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# Separation by Evaporation

Evaporation is the process of the separation of a solid substance that is dissolved in water. The application is based on the fact that solids do not vaporize easily, whereas liquids do. On performing evaporation, the solid substance is left behind as a residue. It is a vaporization technique where the liquid changes into the gaseous phase and residues on the surface.

Evaporation continues until the state of equilibrium is attained. However, in an enclosed space, a liquid will continue to vaporise until air saturation is achieved. In practicality, only a small fraction of the total molecules possess the heat energy that is required to vaporise.

### Factors affecting rate of evaporation

• Pressure: Less pressure implies a greater rate of evaporation. On exerting less pressure, the molecules keep from evaporating themselves.
• Temperature of the substance: On increasing the temperature of the substance, the material heats up and the constituent particles begin moving with greater kinetic energy. This leads to an increase in the rate of evaporation. For example, hot water boils faster than cold water.
• Surface of the substance: Since any substance with a larger surface area will contain more surface molecules per unit of volume, it triggers the potential of the particles to escape, thereby increasing the rate of evaporation.
• Inter-molecular forces: Stronger inter-molecular forces between the molecules of the water in a liquid state will lead to the application of greater forces to allow the particles to escape away. An indicator of the intermolecular forces is provided by the enthalpy of vaporization.
• Flow rate of the atmosphere: The air which is unsaturated with any kind of substances, for instance, fresh air, hovers over the substance, then it will lead to the increase in the concentration of the substance in the air. This increases the rate of evaporation.

### Evaporative Equilibrium

On carrying out evaporation in an enclosed space, the molecules turning into vapour tend to accumulate above the surface of the liquid. With an increase in the density and pressure of the liquid, more of these molecules return back to the surface of the liquid. There is also an increase in the frequency of these returning particles.

At Equilibrium:

The rate of evaporation becomes equivalent to the rate of condensation. As soon as the process of escape of the water molecules become equivalent to the return, the escaping vapour is said to be saturated. The system attains a level where the equilibrium state is directly related to the vapour pressure of the substance At this point, the following factors remain constant:

• Vapour pressure
• Density
• Liquid temperature will occur.

The Clausius–Clapeyron relation conveniently elaborates this relation which lays emphasis on the fact that the rate of evaporation of any liquid holds a relation to the vapour pressure maintained in the enclosed system:

Here, P1 is the vapour pressure associated with time T1 and P2 at T2 respectively.

R is the universal gas constant. ΔHvap is referred to as the enthalpy of vaporisation

### Advantages of Separation by Evaporation

• Manufacture of concentrators: Beverage producers can create concentrates, for instance, juice concentrators. Dairy producers also use the technique of evaporation to use it to dry lactose into powders
• Water containing minerals can be evaporated to extract the metals and minerals which simulates the chemical processes. This can also be used to provide boiler feed.
• Seawater can be evaporated to produce drinking water.
• It can be used to concentrate solution by evaporating the boiling mixture.
• Removal of water from an aqueous solution containing a mixture, for instance, sodium chloride, sodium hydroxide, glycerol, glue and milk

### Disadvantages of separation by evaporation

• Loss of a substance into the atmosphere – One of the substances is usually lost to the atmosphere by performing evaporation. The vapour has to be recondensed to preserve it.
• This process is not suitable for systems where the solids are not soluble in mixtures.
• Loss of heat energy on subjecting the system to the evaporation process.

### Applications of Evaporation

• Separation of common salt from water: A mixture of common salt and water is taken in a petri dish and subjected to constant heating. When the system attains the state of evaporation, the water escapes the surface of the dish while the common salt remains on the dish. The liquid that escapes the surface cannot be recovered back.

• Separation of sugar and water: A mixture of sugar and water can be separated by heating the mixture in a china dish. The sugar remains in the container as a residue while the water evaporates upon heating up to the equilibrium point. If the solvent used is alcohol, the sugar remains dissolved in water.
A mixture of sugar water can be subjected to evaporation to make rock candy.
• Separation of copper sulphate crystals from copper sulphate solutions: The chemical compound of copper sulphate is soluble in water, such that its crystals dissolve in water to form a copper sulphate solution. When we evaporate the mixture, the water evaporates away and the solid copper sulphate crystals are left behind as a residue.
• Separation of a volatile component from a non-volatile one.

### Sample Questions

Question 1: Illustrate the complete stepwise procedure of separation of sodium from sodium sulphate solution.

1. A dish is filled with water and few drops of dilute sulphuric acid are added.

2. The water is heated. Upon boiling, the sodium sulphate powder is added while continuous stirring.

3. The powder is added till it stops dissolving further in the mixture.

4. The solution is filtered and left for cooling.

5. The sodium crystals will slowly separate out.

Question 2: Can evaporation be used to separate dye from water?

Dye is referred to as the coloured component of the ink. Upon performing evaporation, the coloured part of the ink can be filtered out.

Question 3: Give a daily life example to validate the separation by evaporation.

Nail polish applied on the hand evaporates due to the presence of the liquid solution, acetone in it.

Question 4: Illustrate one major disadvantage of separation by evaporation.

It incurs high usage of resources such as capital and energy. It also puts an overhead on the maintenance costs. Difficulty may occur in carrying out the procedure due to large size of apparatus and enhanced temperatures. Some of the other problems are corrosion, scaling as well as foaming.

Question 5. How to separate water from milk.

By Measuring the density of milk and separate water by evaporation till the desired density is achieved.

Question 6. Evaporation in the dairy industry is a preliminary step to which of the following process?

• Drying
• Flavoring
• Watering
• Pasteurization

Drying

Dairy industry evaporation is used for concentration duties such as milk, skim milk and whey. It is also used as a preliminary step to drying. Milk products intended for milk powder are normally concentrated from an initial solids content of 9 – 13% to a final concentration of 40 – 50% total solids before the product is pumped to the dryer.

Question 7. Which evaporators can be used when a low degree of concentration is required?