Properties of Liquids

The various properties of liquids like shape, volume, vapour pressure; surface tension, viscosity etc. can be explained on the basis of Kinetic Molecular Theory Of Liquids which is based on following assumption;

·                     The liquid molecules are relatively close together.

·                     The intermolecular forces of attraction in case of liquids are much larger than in gases.

·                     Unlike gases, liquids have a definite volume although no definite shape.

·                     The molecules are in constant random motion.

·        The average Kinetic energy of molecules in a given sample is directly proportional to absolute temperature.

(1).         SHAPE

Liquid have no definite shape. They take up the shape of vessel in which they are put. This is due to the fact that liquid molecules are in constant random motion.

(2).         LIQUID

Liquids have definite volume. This is due to reason that intermolecular forces of attraction among the liquid molecules are quite strong and unlike gases, the liquid molecules are not completely free to move.

(3).         DENSITY

Liquids have much higher density than gases. This is because liquids molecules are close to each other and hence occupy much smaller volumes than in gases.

(4).         COMPRESSIBILITY

Liquids are much higher density than gases .This is because intermolecular distances between liquid molecules are much smaller than in gases. Thus they have very low compressibility.

(5). VAPOUR PRESSURE

Vapour pressure of a liquid at any temperature may be defined as “The pressure exerted by the vapours present above the liquid in equilibrium with the liquid at that temperature.”

The magnitude of state vapour pressure depends upon the nature of liquid and temperature.

·                     Nature

Non polar or less polar liquids have fairly high vapour pressure e.g. CCl₄, CHCl₃, ethers, acetone etc. This is because such liquids have weak forces of attraction.

On the other hand polar liquids e.g. water, alcohol etc have low vapour pressure because they have strong dipole-dipole interaction between their molecules.

·                     Temperature

The vapour pressure of liquid increases with increase in temperature. With the increase in T, K. E. of molecules increases. Thus with increase in K. E, the bonds (intermolecular attraction) becomes weaker and weaker and liquid molecules gets evaporated easily, which in turn increases vapour pressure.

·                     Boiling Point

Boiling point in terms of vapour, pressure can be defined as “the temperature at which vapour pressure of liquid becomes equal to external pressure usually one atmosphere.”

Thus boiling point depends on the external pressure. Greater the pressure on liquid higher will be its boiling point, lower the external pressure, lower will its boiling point.

So, boiling point of liquids is lower at higher altitudes i.e. Mountains than at sea level.

·                     Freezing Point

Freezing point may be defined as “The temperature at which vapour pressure of liquid state and its solid state is equal.”

(6).         SURFACE TENSION

Surface tension is an important property of liquids which is linked with presence of intermolecular forces in them. It is related with surface so it is a surface phenomenon.

A molecules well within the liquid is attracted equally in all directions by the surrounding molecules. As a result, the net resultant pull is zero. But the same is not true for molecules at surface. The molecules at the surface are not equally attracted by the molecules in all directions, since there is no molecule on the upper side, it is being pulled by the molecules below it. Thus leads to imbalance of forces at the surface of the liquid.

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

 “The tangential force acting along the surface of the liquid at right angles to the surface along one centimeter length of the surface.”

Due to surface tension, the liquids tend to have the smallest possible area for a given volume. In order to increase the area, some work has to be done against the inward pull. Surface tension may also be defined as;

 “The work that must be done to increase the surface area of any liquid by one unit at constant temperature.”

The S. I. unit of surface tension is JM^–2 or NM^–1

In C. G. S unit dyne cm^–1.


The surface tension of liquid depends upon fallowing factors.

Temperature

The Surface tension decreases with increase in temperature. As the temperature increases, the average K. E. increases and hence intermolecular attraction decreases. Thus surface tension decrease. The temperature at which surface tension of liquid becomes zero is known as Critical Temperature.

Nature of Liquids

The surface tension of a liquid is linked with intermolecular forces. The liquids having greater intermolecular forces, higher will be their surface tension. Liquids having lower intermolecular force of attraction, lower will be the surface tension. Polar liquids have greater forces of attraction than non-polar liquids have greater forces of attraction than non-polar liquids so surface tension of polar liquids is higher than non-polar. E.g. Kerosene being non-polar has low surface tension than water which is polar in nature.