Separation and Purification

A. Criteria of Purity

A substance is pure if its made up of only one substance. A mixture is a substance that is made up of two or more substance that are physically mixed. The purity of substances can be determine :

1. Using Chromatography

2. Checking its melting point and boiling point

B. Chromatography

Chromatography is a technique used to :

a. Identify a substance

b. Determine the purity of substance

c. Separate two or more substance in a mixture

Principle of Chromatography

· Different substance have different solubility in the same solvent

· The more soluble substance will dissolve faster into the solvent

· The more soluble substance will then be carried along faster by the solvent and move ahead of the less soluble solutes

There are two types of paper chromatography :

a. Ascending type – solvent moves up the chromatography paper

b. Descending type – solvent moves down the chromatography

Picture 1.1 Types of Paper Chromatography

C. Melting and Boiling Point

Melting and boiling point determination can be used to identify a substances. A pure substances has a definite (fixed) melting point. A pure substances has a definite (fixed) boiling point. The greater the amount of impurities, the lower the melting point of the solid. An impure solid substances will melt over a range of temperatures. Impurities increase the boiling point of a pure liquid. The greater the amount of impurities, the higher the boiling point of the liquid. The liquid boils over a range of temperatures.

Picture 1.2 Impurities Lower the Melting Point

Importance the Purity of Substance

1. Foodstuffs that are impure, i.e. contaminated with harmful substances can cause adverse side effect on the human body and in extreme cases may even lead to death

2. Drugs that are impure can cause adverse allergic reactions that may lead to death

3. Pure silicon is used to make silicon chips for making microprocessors

4. Pure copper is needed to make copper wires used in electrical wiring.

A. Method Separation

Separating a solid from liquid :

a. Filtration

Filtration is used to separate small insoluble solid from liquid such as clay, limestone and sand.

Picture 1.3 Insoluble Solid

Separating limestone and water

Picture 1.4 Filtration Method

Procedure :

· Fold a piece of filter paper and fit it into the filter funnel

· Wet filter paper with a little distilled water

· Pour the mixture into the filter funnel

· The insoluble limestone remain in the filter funnel as residue

· The liquid water passes through the filter paper and is collected to filtrate

· Wash the residue with distilled water

· Dry the limestone with a few sheet filter paper

b. Evaporation

Evaporation to dryness is used to separate a soluble solid from its solution. Common

salt dissolves in water to form a salt solution. To completely remove the water, the salt solution is evaporated to dryness. The salt left behind. This method cannot be used if the solid is unstable to heat. The solid will decompose when heated strongly. Examples of solids that decompose on strong heating :

· Sugar crystal will decompose to give water and carbon

· Blue copper (II) sulfate crystals decompose to form white crystals

Picture 1.5 Sodium Chloride

c. Crystallization

Crystallization is used to separate a soluble solid from its solution. This method should be used for different solubility solids that decompose on heating

Picture 1.6 Copper (II) Sulfate

Separating Crystal Copper (II) sulfate from salt solution

Picture 1.7 Crystallization Method

Procedure :

· The aqueous copper (II) sulfate solution is poured into an evaporating dish.

· The solution is heated to evaporate off some water

· Continue heating until the solution becomes saturated

· Allow the hot saturated solution to cool

· Crystallization occurs and blue crystals of copper (II) sulfate are formed

· Filter to remove the crystals

· Wash crystals with a little cold water and dry the crystals by pressing between layers of filter paper

Separating a Mixture of Solid

a. Dissolution

Dissolution is used to separate a mixture of a soluble and an insoluble solid by using a suitable solvent. The solvent chosen will dissolve one of the solids in the mixture. The insoluble solid can then be remove by filtration.

Picture 1.8 Sand and Salt

Separating sodium chloride and sand with water as a solvent

Picture 1.9 Dissolution Method

Procedure :

· Add distilled water to the mixture and stir

· Sodium chloride will dissolve in the water whereas sand will not dissolve

· Filter the mixture

· The insoluble sand remains in the filter funnel as residue

· The soluble sodium chloride passes through the filter paper and is collected as filtrate

· The filtrate is then evaporated to dryness to obtain the sodium chloride

b. Using Magnet

Metal such as iron, cobalt, nickel and steel are magnetic. Hence these metals can be separated from other non metallic solids.

