Re: Are liquids only possible if there is some atmosphere or pressure?

Dear Geoff:
 
As a general rule liquids evaporate more quickly than solids at room 
temperature (23 degrees C).  If there is a perfect vacuum above the liquid 
all molecules that evaporate never make collisions with other molecules so 
they never bounce back to the surface, and there are only evaporating 
molecules and no molecules condensing.  That means gradually the water 
would disappear and no longer exist on the table.  However a vacuum that 
perfect is very difficult to make.   However if the water is in a closed 
bottle, then the water molecules that evaporate stay in the bottle and 
build up a pressure equal to the vapor pressure of water at 23 degrees C, 
make equilibrium, and the water stays there as water in the bottle.

Some liquids like oils, or mercury, can have very low vapor pressure and 
their evaporation rate is very low, but if they were placed in a perfect 
vacuum eventually the liquid would evaporate and be gone.  It may take a 
few years to be gone but it would finally disappear.

If you want a more detailed explanation read the rest of this message for 
more information!

Every substance, whether solid, liquid, or gas is made up of atoms.   There 
are ~120 elements known, and each one has a specific atom with its name 
(e.g. hydrogen, oxygen, iron, uranium, etc.).  Atoms bind together to form 
molecules containing more than one atom.  The oxygen molecule contains two 
oxygen atoms, and is called O2.  Water, on the other hand, is a molecule 
which contains hydrogen and oxygen, and is called H2O because is has two 
hydrogen atoms and one oxygen atom which bind together to form the water 
molecule.  Iron, called Fe, is an element; and many iron atoms can bind 
together to form a piece of metal.  Now at ordinary temperatures, like room 
temperature which can be about 23 degrees C, oxygen is a gas; water is a 
liquid; and iron is a solid.  If we drop the temperature to absolute zero, 
the oxygen, the water, and the iron will all be solids.  On the other hand 
if we raise the temperature to 5,000 degrees C the iron, the water, and the 
oxygen will all be gases.

At all temperatures above absolute zero these atoms and molecules are in 
motion, and they exert forces between them.  At 23 degrees C the oxygen 
molecules show very little interaction with each other and they move around 
in space and bump into each other all the time but do not stick together.. 
 If we warm up the oxygen they move faster, bump into each other more 
often, and create what is called "a higher pressure";  the "pressure" is a 
measure of the force and frequency with which they bump into each other.  
But all of the oxygen molecules will be in a gaseous state, i.e. no liquid 
or solid will be present because they do not stick to each other.  At room 
temperature the water molecules, however, have a strong force of attraction 
between each other and so they can clump together to form drops of water 
and make a liquid.  However on the surface of the liquid drops water 
molecules are continuously bumping into and sticking to the surface, and 
others are continuously evaporating from the surface and flying away.  If 
there are the same number of water molecules sticking to the surface per 
second as there are water molecules evaporating away from the surface then 
there is an equilibium and the liquid water will always be there but there 
is constant exchange between the molecules in the liquid and the molecules 
in the gas form.  Water is a liquid at room temperatures because although 
the water molecules stick to each other they do not form a rigid structure 
and so can slide among each other and show behavior of a liquid.  Now iron 
molecules, however, stick to each other very tightly at room temperature 
and therefore form a solid structure in which the iron atoms move around 
very little.  The iron atoms stick to each other so strongly that at 23 
degrees C essentially no iron atoms are leaving the surface of the iron 
metal, or evaporating, so the metal iron just sits there changing very 
little.

If you now think of what is happening at the surface of the iron, or the 
water, or the oxygen there is always some evaporation of atoms or molecules 
from the surface and there is always some amount of condensation, or 
sticking of atoms to the surface.  The number of atoms evaporating from the 
surface per second is what is called the "vapor pressure" of the water or 
the iron or the oxygen; at each temperature there is a vapor pressure for 
every material, (water, iron, oxygen. etc).  The lower the temperature the 
lower the vapor pressure;  the higher the temperature the higher the vapor 
pressure.  At 23 degrees C the vapor pressure of oxygen is very very high 
(thousands of atmospheres); the vapor pressure of water is about 0.024 
atmospheres, very moderate; and of  iron is very very low (millionths of an 
atmosphere).   So you can see there is a very big difference among these 
three substances in their behavior at 23 degrees C.  That is why at 23 
degrees C a piece of iron will sit on the table and not disappear because 
the vapor pressure is so low;  a dish of water will slowly evaporate and 
disappear because the evaporation rate is higher than the rate of 
condensation;  and no oxygen will be seen because the vapor pressure is so 
high that it never forms any substance on the surface.  It all depends on 
the temperature, however.  At absolute zero (-273 degreecs C) solid oxygen 
would also stay on the surface.

One more point. Normally the atmosphere contains water, except in the 
desert..As long as there is water (moisture) in the air there is a pressure 
of water which will provide water molecules to bombard the surface.  That 
is why lakes and oceans do not disappear-because of the water that is 
always present in the air.  Water is the only liquid for which this is 
true, however.

I hope this gives you better understanding.

R.  Bersin…..