Volume
Liquids have a definite volume under given temperature and pressure conditions. Though they take the shape of the container, they maintain their volume. The intermolecular forces in liquids are strong and therefore, they do not expand to occupy all the space available (as gases do). A given mass of liquid has a fixed volume. For example, 10 cm3 of water always occupies 10 cm3 whether it is placed in a beaker, a conical flask or a large round bottom flask.
Shape
Liquids have no shape of their own. The molecules in the liquid state are not rigidly fixed to their own sites and take the shape of the container in which they are placed.
Density
The higher density of liquids compared to gases is due to the fact that molecules of liquids are more closely packed than gases. In general, the density of liquids is higher than gases and it decreases with increase in temperature. It has been found that the densities of liquids are about 1000 times greater than the densities of gases under similar conditions. For example, the density of water at 100oC and 1 atm is 0.958 g cm-3 while the density of water vapours at the same temperature and pressure is 0.000588 gcm-3.
Compressibility
Liquids are 105 times less compressible than gases but are about 10 times more compressible than solids. This is due to the fact that the molecules in liquid state are not as closely packed as in solids but are closer to each other as compared to gases. Moreover in the case of liquids, the repulsive forces between molecules prevent the compressibility of liquids. Therefore, large pressure is needed to reduce the volume of a liquid to a significant extent. For example, at 25oC, an increase in pressure from 1 atm to 2 atm decreases the volume of an ideal gas by about 50%, whereas the same change in pressure decreases the volume of water by only 0.0045%.
Fluidity
Liquids flow with greater ease as compared to solids. The ease of flow of liquids depends on the strength of the intermolecular attractive forces e.g. liquids with hydrogen bonding are more viscous and therefore less fluid.
Surface tension
Due to strong intermolecular cohesive forces in the liquid, the interior of the liquid holds the molecules on the surface inwards. This property is described in terms of surface tension and the free surface of the liquid acts as a stretched membrane.
Diffusion and miscibility
Like gases, liquids also diffuse but they diffuse slowly than the gases. The diffusion of liquids is defined as the process of intermixing of the molecules of two or more liquids to form homogeneous mixture solution. This is also the movement of solute particles from higher concentration to lower concentration in a medium.
The rate of diffusion is much slower in liquids than in gases. In the liquid state, molecules are quite close to each other. Therefore, a molecule of the liquid has to undergo a number of collisions with the neighbouring molecules. Thus, there is more obstruction for the movement of the molecules of a liquid. As a result, diffusion takes place slowly. In contrast, in gases, there is very less obstruction to the moving molecules because of large empty spaces available for their movement. Moreover, in liquids there are forces of attraction between individual molecules which hold them together. In other words, this slows down the process of diffusion. However raising the temperature increases the kinetic energy of the molecules. This increase the rate of diffusion of a liquid.Quasilattice structure
The partially ordered structure of liquids is called the quasilattice structure. They do not have a long-range order. Due to the cohesive forces between the molecules in liquids, molecules appear to be under the influence of each other for short-range order.
Evaporation and volatility
Evaporation is the process of conversion of a liquid into its vapours at room temperature. When a liquid is placed in an open vessel, it gradually evaporates and is converted into its vapours. The molecules of liquids escape from the surface inspite of strong intermolecular cohesive forces among themselves. The process of evaporation can be explained on the basis of kinetic molecular theory.
Different molecules in a liquid have different speeds and therefore, have different kinetic energies. There is a certain fraction of the molecules at the surface, which have very high kinetic energies. These molecules can readily overcome the attractive forces in the liquid and escape from the liquid surface into vapours. This process of escaping of molecules spontaneously from the surface of the liquid to vapour state is called evaporation.It may be noted that during the process of evaporation, cooling always occurs. This is because during evaporation the molecules of higher kinetic energy escape. The slow moving molecules are left behind and therefore, the average kinetic energy of the molecules left in the liquid state is lowered, assuming that no heat is supplied from some outside source. The lowering of the average kinetic energy results in a temperature drop in the liquid.
The conversion of a liquid into its vapour at its boiling point is called as vaporization.The tendency of molecules in the liquids to escape from its surface is called volatility. The liquids whose molecules have very little tendency to leave the surface are called non-volatile liquids. Conversely, the liquids whose molecules leave the surface easily are volatile liquids. Volatile liquids evaporate faster.
