Changes of State

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Changes of State
Beta Science
Overview
This PowerPoint examines how matter changes from
state to state.
Changes in state are explained in terms of matter
gaining or losing energy.
Change of State
Change of state: The change of a substance from one
physical state to another.
All changes of state are PHYSICAL CHANGES.
Therefore, the “identity” of a substance does not
change.
Particles of a substance move differently depending on
the state of the substance.
Particles have different amounts of energy when the
substance is in different states.
Therefore, in order to change substances from one state
to another, you must add or remove energy. *
Adding and Subtracting
Energy
Endothermic: when energy is added to a substance as
it changes state.
Means the object was heated-up
Exothermic: when energy is removed from the
substance as it changes state.
Means the object was cooled down
Melting
Melting: the change of state in which a solid becomes a liquid by
adding energy.
Adding energy to a substance increases the temperature of a
substance
As temperature increases the particles begin to move faster. As
soon as a solid hits a certain amount of energy the solid will
melt.
Melting Point: the temperature at which a substance changes
from a solid to a liquid.
Different substances have different melting points.
Gallium melts at 30°C. Normal body temp is 37°C. Therefore,
Gallium will melt in your hand.
Table Salt has a melting point of 801°C so it will not melt in your
hand.
Freezing
Freezing: the change of state from a liquid to a solid.
Freezing point: the temperature at which a liquid
changes from a liquid to a solid.
Different substances have different freezing points.
Freezing and melting occur at the same temperature.
Ex. Liquid water freezes at the same temperature at
which ice melts- 0°C
Evaporation
Evaporation: the change of a substance from a liquid
to a gas.
Can occur at the surface of a liquid that is below it’s
boiling point.
Ex. When you sweat your body is cooled through
evaporation. Water absorbs energy from your skin as it
evaporates. You feel cooler because your body
transfers energy to the water.
Boiling
Boiling: The change of a liquid to a vapor or gas
throughout the liquid.
Vapor Pressure: pressure inside bubbles of boiling water.
Atmospheric pressure: pressure from the outside of the
bubbles.
Boiling occurs when the vapor pressure equals the
atmospheric pressure.
Boiling Point: The temperature at which a liquid boils. Ex.
Water boils at 100°C
Pressure and Boiling
Did you know: water only boils at 100°C at sea level?
In Denver, water boils at 95°C.
Atmospheric pressure varies depending on where you
are in relation to sea level. Atmospheric pressure is
lower at higher elevations.
If you were to boil water on top of a mountain, the
boiling point would be lower than 100°C. Why?
A: because vapor pressure and atmospheric pressure
have to be the same in order for water to boil.
Condensation
Condensation: the change of state from a gas to a
liquid.
Condensation is the reverse of evaporation.
Condensation Point: the temperature at which a gas
becomes a liquid.
The condensation point is the same temperature as the
boiling point at a given pressure.
How Condensation
Occurs
In order for a gas to become a liquid, large numbers
of particles must clump together.
First they must have a strong enough attraction to one
another to overcome their current traveling path.
For this to happen energy is removed from the gas to
slow the movement of the particles.
Because energy is removed in the condensation
process, it is an exothermic change.
Sublimation
Sublimation: the process in which a solid changes
into a gas. Ex. Dry ice
For this to happen, particles must move from being
tightly packed to being spread far apart.
Attractions must be completely overcome.
Particles must gain energy to overcome the attractions
Sublimation, therefore, is an endothermic change.
Change of Temperature vs
Change of State
In order to lose or gain energy, a substance must
either change temperature or change state.
Temperature speeds up particles to help them
overcome attraction.
But the temperature does not change until the change
of state is complete.
Ex. The temperature of boiling water stays at 100°C
until it has all evaporated.
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