Properties of Liquids - II

Boiling

Vapour pressure measures the tendency for the molecules to escape from liquid to the gas phase. At lower temperatures the vapour pressure of a liquid is much lower than the pressure on the surface of the liquid. When the temperature of the liquid is gradually increased, its vapour pressure also increases. Ultimately a stage is reached when the vapour pressure of the liquid equals the pressure of the air above it. At this point, molecules and vapours formed within the liquid can easily rise through the liquid in the form of bubbles and escape into the air. This phenomenon is known as boiling and the temperature at which this occurs is known as boiling point.

Boiling point is the temperature at which the vapour pressure of a liquid becomes equal to the atmospheric pressure. The boiling point, therefore, depends upon the atmospheric pressure and it changes with the change in the pressure above the liquid. As the atmospheric pressure increases, it is necessary to heat the liquid to a higher temperature to make its vapour pressure equal to atmospheric pressure. At high altitudes like Ooty, having lower atmospheric pressure, water boils at a much lower temperature than in the plains like Bangalore where atmospheric pressure is higher.

Generally, boiling points for most of the liquids are specified as normal boiling points. The normal boiling point of a liquid is the temperature at which the vapour pressure of the liquid becomes equal to one atmospheric pressure (standard pressure).

It may be noted that once the boiling starts, the temperature of the liquid remains constant, until the whole of the liquid has vaporized, even though heating is continued.

Problem

10. Why does the temperature of the boiling liquid remains constant even though heating is continued ?

Solution

When a liquid is heated, the heat supplied is consumed to pull the liquid molecules apart against strong attractive forces, which hold them together in the liquid form. The heat energy goes to compensate the loss of energy due to escaping molecules, which have to overcome the attractive forces between the molecules of the liquid. As a result, there is no change in the average kinetic energy of the molecules remaining in the boiling liquid. Since the average kinetic energy is proportional to the temperature (1/2mV² T), therefore, the temperature of the liquid does not change till the whole of liquid has been converted into the vapour state.

Effect of change in external pressure on the boiling point

A liquid may be made to boil at any desired temperature by changing the external pressure.

For example, a liquid may be made to boil at higher temperature by increasing the pressure on its surface, as is done in a pressure cooker. It may be made to boil at a lower temperature than the normal boiling point by reducing the external pressure on it. It is observed that some substances decompose at their normal boiling points and therefore, they cannot be heated up to their normal boiling points. They are made to boil at reduced temperatures. This principle is used in purifying less stable liquids by distillation under reduced pressure. This process is called vacuum distillation..

Difference between evaporation and boiling

Although boiling and evaporation are similar processes, yet they differ in few aspects. The main points of difference between evaporation and boiling are given below:

Surface tension

Surface tension is also related to the intermolecular forces in the liquid. The molecules in liquids are held closely and hence attract each other. A molecule in the bulk of the liquid is attracted equally on all sides so that the net attractive pull on the molecule is zero. However, a molecule at the surface is subjected only to the attractive forces of the molecules below it, as there are no molecules above it. Therefore, surface molecules experience a resultant downward attractive force from within the liquid, which tends to make the surface area of the liquid as small as possible. This causes the molecules at the surface to be pulled inwards and so there is always some residual imbalance force acting on the surface of the liquids. This is called surface tension. Whenever we want to stretch the surface, work has to be done against surface tension. Surface tension may be defined as:

'Force per unit length acting perpendicular to the tangential line on the surface'. It may also be defined as 'the work done to increase the free surface area of any liquid by one unit at constant temperature and pressure.'

The SI unit of surface tension or force per unit length is Nm^-1. Since this unit is too big for any practical purpose we use a smaller unit mN m^-1 (millinewton meter^-1).

Fig: 2.10 - Forces at the surface and interior of the liquid

Effect of temperature

Surface tension decreases with rise in temperature, almost linearly. The decrease of surface tension with increase in temperature results because the kinetic energy (or speeds) of the molecules increases. Thus, the strength of intermolecular forces decreases resulting in the decrease of surface tension also. For example, clothes are washed more efficiently in hot water than in cold water due to decreased surface tension in hot water.

