Expert: Profile Closed - 1/6/2010

Question
QUESTION: Hello again,
I have decided to change my experiment entirely if you remember from my previous question. My understanding is that capillary action is the result of adhesion of water's polarity on the solid glass tube. IF you could explain this a bit more in detail using the intermolecular forces such as hydrogen bonding and dipole attractions. My experiment will now be the speed of capillary action using the 3 compounds containing hydrogen bonds: H2O, NH3, and HF. Is an experiment that will produce results? Again, a quick response would be very much appreciated. Thanks mate.

ANSWER: Hi there, and thanks for your question.

Your explanation of capillary action is not quite correct, but involves all the right topics.

Capillary action is not due to the bonding of water to the glass tube by polar action; that would tend to slow down the flow of any liquid. In addition, capillary action takes place in a huge number of different environments (think of a towel soaking up water or of transpiration in a plant).

The reason that surface tension and the polarity of water are important are that they affect the behavior of the water itself. Water molecules, instead of flying around individually, are pulled together by hydrogen bonds. Hydrogen bonds are a form of dipole-dipole bond, and are particularly easy to form in water, because the water molecule is polar - the electonegative oxygen pulls electron density away from the hydrogen

Because of the intermolecular (Hydrogen) bonding, water molecules are pulled towards each other; you can see this when you pour a small amount of water on a surface and observe the water sticking together in a blob rather than spreading out to form the thinnest surface that would otherwise be possible.

This tendency of water to pull itself inwards results in the characteristic meniscus seen when water is put in a glass; the water surface is concave, lower in the center of the glass than it is on the outside. This is because there are more water molecules in the center, so a greater force is exerted on the surface water molecules by the hydrogen bonds from the rest of the solution. The water on the outside is in contact with fewer of the bulk water molecules, so is attracted less strongly. This means it is able to reach higher up the sides of the glass.

To summarize so far; water forms a concave surface because the surface molecules in the center are pulled down by the hydrogen bonds in the rest of the solution. The water at the sides is less affected because not so much of the bulk solution is in contact with the surface, so it remains at a higher level.

Because water tends to stick together, the situation where water has a concave surface is not stable - the bulk of the water moves upwards to try and level out the surface. As the water molecules at the side are hydrogen-bonded to those in the center, you find that as the sides move up, a force is generated that pulls the bulk of the liquid with it.

Of course, as soon as the bulk of the water approaches the level of the meniscus at the side, the meniscus moves higher, for the reasons described in the paragraph above the last one. So there is a continual process of the meniscus pulling water from the center upwards, and then water from the meniscus seeking a higher level. This continues until the force of gravity pulling downwards on the water is greater than the force generated by the movement of the meniscus, and at that point the level of water will stabilize in the tube.

This effect is normally only seen in narrow tubes, because in narrow tubes, there is a greater similarity between the number of water molecules at the side and the number on the center. As the tube gets wider, there are comparatively more and more water molecules at the center; the molecules at the side have a harder time pulling up the bulk layer to form the meniscus, so the level doesn't rise as high.

Hope that helps; Capillary action is a bit tricky to get your head around, so if I've missed anything or not explained anything well, let me know. In summary, the water's attraction to itself is what pulls the liquid up the tube, not an attraction between the water and the tube.

Your experiment would be theoretically interesting, but in practical terms is not feasible unless you are specially trained and in a uni lab. Remember that both ammonia and HF are gases at room temperature, so you would have to do the experiments with solutions of ammonia and HF in water - which is not ideal, because how will you know how much the surface tension of the water affects the behavior of the solution? Ammonia is a toxic substance, but HF is extremely dangerous - it etches glass, causes deep burns, and can poison you if absorbed through the skin - special burn kits and risk assessments are needed before HF or its solutions are used.

If you don't have the necessary kit, and easier method would be to use a sample of water and then samples of water with different amounts of washing-up liquid in. The effect of washing-up liquid is to lower the surface tension of the water, so you can easily create a number of solutions of varying surface tension.

Hope this helps; once again, follow-ups are welcome.



