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Physical Science - Ch. 3.3

Phase Changes

Sarah Gleason

on 23 September 2013

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Transcript of Physical Science - Ch. 3.3

Chapter 3.3
Phase Change
Characteristics of a Phase Change
Melting and Freezing
When there are at least two states of the same substance - scientist describe them as "phases"
During phase changes energy is transferred between a substance and its surroundings
Molecules are what makes up MOST substances
Phase Change: is a reversible PHYSICAL change that occurs when a substance changes from one state of matter to another
Temperature and Phase Changes
Temperature changes are one way to recognize a phase change
Specific temperatures cause certain phase changes (like freezing and boiling for water)
** BUT the temperature of a substance does not change DURING the phase change
As the substance goes through the phase change from a liquid to a solid notice that the temperature remains the SAME
The energy is either absorbed or released
This is when energy is ABSORBED from the systems' surrounding
*An example would be of ice melting - absorbing the energy or heat causes the ice to melt
making the temperature warmer
When a system releases energy into its' surrounding area
*An example is freezing liquid - releases energy and heat to the surrounding areas
Makes the temperature cooler
This is the amount of energy that is absorbed during a endothermic reaction
IT'S another term for melting
Example: When water melts its absorbs 334 Joules of energy - that is water's heat of fusion
**The amount varies between substances
The arrangement of molecules IN WATER becomes less orderly as water melts and more orderly as water freezes
In a substance like ice - the attraction between molecules are what keep the molecules in a fixed and orderly positions (like solids have)
FOR EXAMPLE: as ice gain energy (heat) the molecules vibrate quickly - giving them enough energy to move from their fixed positions
When the molecules have enough energy to move - melting is complete
This is when energy is RELEASED from a substance and flows into the surrounding area
Example: When water is placed in a freezer, the molecules slow down - energy is released in freezer
They become slow enough to have a force of attraction on each other
Molecules are then drawn in close to each other in an orderly arrangement (solid)
Vaporization and Condensation
These processes depend on substances that change from a liquid to a gas to a liquid over and over continually.
Example: These phase changes allow energy to flow from the inside of a fridge to the outside
This is a phase change where a liquid changes into a gas.
Energy is absorbed (endothermic)
Example: Liquid water changing into water vapor (gas) when heated to a boiling point
This is the amount of energy needed for vaporization to occur
This amount varies between substances
This takes place at the surface of a liquid - and occurs at temperatures BELOW the boiling point
Process that changes a liquid substance to a gas at temperatures BELOW boiling point
The molecules at the surface must begin to move fast enough to escape the liquid and become a vapor
**bigger the container FASTER the evaporation
Pressure that is caused by the collisions of molecules in the vapor (gas has more collisions than a liquid)
The higher the temperature the higher the vapor pressure (High temp. = more collisions)
*another form of vaporization
When a liquid becomes a gas - at or above the boiling temperature (also not just the surface)
*Based on the KINETIC THEORY as the temperature increase so does molecules movement and collisions
some of the water then attempts to escape as vapor
which forms bubbles that you see when water boils
Boiling Point and Atmospheric Pressure
Atmospheric pressure can determine boiling point! (Or Altitude)
For example: water boils at lower temperatures in higher altitudes
In Colorado water boils at 95 C instead of 100 C
This is a phase change when a substance changes from a liquid to a gas to a liquid
Example: when you take a shower and you see condensation on the mirror
The water from the shower evaporated and then settled and cooled on the mirror changing back into a liquid (or condensation)
Phase change that goes from solid to a gas or vapor WITHOUT CHANGING TO A LIQUID FIRST
Example: Dry Ice (endothermic change)
Phase change where a gas or vapor changes directly to a solid WITHOUT BECOMING A LIQUID FIRST
Example: Frost on a window (exothermic change)
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