Separation of Mixtures

Separation of Mixtures

At the end of the lesson you will be able to

·        define  key  terms  such  as  solute, solvent, solution, filtration, filtrate, distillation, distillate, centrifugation, and chromatography

·        analyse and select appropriate methods for separating a given mixture, based on certain difference in physical properties

·        describe appropriate methods of separating a given mixture

·        perform simple experiments involving separation of mixtures

·        identify and assemble the suitable set of apparatus used for separating the components of a given mixture

·        explain the basic principles involved in filtration, centrifugation, distillation and chromatography

·        gather information about the industrial applications of the different techniques of separation

1. Introduction

A mixture as you know contains more than one substance in which the components can either be elements or compounds or both. We separate the components of a mixture very often as they contain useful substances mixed with harmful or unwanted substances which have to be removed. The choice of a particular method to separate components of a mixture will depend on the properties of the components of the mixture as well as their physical states.

2. Separation of solid – liquid mixtures

Before we talk about the separation methods let us recall briefly some aspects of solubility of solid and liquid. When a solid is added to a liquid, either the solid will dissolve in the liquid or not.

·        When the solid dissolves in the liquid, it is said to be soluble i.e. Solid (solute) + Liquid (solvent) Solution.

·        When the solid does not dissolve in the liquid, it is said to be insoluble.

Separation of insoluble solids from liquids

Filtration and Decantation: You are already familiar with these methods. The illustrations given below will help you to recall these important techniques.

Centrifugation: is is used to separate very ne and tiny particles of solid which do not settle down easily in a liquid. The mixture taken in a centrifuge tube is centrifuged (by rotation) in a centrifuging machine, so that the solid gets deposited at the bottom of the tube and the clear liquid (supernatant) is decanted. E.g. this is used to separate plasma (the liquid) from blood.

Separation of soluble solids from liquids

Evaporation and crystallisation: This is used to separate the dissolved solute from the solution. The solution is heated slowly so that the liquid (solvent) evaporates leaving behind the solid as crystals. E.g. Separation of salt from sea water (by solar evaporation in saltern).

Salterns in Tuticorin of Tamil Nadu

Simple distillation: This method is used to separate two liquids whose boiling points differ by more than 25 K. Also by this method, brackish water can be distilled.

Procedure: A distillation flask is fixed with a water condenser. A thermometer is introduced into the distillation flask through a one-holed stopper. The bulb of the thermometer should be slightly below the side tube.

The brackish water (sea water) to be distilled is taken in the distillation flask and heated for boiling. The pure water vapour passes through the inner tube of the condenser. The vapours on cooling condense into pure water (distillate) and are collected in a receiver. The salt are left behind in the flask as a residue.

3. Separation of liquid – liquid mixtures

a) Type I – Miscible liquids

Fractional distillation: To separate two or more miscible liquids which do not differ much in their boiling points (difference in boiling points is less than 25 K) fractional distillation is employed.

Example: Refining of petroleum product by fractional distillation.

Applications of fractional distillation

Fractional distillation is used in petrochemical industry to obtain different fractions of petroleum, to separate the different gases from air, to distil alcohols etc.

b) Type II: Immiscible liquids

Mixtures of two immiscible liquids are separated by using a separating funnel.

Examples: Mixture of water and oil, Mixture of water and kerosene.

Two immiscible liquids can be separated by solvent extraction method, which is also called as liquid – liquid extraction method. is method works on the basis of difference in solubility of two immiscible liquids in a suitable solvent. Solvent extraction method is used in soap, pharmaceutical and petroleum industries.

Separation of mixture containing volatile and non-volatile solids

(i) Sublimation: Certain solid substances when heated change directly from solid to gaseous state without attaining liquid state. The vapours when cooled give back the solid substance. This process is known as sublimation. Examples: (a) Iodine (violet vapours) (b) Camphor, (c) Ammonium chloride etc.

The powdered mixture of Ammonium chloride and sand is taken in a china dish and covered with a perforated asbestos sheet. An inverted funnel is placed over the asbestos sheet as shown in the gure. The open end of the stem of the funnel is closed using cotton wool and the china dish is carefully heated. The pure vapours of the volatile solid pass through the holes in the asbestos sheet and condense on the inner sides of the funnel. The non-volatile impurities remain in the china dish.

Separation of mixture containing volatile and non-volatile solids

Before we discuss the technique we will take a look at two important terms that chromatography involves: Absorption and Adsorption.

Absorption is the process in which the substance is dissolved throughout the bulk of another substance. For example a paper (absorbent) soaks up or absorbs water.

Adsorption is the process in which particles of a substance (it could be gas, liquid or dissolved solid) adhere to a surface of another substance.

For example: charcoal adsorbs gases on its surface. Charcoal is called the adsorbent and the gas is called the adsorbate.

Chromatography is a separation technique. It is used to separate different components of a mixture based upon their different solubilities in the same solvent.

There are several types of chromatography; based on the above basic principles. It involves separation of mixtures by allowing the constituents of the mixture to move between two phases namely

I. Mobile phase

II. Stationary phase

The simplest type is paper chromatography. Here, the stationary phase is the chromatography paper and the mobile phase is the solvent. For example, to separate the different-coloured dyes in a sample of ink, a spot of the ink (e.g. black ink) is put on to a piece of chromatography paper. is paper is then set in a suitable solvent as shown in Figure. The black ink separates into its constituent dyes. As the solvent moves up the paper, the dyes are carried with it and begin to separate. They separate because they have different solubility in the solvent and are adsorbed to different extents by the chromatography paper. The chromatogram shows that the black ink contains three dyes.

We can also draw important inferences from a numerical measurement called Rf (Retention factor) values using the obtained chromatograms. Rf value is defined as the ratio of the distance travelled by the solute spots to the distance travelled by the solvent.

Applications

Chromatography is used extensively in medical research and forensic science laboratories to separate a variety of mixtures. For example, protein samples are separated by electrophoresis in medical research laboratories.