Intermolecular forces are stronger in liquid state than in gaseous state. Molecules in liquids are so close that there is very little empty space between them and under normal conditions liquids are denser than gases.
Molecules of liquids are held together by attractive intermolecular forces. Liquids have definite volume because molecules do not separate from each other. However, molecules of liquids can move past one another freely, therefore, liquids can flow, can be poured and can assume the shape of the container in which these are stored. In the following sections we will look into some of the physical properties of the liquids such as vapour pressure, surface tension and viscosity.
The process by which molecules of a liquid go into the vapour state from the surface of the liquid at any temperature below the boiling point of the liquid, is called evaporation.
During their motion, they collide with each other, with the walls of the container, and with the surface of the liquid itself. When these molecules collide with the surface of the liquid, they are attracted back to the liquid, called condensation.
Initially the rate of evaporation is higher as compared to the rate of condensation,then rate of condensation increases gradually untill the rate of evaporation becomes equal to the rate of condensation.
Vapour pressure is defined as, "The pressure exerted by the vapour in equilibrium with its liquid at a given temperature".
The molecules on the surface experience a net downward attractive force towards the interior of the liquid due to the molecules below it. Thus, there is a tendency on the part of the surface to molecules to go into the bulk of the liquid.
The attractive forces have to be overcome in bringing the molecules from the bulk to increase the surface of the liquid. The energy required to increase the surface area of the liquid by 1unit is known as the surface energy. The units of surface energy are joule/meter2.
Surface tension is defined as the force acting at per unit length perpendicular to the surface of liquid.
Liquids naturally tend to have the minimum number of high-energy molecules on the surface to increase the stability.
To minimise the surface area as much as possible, the molecules of liquids gets the shape of a sphere.
The magnitude of the surface tension of a liquid is directly proportional to the attractive forces between the molecules. Thus larger the attractive force, the higher is the surface tension of a liquid.
Surface tension decreases with an increase in temperature.
Viscosity is a measure of resistance to flow in liquids, which arises due to the internal friction between the layers of the fluid as they slip past one another when the liquid flows.
When a liquid flows over a fixed surface, the layer of molecules in immediate contact with the surface is stationary, while the upper layers move with different velocities.
The velocity increases with the distance of the layers from the fixed layer. This implies that the uppermost layer of molecules has the greatest velocity.
A force maintains the flow of layers, which is directly proportional to the area of contact of the layers and the velocity gradient.
This implies that the force required for maintaining the flow of layers is a function of the velocity gradient and the area of the layers.
The coefficient of viscosity is defined as the tangential force per unit area required to maintain the unit difference of velocity between two layers unit distance apart.
An increase in viscosity decreases the flow of a liquid.
An example of a highly viscous liquid is the glass. Glass is so viscous that it would take years to move a few inches.
Increase in temperature decreases the viscosity of liquids.
If the velocity of the layer at a distance dz is changed by a value du then velocity gradient is given by the amount . A force is required to maintain the flow of layers. This force is proportional to the area of contact of layers and velocity gradient i.e.