Wet Water?

[hazelm]

In a town where I used to live, each year the high school chemistry teacher started the year with one question for the class.  He told them that, at the end of the year he expected them to be able to answer one question.  Why is water wet? 

 

Assuming it is a valid question which it should be, why is water wet?  Does someone have a valid answer?  Thank you.

[exchemist]

In a town where I used to live, each year the high school chemistry teacher started the year with one question for the class.  He told them that, at the end of the year he expected them to be able to answer one question.  Why is water wet? 

 

Assuming it is a valid question which it should be, why is water wet?  Does someone have a valid answer?  Thank you.

Golly! This is so open-ended it sounds like an Oxford Entrance exam question. (I recall one which asked:"How could you tell that a Martian was playing?")

 

What is meant by "wet"?

 

First I suppose, that the substance is a liquid, rather than a solid or gas, at room temperature.

 

Second, that it creates the sensation of "wetness", by which I suppose we mean several things:

- a high surface tension that makes clothes cling to the skin,

- a coldness, due to the heat abstracted by evaporation, which in turn means:

- a significant rate of evaporation at room temperature and

- a high enough latent heat of vaporisation to make the evaporation cause a significant cooling effect.

 

There is in fact a rather glib, two word reason for all the above, which is "hydrogen bonding".

 

But now, I suppose, one has to explain why it is that hydrogen bonding does all this and, hardest of all, what exactly hydrogen bonding is.

 

But I'm not getting into all that unless there is an audience for it!    

[hazelm]

Golly! This is so open-ended it sounds like an Oxford Entrance exam question. (I recall one which asked:"How could you tell that a Martian was playing?")

 

What is meant by "wet"?

 

First I suppose, that the substance is a liquid, rather than a solid or gas, at room temperature.

 

Second, that it creates the sensation of "wetness", by which I suppose we mean several things:

- a high surface tension that makes clothes cling to the skin,

- a coldness, due to the heat abstracted by evaporation, which in turn means:

- a significant rate of evaporation at room temperature and

- a high enough latent heat of vaporisation to make the evaporation cause a significant cooling effect.

 

There is in fact a rather glib, two word reason for all the above, which is "hydrogen bonding".

 

But now, I suppose, one has to explain why it is that hydrogen bonding does all this and, hardest of all, what exactly hydrogen bonding is.

 

But I'm not getting into all that unless there is an audience for it!    

What you call hydrogen bonding is really what I was guessing.  I did not know it as "hydrogen bonding", of course, but was describing that in lay terms.  Then, much later, I thought about how it can cling to our warm skin.   

 

As for the "open end",  maybe that is so they'll really write a good paper about it.  A good paper would be talking about the interpretations that you list.  A  short "because it bonds" or because it clings" would not get the student much of a grade.

 

I'm just talking.  Do I have it right even if not technical?  Thank you.

[OceanBreeze]

Golly! This is so open-ended it sounds like an Oxford Entrance exam question. (I recall one which asked:"How could you tell that a Martian was playing?")

 

 

 

Whoa! I'm fairly sure I know why water is wet; but I have no idea how to tell that a Martian was playing. Now, that is information that might come in handy some day soon. Please do tell.

[exchemist]

Whoa! I'm fairly sure I know why water is wet; but I have no idea how to tell that a Martian was playing. Now, that is information that might come in handy some day soon. Please do tell.

Oh that was in a practice paper, not the one I sat. What they were after, I imagine, was an exploration of how "play" could be defined and thus recognised, in the absence of any points of cultural contact.  Rather a tricky one, eh? But thought-provoking.      

[exchemist]

What you call hydrogen bonding is really what I was guessing.  I did not know it as "hydrogen bonding", of course, but was describing that in lay terms.  Then, much later, I thought about how it can cling to our warm skin.   

 

As for the "open end",  maybe that is so they'll really write a good paper about it.  A good paper would be talking about the interpretations that you list.  A  short "because it bonds" or because it clings" would not get the student much of a grade.

 

I'm just talking.  Do I have it right even if not technical?  Thank you.

Well yes, to the extent that "wet" would first need to be defined. As for hydrogen bonding, in a way it is easy for a chemist to put that forward for almost anything to do with water, as it explains so much about its properties.

