Liquid State: There are three physical states of matter: solids, liquids, and gases. The qualities of each of the three states of matter are different. Did you know that liquid may be thought of as a bridge between the solid and gaseous states? The liquid state is quite similar to the previous two stages. We shall learn more about the liquid state of matter and its characteristics in this post.

What is Liquid State?

The observed macroscopic properties of liquids are based on a theory called the Kinetic Molecular Theory of Liquids.

The main postulates of the theory are:

1. Liquids are composed of particles called molecules.
2. The intermolecular forces of attraction among the molecules of a liquid are quite appreciable.
3. As compared to gas, the molecules in a liquid are relatively close to each other.
4. The molecules are in constant random motion.
5. The average kinetic energy of molecules in liquid is directly proportional to the absolute temperature.

Characteristics of Liquids

Let us see some of the characteristics of the liquid:

1. Liquids do not Have a Fixed Shape

Liquids do not have a fixed shape and take up the shape of any container in which these are put. This is because the interparticle forces are not very strong, and the particles present in one layer can slide over the other layer quite easily. As a result, a liquid takes up the shape of the container in which it is kept.

2. Liquids Occupy a Definite Volume

Even though the liquids do not have a definite shape, they do have a definite volume. Similar to a solid, a liquid cannot be compressed by applying pressure. The interparticle forces in the liquids are so strong that the applied pressure cannot overcome these.

3. Liquids have Flexibility and not Rigidity

The liquids have fluidity and not rigidity, i.e., they tend to flow. It is due to lesser intermolecular forces that are present in the liquids. However, different liquids differ in their relative fluidity. For example, water flows at a faster rate than honey.

4. Liquids have a Lesser Density

Liquids are generally light as compared to solids. This is because of the more significant number of interparticle spaces in the liquid state than the solid state of the same substance.

Exception- Ice (solid-state) floats over water (liquid state). Both are chemically the same and made of the same molecules, but the structure of ice is more porous than that of water. Therefore, for a given mass, the ice volume is more than that of water, and its density is comparatively less. As a result, ice floats over water.

5. Kinetic Energy of Particles is More than the Solid State

The particles in the liquid state are less closely packed as compared to the solid state. As a result, the interparticle forces are weaker. Therefore, the kinetic energy of the particles in the liquid form is more than in the solid state. It further increases with the temperature rise.

6. Particles in the Liquid State can Easily Diffuse

Due to lesser intermolecular forces of attraction, the particles in a liquid state can diffuse more readily than the solid-state. The property helps in the intermixing of certain liquids. For example, water and alcohol are liquids and can easily mix to form a liquid mixture or solution.

When the physical conditions of a liquid change, the basic character of the liquid also changes. Some more characteristics that liquids show are as follows:

Evaporation

Evaporation may be defined as the process of conversion of a liquid into its vapours.

The molecules of a liquid are in a state of constant motion and possess kinetic energy, but all the molecules do not have the same kinetic energy. It may vary from very small to very high value. As a result, some molecules that have a higher value of kinetic energy present at the surface of the liquid will overcome the attractive forces and will escape into space above the liquid surface and appear as vapours.

Factors Affecting Rate of Evaporation

1. Nature of the liquid- The rate of evaporation depends on the intermolecular forces of attraction. The greater the magnitude of forces, the lesser will be the rate of evaporation.
2. Temperature- As the temperature increases, the rate of evaporation becomes faster.
3. Surface area- The larger the surface area, the greater will be the rate of evaporation.

Vapour Pressure

The pressure exerted by the vapours in equilibrium with the liquid at a given temperature is known as the vapour pressure. It results simply because of the kinetic phenomenon and is, therefore, independent of the amount of liquid. It depends mainly upon the temperature of the liquid.

Factors Affecting Vapour Pressure

1. Nature of the Liquid- The vapour pressure depends on the magnitude of the intermolecular forces of the liquid. When these forces are weak, the molecules will readily change into vapour, and the equilibrium vapour pressure will be high. However, if these forces are strong, then the molecules will not change into vapours, and consequently, the vapour pressure will be comparatively low.
2. Temperature of the Liquid- The vapour pressure of a liquid increase with the rise in temperature as the average kinetic energy also increases.

The Boiling Point of Liquids

The boiling point of a liquid is the temperature at which the vapour pressure of the liquid is equal to the surrounding atmospheric pressure. Thus, the boiling point is not a result of the heating process; instead, it is the outcome of the increasing vapour pressure.

Factors Affecting the Boiling Point

Pressure- When the external pressure is:

1. less than \(1\) atm, the boiling point of the liquid is lower than the normal boiling point.
2. Equal to \(1\) atm, the boiling point of the liquid is called the normal boiling point.
3. Greater than \(1\) atm, the boiling point of the liquid is greater than its normal boiling point.

Type of Molecules: If the intermolecular forces between the molecules are-

1. Relatively high, the boiling point will also be relatively high.
2. Relatively weak, the boiling point will also be relatively weak.

Surface Tension of Liquids

The molecules in a liquid experience an equal intermolecular force from all sides, but the molecules present on the surface of the liquid behave differently than those present in the interior. The molecules present on the surface of the liquid experience tension because of the inward pull by the molecules. The phenomenon is known as surface tension.

The inward pull tends to contract the surface of the liquid. As a result, the surface behaves like a stretched membrane.

