Evaporation and Condensation
Directions: Read, highlight, and once you are finished reading, go to schoology.com and answer the
questions.
Remember to go to schoology.com by first going to wcsoh.org and selecting schoology at the top of the
page!
Evaporation
When liquids turn into a gas, we call this evaporation. The temperature where a liquid turns into a gas is
called the boiling point. Every substance on earth has a different boiling point. This should sound very similar
to what you read for melting points. Water, for example, has a boiling point of 212oF (100oC)
a temperature special to water only.
As a liquid approaches its boiling point, the particles absorb more and more energy making
them vibrate faster. Here’s the weird part though. When liquids reach their boiling point,
the particles don’t continue to vibrate faster. You saw this when we boiled water (and when
we melted ice). The heat instead goes into break the attraction the particles have to each other.
When the liquid particles gain enough energy to overcome the attraction holding them together, they
become a gas. Once they become a gas, the molecules are free to move faster and faster. Instead of just
sliding over each other like they do in a liquid, they now have the ability to fly around attracted to nothing
but themselves.
Because molecules that are evaporating are absorbing heat, we refer to evaporation as an endothermic
process. Remember, endo means “to enter” while thermic means “heat.”
Liquid can also turn into a gas at temperatures lower than the boiling point. Evaporation occurs because
some particles have enough energy to escape as a gas from the surface of the liquid even though the whole
liquid hasn’t heated up yet.
This is why even though it is cold outside and far below water’s boiling point that the pavement can be wet
with snow melt in the morning, but by early afternoon the pavement has dried. If you stick your hand in the
water, it will be really cold. The surface molecules though get enough energy to evaporate.
Boiling vs. Evaporation
Boiling
Evaporation
Occurs only at the boiling
point
Occurs all the time
Occurs throughout the
liquid
Occurs only at the
surface of the liquid
Occurs rapidly
Occurs slowly
Condensation
The opposite of evaporation is condensation. In this case a substance changes from a gas
back to a liquid. It does this by losing heat energy.
For water, condensation happens at 212oF (100oC). That same weird thing happens during
condensation. The temperature stops until all of the gas has condensed turning into a liquid
and then it is free to drop more.
If that temperature seems familiar, it should because condensation and evaporation happen at the same
temperature. The difference is that evaporation is endothermic, but condensation is exothermic. Remember
that because the molecules are giving up their heat (getting colder), we refer to condensation as an
exothermic process (exo means “to exit” while thermic means “heat”).
Complete the following chart for Water in degrees Celsius
Evaporation/Boiling Condensation
Temperature
where it occurs?
Endothermic or
Exothermic
change?
Temperature
climbing or
staying the same?
Give an example
where you have
seen each change
of state occur.