 

In essence, because of H-bonds, the forces between water molecules are much stronger (of the order of 20x) than intermolecular forces typically are. Some of the results are that:

- water molecules stick together, giving water a very high boiling point for such a small molecule. For comparison CO2, whose molecules have more than twice the mass, "boils" (sublimes) at -80C;

- it has a very high Latent Heat of Vaporisation - the amount of heat absorbed when a unit mass of water evaporates - because it take a lot of energy to break apart the H-bonds and allow the individual molecules to fly away from each other as molecules do in the gas phase;

- water molecules will stick not only to each other but to anything with oxygen atoms on the surface, e.g, glass, human skin and most natural fabrics, "wetting" them. Modern waterproof coatings are designed not have any "polar" molecules or polarising atoms in them, in order not to provide anything for the H-bonds to grab hold of.

 

As to what H bonds really are, this is still not fully modelled, I think. It is something to do with:

(a) the partial charge separation in O-H, whereby oxygen has a partial -ve charge and H a partial +ve charge, creating a "dipole that can attract other dipoles by electrostatic attraction,

(b )   the two "lone pairs" of non-bonding electrons on oxygen, which point in two specific directions, causing H-bonds to form along two axes (which would not be true if it were just an electrostatic interaction between dipoles) and

(c ) the small charge on the hydrogen nucleus (just +1), which seems to allow some sort of weak multicentre bond to be set up along an O-H....O axis.

 

But I am out of date on this now. H bonds are really very interesting to the chemist who is interested in unconventional bonding, i.e.the sort that does not fit the simple "ball and spring" model of high school chemistry.   :)      

Edited August 20, 2018 by exchemist

[hazelm]

Fascinating.  Thank you.   If I may use a world lightly, water drops are "magnetic".  Put two drops of water very close to each other and they will fall together and bond.  They have to be close, of course, and I imagine on certain surfaces.

 

You said "Modern waterproof coatings are designed not have any "polar" molecules or polarising atoms in them, in order not to provide anything for the H-bonds to grab hold of."

 

A couple of years ago, I went shopping to find a new raincoat. I wanted a waterproof coat.  water proof!  After several shops i had learned:  Men's raincoats are water proof.  Women's raincoats are water resistant.  Hmmm?  Not hard to analyze that situation. 

Edited August 20, 2018 by hazelm

[exchemist]

Fascinating.  Thank you.   If I may use a world lightly, water drops are "magnetic".  Put two drops of water very close to each other and they will fall together and bond.  They have to be close, of course, and I imagine on certain surfaces.

 

You said "Modern waterproof coatings are designed not have any "polar" molecules or polarising atoms in them, in order not to provide anything for the H-bonds to grab hold of."

 

A couple of years ago, I went shopping to find a new raincoat. I wanted a waterproof coat.  water proof!  After several shops i had learned:  Men's raincoats are water proof.  Women's raincoats are water resistant.  Hmmm?  Not hard to analyze that situation. 

Well, don't expect me to endorse the use of the term "magnetic", as it is electrostatic attraction that lies behind it, rather than magnetism. But yes water has a strong affinity for itself and also many types of surface, if there are atoms in the surface which can take part in forming hydrogen bonds, usually oxygen.

[hazelm]

Well, don't expect me to endorse the use of the term "magnetic", as it is electrostatic attraction that lies behind it, rather than magnetism. But yes water has a strong affinity for itself and also many types of surface, if there are atoms in the surface which can take part in forming hydrogen bonds, usually oxygen.

Not at all.  As I said, I used it lightly.  Why?  Because I did not know it was "electrostatic attraction".  :-)  Thank you for that.

[OceanBreeze]

Well yes, to the extent that "wet" would first need to be defined. As for hydrogen bonding, in a way it is easy for a chemist to put that forward for almost anything to do with water, as it explains so much about its properties.