Factors Affecting Surface Tension of Liquids

1. The temperature of the liquid- Surface tension decreases with the temperature rise. This is because on increasing the temperature, the kinetic energy will increase, which will result in the decrease in the intermolecular forces of attraction and thus will decrease the surface tension.
2. Nature of the liquid- The surface tension depends on the intermolecular forces of attraction present in the molecules of a particular liquid. The greater the magnitude of force, the more will be the value of surface tension.

Viscosity

All the liquids do not flow at the same speed. The liquids like water, alcohol, kerosene, oil, etc., flow rapidly while others like honey, glycerol, castor oil, etc., flow very slowly. Thus, viscosity may be defined as the internal resistance to the flow possessed by a liquid.

This internal resistance is due to different intermolecular forces of attraction present in different liquids. If these forces are strong, then the liquid is more viscous, and if the forces are weak, the fluid’s viscosity is less.

When a liquid flows, the layer of the liquid in immediate contact with the surface is stationary; with the increase in distance from the fixed surface, the velocity of the layers increases. This type of flow in which there is a regular gradation of velocity is called laminated flow. The layer present immediately below tries to retard the velocity while immediately above the layer accelerates it.

Coefficient of Viscosity

The force that is needed to maintain the flow in these three different layers depends upon two factors- the area in contact (A) and velocity gradient \(\frac{{{\rm{du}}}}{{{\rm{dx}}}}{\rm{.}}\)

If the velocityof the layer at a distance \({\rm{dx}}\) is changed by a value \({\rm{du}}\) then the velocity gradient is given by the amount of \({\rm{du/dx}}{\rm{.}}\) Some force is needed to maintain the flow of layers.

Force \({\rm{(f)}} \propto {\rm{(A)}}\)

Force \({\rm{(f)}} \propto \frac{{{\rm{du}}}}{{{\rm{dx}}}}\)

Force \({\rm{(f)}} \propto {\rm{A \times }}\frac{{{\rm{du}}}}{{{\rm{dx}}}}{\rm{ = \eta A}}\frac{{{\rm{du}}}}{{{\rm{dx}}}}\)

Hence, \(\eta \)is called the coefficient of viscosity.

The viscosity coefficient may be thus defined as the force applied per unit area to maintain a unit relative velocity between the two layers of a liquid at a unit distance.

Factors Influencing Viscosity of Liquids

1. Temperature- The viscosity of liquid decrease with an increase in temperature.
2. Nature of liquid- The liquids that have a stronger intermolecular force of attraction have more viscosity.
3. Molecular mass- Viscosity increases with the increase in the molecular mass.

Metal in Liquid State

Generally, metals are solid at room temperature, except for some metals that are liquid at room temperature.

Four metals occur as liquids below \({\rm{4}}{{\rm{0}}^{\rm{o}}}{\rm{C}}\)

1. Mercury (Hg)
2. Caesium (Cs)
3. Gallium (Ga)
4. Rubidium (Rb)

Out of the four metals, only mercury is the stable liquid elemental metal at room temperature. The other three metals melt just above room temperature.

Non-Metal in Liquid State

Bromines (Br) is the only non metal in liquid state at room temperature.

Summary

A liquid is a nearly incompressible fluid that adapts to the shape of its container while retaining a (nearly) constant volume independent of pressure. The volume is fixed if the temperature and pressure stay constant. When a solid is heated past its melting point and the pressure exceeds the triple point of the material, it becomes liquid. While intermolecular (or interatomic or interionic) forces remain, the molecules have enough energy to move about and the structure is mobile.

Through this article, we studied the kinetic theory of liquids and their physical properties. We also discussed the characteristics of fluids like boiling point, vapour pressure, surface tension and viscosity. We also named the metal and non-metal, which occur in a liquid state.

FAQs on Liquid State

Q.1. What is the name of the metal that exists in the liquid state below \({\rm{4}}{{\rm{0}}^{\rm{o}}}{\rm{C}}\)?
Ans: Four metals occur as liquids below \({\rm{4}}{{\rm{0}}^{\rm{o}}}{\rm{C}}\)
Mercury (Hg)
Caesium (Cs)
Gallium (Ga)
Rubidium (Rb) 

Q.2. What is the name of the metal that exists in the liquid state at room temperature?
Ans: The metal that exists as a liquid at room temperature is mercury \(\left( {{\rm{Hg}}} \right)\).

Q.3. What is the liquid state of matter?
Ans: Some of the characteristics of the liquid state of matter are as follows:
1. The particles in the liquids are less closely packed.
2. The intermolecular force of attraction is weaker than the solids but stronger than the gases.
3. They have fixed volume but no fixed shape.
4. They cannot be compressed. Large pressure is required to compress the liquids.
5. Liquids have lesser densities than solids.
6. Liquids are not rigid. They are fluid. 

Q.4. Which non-metal exists in the liquid state?
Ans: The non-metal that exists in the liquid state at room temperature is Bromine \(\left( {{\rm{Br}}} \right)\).

Q.5. What is the difference between boiling and evaporation?
Ans: The major difference between boiling and evaporation is that evaporation is a surface phenomenon, whereas boiling is a bulk phenomenon- it happens over a large mass of the liquid. 

Q.6. What is common between the state of the liquid and the gaseous state?
Ans: The common properties of gases and liquids are- no defined shape and the property to flow.

Learn About Gaseous State

We hope this detailed article on Liquid State proves helpful to you. If you have any questions regarding the article, or in general, about the liquid state, please ping us through the comments section, and we will get back to you as soon as possible.