Reason for nature of surface tension According to a high school textbook, stretching of bonds near the surface of the fluid causes a force acting on neighbouring molecules of fluid between themselves like small small 'springs' which try to reduce the surface area of the fluid, and this force is called surface tension.
My doubt:
What is really meant by "stretching between bonds near the surface..." and how does this stretching occur just at the surface of the fluid and not anywhere else such as in the bulk of the fluid and what phenomenon causes this "stretching"?
Please explain in simple non highly mathematical terms.
 A: The explanation you describe, based on how the bonds between molecules rearrange at the surface, is very common but it's a poor explanation of the physics involved. The interface between, for example, water and air is complex and dynamic with the water molecules continuously rearranging themselves at the surface. Trying to explain the behaviour using simple ides of bonds between neighbouring molecules is not very helpful.
Instead surface tension can be simply explained as a surface energy. That is there is an energy energy of $72$ milli-Joules per square metre associated with an air-water interface. Note that this number is the same as the surface tension of $72$ milli-Newtons per metre, and as we'll see in a moment this is not a coincidence.
Why a surface energy exists is not hard to explain. Water molecules form quite strong hydrogen bonds with neighbouring water molecules. If you imaging taking some quantity of water and pulling it into two parts then you need to put in work to pull the hydrogen bonds apart, and this work goes into the surface energy.

The work done to pull apart the water goes into the surface energy associated with the newly created (shown in red) air-water surface.
To see how the surface energy is related to the surface tension consider this experiment. Imagine we have a water film trapped inside a U shaped wire, and we have a rod we can slide along the U:

If the surface tension of the film is $\gamma$, and the length of the sliding rod is $\ell$, then the force on the rod is:
$$ F = \gamma \ell $$
Now pull the rod a distance $dx$ and the work we do is force times distance i.e.:
$$ dW = \gamma\ell dx $$
By moving the rod we have created a new area:
$$ dA = \ell dx $$
So the work per unit area is:
$$ \frac{dW}{dA} = \frac{\gamma\ell dx}{\ell dx} = \gamma $$
If we associate the work per unit area with the surface energy of the newly created surface we find the surface energy is the same as the surface tension.
The point of this is that although the way the molecules are arranged at the surface is complicated, and will be different for every different type of liquid, the surface energy is a nice simple concept. We don't need to worry about how the molecules are arranged. We know that creating new surface always requires an energy and this is why it takes work, i.e. an applied force, to create the new surface.
A: I don't think bond stretching is actually the physical explanation. Water is a polar molecule, as a result of non-uniform distribution of electron charge within the water molecule, which is due to the angle between the two hydrogen atoms of the water molecule. One side of the water molecule is "more negative" than the opposite side. This means there is electrostatic attraction between the "more negative" side of one water molecule to the "less negative" side of another water molecule, as a result of the nonuniform distribution of electronic charge.
Surface tension is merely the consequence of this electrostatic attraction of water molecules for each other. Every water molecule in the body of the liquid is surrounded by other water molecules, so the electronic attraction is omnidirectional, which means the net attraction is zero.
Water molecules on the surface, however, only experience attraction to other water molecules within the body of the liquid, so the attraction is in the direction of the body of the liquid, and the net attraction is non-zero: we experience this as surface tension.
A: The molecules in a liquid surface experience attractive forces to each other within the plane of the surface. This is true regardless of the chemical nature of the liquid. The attractive forces are secondary bonding forces, not primary (chemical) bonding forces. They include ion-dipole, dipole-dipole, and hydrogen bonds. For dipole bonds, sub-categories include permanent dipoles and fluctuating dipoles. Most are non-directional ... the same in any direction. The one significant exception is hydrogen bonding, where some orientation is required for the bond to be effective. In water, this is along an O-H axis in one water molecule pointing toward the lone pair of electrons on the oxygen in the opposing H-O-H molecule.
In the simplest of picture for surface tension, orientation effects are ignored as being averaged out over the entire surface plane.
In a physical picture, as you pull along a zero-thickness but finite-length line at the surface, you stretch the bonds between the molecules in that surface plane. The force that you need in the surface plane to pull the surface molecules apart is the surface tension. Think of a rubber band. Pull on the rubber band. Imagine making the rubber band thinner and thinner until you have it as only one layer thick. You still need a force to pull on the molecules in that "infinitesimally thin" rubber band.
The attractive forces between liquid molecules are the same throughout the entire bulk liquid. In the bulk system, we would translate surface tension into an equivalent as the isotropic pressure needed to expand a finite-sized, zero-surface (spherical) volume of the liquid. In isotropic expansion of a container volume, we act to stretch all the secondary bonds between the liquid molecules uniformly in all directions. The force required is related to a measure of the average secondary bond force constant between the molecules.
How does the stretching occur just at the surface and not within the bulk? This is the most insightful question. The picture of surface tension imagines a plane with essentially no thickness. Any work (stretching) forces we apply in that hypothetical plane are not transmitted to the bulk below it. In reality, the unique feature of a liquid compared to a solid is that liquid molecules will renew themselves in the surface from the bulk when we have stretched the real surface plane too far.
So in summary, to picture surface tension in liquids requires you to think only about doing an "infinitesimal stretch" of a "zero thickness" surface plane. But, in real liquids, pulling on the surface plane brings in real factors. The surface plane has a real thickness, the molecules in the surface plane may have different (preferred) orientations relative to the bulk, the surface plane is probably closer to the bulk on average than other equivalent planes in the bulk liquid, and the effects of pulling on the surface will propagate to the underlying (bulk) layers, specifically to a point where the liquid surface will at some point renew itself with new molecules.
