The Liquid State: Vapour Pressure, Surface Tension, Viscosity

The Liquid State: In the liquid state, intermolecular forces are stronger than in the gaseous state. Liquid molecules are so close together that there is very little empty space between them. Thus liquids are denser than gases under typical conditions. Liquid molecules are held together by intermolecular forces that are attractive. Because molecules do not split from one another, liquids have a defined volume. Liquid molecules, on the other hand, mayly move past one another, allowing them to flow, pour, and take on the shape of the container in which they are held. In the sections below, we’ll look at some of the physical features of liquids, such as vapour pressure, surface tension, and viscosity.

Know About Colligative Properties Here

Vapour Pressure

A liquid evaporates when it is placed in an open vessel. The molecules in the liquid are moving with different kinetic energies. Molecules with higher-than-average kinetic energy are able to overcome the intermolecular interactions that keep them in the liquid. As vapour, these energetic molecules escape from the liquid surface. Vaporisation or evaporation is the process by which the molecules of a liquid become gaseous (vapours). Condensation is the process by which gas molecules transform into liquid molecules. When a liquid is enclosed in a closed vessel, high-kinetic-energy molecules escape into space above the liquid. Some molecules in the gas phase impact the liquid surface and are recovered as the number of molecules in the gas phase grows (condensation). A stage comes when the number of molecules escaping from the liquid is equal to the number of molecules returning to the liquid. In other words, the rate of evaporation and condensation are exactly equal. At the given temperature, a dynamic equilibrium is created between the liquid and the vapour.

With the passage of time, the concentration of vapour in the region above the liquid will remain unchanged. As a result, at equilibrium, the vapour will impose a definite pressure. The pressure exerted by the vapours, which are in equilibrium with the liquid at a certain temperature, is known as the vapour pressure of a liquid. The vapour pressures of various liquids vary greatly based on the liquid’s identity as well as its intermolecular forces. Because ethanol has a weaker hydrogen bond than water, it evaporates faster. As a result, we expect ethanol to have a higher vapour pressure at a given temperature than water.

Effect of Temperature on Vapour Pressure

The vapour pressure will rise as the temperature of the liquid rises. This is because as the temperature rises, the kinetic energy of the molecules in the liquid increases, causing them to break away from the liquid surface. As a result, before the equilibrium is restored, the concentration of vapour molecules will rise. The average kinetic energy of the vapour molecules will also increase as the temperature rises. The temperature has a direct relationship with both vapour concentration and kinetic energy. As a result, each rise in temperature leads to an increase in vapour pressure. From the experimental curves shown below, it is clear that for both ethyl alcohol and water, the vapour pressure rises with an increase in temperature.

Effect of Vapour Pressure on Boiling Points

Tiny bubbles occur in a liquid when it is heated. These ascend to the surface of the liquid and burst. The boiling point of the liquid is the temperature at which it occurs. Consider the case of a single bubble. The liquid evaporates inside it, and the bubble’s vapour pressure holds it in shape. The atmosphere’s pressure on the liquid top, on the other hand, tends to collapse the bubble. The vapour pressure in the bubble equals the air pressure as it rises to the surface. As a result, the bubble bursts. As a result, the boiling point of a liquid can be defined as the temperature at which the liquid’s vapour pressure equals the atmospheric pressure.

The boiling points are stated at \(760 \,\text {torr}\) since air pressure varies with altitude and other factors (\(1 \,\text {atm})\). As a result, the normal boiling point of a liquid is defined as the temperature at which the liquid’s vapour pressure is \(760 \,\text {torr} (1 \,\text {atm})\). As seen in Fig., ethanol has a boiling point of \(78^\circ {\rm{C}}\) while water has a boiling point of \(100^\circ {\rm{C}}\). The boiling point of a liquid can be decreased by using a vacuum pump to reduce the external pressure. At a lower temperature, the liquid’s vapour pressure equals the external pressure. Raising the external pressure can raise the boiling point of a liquid. At a higher temperature, the liquid’s vapour pressure equals the external pressure. This is how a household pressure cooker works. The pressure within the cooker is kept above one atmosphere, and the liquid inside boils at a temperature higher than \(100^\circ {\rm{C}}\). As a result, the food is prepared in less time.

Surface Tension

The intermolecular forces of attraction give liquids this feature. The molecules around a molecule in the centre of a liquid attract it in all directions equally. A molecule on a liquid’s surface is only attracted sideways toward the interior. Because the forces on the sides are balanced, the surface molecule is only dragged inward into the liquid. As a result, the surface molecules have a tendency to migrate into the liquid’s bulk. In order to have the fewest number of molecules at the surface, the liquid surface is under stress and tends to contract to the smallest possible area. Drops of liquid in air adopt spherical shapes because a sphere has the smallest surface area for a given volume. The force in dynes acting along the surface of a liquid at right angles to any line \(1 \,\text {cm}\) in length is known as surface tension \((\text {ϒ})\).

