Changes Of State Ch3.3 8th Pdf

CHANGES OF STATE Chapter 3 Section 3

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 or

gains energy

Energy & Changes of State  A change in a substance from one physical form to another  Particles in a substance move differently depending on the

state of the substance

 Ex: particles in liquid water have more energy than particles

in ice, steam has the most energy

 To change state: add or remove energy

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 30˚C (melts in your hand); NaCl melts

at 801˚C.

Adding Energy  For a solid to melt, particles need to overcome the

attractions between them  Energy is added

 Melting is endothermic because energy is gained by the

substance as it changes state

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

Freezing: Liquid to Solid  The temperature at which a liquid changes into a solid is the

liquid’s freezing point  The reverse process of melting

 *Freezing and melting occur at the same temperature:  Liquid water freezes at the temperature which ice melts – 0˚C

Removing Energy  Removing energy will cause particles to lock into place

 Freezing is an exothermic change because energy is removed

from the substance as it changes state

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

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 sweat – your body transfers energy to the water to cool itself)

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 100˚C at sea level (due to atmospheric pressure)

 Atmospheric pressure is lower at higher elevations  Ex: In Denver, water boils at about 95˚C (1.6km above sea

level)

Condensation: Gas to Liquid  Condensation and evaporation are the reverse of each other

 Condensation point is the temperature at which a gas

becomes a liquid (same temperature as the boiling point at a given pressure)  Large numbers of particles must clump together when their

attraction overcomes their motion  Energy is removed from the gas (exothermic change)

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

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 substance’s

particles  Temperature remains constant until the change of state is

complete

Quick Quiz  What 2 changes of state are endothermic?

 Do the particles of a substance move faster or slower as the

substance is heated?  When water evaporates, has a chemical change or a physical

change occurred?