Post on 25-Jan-2022
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Chp 3.3 -‐ Phase Changes
Pg. 84-‐91
Characteris6cs of Phase Changes • When at least two states of a substance are present, each state is described as a phase
• Phase change-‐ a reversible physical change that occurs when a substance changes from one state of maCer to another
• Mel6ng, freezing, vaporiza6on, condensa6on, sublima6on, and deposi6on are six common phase changes.
Temperature and Phase Changes • The temperature of a substance does not change during a phase change it remains the same un6l the phase change is complete – The temperature at which a substance freezes (freezing point) is iden6cal to the temperature at which it melts (mel6ng point)
Energy and Phase Changes • During a phase change energy is transferred between a substance and its surroundings
• Energy is either absorbed or released during a phase change
• Temperature and kine6c energy are directly related – As temperature increases the kine6c energy of the par6cles increase
Energy and Phase Changes
• The amount of energy absorbed depends on the substance – One gram of water absorbs 334 Joules of energy as it melts = its heat of fusion
• The heat of fusion or heat of “mel6ng” – Varies from substance to substance
Phase Changes that REQUIRE Energy (Endothermic)
1. Melting 2. Vaporization 3. Sublimation
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Mel6ng • The arrangement of molecules in water becomes less orderly as water melts and more orderly as water freezes
• Phase change in which a substance changes from a solid to a liquid
• Mel6ng – Heat flows from air to ice – Ice gains energy and molecules vibrate un6l they gain enough energy to overcome the aCrac6ons and move away
– When all can move mel6ng is complete
Vaporiza6on • Vaporiza6on -‐ the phase change in which a substance changes from a liquid to a gas
• Vaporiza6on is an endothermic process – A substance must absorb energy in order to change from a liquid to a gas
Vaporiza6on • Evapora6on
– takes place at the surface of a liquid
– The process that changes a substance from a liquid to a gas at temperatures below the substance’s boiling point
• Water evaporates and vapor collects above the liquid in a closed container – Vapor Pressure -‐ caused by collisions of the vapor and the walls of the container
– increases as the temperature increases
Boiling • Is when the vapor pressure and atmospheric pressure become equal the water boils. Occurs throughout the liquid
• Heat a pot of water, As temperature increases water molecules move faster and faster – When it reaches 100 degrees Celsius some molecules below the surface overcome the aCrac6on forces and form bubbles of water vapor
– When the bubbles reach the surface they pop and release water vapor into the air
Sublima6on
A phase change in which a substance changes from a solid to a gas or vapor
• Endothermic • Example is dry ice
Goes from solid carbon dioxide to a gas The gas causes water vapor to condense and that’s how it forms clouds or fog
Phase Changes that RELEASE Energy
(Exothermic)
1. Freezing 2. Condensation 3. Deposition
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Freezing • Freezing – change from a liquid to a solid – Energy flows from the water to the air in a freezer and the water cools down
– As kine6c energy of molecules decrease, the move more slowly
– ACrac6ons between molecule take effect
– When all molecules have been drawn into orderly arrangement freezing is complete
Condensa6on • Condensa6on -‐ the phase change in which a substance changes from a gas or vapor to a liquid
• An exothermic process • Responsible for the morning dew on grass and the “cloudy” mirror a_er you take a shower.
Deposi6on
• Deposi6on -‐ phase change in which a gas or vapor changes directly into a solid without first becoming a liquid – Exothermic – Opposite of sublima6on – Causes frost on cold window glass
The End