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science chapter 23
change of phase
Terms in this set (29)
Matter exists in 3 phases & can change from one to another
1) Solid (ex: ice)
2) liquid (ex: water)
3) gas (ex: steam or vapor)
how does matter change from one phase to another?
if you add energy, the rigid molecular structure breaks down -- and a solid becomes a liquid or a liquid becomes a gas
what determines whether matter is in solid, liquid or gas form?
a change of phase from liquid to gas
why is evaporation a cooling process?
molecules evaporate when other molecules bump into them and cause them to gain kinetic energy. When some molecules gain KE & evaporate, the molecules that bump into them lose KE. As a result, the average KE of the molecules that remain behind in liquid form is lower (and so the temperature of the liquid falls - this is why evaporation is a cooling process)
why does sweat cool the body?
When we overheat, we perspire. Evaporation of sweat cools us and helps us maintain a stable body temperature. Animals that don't sweat have to cool themselves (ex: pigs in mud)
Changing gas to a liquid (ie: opposite of evaporation)
example of condensation
drops of liquid on outside of a soda can. Water vapor molecules collide with the cooler soda can. When they collide with the slow-moving molecules on the cold soda can, they lose so much KE that they turn to a liquid
why is condensation a warming process?
KE lost by condensing gas molecules warms the surface of what they hit
holds the most water vapor it can at any given temperature
indicates how much water vapor is in the air vs. limit for that temperature.
• At relative humidity of 100%, air is saturated
does warm or cool air hold more water vapor?
More water vapor is required to saturate warm air than cool air (because when molecules are warm they move faster - and they are less likely to combine and condense because fast-moving molecules rebound off each other)
what is a cloud?
Warm air rises and expands. As it expands, it cools and water vapor condenses. This forms a cloud.
what is fog?
a cloud formed near the ground
Why do you feel chilly when you emerge from the shower into a dry room?
Because the water is evaporating off of you (evaporation is a cooling process - remember sweat)
Occurs when evaporation and condensation occur at equal rates
If evaporation exceeds condensation
a liquid is cooled
if condensation exceeds evaporation
a liquid is heated
evaporation that occurs below the surface of water (it causes bubbles)
• At boiling temperature the pressure of the vapor in the bubbles is so great it is higher than the pressure of the surrounding water
what happens to the boiling temperature in the mountains, where the atmospheric pressure is lower?
it goes down. (In Denver CO the boiling temperature is only 95 degrees Celsius)
why is boiling a cooling process?
Like evaporation, boiling is a cooling process (this means that energy is leaving the water through evaporation as quickly as energy is added to the water via the heat from the stove flame... if cooling didn't take place, the temperature of the boiling water would continue to rise)
change from liquid to solid
• Occurs when energy is withdrawn from a liquid, the molecules slow down and get closer to one another
why does adding salt cause ice to melt?
The salt ions get in the way of ice crystals forming
ice normally melts at 0 degrees but if pressure is applied to the ice, it melts a little bit and the melting point is lowered. When ice is melted under pressure and then it freezes again it is called regelation
• To make a snowball, you use regelation
• When you compress the snow, you cause a slight melting which helps to bind the snow into a ball
Energy must be _________ to change the phase of water from solid to liquid to gas
Energy must be __________ to change phase from gas to liquid to solid
What happens to the temperature of water when ice is melting?
When ice melts (ie: changes from a solid to a liquid), more and more energy is added but the temperature doesn't rise until the ice is completely melted.
• Once it is melted, for every 1 calorie of energy added to water, the temperature rises by 1 degree Celcius
• It takes a lot of calories of energy (about 80) to fully melt 1 gram of ice
what happens to the temperature of boiling water when the water is vaporizing
it stays the same until all the water turns to vapor (and then it rises)
How a refrigerator works
• Liquid is pumped into a cooling unit, where it is forced to evaporate - this is a cooling process
• The gas is directed outside to coils in back. As it condenses heat is given off to the surrounding air
• The condensed liquid is pumped into the cooling unit where it is forced to evaporate - which cools the unit - etc.
• An A/C works the same way
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