Liquid state, viscosity and surface tension

Introduction

Liquid is a state of matter having a wide variety of physical and chemical properties. Both the liquids and gases are called fluids because of their common feature- their ability to flow. However, at the molecular level, a liquid is different from that of a gas. Liquids exhibit different properties and behaviours based on their forces of attraction and physical properties. This post is designed to understand the properties of liquid such as viscosity and surface tension.

1. Introduction

2. Properties of liquid state

   2.1 Strong Intermolecular forces

   2.2 Definite Volume and density

   2.3 Free-flowing and shapeless

3. The polarity of a water molecule

4. Surface tension

5. Viscosity

   5.1  Mechanism of viscosity

   5.2  Coefficient of Viscosity

   5.3 Effect of Physical Changes in Viscosity

 

Properties of liquid state

  1. Strong Intermolecular forces: Liquids exhibit strong intermolecular force than the gases but they are weaker than the solids.  The strong force of interaction between the liquid molecules is due to the lesser space shared between the molecules.

  2. Definite Volume and density: Liquids, unlike gases, occupy a limited space and they have a definite volume because of the reason that they have low space between the molecules. Under normal physical conditions, the molecules of a liquid seldom separate from one another. Not only are liquids denser than gases but are also less compressible than them.

  3. Free-flowing and shapeless: Liquids take the shape of the container in which they are stored. Due to the free-flowing of molecules, they exhibit their ability to flow.

When we talk about a unique liquid, water comes in the first place. Water is the most important, primary and richly found liquid source on the planet than any other liquid. Some of the basic features of water are discussed here.

 

The polarity of a water molecule

Consider a water molecule -a unique liquid matter. Water is life, without which none of the organisms survive. Sources of water include; lakes, rivers, ponds, streams, oceans, and boreholes. Ice or solid water dominates the vast polar regions of the world. Water vapour from the evaporation of surface water and steam spouted out of geysers reach the atmosphere. In the water molecule, each O-H covalent bond is highly polar. Oxygen attracts the O-H covalent bond with a slightly negative charge because of its greater electro-negativity.  In the water molecule, the hydrogen atom is less electronegative resulting in a slightly positive charge. Water molecule has a bond angle of 105◦ ,  therefore the two O-H bond polarities won’t cancel.

The polar molecules are attracted to one another by dipole interactions. In water, however, the attraction is the result of intermolecular hydrogen bonds. The water molecules can change thier positions as they can slide over one another easily. Because of the hydrogen bonding, most water molecules do not have enough kinetic energy at room temperature. Many unique and essential properties of water -including its high surface tension, low vapor pressure, and high specific heat capacity result from its hydrogen bonding.

 

Surface tension

The surface of water behaves like skin. Its ability to form hydrogen bonds explain the surface tension of water. At the surface of the liquid, however, water molecules experience an uneven attraction. The water molecules contain a hydrogen bond one on each side. The particles are not attracted to the air because they cannot form hydrogen bonds with air molecules. As a result, water molecules at the surface tend to be drawn into the body of the liquid. The inward force that tends to reduce the surface area of a liquid is called surface tension. The liquid that has strong intermolecular attractions has a high surface tension. Higher the surface tension, more nearly spherical is the drop of that particular liquid. Very important, the surface tension of a liquid tends to hold a drop of liquid in a spherical shape. At the same time, the force of gravity tends to flatten the drop.  Furthermore, the hydrogen bonding between the water molecules also explains water's unusually low vapour pressure. The vapour pressure of a liquid is caused by molecules that escape the surface of the liquid and enter the gas phase. The tendency of molecules to escape is low since they are held to one another by hydrogen bonds.

 

Mechanism of viscosity

It is the state of being thick, sticky, and semi-fluid inconsistency, due to internal friction. The viscosity is nothing but the amount of resistance experienced by a fluid while it flows on a surface. Higher the resistance more viscous they are. The main reasons for their viscous flow of a liquid are the intermolecular forces and internal friction between the molecules. While a liquid flows, the molecules in contact with the surface of the liquid are stationary, whereas the upper layer tends to move. The velocity of the moving layer increases with the distance of layers from the stationary layer. This indicates that the farther the moving layer, the quicker it moves and this is termed as Laminar flow. The layer nearest to the surface retards the movement of the top layer resulting in viscosity in liquids.

 

Coefficient of Viscosity

Since viscosity is the resistance against the liquid to flow, the layers require a force to get away from the resistance present between the layers. Such a force help to maintain the flow of molecular layers and is directly proportional to the area of contact of layers and the velocity gradient. The ratio of changed velocity (du) and original velocity (dx) is termed as the velocity gradient.

Now, F ∝ A

F ∝ du/dx ( velocity gradient)

F ∝ A(du/dx)

⇒ F= η A(du/dx)

Here, η is the proportionality constant which is known as the coefficient of viscosity. The unit of η in the SI system is 1 Newton second per sq.metre (1 N sm-2 ) or Pascal-second (Pa s), whereas in the CGS system the unit of η is Poise.

 

Effect of Physical Changes in Viscosity

The viscosity of the aqueous substance decreases with the increasing temperature. At high temperature, the force of attraction between the molecules decreases and due to the high kinetic energy in the molecules the intermolecular forces slip past each other in the layer. This results in decreased viscosity.



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