i've just got some basic questions on water...

1.Why is the water molecule a polar molecule?
2.What's the hydrogen bonding between:
a) water molecules
b) ammonia molecules

3. what is dipole-dipole forces?

i'm just a bit confused right now... >__<
thanks if you can help! ^__^


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in all these years, I've never heard of a "pipole"!
(A pie pole? lol )

Water is a polar molecule because of
(a) the fact that oxygen is more "electronegative" than hydrogen (ie, it greedily pulls away electrons towards itself away from the hydrogens), thus creating a dipole along each O-H bond
(b) the shape of the water molecule (it's bent)

3. Should be called a dipole-dipole force.

yeah sorry. that was a typo there... it's supposed to be dipole-dipole forces... ^^;;

sorry about that...!

Ah, dipole forces.....

Well, first you need to understand electrostatic forces

There are two types of electric charge postive, and negative. (Don't ask why - there just are ;-)

We say that an electric charge is surrounded by an "electric field".
"Fields" are rather strange and magical things. If you put another electric charge inside a field, then that charge will experience a "force". (We don't mean Star Wars here.... by a "force", we simply mean a "push" or a "pull".)
So, if you place two LIKE electrical charges near each other (two +ve charges, or two -ve charges), they're both sitting inside each other's overlapping electric fields, and they both REPELL each other - magically experience a force PUSHing them apart. (How do they push each other, even though they're not touching? How do they "know" the other charge is there, to be able to "push" it? It's best not to ask questions like this, as you will only get dizzy.... -)

And, if you place two UNLIKE electric charges (+ve and -ve) near each other, they're both sitting inside each other's electric field and they both ATTRACT each other - magically experiencing a force PULLing them together. (Again, it's best not to wonder how the charges know that each other is there, and how they manage to pull each other without touching. I mean, if I sat down in the same room as you and pulled you across the room towards me simply by LOOKING at you, without even touching you, you'd be a bit surprised...... but hey, it's the crazy werld of electrics, they just use The Force and stuff.)

that's electrostatic attraction and repulsion.
If you understand that bit, we can go on to talk about dipoles.....

okay! ^___^ i get what you mean... it's sorta like magnets, that have positive and negative ends right? i get that! okay, now i feel confident that we can go on to pipole-pipole forces...! ^___^

Right okay already.....

Yes, the pole of a magnet also has a mysterious "field" around it,
where other electromagnetic things will experience a "force".
You can pick magnets up and play with them, it gives a good real world example of this kind of thing happening.

so, if you are comfortable with the concept that a point electric charge has a "field" around it, where other electric charges will experience a force (either attracted or repelled...)

Imagine then a situatation where there's not just one electric charge at a point in space,
but instead there's a "charge separation" - some positive charge over in one place, and a little distance away, there's a negative charge.
(For example, along a Oxygen - Hydrogen bond in a water molecule....
the O is electronegative, and so greedily grabs more of it's fair share of the electrons, leaving it "delta negative", and the H has the electrons pulled away from it, so is left "delta positive")
("delta" in this context just means "a little bit")

The oxygen is a little bit more negative by just the opposite amount that the hyrogen is a little bit more positive
(maybe imagine it like a kind of electric see-saw? the oxygen is tipped down negatively just as much as the hygrogen is tipped up positively)

ANYWAYS - this kind of arrangement, where you have delta positive in one place, and a corresponding delta negative at a fixed distance a little bit away in another place is called a "DIPOLE".

ANNNNND - dipoles have electric "fields" around them in just the same way as single electric charges at a point do.
So +ve or -ve electric charges, which come close to the Dipole, so that they are inside it's "field", will either be attracted or repelled.
*AND* other *dipoles* which come close to a dipole will also interact with *their* fields and be attracted or repelled.

(this is the "DIPOLE - DIPOLE" interaction you asked about)

Each water molecule has TWO dipoles in it! - one along each O-H bond.

So there will be 2 dipole fields around each H2O molecule,
and thus there will be dipole-dipole interactions between the water molecules all over the place -)

If you understand the principles behind that,
we could go on to look at equations which describe what directions and strengths the fields and forces from point charges and dipoles are....
....but frankly it's been years since I looked at them and I can't remember them anymore ;-)

Thankx! ^____^

thankx so much for that short lesson...! i now get dipole-dipole forces... but now i don't get the dissolution in water of:
* an ionic substance and
* a covalent substance...

please explain... ^^ thanks! ^^

if a solid dissolves in a liquid,
instead of just sitting there, insoluble,
it means the net energy is lower in solution,
rather than the separate solid and liquid.

Likewise, if two liquids are miscible,
rather than just sitting there, one floating in a separate layer on top of the other one,
it means the net energy is lower as a mixture.

Now, why would the energy be lower?
Well, you have to think about what the bonding and INTERmolecular interaction is like, first in the separate chemicals,
and then afterwards in the solution or mixture.

If the bonding is stronger and more extensive in the solution/mixture,
then that's generally gonna be lower energy, and thus the substances will dissolve/mix.

So, if specifically,
you want to look at what WATER will do with regards dissolves/mixing with with ionic or covalent substances,
you need to look at what types of intermolecular forces there could potentially be between the H2O molecules and the other substance.

Now, as I hope you know, although it's a neutral, uncharged, covalent species (thus can do weak Van der Waals intermolecular bonding), WATER is also a "polar" molecule, with two DIPOLES in it,
and it can also participate in HYRDROGEN BONDING

So - when you consider how water would mix with another substance,
ask could that other substance participate in HYDROGEN BONDING with the H2O...?
Or could it interact with the electric DIPOLES in H2O...?

Ahhhh... thankx! ^^

now that i understand that...
i have another question, what would you classify chalk as? i know it has to be either a:

*insoluble molecular or
*covalent network structure or
*a substance with large molecules

thankx for all the help so far! :D

come now, I'm sure you can type "chemical composition" and "chalk" into Google and find out what it's made of... -)

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