Oxygen Form(S)?

[hazelm]

When two oxygen atoms bond, they become a molecule of gas.  This sounds as though a change from another form took place.  So, what form did pure oxygen have before it bonded?  I thought oxygen was always a gas.  It is part of other compounds - yes - but isn't it there as a gas?  As I think of water, though, I must be wrong.  Hydrogen and oxygen combine to become a liquid.  I'm confused.  Either one or both of those elements change,  Yet, oxygen is called a gas in explaining how fish breathe.

 

Theodore Gray - in The Elements - says  "In elemental collections, pure oxygen can only be represented by a seemingly empty bottle."  That indicates to me that the pure element is a gas.

 

Theodore Gray also says, "At 183° C, oxygen is a beautiful pale blue liquid." He is talking a about pure oxygen but something has been done to it.  How I don't know.

 

I have a feeling there is a story her worth reading.  So, again, can pure oxygen be in any other form than gas?

 

 

[exchemist]

When two oxygen atoms bond, they become a molecule of gas.  This sounds as though a change from another form took place.  So, what form did pure oxygen have before it bonded?  I thought oxygen was always a gas.  It is part of other compounds - yes - but isn't it there as a gas?  As I think of water, though, I must be wrong.  Hydrogen and oxygen combine to become a liquid.  I'm confused.  Either one or both of those elements change,  Yet, oxygen is called a gas in explaining how fish breathe.

 

Theodore Gray - in The Elements - says  "In elemental collections, pure oxygen can only be represented by a seemingly empty bottle."  That indicates to me that the pure element is a gas.

 

Theodore Gray also says, "At 183° C, oxygen is a beautiful pale blue liquid." He is talking a about pure oxygen but something has been done to it.  How I don't know.

 

I have a feeling there is a story her worth reading.  So, again, can pure oxygen be in any other form than gas?

OK Hazel, first of all oxygen atoms are, by themselves, highly reactive things, as they want to form bonds to something. This is because they have only 6 electrons in their outer shell and greater stability comes with 8 (for reasons of quantum mechanics that we need not go into).

 

Normally, an oxygen atom encountering another oxygen atom will bond to it, with a double bond: O=O, or O2. Each bond involves 2 electrons, allowing a participating atom to share one of its electrons with one from the other atom. By forming 2 bonds each atom now has a share in 8. The energy of this combination is lower than that of the isolated atom, which is what we mean when we say that the resulting molecule is "more stable" than single atoms.  

 

(There is also another way for oxygen atoms to combine, into threes to produce ozone. This looks like this: O=O->O, or O3, where the arrow shows that a single bond to the 3rd atom can be made by sharing 2 electrons, but both this time coming from the central atom. This is called a "dative" bond as it involves one atom "giving" both of the electrons needed for the bond. Ozone is not as stable as diatomic oxygen however.)

 

Both O2 and O3 are gases at room temperature and pressure. Both can dissolve to some degree in water, which is how fish can use it for respiration. 

 

Both O2 and O3 will condense to a liquid state if cooled sufficiently. Normal diatomic oxygen, O2, condenses at -183C, forming a blue liquid. (This is something I used to be terrified of seeing when I was doing Part II of my chemistry degree, as I worked with a vacuum line involving a liquid nitrogen trap. You should never pass gas containing oxygen through such a trap for fear of getting liquid O2, as it is highly reactive and can easily blow up your apparatus if it encounters anything it can oxidise!)

 

If you cool oxygen further, the blue liquid will freeze, at - 219C. So oxygen can exist in all 3 standard phases : solid, liquid or gas, depending on the conditions. 

 

When elements combine to form molecules, it is the behaviour of the molecules that determines whether the substance is solid, liquid or gas. The oxygen in water molecules (H-O-H) is not a gas, it has become an inextricable part of water molecules. Water can be a solid, liquid or gas, depending on how much energy its molecules have got, in relation to the intermolecular forces (weaker than full chemical bonds but still significant) that tend to hold the molecules together.

 

In a solid, the molecules are held by these intermolecular forced in a fixed arrangement. When they are given more energy by heating they cannot move from one place to another or rotate, just vibrate a bit....until the melting point is reached. At that stage most have enough energy to break free of the forces holding them in position and can rotate and move, but they cannot escape the collective attraction of their neighbours. A few however have enough energy to escape their neighbours and fly freely into the space above the liquid. These are in the vapour or gas phase. As the liquid is heated towards its boiling point a greater and greater proportion of the molecules have enough energy to break free. We say the "vapour pressure" goes up.

 

At the boiling point the vapour pressure become equal to that of the atmosphere the molecules are escaping into. This allows bubbles of gas to develop in the body of the liquid...which is what boiling is. 

Edited March 19, 2018 by exchemist

[hazelm]

Oh - last things first.  I got Mr Gray's comment wrong.  I should have typed -183 C.  Very important.  Quite a difference.  Thank you. 