 

In essence, because of H-bonds, the forces between water molecules are much stronger (of the order of 20x) than intermolecular forces typically are. Some of the results are that:

- water molecules stick together, giving water a very high boiling point for such a small molecule. For comparison CO2, whose molecules have more than twice the mass, "boils" (sublimes) at -80C;

- it has a very high Latent Heat of Vaporisation - the amount of heat absorbed when a unit mass of water evaporates - because it take a lot of energy to break apart the H-bonds and allow the individual molecules to fly away from each other as molecules do in the gas phase;

- water molecules will stick not only to each other but to anything with oxygen atoms on the surface, e.g, glass, human skin and most natural fabrics, "wetting" them. Modern waterproof coatings are designed not have any "polar" molecules or polarising atoms in them, in order not to provide anything for the H-bonds to grab hold of.

 

As to what H bonds really are, this is still not fully modelled, I think. It is something to do with:

(a) the partial charge separation in O-H, whereby oxygen has a partial -ve charge and H a partial +ve charge, creating a "dipole that can attract other dipoles by electrostatic attraction,

(b )   the two "lone pairs" of non-bonding electrons on oxygen, which point in two specific directions, causing H-bonds to form along two axes (which would not be true if it were just an electrostatic interaction between dipoles) and

(c ) the small charge on the hydrogen nucleus (just +1), which seems to allow some sort of weak multicentre bond to be set up along an O-H....O axis.

 

But I am out of date on this now. H bonds are really very interesting to the chemist who is interested in unconventional bonding, i.e.the sort that does not fit the simple "ball and spring" model of high school chemistry.   :)      

 

 

I have always been interested in the properties of the stuff that I spend half my life floating around on top, and sometimes under.

 

I find a picture to be very helpful in understanding hydrogen bonding, from this link:

 

 

"A molecule of water has two hydrogen atoms. Both of these atoms can form a hydrogen bond with oxygen atoms of different water molecules. Every water molecule can be hydrogen bonded with up to three other water molecules"

 

A picture also helps to understand wettability, from Wiki:

 

 

 

"The contact angle (θ), as seen in Figure 1, is the angle at which the liquid–vapor interface meets the solid-liquid interface. The contact angle is determined by the balance between adhesive and cohesive forces. As the tendency of a drop to spread out over a flat, solid surface increases, the contact angle decreases. Thus, the contact angle provides an inverse measure of wettability"

[exchemist]

I have always been interested in the properties of the stuff that I spend half my life floating around on top, and sometimes under.

 

I find a picture to be very helpful in understanding hydrogen bonding, from this link:

 

 

"A molecule of water has two hydrogen atoms. Both of these atoms can form a hydrogen bond with oxygen atoms of different water molecules. Every water molecule can be hydrogen bonded with up to three other water molecules"

 

A picture also helps to understand wettability, from Wiki:

 

 

 

"The contact angle (θ), as seen in Figure 1, is the angle at which the liquid–vapor interface meets the solid-liquid interface. The contact angle is determined by the balance between adhesive and cohesive forces. As the tendency of a drop to spread out over a flat, solid surface increases, the contact angle decreases. Thus, the contact angle provides an inverse measure of wettability"

Interesting, thanks for this. I looked at your first link and there are a couple of points I would take issue with. 

 

Firstly, it implies that the H bond is just an electrostatic phenomenon, arising from the interaction of dipoles.  This is a simplification and not strictly true. The H bond has directional character, due to involvement of the "lone pairs" of electrons on the electronegative atom (Oxygen in the case of water).

 

Secondly, it states, wrongly, that a water molecule can form H bonds with up to 3 neighbours. In fact it is up to 4. There are 2 lone pairs on each oxygen, to which neighbouring H atoms can bind, and two H atoms which can bind to 2 neighbouring oxygen atoms. This in fact takes place in the structure of ice. (The directional character of H bonds explains why water explains on freezing: there is more energy released by forming this relatively open network of H bonds than there would be by simply packing the molecules as closely together as possible.)   

[hazelm]

Thank you both but I am puzzled about this:  "Both of these atoms can form a hydrogen bond with oxygen atoms of different water molecules."  How do they bond?  What puts and holds them together.  Electrostatic attraction?

 

Be back later to learn all about it.  Thanks.

[OceanBreeze]

Interesting, thanks for this. I looked at your first link and there are a couple of points I would take issue with. 

