Heat Science: Change of State (part-3)

Difference between unsaturated and saturated vapors

When the volume is increased, observation shows that Boyle’s law and Charles’s law are approximately obeyed when the pressure values are not near the saturation vapor pressure value for the given temperature. Thus unsaturated and saturated vapors of the same substance have different properties and they differ from one another in the following respects:

(1)    A saturated vapor is one which is in contact with its liquid in a closed space. The pressure exerted by a saturated vapor is always constant for a given temperature of the liquid, even though the vapor undergoes changes in volume. Experiment shows that the saturation vapor pressure of a liquid ‘varies’ with its temperature.

(2)    An unsaturated vapor at a given temperature is one which exerts less than the saturation vapor pressure for that temperature and is obtained when no liquid is in contact with it in a closed space. The pressure of a given mass of unsaturated vapor depends on its volume as well as its temperature and experiment shows that it obeys Boyle’s law and Charles’s law approximately.

Adiabatic transformation of saturated vapor

Behavior of saturated vapor undergoing adiabatic change is very interesting. For a vapor whose specific heat is negative, adiabatic compression might bring about such a large increase in temperature that the vapor becomes unsaturated. In general, however, the specific heat is a small positive quantity and a rise in temperature, unaccompanied by change over to instauration results.

When a saturated vapor undergoes adiabatic expansion, its temperature falls which produces super saturation. A part of the vapor is, therefore, at once condensed giving rise to a cloud of liquid droplets. This phenomenon has been utilized in Wilson’s cloud chamber.

Vapor density

The vapor density is generally taken to be the ratio of the mass of a given volume of the vapor to an equal volume of dry air at the same temperature and pressure. The chemist uses hydrogen instead of air as the standard and since by Avogadro’s hypothesis equal volumes of all gases at the same temperature and pressure contain equal number of molecules, the vapor density is in effect a comparison between the mass of a molecule of hydrogen. For this reason the determination of vapor densities is of considerable interest in physical chemistry and various methods for measuring it have been devised.

Dalton’s laws of vapor pressures

In the following laws, known as Dalton’s laws, we shall see that presence of other gases and vapors does not affect in any way, the behavior of vapor.

Law 1: The saturation pressure of a vapor in closed space depends only on its temperature and independent of the volume or of the presence of other gases and vapors with which it can not react chemically.

Law 2: The total pressure exerted by a mixture of gases and vapors (which do not react chemically with each other contained in the same vessel, is equal to the sum of the pressures which each would separately exert, if it alone occupied the space filled by the mixture.

Law 2 is known as Dalton’s law of pressure. The law applies rigorously to gases and vapors which obey Boyle’s law. As no real gas strictly obeys this law, Dalton’s law is also approximate to that extent.

Superheated liquid and boiling by bumping

If pure water, which has been previously boiled to drive away dissolved air be heated in a perfectly clean flask, sometimes it is seen that although the thermometer inserted in it shows a temperature of 103°C or 104°C, boiling does not start. Water then said to be superheated.

If a glass rod is now introduced, at once boiling starts with great vigor and the temperature drops down to 100°C, its normal boiling point. Boiling continues for sometime and then stops, the surface is perfectly calm and the temperature rises a few degrees above the normal boiling point. Suddenly a large quantity of vapor bursts on the surface with almost explosive violence. This is known as boiling of bumping.

A change of state (boiling or freezing) requires a start to set in. Water can be super cooled a few degrees below below 0°C without freezing taking place. If a small crystal of ice is dropped, at once freezing starts with great rapidity and temperature rises to 0°C. The ice particle forms a nucleus around which ice crystal grows.

Similarly, superheated liquid requires some nucleus for boiling to set in. if v feather or a glass rod is dipped in superheated water, at once boiling starts with great vigor. The film of air on the glass rod or feather supplies the nucleus for boiling to set in.

Boiling by bumping is prevented by providing the liquid with sufficient nuclei. If a few pieces of charcoal or glass beads be taken inside the vessel, the liquid boils steadily without bumping. Like super cooling, super heating, also is an unstable phenomenon.

The spheroidal state

When drop water is placed upon a very hot metal plate, it neither spreads out nor boils but assumes the form of a spheroid (globules) which rolls on its surface. The drop is then said to be in a spheroidal state in which it is not really in contact with the hot plate, but is supported on a cushion of its own vapor. The region in between the drop and the plate is occupied by a layer of water vapor which is heated much heated much above the boiling point. The drop is supported by the pressure of this superheated vapor. The layer also serves as a thermal insulation between the drop and the hot plate so that the temperature of water does not rise above 98°C. Many other liquids are capable of assuming the spheroidal state and liquid air shows the phenomenon well on surfaces at ordinary room temperature. Liquid drops may float in the same way on the surface of another liquid when the latter is at a temperature higher than the boiling point of the former.