Picture 1.10 Metal

Separating iron filings and sulfur powder

Procedure :

· Place the mixture in evaporating dish

· Use a bar magnet to stir the mixture

· The iron filings will stick to the magnet

· Scrap off the iron filings from the magnet

· The sulfur powder remains in the dish

C. Sublimation

Sublimation is when these substances are heated, they change straight from solid state into a vapor without becoming liquid.

Particle 1.11 Ammonium Chloride

Separating sand and iodine can be separated using the sublimation technique

Picture 1.12 Sublimation

Procedure :

· Place the mixture in an evaporating dish covered with an inverted filter funnel

· Gently heat the mixture

· The iodine changes into a gas directly on heating

· The purple fumes of iodine rise and condense on the cold funnel as iodine crystals

The non-volatile sand remains in the evaporating dish

Separating a Liquid from Solution

a. Simple Distillation

A solution is formed by dissolving a solute in a solution to make a solution. Simple distillation is a process of liquid boiled and the vapor produced is condensed back to a liquid. Simple distillation carried out to separate a volatile liquid solvent from a non-volatile. A volatile liquid has low boiling point and the non-volatile has high boiling point

Picture 1.13 Simple Distillation Salt and Water

Separating salt solution by simple distillation

Picture 1.14 Simple Distillation Method

Salt has Boiling point1413^oC

Water has Boiling point 100^oC

Procedure :

· The distillation flask should not be more than half full

· Add a few pieces of boiling chips to smooth the vaporization of the liquid

· Ensure the bulb of the thermometer is opposite to the side arm exiting from the flask

· Heat the flask on a wire gauze or sand tray so that heat is not concentrated on a single spot

· Heat the solution until it boils

· The water vaporizes

· The vapor rises up the flask and enters the condenser.

· In the condenser, the water vapor condenses into liquid water

· Pure water is collected as the distillate in a collecting vessel

· The collecting vessel must not be sealed to the condenser to avoid explosion

· The salt remains in the distillation flask

b. Fractional Distillation

Fractional distillation is used to separate two or more miscible liquid with different boiling points. A fractionating column filled with glass beads is attached to the distillation flask. As the vapors rise up in the fractionating column, the boiling and condensation process occur repeatedly on the glass beads. The liquid with the lowest boiling point is distilled out first.

Application

Fractional distillation is used in the industries :

To separate petroleum into various useful fractions such as petrol, diesel, kerosene, lubricating oil and bitumen
To obtain nitrogen from liquid air to be used in the Haber process for the manufacture of ammonia
To obtain wine, beer, whisky, and other liquor from fermented liquor

Picture 1.15 Fractional Distillation

Separating of methanol (b.p 65^oC) and ethanol (b.p 78^oC) can be separated using fractional distillation.

Procedure :

· The distillation flask should not be more than half full

· Add a few pieces of boiling chips

· The liquid mixture is heated and brought to boil.

· Both ethanol and water change to vapor

· The more volatile ethanol vaporizes more easily, producing more ethanol vapor in the vapor phase

· The vapor rise up the fractionating column

· The glass beads provide large surface area so that vapor can undergo repeated condensation and vaporization as they rise up the fractionating column

· The first vapor to reach the top of the fractionating column is the lowest boiling point.

· Methanol will distil over when the thermometer reads a constant 65^oC.

· When all the methanol has distilled out, the temperature will rise again

· The receiver is changed to collect ethanol which will distil over at 78^oC

Picture 1.16 Boiling Point Methanol and Ethanol

Separating a Mixture of Immiscible Liquid

a. Using Funnel

Two or more component of liquid that do not dissolve in each other. These liquid are said to be immiscible. A mixture of two immiscible liquids will separate into two layers on standing . The less dense liquid will form the top layer, whereas the denser liquid will be the lower layer. A separating funnel is used to separate two immiscible liquids.