The effect of temperature on surface tension is given by Eotvas equation

where, g = surface tension, k = constant, d = density

M = Molecular mass, T_c = critical temperature; T= temperature

As 'T' approaches critical temperature, the surface tension becomes zero. At this stage the meniscus between the liquid and vapour disappears.

Nature of liquid

Primarily surface tension of a liquid is governed by the strength of intermolecular attractive forces. Therefore, the magnitude of surface tension is a measure of intermolecular attractive forces. For instance the order of strength between inter molecular forces as well as the surface tensions of water, ethyl alcohol and ether are in their respective orders:Water (72.8)> Ethyl alcohol (22.3) > Ether (17.0).

Impurities present in the liquid

Impurities affect surface tension appreciably. It is observed that impurities, which tend to concentrate on the surface of liquids, compared to its bulk lower the surface tension. Substances like detergents, soaps, alcohol lower the surface tension of water, while inorganic impurities present in the bulk of a liquid such as NaCl tend to increase the surface tension of water.

Pressure

Increase of pressure on the surface of a liquid increases the surface tension. Such effects are not large.

The following two important phenomena are due to surface tension:

Spherical shape of drops

The liquid drops have nearly spherical shapes. Because of surface tension, the free surface of a liquid tends to attain minimum surface area. Since the sphere has minimum surface area for a given volume of liquid, the liquid tries to adopt spherical shape. Examples are water droplets or mercury globules.

Capillary action

When one end of a capillary tube is put into a liquid that wets glass, the liquid rises into the capillary tube to a certain height and then stops. The rise of liquid in a capillary is due to the inward pull of surface tension acting on the surface, which pushes the liquid into the capillary tube. This phenomenon is called capillary action. The rise of liquid in the capillary is very important. For example, water below the surface of the Earth rising to the plants through the roots, oil rising into the wick of an oil lamp, ink rising in a blotting paper, are all examples of capillary action.

Viscosity

Certain liquids flow faster than others. Water and kerosene oil flow rapidly while honey and castor oil flow slowly. The cause of different rates of flow of liquids may be easily understood by considering flow of liquid in a beaker. If water is stirred in a beaker with a rod and left undisturbed for some time, its swirling motion stops after some time. The faster moving outer layer adjacent to the edge of the beaker comes to stop first. The slower moving layer near the center, which stops last, is pulling it back. Different layers in a liquid move over one another with different speeds in the direction of the flow of the liquid. However, due to intermolecular forces, there is resistance of one layer to the other layer. These internal self-governing forces tend to oppose any free motion. This resistance to the flow of a liquid is termed as viscosity.

'The internal resistance to flow in liquids, which, one layer offers to another layer trying to pass over it is called viscosity'.

The viscosities of liquids are compared in terms of coefficient of viscosity (h). This is defined as the tangential force per unit area required to maintain a unit difference in velocities.

The units of viscosity are poise (P), where 1P = 1 g cm^-1 s^-1. In S.I. unit, 1P = 0.1 Nsm^-2.

Viscosity is also related to the intermolecular forces in the liquid. If the intermolecular forces are large, viscosity will be high. For example, water has higher viscosity than methyl alcohol because intermolecular forces in water are more than that in methyl alcohol.

With rise in temperature viscosity of a liquid decreases. This increases the average kinetic energy and so the intermolecular forces can be easily overcome.

Liquids with extensive hydrogen bonding show higher viscosity due to an increase in size and mass of the molecule.

For example, glycerol and sulphuric acid show higher viscosity than water due to hydrogen bonding.

Problem

11. Which of the liquids in each of the following pairs has a higher vapour pressure.

(a) Alcohol, glycerine (b) Mercury, water (c) Petrol, kerosene.

Solution

Liquids having higher vapour pressure are:

(a) Alcohol (b) Water (c) Petrol.