to the nuclei of another atom or atoms

  • There is a competing force which tends to break things up, that is the motion of molecules from their temperature. The higher the temperature, the more things break up.


Three states of matter

Three States of Matter

  • Liquid

    • SOME bonds between molecules are broken

    • Shape is defined by container, if any, and with a surface

    • Volume is fixed

    • Molecular movement is fast, but short distance

  • Solids

    • All molecules are bonded

    • Shape is rigid, with a defined surface

    • Volume is fixed

    • Each molecule stays in one place

  • Gas

    • All bonds between molecules are broken

    • Shape is defined by container, if any

    • Volume is defined by container, if any

    • Molecular movement is rapid and random


Change of state

Change of State

Solid Liquid  Gas

Sublimation

Freezing

Condensation

Evaporation

Melting

 

  • It takes energy (heat, warmth) to break the bonds

  • The temperature ( = kinetic energy) that the bond breaking occurs reflects the energy of the bond

  • The melting temperature = freezing temperature


Evaporation and boiling p oint

Evaporation and Boiling Point

Evaporation of a liquid occurs at any temperature

How quickly a liquid evaporates depends on the temperature

It occurs ONLY at the surface of the liquid

Boiling is different:

It occurs at one temperature

It occurs throughout the liquid

It occurs when the pressure of the evaporating liquid equals or is greater than the air pressure pushing down on the surface of the liquid

(Vapor Pressure > Atmospheric Pressure)


Vapor pressure continued

Vapor Pressure,continued

pressure

As more and more gas molecules evaporate, the pressure increases.

But as more and more molecules are in the gas phase, some will begin to condense.

Eventually, the rate of molecules evaporating and condensing will be equal to each other

The pressure inside the container will get to a new constant pressure

This is the vapor pressure of this liquid. It will be constant for this temperature

time


Vapor pressure and temperature

Vapor Pressure andTemperature

pressure

At any one temperature, a liquid will have a constant vapor pressure

Molecules will constantly be moving in and out of the gas phase

But as temperature increases, more molecules will start evaporating, and the vapor pressure will increase

Temperature

HEAT


Vapor pressure and temperature continued

Vapor Pressure and Temperature, continued

Vapor Pressure depends on the temperature AND the liquid being heated.

Some liquids, like water, evaporate fairly easily and have a moderate vapor pressure

Others, like acetone, evaporate very easily, and have a very high vapor pressure

Others, like olive oil, have a low vapor pressure, because it is quite difficult for the molecules to evaporate

Compare the vapor pressure at any one temperature (say room temperature)

Or compare the temperatures needed to reach atmospheric pressure

Room Temp.

1.0 atm

oil

water

Pressure

acetone

Temperature


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