[Byrne]

I'm having troubles explaining this statement:

"The assumption that there is no force of attraction between gas particles cannot be true.  If it was, gases would never condense to form liquids."

[Bakegaku]

I'm not entirely sure what you need help with, but I'll try.

The molecules [or atoms] in a gas need a force between them to want to go near eachother.  Otherwise they would never get close enough to become a liquid.

Say you have around a hundred metal balls.  If you were to dump ordinary metal balls on a flat surface then would roll around and spread out without going near eachother.  However, if we make a force between them by making the metal ball magnetic, then if you dump them onto a table they may begin moving away from eachother, and then come together.  The metal balls are the molecules in a gas; in the first scenario there are no forces between them, in the second one there are.  If the balls were not magnetic, then they would not come together into a single mass just as if there were no forces between molecules/atoms then they would never come together and form a liquid.

I hope you understand this.

[Chrataxe]

Lol...good analogy.

You have to think of states of matter as a matter of energy.  The reason that everything doesn't exist as a solid is b/c there is enough energy to break the intermolecular forces holding the atoms together.  If you put energy into a solid, say heat, it will break some of the intermolecular forces and the molecules will not be held together as well and it will be a liquid.  If you add more energy, you will continue to break the forces holding the molecules together and it will become a gas.  Now, it is commonly said gases in the air will not react with each other b/c the energy is so great that all intermolecular forces are being broke.  But, as we all know, if you were to boil water in a Hickman-Hinkle still, the steam will reach the top of the cooler glass and recondense as a liquid.  The statement is saying if there is no interaction, how is it possible?  Its possible by lowering the energy around it.

Not sure if that answers your question either....

[jdurg]

Now, it is commonly said gases in the air will not react with each other b/c the energy is so great that all intermolecular forces are being broke.  

Actually, I've never heard that said.  Hydrogen and oxygen are gases and react quite readily.  All combustion reactions occur in the gas phase.  A great deal of chemical reactions occur when the reactants are gases.  

[Bakegaku]

Actually, I've never heard that said.  Hydrogen and oxygen are gases and react quite readily.  All combustion reactions occur in the gas phase.  A great deal of chemical reactions occur when the reactants are gases.  

I think he means the gas particles will not be attracted to eachother.  Perhaps it's just a poor choice of wording...

[Chrataxe]

Indeed bakegaku.  Funny how one word makes a difference.  I meant to say interact instead or react.  But, even still, atmospheric gases do not react with each other...and H and O need a flame.  

[Donaldson Tan]

"The assumption that there is no force of attraction between gas particles cannot be true.  If it was, gases would never condense to form liquids."

We assume there is no intermolecular attraction/repulsion between gas particles. This is because the gas particles do not cluster together. When the gas particles begin to cluster together, this phenomena is called vapour-liquid transition. This is whereby our assumption holds no longer.

Gas particles move in a random fashion, aka Brownian Motion. This random fashion of movement is the experimental evidence why we can assume there is zero or negligible attractive forces between the gas particles.

To simply put it, the assumption holds true for most cases, with certain exceptions. eg. saturated conditions.