 

Secondly, it states, wrongly, that a water molecule can form H bonds with up to 3 neighbours. In fact it is up to 4. There are 2 lone pairs on each oxygen, to which neighbouring H atoms can bind, and two H atoms which can bind to 2 neighbouring oxygen atoms. This in fact takes place in the structure of ice. (The directional character of H bonds explains why water explains on freezing: there is more energy released by forming this relatively open network of H bonds than there would be by simply packing the molecules as closely together as possible.)   

 

Yes, you are right. But I have to admit to not quoting the full paragraph. The next sentence is :

“However, because hydrogen bonds are weaker than covalent bonds, in liquid water they form, break, and reform easily. Thus, the exact number of hydrogen bonds formed per molecule varies”

So, it seems they were only talking about liquid water, not ice.

 

Here is a graph that shows the number of hydrogen bonds around each water molecule versus temperature:

 

 

 

 

It appears that the tetrahedral arrangement is only stable at around 250 Kelvin, well below freezing.

 

That same source even illustrates five-coordinated hydrogen-bonded water:

 

 

But goes on to say that “no stable water cluster (for example within a crystal structure) has been found with the central water molecule 5-coordinated by hydrogen bonding to five water molecules”

 

Interesting!

[exchemist]

Yes, you are right. But I have to admit to not quoting the full paragraph. The next sentence is :

“However, because hydrogen bonds are weaker than covalent bonds, in liquid water they form, break, and reform easily. Thus, the exact number of hydrogen bonds formed per molecule varies”

So, it seems they were only talking about liquid water, not ice.

 

Here is a graph that shows the number of hydrogen bonds around each water molecule versus temperature:

 

 

 

 

It appears that the tetrahedral arrangement is only stable at around 250 Kelvin, well below freezing.

 

That same source even illustrates five-coordinated hydrogen-bonded water:

 

 

But goes on to say that “no stable water cluster (for example within a crystal structure) has been found with the central water molecule 5-coordinated by hydrogen bonding to five water molecules”

 

Interesting!

The way I read this is that 5-coordination involving H-bonding requires a bifurcated H-Bond to two of the molecules, I.e. H joined to not 2 but 3 O atoms, which is a lot weaker than a normal H-bond. All these bonds are examples of multi-centre bonding, i.e. not like the normal classical covalent bonding model.

 

Having read a few of these web articles, it is interesting that an exact description of H-bonding is still open to debate and further research, in spite of its ubiquity and importance in areas such as biochemistry.  

 

Multicentre bonding is quite cool - a reminder that the MO method of treating bonding is really the way to do it, though normally we don't bother. Boron is a great exponent of it........   

[exchemist]

Thank you both but I am puzzled about this:  "Both of these atoms can form a hydrogen bond with oxygen atoms of different water molecules."  How do they bond?  What puts and holds them together.  Electrostatic attraction?

 

Be back later to learn all about it.  Thanks.

It is a mixture of (i) electrostatic attraction, due to the polarisation of the normal O-H bond (it is slightly -ve at the O end and slightly +ve at the H end), and (ii) a covalent bond, involving one of the nominally non-bonding "lone pairs" of electrons on the oxygen atom.

 

So you get some of this:

 

O(δ-) - H(δ+)  ->  <-  O(δ-) - H(δ+) where the δ+ and δ- attract each other electrostatically, which I have shown by the arrows showing an attractive force

 

AND

 

some of this: 

 

 

O-H.........O-H, where "......."  signifies that a pair of electrons on oxygen are slightly shared with the H at the other end of the dotted line, making a weak covalent bond. 

[hazelm]

It is a mixture of (i) electrostatic attraction, due to the polarisation of the normal O-H bond (it is slightly -ve at the O end and slightly +ve at the H end), and (ii) a covalent bond, involving one of the nominally non-bonding "lone pairs" of electrons on the oxygen atom.

 

So you get some of this:

 

O(δ-) - H(δ+)  ->  <-  O(δ-) - H(δ+) where the δ+ and δ- attract each other electrostatically, which I have shown by the arrows showing an attractive force

 

AND

 

some of this: 

 

 

O-H.........O-H, where "......."  signifies that a pair of electrons on oxygen are slightly shared with the H at the other end of the dotted line, making a weak covalent bond. 

Thanks much.