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## SUBLIMATION

Sublimation is the process in which a gas without passing through the intermediate liquid state. Such materials are called sublime. This due to the fact that if a solid can at some temperature exert a vapour pressure equal to the external pressure, it will on heating to this point, pass direct to vapour, and reverse changes will occur on cooling the vapour.

This is called true sublimation (Type-1).

On the other hand, if a solid cannot at any temperature exert a pressure equal to the external pressure, it will, when heated, first liquefy and later, when vapour-pressure of the liquid equals the external pressure, the liquid will boil, and the reverse changes will occur on cooling the vapour. This is categorized as Type-2 sublimation.

The sublimation curve represents the vapour pressure curve of the solid phase, the distillation curve and the vapour pressure curve of the liquid phase, the melting point curve showing the temperature at which the solid melts under increasing pressure.

The point of intersection of these curves is called the triple-point, and the point where the vertical line from the triple-point cuts the abscissa-axis is called the triple-point pressure.

It is evident that if the pressure on a sold is lower than the triple-point pressure, the solid cannot exist as a liquid. Hence, upon heating, the sold must pass directly to vapour showing true sublimation of Type-1 represented by inner dotted lines.

Conversely, if the pressure on the solid is greater than the triple-point pressure, the solid can exist as a liquid. Thus upon heating, the solid will melt to form a liquid and at a higher temperature this liquid will pass into vapour, showing sublimation of type-2 .

The above discussion can very well be stated in two obvious facts:

a) When the vapour of the substance exerts a pressure less than the triple-point pressure, it must, on condensation, pass directly into solid by passing liquid phase.

b) When the vapour of the substance exerts a pressure greater than triple-point pressure, it must, on condensation pass to liquid phase which on further cooling solidifies.

The sublimation (Type-1) and the sublimation (Type-2) can be exemplified by the sublimation of arsenic trioxide and the iodine.

Arsenic trioxide undergoes true sublimation (Type-1). The triple-point pressure of arsenic trioxide is slightly greater than atmospheric pressure (760 mm of Hg), i.e. one atmosphere.

Hence, when heated, solid arsenic trioxide yields a vapour pressure equal to the atmospheric pressure and therefore passes directly to vapour. The vapour of the arsenic triode exerts a pressure less than the triple-point pressure, consequently upon cooling, the vapour passes direct to solid. Arsenic trioxide therefore undergoes true sublimation Type-1).

In case of iodine, the triple-point pressure of iodine is 91 mm of Hg. When it is heated, solid iodine cannot exert a vapour pressure equal to the atmospheric pressure. Consequently, iodine melts (at 114°C) at triple-point pressure, to form a liquid which (at 184°C), exerts a vapour pressure equal to atmospheric pressure (760 mm of Hg) and therefore boils. The phase changes on cooling are dependent on the pressure of the iodine vapour when condensation takes place. Thus, if the space is saturated, with iodine vapour, the latter will on cooling, condense to form a liquid which will solidify to form solid iodine. These phase changes will occur because under the conditions stated, the iodine vapours exerts a pressure greater than the triple-point pressure, and thus Type-2 'sublimation occurs.

Que. Describe in brief about sublimation.
Ans.

Sublimation:

﻿﻿﻿Sublimation is the transition of a substance directly from the solid to the gas phase, without passing through the intermediate liquid phase.

﻿﻿﻿It is an endothermic process that occurs at temperatures and pressures below a substance's triple point in its phase diagram, which corresponds to the lowest pressure at which the substance can exist as a liquid.

﻿﻿﻿The reverse process of sublimation is deposition or desublimation, in which a substance passes directly from a gas to a solid phase.

﻿﻿﻿Sublimation has also been used as a generic term to describe a solid-to-gas transition (sublimation) followed by a gas-to-solid transition (deposition).

﻿﻿﻿The term sublimation refers to a physical change of state and is not used to describe the transformation of a solid to a gas in a chemical reaction. For example, the dissociation on heating of solid ammonium chloride into hydrogen chloride and ammonia is not sublimation but a chemical reaction.

﻿﻿﻿Sublimation is caused by the absorption of heat which provides enough energy for some molecules to overcome the attractive forces of their neighbours and escape into the vapour phase.

﻿﻿﻿Since the process requires additional energy, it is an endothermic change.
The enthalpy of sublimation (also called heat of sublimation) can be calculated by adding the enthalpy of fusion and the enthalpy of vaporization.

Que. Give the statement and postulates of kinematic molecular theory of ideal gases.
Ans.

• Maxwell and Boltzmann in 1859 developed a mathematical theory to explain the behaviour of gases. It is based on the concept that gas is made of a large number of molecules in a perpetual motion. This theory is therefore called as kinetic theory of gases.
The theory is based on the following assumptions:

1. ﻿﻿﻿A gas consists of extremely small discrete particles dispersed throughout the container.

2. ﻿﻿﻿Gas molecules are in constant random motion with high velocities.

3.﻿﻿﻿Gas molecules can move freely, independent of each other.

4. ﻿﻿﻿There is no loss of kinetic energy of a molecule during a collision.

5. ﻿﻿﻿The pressure of a gas is due to the striking of gas molecules on the walls of the container.

6. The average kinetic energy molecules is the same at a given temperature.

Que. Write the characteristics of gas.
Ans.

Characteristics of gas: A gas consists of molecules separated wide apart in empty space.

They have the following characteristics:

• ﻿﻿﻿They have infinite expansibility. They expand to occupy the entire volume of the container in which they are stored.
• ﻿﻿﻿They can be compressed by application of pressure.
• ﻿﻿﻿They exert pressure on the walls of the container in all directions.

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