The liquid state is the intermediate state between gaseous state and solid state. In liquid state particles are close together in comparison to those in gases but they are relatively loose in comparison to solids. Similarly, the interparticle forces are stronger than those in gases but relatively weaker than solids. Unlike solids, the particles in liquids do not occupy fixed positions. They possess translatory motion but it is not as vigorous as in gases. At the same time the particles cannot be separated from one another. Let us discuss some important characteristics of liquids.
SHAPE AND VOLUME
Since the liquid molecules are quite close but they do not occupy fixed positions. therefore liquids do not have definite shape. But they do occupy definite volume because liquid molecules are not free to occupy whole space available to them. Liquids take up the shape of the container because the molecules are still free to move about. The slow movement of liquid molecules also give them the tendency to flow.
DENSITY AND COMPRESSIBILITY
In liquids, molecules are quite closely packed, therefore, their densities are much higher than the densities of gases under comparable conditions. For example, density of water at 373 K and at 1 atmospheric pressure is 0.958 g cm-3, while that of water vapour at the same temperature and pressure is equal to 0.000588 g cm-3.
The space between the molecules of liquid is reduced almost to a minimum by the intermolecular attractions, therefore, liquids are much less compressible than gases. A change in pressure has almost no effect on the densities of the liquids.
Diffusion involves movement of molecules from one place to another. In liquids, since molecules can move from one place to another they show diffusion, however they diffuse rather slowly. The slow diffusion in liquids can be explained in terms of smaller intermolecular spaces and restricted motion of molecules. The· diffusion in liquids can be demonstrated by releasing a drop of ink in water. At first there is a sharp boundary between ink cloud and water, eventually the colour spreads uniformly throughout the water.
FLUIDITY AND VISCOSITY
As pointed out earlier that liquids have definite volume and also possess tendency to flow. Therefore, they are able to transmit the same pressure from one point to another during flow. This pressure transmitting property of liquids is used in break system of cars, lorries and moving parts of heavy machinary in industry. This system of breaks is referred to as hydraulic system.
Although liquids have tendency to flow but the intermolecular forces also provide some resistance to flow. This resistance to flow is called viscosity. Now higher the magnitude of inter-particle forces higher is viscosity of the liquid. Some examples of more viscous liquids are honey, glycerine, caster oil, etc.
When a liquid is placed in an open vessel, it slowly escapes into gas phase, eventually leaving the vessel empty. This phenomenon is known as evaporation. It may be noted that in a given sample of liquid, the speed and kinetic energy of all the liquid molecules is not same. Some molecules move fast while other may be slow. In fact, they move with ever changing speeds. The fast moving liquid molecules having sufficient energy needed to overcome the intermolecular attractive forces can escape into gas phase if such molecules happen to come near the surface.
Evaporation causes cooling. This is due to the reason that the molecules which undergo evaporation are high energy molecules, therefore the kinetic energy of the molecules, which are left behind is less. Since the remaining molecules have lower average kinetic energy, then temperature must be lower.
The rate of evaporation increases with
(a) increase of temperature
(b) increase in surface area of liquid and
(c) blowing the current of air across the surface of liquid.
CONCEPT OF VAPOUR PRESSURE
If a liquid is placed in a closed vessel, the molecules entering the gas phase cannot escape. The vapour formed by escaping of particles ·of liquid in a close vessel exerts a pressure called vapour pressure. The molecules in vapour phase in their random motion may strike the liquid surface and are captured. Thus, when a liquid is taken in closed vessel, two opposing processes, evaporation and condensation, take place simultaneously. The rate of condensation increases as the number of molecules in the vapour phase increases. Eventually, the rate of condensation becomes equal to the rate of evaporation. This state in which there is dynamic balance between evaporation and condensation is the state of equilibrium (Fig. 14.6)
When equilibrium is reached, there is no further change in the number of particles in the vapour phase. The molecules in the vapour phase exert pressure in the closed vessel which is called vapour pressure. However, at the equilibrium point this pressure is known as equilibrium vapour pressure or saturated vapour pressure. At a given temperature, saturated vapour pressure of a liquid is defined as the pressure exerted by the vapours of the liquid in equilibrium with liquid at that temperature.
The saturated vapour pressure of a liquid depends upon the following factors:
(a) Nature of liquid. More volatile liquids have higher vapour pressures.
(b) Temperature. The saturated vapour pressure of the liquid increases with increase in temperature because at higher temperature larger number of molecules escape into vapour phase. Fig. 14.7 shows temperature dependence of saturated vapour pressures of some liquids.
Boiling is a special case of evaporation, it is the rapid conversion of a liquid into vapour phase by means of formation of bubbles. During boiling the evaporation is not restricted to surface only but takes place throughout the bulk of the liquid. The process of boiling can be understood as follows:
When a liquid is heated in open vessel, its saturated vapour pressure increases. At the temperature, where the equilibrium vapour pressure is lower than atmospheric pressure the vapours formed in the bubbles inside the bulk of the liquid remain trapped and the evaporation takes place only from the surface. As the temperature is increased, the vapour pressure increases. When the saturated vapour pressure becomes equal to atmospheric pressure, the bubbles of vapours formed within the liquid can freely rise to the surface and can escape into air. Evaporation taking place like this is called boiling (Fig. 14.8) and the temperature at which boiling takes place is called boiling point. Thus, boiling point of a liquid may be defined as the temperature at which the saturated vapour pressure of the liquid becomes equal to atmospheric pressure
From the above discussion it is clear that boiling point depends upon atmospheric pressure. The boiling point of the liquid at one atmosphere pressure is called normal boiling point. At normal boiling point vapour pressure of the liquid is equal to one atmosphere. The boiling point of the liquid at one bar (100 kPa) pressure is called standard boiling point. Normal boiling points can be determined from the vapour pressure-temperature curves as shown in Fig. 14.7.
Effect of Changing the external Pressure on Boiling point
The boiling point of a liquid can be altered by changing the pressure over the liquid. The higher the pressure over the liquid, higher is the boiling point A liquid may be made to boil at lower temperature by reducing the prevailing pressure. This fact finds application in purification of liquids, which decompose at their normal boiling points, by distillation under reduced pressure. Under reduced pressure the liquid distils over at some temperature lower than the normal boiling point. The effect of pressure on boiling point also explains why it takes longer to cook food at higher altitudes. At mountains the atmospheric pressure is lower and water boils at a lower temperature.
Difference Between Boiling and Evaporation
Boiling and evaporation both involve conversion of liquid into vapour and appear to be similar. However, they differ in some respects. The main points of differences are given in tabular form.
1. Evaporation occurs at the 1. It involves the formation of
surface of liquid. bubbles even below the
surface within the bulk of
2. It occurs spontaneously at 2. It occurs only at a specific
all temperatures. temperature at which vapour
pressure equals the imposed
pressure on the liquid
3. It is a slow phenomenon. 3. It is a rapid phenomenon.