 

The rest will take longer and I appreciate your explanation.  Just need to finish it and think. 

[hazelm]

OK Hazel, first of all oxygen atoms are, by themselves, highly reactive things, as they want to form bonds to something. This is because they have only 6 electrons in their outer shell and greater stability comes with 8 (for reasons of quantum mechanics that we need not go into).

 

Normally, an oxygen atom encountering another oxygen atom will bond to it, with a double bond: O=O, or O2. Each bond involves 2 electrons, allowing a participating atom to share one of its electrons with one from the other atom. By forming 2 bonds each atom now has a share in 8. The energy of this combination is lower than that of the isolated atom, which is what we mean when we say that the resulting molecule is "more stable" than single atoms.  

 

(There is also another way for oxygen atoms to combine, into threes to produce ozone. This looks like this: O=O->O, or O3, where the arrow shows that a single bond to the 3rd atom can be made by sharing 2 electrons, but both this time coming from the central atom. This is called a "dative" bond as it involves one atom "giving" both of the electrons needed for the bond. Ozone is not as stable as diatomic oxygen however.)

 

Both O2 and O3 are gases at room temperature and pressure. Both can dissolve to some degree in water, which is how fish can use it for respiration. 

 

Both O2 and O3 will condense to a liquid state if cooled sufficiently. Normal diatomic oxygen, O2, condenses at -183C, forming a blue liquid. (This is something I used to be terrified of seeing when I was doing Part II of my chemistry degree, as I worked with a vacuum line involving a liquid nitrogen trap. You should never pass gas containing oxygen through such a trap for fear of getting liquid O2, as it is highly reactive and can easily blow up your apparatus if it encounters anything it can oxidise!)

 

If you cool oxygen further, the blue liquid will freeze, at - 219C. So oxygen can exist in all 3 standard phases : solid, liquid or gas, depending on the conditions. 

 

When elements combine to form molecules, it is the behaviour of the molecules that determines whether the substance is solid, liquid or gas. The oxygen in water molecules (H-O-H) is not a gas, it has become an inextricable part of water molecules. Water can be a solid, liquid or gas, depending on how much energy its molecules have got, in relation to the intermolecular forces (weaker than full chemical bonds but still significant) that tend to hold the molecules together.

 

In a solid, the molecules are held by these intermolecular forced in a fixed arrangement. When they are given more energy by heating they cannot move from one place to another or rotate, just vibrate a bit....until the melting point is reached. At that stage most have enough energy to break free of the forces holding them in position and can rotate and move, but they cannot escape the collective attraction of their neighbours. A few however have enough energy to escape their neighbours and fly freely into the space above the liquid. These are in the vapour or gas phase. As the liquid is heated towards its boiling point a greater and greater proportion of the molecules have enough energy to break free. We say the "vapour pressure" goes up.

 

At the boiling point the vapour pressure become equal to that of the atmosphere the molecules are escaping into. This allows bubbles of gas to develop in the body of the liquid...which is what boiling is. 

Aha!  When you got to the last part  - about the three stages of water, it all came clear.  We learned it as "depending on temperature"  but temperature is what controls what happens to the water.   And that explains Mr. Gray's comment which I quoted.

Sometimes we forget the simplest things, don't we?  Or, we fail to spread the wings far enough to take in other things.

 

Again, thank you.  I am back on track now. 

[exchemist]

Aha!  When you got to the last part  - about the three stages of water, it all came clear.  We learned it as "depending on temperature"  but temperature is what controls what happens to the water.   And that explains Mr. Gray's comment which I quoted.

Sometimes we forget the simplest things, don't we?  Or, we fail to spread the wings far enough to take in other things.

 

Again, thank you.  I am back on track now. 

Good, glad to be of help.

 

By the way, before any physical scientists reading this thread object, I should say my description of both O2 and O3 is a bit simplistic. To a chemist, the more interesting part is why O2 has unpaired electrons - something that is not accounted for my explanation in terms of 2 electron bonds - and why ozone has a symmetrical structure, in spite of the bonding arrangement I showed suggesting a single bond on one side and and a double one on the other.  You need to use more a sophisticated model of chemical bonding to account for these findings (what is called Molecular Orbital theory). 

 

Which illustrates a point about the philosophy of science that I never tire of making. In science we make explanatory and predictive models of the physical world. We quite often use different models of the same thing, depending on what we are trying to do. We do not pretend that these models are exact depictions of reality and we use each one conscious of its limitations. I have found that some non-scientists have difficulty with this modus operandi, expecting science to offer some form of ultimate truth. Science cannot do that.   

[DaveC426913]

I have found that some non-scientists have difficulty with this modus operandi, expecting science to offer some form of ultimate truth. Science cannot do that.   

It can provide a truth, it's just that, like most sciency things, it doesn't fit in a sound bite or a caption to a pretty picture.

There's no substitute for actually learning to science.

 

Something that tends to escape most of (that other) SciFo's members...

Edited May 18, 2018 by DaveC426913