# Cohesion of water

1. Jun 10, 2005

### pete worthington

Anyone know the value for the cohesive force of water (STP) ? I am having difficulty finding this info even thgough I have made a solid attempt to look for these value(s). I assume the units would be in force/area.

2. Jun 16, 2005

### redwraith94

I would imagine that it is one of those values that is also dependant on whatever material that the water is adhering to. But I am thinking that you mean adhesive force?

Cohesive I would imagine could be extrapolated somehow from boiling point, and heat capacity...or something?

3. Jun 16, 2005

### Astronuc

Staff Emeritus
4. Jun 17, 2005

### Gokul43201

Staff Emeritus
Cohesion in water comes from hydrogen bonding. The bond energy of H-bonds in water is roughly 5 KCal/mol. Someone might know this number more accurately.

$$5KCal/mol \approx 21KJ/18cc = 21kN \cdot m/18*10^{-6} m^3 \approx 10^9 N/m^2$$

5. Jun 18, 2005

### pete worthington

The cohesion value was higher than I expected, perhaps that is due to me asking the wrong question:

The question: I have two acrylic plates that are 4 1/2 inches in diameter and 3/4 inch thick. They have handles on them. By placing a small quantity of water between the plates and pressing/turning them together you can drive most of the air bubbles out. Then you try to pull the plates apart. This is similar to magdeburg plates where you draw a vacuum between the plates or spheres. The question I have is whether the plates finally seperate due to cohesion forces of the water or adhesion forces between the acrylic and water. I've been told that the cohesion force would be smaller. It doesn't appear that surface tension is a factor here since the water is sandwiched between the two plates. Any ideas here on identifying this force and how I can calculate the theoretic value for the pull-out force?

6. Jun 19, 2005

### Gokul43201

Staff Emeritus
This may be true, but it can not be a blanket statement - this WILL depend on that the material of the plates is.

The number I calculated above is approximately the mean value of the intermolecular attraction. However, to achieve "cleavage", one need only break the weakest bonds (remember, the molecules are in constant motion, so there will be local thermal fluctuations which change the H-bond strength locally, on a continuous basis). If the H-bond strengths follow a Gaussian distribution, there will always be sufficient number of weak enough bonds that cleavage can be achieved at much lower stresses than the average value.

How does one determine this number ? I haven't a clue, but I'm guessing the pull out force will tell you what it is, if the premise is correct.

7. Jun 20, 2005

### redwraith94

If the apparatus is completely sealed w/ no air in it, then I would think that simply the area of the plates divided by whatever your local atmospheric pressure is (standard @ sea level is 14.69 psi, 101.325 kp)...unless I am not understanding you setup correctly.

8. Jun 23, 2005

### pete worthington

You are understanding this correctly redwraith94. This topic was being discussed recently in General or Classical Physics under "book on table".

The question was whether atmospheric pressure is acting uinder a book. I extended the question to whether it would be acting along the water seam between these two plates.

In class we set up these plates with some rigging and were able to lift almost 150 pounds. This was roughly 80 % of the force due to atmosheric pressure acting over the 4.5 inch diameter plates. We noticed that as the loading increased a vacuum "bubble was forming at the center of the plates, it grew to 1.5 inch diameter.

The question I then posed was whether the water was acting as a seal much like an o-ring paced between the plates and drawing a vacuum (vandeburg plates). I hypothesized that there was a seal with a pressure gradient formed as you load the plates. Anotherwards, with zero load atmosheric pressure may act along the water seal between the plates, but as you load the plates a vacuum is formed.

My plan is to make plates that do not deform. Is it the deformity (bending) that creates the vacuum? They make these plates out of polished metal also. Science Kit sells these (www.sciencekit.com) item #47416-00 is called an economy cohesion disc (I have this one) and item # 46523-00 is called Forces of Cohesion Demo and is made of metal plates.

The later desribes the physics as "direct condensed phase molecular contact".

I am not seeing how surface tension is at work here. Vanderwalls forces- hydrogen bonding make more sense. I am trying to get a feeling of whether it is atmospheric pressure or cohesion forces or a combination of them. I sense that it is the later.

I am high school physics teacher with a mech engineering degree. The chem teacher at my high school has not been very helpful. Next fall I plan to make some plates (metal and lexan) and tap vacuum ports at diffrent radii so that we can experimentally determine the vacuum as a function of load at different positions.
Thanks for staying with this thread.
Pete W