Changes of State Ch3.3 8th PDF

8/14/2019 Changes of State Ch3.3 8th PDF

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Chapter 3 Section 3

CHANGES OF STATE


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Objectives

Describe how energy is involved in changes of state

Describe what happens during melting and freezing

Compare evaporation and condensation

Explain what happens during sublimation

Identify 2 changes that can happen when a substance loses orgains energy


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Energy & Changes of State

A change in a substance from one physical form to another

Particles in a substance move differently depending on thestate of the substance

Ex: particles in liquid water have more energy than particlesin ice, steam has the most energy

To change state: add or remove energy


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Melting: Solid to Liquid

The temperature at which a substance changes from a solid to

a liquid is the melting point

Melting point is a physical property

Different substance have different melting points

Ex:Gallium melts at 30C (melts in your hand); NaCl melts

at 801C.


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Adding Energy

For a solid to melt, particles need to overcome the

attractions between them

Energy is added

Melting is endothermicbecause energy is gained by the

substance as it changes state

http://www.saskschools.ca/curr_content/chem30_05/graphics/1_graphics/endothermic.gif


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Freezing: Liquid to Solid

The temperature at which a liquid changes into a solid is the

liquidsfreezing point

The reverse process of melting

*Freezing and melting occur at the same temperature:

Liquid water freezes at the temperature which ice melts 0C


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Removing Energy

Removing energy will cause particles to lock into place

Freezing is an exothermic change because energy is removedfrom the substance as it changes state

http://www.saskschools.ca/curr_content/chem30_05/graphics/1_graphics/exothermic.gif


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Evaporation: Liquid to Gas

Evaporation is the change of a substance from a liquid to a

gas

Can occur at the surface of a liquid that is below its boiling

point (ex: when you sweatyour body transfers energy to

the water to cool itself)


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Boiling & Evaporation

Boiling is the conversion of a liquid to a vapor when the

vapor pressure of the liquid equals the atmospheric pressure

The effects of pressure on boiling point:

Water boils at 100C at sea level (due to atmospheric pressure)

Atmospheric pressure is lower at higher elevations

Ex: In Denver, water boils at about 95C (1.6km above sealevel)


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Condensation: Gas to Liquid

Condensation and evaporation are the reverse of each other

Condensation point is the temperature at which a gasbecomes a liquid (same temperature as the boiling point at a

given pressure)

Large numbers of particles must clump together when theirattraction overcomes their motion

Energy is removed from the gas (exothermic change)


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Sublimation: Solid to Gas

Sublimation is the change in state in which a solid changes

directly into a gas

Attractions between particle must be completely overcome

The substance must gain energy for the particle to overcome

their attractions: endothermic change

http://www.ice-agency.com/ice-dry-ice/p-dryice.jpeg


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Change of Temperature vs.

Change in State

When substances lose or gain energy, either the temperature

change or the state changes

Temperature is related to the speed of the substances

particles

Temperature remains constant until the change of state iscomplete


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Quick Quiz

What 2 changes of state are endothermic?

Do the particles of a substance move faster or slower as thesubstance is heated?

When water evaporates, has a chemical change or a physical

change occurred?

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Changes of State Ch3.3 8th PDF