World Builders™
World Builders™
Session Five  --  Seaweeds
Session Five  --  Seaweeds

Atmospheric Pressure and Evaporation

Atmospheric Pressure and Evaporation

Atmospheric pressure affects the rate of evaporation.


Liquids evaporate.  We have all noticed this with water, gasoline, and cleaning fluids.

What happens when liquids evaporate?


eviau2001: You know that when you are up on the top of a high mountain, there is less atmospheric pressure and water boils at a lower temperature?

eanbug: I know that the different atmospheric pressure would effect the weather.

eviau2001: Yes. At low pressure water will evaporate quickly at lower temperatures than here.

dyyanbug: which would mean it affects the evolution of life and dealing with storms, rains, etc. is that what you meant?
eviau2001: Boiling is just very enthusiastic evaporation.

eviau2001: The molecules in water are all jumping around in there. The warmer the water is, the faster the molecules jump.

eviau2001: Some of them jump right out of the water and into the air! Evaporation!

eviau2001: But the armospheric pressure pushes down on the water so that fewer molecules can escape.
eviau2001: Reduce the pressure and evaporation will happen faster.
eviau2001: Reduce it a lot and you may not have standing water.
dyyanbug: the atmospheric pressure we have currently isn't so low that the stmosphere will eventually disappear, do we?

eviau2001: With an atmosphere, the small molecules escape to space. Hydrogen is a small atom.

eviau2001: Big atoms cannot escape as readily.
eviau2001: The gravity of the planet affects which molecules escape.
eviau2001: how fast the planet spins affects that too.
oscar2465: I will do the changes as soon as possiable
eviau2001: You've heard the news talk about escape velocity?
eviau2001: That is how fast something has to be travelling to escape from the earth.
eviau2001: Baseballs do not escape.
eviau2001: Bullets do not escape.
eviau2001: But space ships do!
eviau2001: A small planet has less gravity than a large one, so the atmosphere can escape more rapidly.
eviau2001: Mars has only one hundredth of the atmosphere that we have here.
eviau2001: Most of the gases escaped into space.
eviau2001: On earth the oxygen and carbon dioxide will probably never be able to escape, but if all the hydrogen goes, what will happen to the water?
oscar2465: It will evaporate.
eviau2001: Good. Will it condense into rain?
dyyanbug: it's bigger than hydrogen. It wouldn't escape, right?
dyyanbug: so wouldn't it condense?
oscar2465: yes. only the h2 will become a gas and escape.
eviau2001: We would thin not. Good. But when the water gets high into the atmosphere, the sunlight causes it to photo disociate.
eviau2001: What is water made of?
oscar2465: h2o
dyyanbug: oh, the diassociated hydrogen would escape.
eviau2001: Yes, Oscar, you are right The sunlight splits the hydrogen from the oxygen.
dyyanbug: and the disasociate oxygen?
eviau2001: "Bye Bye!" says the hydrogen as it flies off into space!
eviau2001: But the oxygen stays behind because it is heavier.
eviau2001: However, oxygen is a very friendly element that will combine with just about anything.
dyyanbug: ok, got it.
eviau2001: Maybe it will attach itself to rocks.
dyyanbug: if iron is in it, it will rust.
eviau2001: But your planet eventually has no water and so nothing can live there.
dyyanbug: red rocks, Mars.

eviau2001: Yesm a rusty red planet! Good!
eviau2001: So now we will all be wondering if there is water frozen into ice under the soil of Mars.
eviau2001: Will there be life there? Nobody knows!
eviau2001: It is so cold on Mars that water can sublime (evaporate directly from a solid) from ice.
eviau2001: But a lot of the ice is from frozen carbon dioxide, which is a heavy molecule made of oxygen and carbon.
eviau2001: Heavy for a gas, anyway. Metals are much heavier.
eviau2001: The periodic table shows which molecules are heavy and which are light.
eviau2001: The light molecules are cooked in the stars, and most of the molecules necessary for life are light and abundant.
eviau2001: Zinc and lead and gold are heavier and more rare.

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© 1996,1997, 1998, 1999, 2000, 2002, 2003.   Elizabeth Anne Viau. All rights reserved. This material may be used by individuals for instructional purposes but not sold. Please inform the author if you use it at .