Factors Affecting the Rate of Evaporation
Rate of evaporation depends upon the following factors:
Factors and Heat of Vapourisation
Temperature
The rate of evaporation increases as the temperature of a liquid is increased, as it is an endothermic process. For example, a glass of hot water evaporates more rapidly than a glass of cold water.
Strength of intermolecular forces
The ease of evaporation of a liquid is related to the strength of the attractive forces between the molecules in the liquid. In polar liquids cohesive forces are strong while in non-polar liquids the cohesive forces are very weak and the molecules escape easily. For example, ether evaporates more rapidly than ethyl alcohol while ethyl alcohol evaporates quicker than water.
Surface area
The larger the exposed surface area of the liquid the greater is the number of molecules escaping from its surface.
Heat of vapoursation
The amount of heat required to evaporate 1 mole of a given liquid at a constant temperature is known as the heat of evaporation or heat of vapourisation.
For example, when one mole of water is completely vapourised at 25C, it absorbs 44.180 kJ of energy. This may be written as:
Vapour pressure
If a liquid is placed in an open vessel, there is complete escape of molecules from the liquid to the atmosphere. If a liquid is allowed to evaporate in a closed vessel, the molecules escaping from the liquid surface are collected in a vapour state, above the surface of the liquid within the container. Due to vapoursation the level of liquid decreases. After some time, the level of liquid does not change further and becomes constant.
Fig: 2.9 - Attainment of equilibrium in evaporation of liquid
This behaviour of liquid can be explained as follows. Initially, the system contains only liquid, and air above it. The air above the liquid is completely evacuated and the vessel is sealed, so there is nothing above the liquid. As evaporation starts, the molecules from the surface of liquid escape into vapour state, in the confined space above. Therefore, the level of liquid falls.Then starts the process of. This is the reconversion of vapour into liquid. Initially, escaped molecules move randomLy in all directions and collide with one another within the confine of the walls and liquid surface of the container. As more and more molecules enter the confined space, some slow-moving molecules are pushed back. They collide with the surface of the liquid and get recaptured by the intermolecular attraction, to reconvert into liquid.
In the initial stages, the rate of evaporation is more than the rate of condensation because only small number of molecules are present in the gaseous state. The rate of condensation thereafter gradually increases as the number of molecules in the gaseous phase increases. Finally, a stage is reached when the rate of the two opposing processes is the same.The state where the rate of evaporation becomes equal to the rate of condensation is called a state of dynamic equilibrium. In such a state, although the amount of liquid level in the vessel does not change, evaporation has not stopped and the system is not at rest. In fact, the number of molecules, which escape from the liquid to the gaseous phase (due to evaporation), becomes equal to the number of vapour molecules that return to the liquid.
The molecules in the vapour phase begin to exert pressure. As evaporation reaches the equilibrium state, the concentration of the molecules in the vapour state becomes constant. The pressure exerted by the molecules in the gaseous phase in equilibrium with its liquid is called equilibrium vapour pressure of the liquid or its vapour pressure.may be defined as 'the pressure exerted by the vapours in equilibrium with its liquid at a given temperature'. Vapour pressure is a characteristic property of a liquid at a given temperature. It does not depend upon the amount of the liquid or the vapour phase. The magnitude of vapour pressure depends upon the following three factors:
Nature of liquid
The vapour pressure of a liquid depends upon the nature of the liquid. When intermolecular forces are weak, the molecules tend to escape readily and therefore, the equilibrium vapour pressure is high. Vapour pressure of liquids provides an indication of the magnitude of intermolecular forces. Ether and alcohol evaporate faster than water. Hence, ether and alcohol have higher vapour pressure than water at a given temperature. The higher vapour pressure of ether and alcohol as compared to water is due to the fact that there are weaker attractive forces in ether and alcohol.
Temperature of the liquid
The vapour pressure of a liquid increases with increasing temperature. As the temperature increases, the number of molecules, which have larger kinetic energy to overcome the intermolecular cohesive forces, increases. As a result more molecules escape from the surface of the liquid and concentration of molecules in the vapour phase increase. Hence, vapour pressure of the liquid increases.
Presence of impurities
Impurities affect the vapour pressure of a liquid appreciably. The non-volatile impurities lower the vapour pressure of a liquid.