---------- FOLLOW-UP ----------

QUESTION: Thank you for the very in depth answer. I would just like to clarify the reason as to why water rises in the tube as i am not sure if i fully understand. Because the water stick to itself, as the concave tries to level out, it instead forces the water on the sides up higher creating the effect of capillary action. This must mean however that the forces holding the sides up are stronger than the effect of gravity right? Since if it wern't gravity would simply pull the sides down into the middle and equalize the level of water. The experiment won't be a problem i have all the necessary precautions put in place.

ANSWER: Hi, thanks for the follow-up;

The start of the capillary action is the natural curve in water, with the sides higher than the center (because of the surface tension).

The sides are higher than the center, but because the sides are attached to the center by hydrogen bonds, they exert a pulling force which pulls the bulk of the water further up the tube.

As this water reaches the higher level, a new meniscus forms - again, because of the natural curve of the water owing to hydrogen bonds.

This new meniscus pulls the water towards it, as before, and the process repeats itself.

It goes on repeating until the force of gravity pulling the water down is equal to the force of surface tension holding the water up. At this point, the water in the tube stops moving and a point of balance is reached.

Up to that point, the surface tension is stronger than the effect of gravity, which is why the water is able to move up. Beyond that point, gravity is the stronger force, so the water does not move any further.

Hope that helps, but again, follow-ups are welcome.

I don't want to interfere with your work, and so I offer the advice re. safety in the spirit of sharing information, but most institutions would only consider allowing the use of HF where it is absolutely necessary, and in general, surface tension experiments would not come in to this class. The toxic and corrosive effects of HF are exceptional and so its use is only justified where there is absolutely no other possible course of action. Anyway, whatever practical work you choose to do, I hope it goes well and that you get some interesting results.

Best wishes, George

---------- FOLLOW-UP ----------

QUESTION: The start of the capillary action is the natural curve in water, with the sides higher than the center (because of the surface tension).

This is still confusing me. I am sorry to keep troubling you with this but i still don't seem to fully understand. What exactly is surface tension? And is  the attraction between glass and water important at all in this process? What i don't understand is why the sides of the water are higher than the concave middle. And rethinking my experiment, would it be better to simply measure the height that each one reaches since different bond strengths will cause the solutions to reach different heights?

Answer
Hi there, no problem, as I said, this is a weird topic.

Firstly, the glass is not important at all in the process except as a container for the column of water; other materials work equally well.

The curve in the water was the bit I waffled on about in my long answer;

You're ok with the fact that water molecules all pull towards each other because of the hydrogen bonds? Well imagine a drop of water on a flat surface. The molecules in the center experience about an equal pull in all directions, because there are a roughly equal number of water molecules on all sides; therefore they don't move.

Now consider a similar molecule on the outside edge of the drop. There are lots of central molecules pulling the outside molecule inwards, and none on the other side to pull it outwards, so all the outer molecules are pulled inwards; this results in the formation of a blob of water, rather than the liquid spreading out in to a thin layer.

A similar logic explains how the meniscus forms when water is put in a container. This time, we'll look as two molecules on the surface of the water in, say, a wineglass. The water molecule at the center of the surface experiences a pull towards the bulk of the water molecules in the glass, and so dips downwards. The side molecule does experience a slight attraction, but because the side molecule is further from the bulk of the water, the attraction is less. This means there is a stronger downwards force on molecules on the center of the surface that there is on those at the sides, so the center of the surface is pulled downwards, leaving the outer part of the surface at a higher level.

To understand the fact that central surface molecules have greater contact with the bulk of the water than the ones at the side, consider a swimming pool. You can sit at the side and just dip your legs in, but if you are in the center of the pool, you're surrounded by water on all sides, and you become part of the bulk of the water rather than a little bit at the edge.

Once the meniscus has formed, the action I described before of a continual rise of the bulk, pushing the meniscus higher, in turn causing the bulk to be pulled up again etc etc, explains the rise of liquid in a capillary tube.

Re.. the experiment; if I was studying this topic, I would choose, say, pure water, saltwater ethanol and vegetable oil as being good examples of liquids with hugely different bonding, which would illustrate nicely the role that intermolecular bonds play in capillary action.

Hope this helps, but again, follow-ups are welcome.