# Sublimation

The transition of solid-phase into gaseous phase is sublimation. The matter does not undergo a liquid phase during this phase transition, that is the solid directly turns into a gas. In the sublimation process, this reaction is an endothermic reaction, as the chemical bonds between molecules are broken down in order to release them into the air.  Therefore the energy is released when chemical bonds form energy should be given in order to break them. Therefore it is an endothermic reaction.

The energy which is calculated is the enthalpy of sublimation. Sublimation only occurs at pressures and temperatures below the triple point of a substance. The pressure and temperature at which the substance will exist in all three phases that is a solid phase, gaseous phase, and the liquid phase are the triple point of a substance.

Solid water sublimates change into gas with a  temperature increase directly, below the triple point. The examples of sublimation are, at room temperature and pressure, the dry ice turns into gaseous carbon dioxide. The organic compound, naphthalene, sublimes very easily at standard pressure and temperature.

Sublimation

## Process of Sublimation

When some molecules absorb heat energy, they are at a much higher energy state than their neighbours, hence overcoming the force of attraction and therefore escape into the vapour phase. It is called an endothermic reaction because of the additional energy required. Then sublimation is defined as heat or energy required to change the state from solid to gas. It is expressed as   $$\frac{KJ}{mol}$$  or even $$\frac{KJ}{kg}$$ .

This method is an excellent purification method because, in this process of Sublimation, only a limited number of solids are capable of sublimation. It is also a very good method for purification and separation, with a contaminated solid having non-volatile impurities. The heating of an impure solid takes place in a vessel while keeping it in contact with the cold surface. Therefore the volatile solids sublime and attach to the above cold surface, and the impurities remain below. As there is no waste generation and also there is no use of solvents, this is a very eco-friendly process. The only limitation is that it is not very useful in the separation of volatile solids from one another.

### Sublimation versus Evaporation

Both evaporation and sublimation items are useful in describing phase transitions of matter. The properties of matter are uniform in the phase of matter. Solid, liquid and gases are the three main phases of matter. The conversion of the phase of matter from one phase to another is a phase transition. In which the transition of matter from a solid phase to a gaseous phase is sublimation, whereas the transition of matter from liquid to the gaseous phase.

In sublimation, the initial phase is solid-phase whereas in evaporation the initial phase is a liquid phase. Also, in sublimation, the enthalpy gives the amount of energy that is required for sublimation to occur on the other hand in evaporation, then till we give the amount of energy that is required for a position to occur.

### Examples of Sublimation

One of the best examples of sublimation is dry ice, the frozen form of carbon dioxide. There is a direct change of phase of dry ice, from solid-state to gaseous state, when dry ice is exposed to air. It is visible as fog. The gaseous state of frozen carbon dioxide is more stable than its solid-state.

An organic compound, Naphthalene is also an example of sublimation. It is found in pesticides such as mothball. The presence of non-polar molecules sublimes this organic compound, which is held by Van Der Waals intermolecular forces. The naphthalene sublimes to vapours at 176F and while it desublimates at cool temperatures and forms needle-like crystals.

This technique is also in use in pharmaceutical companies. After the removal of water from the material, it is being stored in a vessel and thus the material is easily stored and shipped to different locations. It can be brought back to its original form at the destination. Measles virus vaccine, typhoid vaccine are examples of such products. This technique is also useful in manufacturing raw materials for pharmaceutical products.

### Applications of Sublimation

The inkjet printers are now replaced by dye-sublimation printers. The printouts are ready to use as soon as they exit the printer because the prints dry faster. These printers have easy maintenance and use minimal moving parts. There is also a wide application in the textile industry, printing of synthetic fabrics like polyester takes place by the process of dye sublimation. This technique is also useful in the manufacturing of T-shirts, flags, and banners. The advantage is that the colours in printing are extremely brilliant because of the bonding of the dye to the synthetic fibres.

Many decorative works, wash resistant, scratch proof images can be obtained with the current technology. A variety of products such as pens, bags, coffee mugs are printed with a dye sublimation at less cost. It is eco-friendly and safe and there is no production of waste in this process.

There are practical applications of sublimation in forensic sciences. To purify the volatile compounds, the purification method in use by chemists is sublimation. One important use of sublimation is in the frozen food industry which is called freeze-drying. This sublimation from the solid phase to the gas phase of frozen water in the material takes place when we reduce surrounding pressure. No heat is involved during the removal of water hence it is different from evaporation. As there is the use of very low temperatures, a high-quality product is obtained. The structure of the product is also maintained and after rehydration, excellent quality is obtained. When the food product needs to be preserved for a long time, this technique comes into use. Such as when astronauts go out to outer space, space organizations such as NASA, ISRO provide quality food using this technique.

Key Points

• The transition to a gaseous state, from a solid-state, requires a transition of the liquid state, from a solid-state and liquid state to a gaseous state.
• Solids directly sublime into the air if they possess sufficient vapour pressure at a particular temperature.
• In our day to day life, we experience various examples of sublimation.

Q1. What do you mean by the process of sublimation?

Answer: Sublimation includes freezing, melting, and evaporation. It is a shift in a state of matter. Without even undergoing the liquid phase, the substance transforms from a solid to a gas by sublimation. For example dry ice, heavy CO2.

Q.2. What are the things that sublimate?

Answer: Solids, including arsenic, water, iodine, and solid carbon dioxide (dry ice), sublimate at normal temperatures and pressures.

Q.3. What is the sublimation technique of separation?

Answer: Sublimation isolates a mixture of solids such as sublime. Many liquids change instantly,  from solid to vapour phase on heating, without going through the liquid state. This process is sublimation.

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