Question

A) Name two intermolecular forces that exist between $HF$ molecules in liquid state?
B) Name the two phenomena that can be explained on the basis of surface tension?

Answer 1

Remember the electronegativity of fluorine. It is the highest electronegative atom of the periodic table. And understand the role of intermolecular forces in liquid state.
Surface tension leads to various phenomena. It is the property which is experienced by almost all liquids. And surface tension is the reason for many useful applications of liquids on a universal basis.

Complete step by step answer:
Let us first solve the A section;
A) Some of the intermolecular forces that exists in nature are
1. Dipole Dipole force
2. Hydrogen bonding force
3. Van Der Waals force
Let us dive deep into the above- mentioned intermolecular forces;

Dipole - Dipole forces are intermolecular forces that exist / arises due to attraction between partial positive charge of one molecule to partial negative charge of the other molecules.
It arises due to difference in electronegativity in atoms of the molecules. It can also be said that greater the electronegativity difference , more is attraction, in turn stronger are dipole - dipole forces.
Hydrogen bonding is a special case of dipole dipole interaction in which an attractive force is developed when a hydrogen atom is linked to another atom which is comparatively more electronegative than hydrogen atom. Since the electron density around electronegative atoms is maximum , hydrogen bonding is maximum.
Vander Waals forces are weak intermolecular forces which arise due to attractive or repulsive forces between atoms , molecules and compounds. The magnitude of the van der Waals force increases with increase in molecular size.

The intermolecular forces that exist between $HF$ molecules in liquid state are mainly dipole-dipole interaction and inter hydrogen bonding between hydrogen and fluorine.
Now, let us look at part B;
B) Here, let us first understand the concept of surface tension;
The free surface of a liquid behaves like a stretched membrane and tries to minimize surface area, and this property of liquid is called surface tension. It can also be defined as the amount of work done in increasing the free surface of liquid at rest by unity at constant temperature.
The phenomenon that are explained on the basis of surface tension are as follows :
a. Capillarity
b. Spherical shape of liquid drops
Let us learn about this in detail;
a. Capillarity
This is the phenomenon of fall or rises in liquid level due to action of capillary is capillarity.
It depends upon the nature of liquid, radius of the capillary tube and angle of contact.

b. Spherical shape of liquid drops is due to surface tension as a sphere has minimum surface area for a given volume in geometry.
In a similar way, raindrops are assumed to be spherical shapes.

Note:
Fluorine is the most electronegative element in the long form of periodic table with electronegativity $\,4\,$ while that of Hydrogen atom is $\,2.2\,$ . Also, there are two types of hydrogen bonding that take place. They are inter and intramolecular hydrogen bonding.
Surface tension is a molecular phenomenon and arises due to electromagnetic forces. Surface tension is a tensor quantity as it has no specific direction.