Properties of Liquid – Vapour Pressure and Viscosity

Liquids belong to one of the three phases of matter. The other two are solids and gases. The nature of the liquids helps us understand their function, composition and structure. It is essential to study the properties of liquids, as they help you learn about their chemical makeup and how it affects the liquid’s behaviour and appearance. Liquids have a definite volume and an indefinite shape, and are impacted by the intermolecular forces present in them. Some significant properties of liquids are surface tension, vapour pressure, viscosity, and capillary action. In this section, we’ll discuss the two critical properties of liquid – vapour pressure and viscosity.    

Properties of Liquid 

• The general nature of liquids is that they are not rigid. Despite having no definite shape, they have a definite volume.
• The presence of intermolecular forces in liquids makes it stronger and much denser when compared to gases.
• The intermolecular forces are responsible for holding the molecules together. Since the molecules are tightly held in the liquid, they have a definite volume. 
• One of the important properties of liquids is their ability to flow freely, enabling them to take the shape of the object in which it is stored. 

Vapour Pressure

• The pressure that is exerted by the evaporation of the liquids is defined as vapour pressure. For example, when you fill half of an empty container with a liquid, the liquid will evaporate after some time to reach the remaining volume of the container. The vapour pressure increases in the beginning as the pressure exerted by the vapour settled in the container increases. The vapour pressure will become equilibrium vapour pressure when equilibrium is achieved between the liquid and the vapour phase.
• By heating the liquid continuously inside a closed vessel, the vapour pressure increases. Initially, the density of the liquid is higher but as temperature increases, the density of liquid decreases and the density of vapour increases. The temperature at which the density of the vapour and the liquid becomes equal is known as critical temperature.
• The liquid evaporates when you heat it in an open container. Here, the evaporation occurs freely. This is known as boiling. 
• The normal boiling point (1 atm pressure) is different from the standard boiling point (1 bar pressure). For example, the normal boiling point of water is 100 ℃ and the standard boiling point of water is 99.6 ℃. 

Characteristics of Vapour Pressure

• Vapour Pressure is a measure of the ability of a material to transform into a gaseous or vapour state.
• Vapour Pressure increases with an increase in temperature.

Viscosity

• Viscosity is defined as the measure of a liquid’s resistance to flow that happens due to the friction between the different layers of the liquid as they move. 
• Viscosity is dependent on the intermolecular forces of attraction present within a liquid. The stronger the intermolecular force of attraction, the higher the viscosity. 

For example, honey is denser when compared to water. Hence, honey has a higher viscosity and water has a lower viscosity. 

Characteristics of Viscosity 

• Unlike vapour pressure, viscosity decreases with an increase in temperature. 
• The flow of the liquid is generally slower when the viscosity of the liquid is higher. The higher viscosity is due to van der Waals force and hydrogen bonding. 

Coefficient of Viscosity 

Intermolecular forces are stationary in liquids flowing over a fixed surface. The upper layer of the flowing liquid is responsible for accelerating the flow, and the layer below it retards the flow. As the distance of the upper layer from the fixed layer increases, the velocity of the upper layer increases. The type of flow in which there is a change in velocity from one layer to another layer is known as laminar flow. 

Let us consider that the velocity of a layer placed at a distance dz is changed to du. 

The velocity gradient is given by dudz. 

To maintain the flow of the layers, we need force.

Force is proportional to the area of contact and the velocity gradient. The area of contact is denoted by A, and the velocity gradient is dudz . 

F ∝ A dudz

We can write the above equation as 

F = A dudz

Where = proportionality constant. This is the coefficient of viscosity. 

For viscosity coefficient, A = unit area 

dudz = unity = the measure of viscosity

The unit of coefficient of viscosity is poise. 

The SI unit of the coefficient of viscosity is 1 Newton second per square metre (N s m-2) 

Conclusion

The above article would have given you a clear understanding of the nature of the liquids and their two important properties – vapour pressure and viscosity. 

When studying the vapour pressure of the liquid, we covered the two significant concepts – equilibrium vapour pressure and the critical temperature, along with the characteristics of vapour pressure. The viscosity of the fluid, laminar flow, characteristics of viscosity, and the coefficient of viscosity are also detailed.