Units of Surface Tension

As included in the above definition, the unit of surface tension in the CGS system is dynes per centimetre (dyne \({\rm{c}}{{\rm{m}}^{ – 1}}\)). In the SI system, the unit is Newton per metre \(({\rm{N}}{{\rm{m}}^{ – 1}})\). Both these units are related as \(1\) dyne \({\rm{c}}{{\rm{m}}^{ – 1}} = 1\,{\rm{m}}\,{\rm{N}}{{\rm{m}}^{ – 1}}\)

Effect of Temperature on Surface Tension

A change in temperature produces a change in a liquid’s surface tension. When the temperature rises, the kinetic energy of liquid molecules rises \(\left( {{\rm{KE}} \propto {\rm{T}}} \right)\), resulting in a decrease in intermolecular forces. It causes the inward pull on the liquid’s surface to perform less effectively. In other words, as the temperature rises, the surface tension drops.

Viscosity

A liquid can be thought of as a collection of molecular layers stacked one on top of the other. A liquid flows when a shearing force is applied to it. The friction forces between the layers, on the other hand, provide resistance to this movement. A liquid’s viscosity is a measurement of its frictional resistance. Let’s have a look at a liquid that is flowing on a glass surface. The velocity of the molecular layer in contact with the stationary surface is zero. The succeeding layers above it travel in the direction of the flow at increasing speeds.

Consider the following two adjacent moving layers of a liquid. Let’s say they’re separated by dx and have a velocity differential of dv. The friction force \((\text {F})\) that resists relative motion between the two layers is proportional to the area A and the velocity difference dv but inversely proportional to the distance between the layers.

That is:

\({\rm{F\alpha A}}\frac{{{\rm{dv}}}}{{{\rm{dx}}}}\)

or,

\({\rm{F\eta A}}\frac{{{\rm{dv}}}}{{{\rm{dx}}}}\)

or,

\({\rm{\eta  = }}\frac{{\rm{F}}}{{\rm{A}}} \times \frac{{{\rm{dv}}}}{{{\rm{dx}}}}\)

The proportionality constant is \({\rm{\eta }}\) (Greek letter eta). The Coefficient of Viscosity, or simply viscosity of a liquid, is what it’s called. For a given liquid at the same temperature, \({\rm{\eta }}\) has a fixed value. It can be described as the force of resistance per unit area that maintains a unit velocity difference between two layers of a liquid at a unit distance from each other.

Units of Viscosity

The dimensions of the coefficient of viscosity \(({\rm{\eta }})\) may be derived from equation \((2)\).

\({\rm{\eta  = }}\frac{{\rm{F}}}{{\rm{A}}} \times \frac{{{\rm{dv}}}}{{{\rm{dx}}}} = \frac{{{\rm{force}}}}{{{\rm{area}}}} \times \frac{{{\rm{distance}}}}{{{\rm{velocity}}}}\)

\({\rm{\eta }} = \frac{{{\rm{mass}} \times {\rm{length}} \times {\rm{tim}}{{\rm{e}}^{ – 2}}}}{{{{\left( {{\rm{length}}} \right)}^2}}} \times \frac{{{\rm{length}}}}{{{\rm{length/time}}}}\)

\( = {\rm{mass}} \times {\rm{lengt}}{{\rm{h}}^{ – 1}} \times {\rm{tim}}{{\rm{e}}^{ – 1}}\)

The \(\text {SI}\) unit is \({\rm{kg}}\,{{\rm{m}}^{ – 1}}{{\rm{s}}^{ – 1}}\). One poise is equal to one-tenth of the \(\text {SI}\) unit i.e.

poise \( = 1\,{\rm{g}}\,{\rm{c}}{{\rm{m}}^{ – 1}}\,{{\rm{s}}^{ – 1}} = 0.1\,{\rm{kg}}\,{{\rm{m}}^{ – 1}}\,{{\rm{s}}^{ – 1}}\)

^Summary

1. In the liquid state, intermolecular forces are stronger than in the gaseous state.
2. The pressure exerted by the vapours, which are in equilibrium with the liquid at a certain temperature, is known as the vapour pressure of a liquid.
3. The force in dynes acting along the surface of a liquid at right angles to any line \(1 \,\text {cm}\) in length is known as surface tension \((\text {ϒ})\).
4. A liquid can be thought of as a collection of molecular layers stacked one on top of the other. A liquid’s viscosity is a measurement of its frictional resistance.

FAQs on The Liquid State

Q.1.  What is the vapour pressure of a liquid?
Ans: The pressure exerted by the vapours, which are in equilibrium with the liquid at a certain temperature, is known as the vapour pressure of a liquid. The vapour pressures of various liquids vary greatly based on the liquid’s identity as well as its intermolecular forces.

Q.2. What is the effect of temperature of vapour pressure? 
Ans: The vapour pressure will rise as the temperature of the liquid rises. This is because as the temperature rises, the kinetic energy of the molecules in the liquid increases, causing them to break away from the liquid surface. As a result, before the equilibrium is restored, the concentration of vapour molecules will rise. The average kinetic energy of the vapour molecules will also increase as the temperature rises. The temperature has a direct relationship with both vapour concentration and kinetic energy. As a result, each rise in temperature leads to an increase in vapour pressure.  

Q.3. What is the relation between the boiling point and vapour pressure of a liquid?
Ans: The boiling point of a liquid can be defined as the temperature at which the liquid’s vapour pressure equals the atmospheric pressure.

Q.4. What is the surface tension of a liquid?
Ans: The force in dynes acting along the surface of a liquid at right angles to any line \(1 \,\text {cm}\) in length is known as surface tension \((\text {ϒ})\).