6.3 Separating Mixtures

General notes on mixtures separation

Filtration

• The choice of the method of separation depends on the nature of the substances being separated

• All methods rely on there being a difference of some sort, usually in a physical property such as boiling point between the substances being separated

Filtration

• Used to separate an undissolved solid from a mixture of the solid and a liquid / solution ( e.g. sand from a mixture of sand and water). Centrifugation can also be used for this mixture

• Filter paper is placed in a filter funnel above another beaker

• The mixture of insoluble solid and liquid is poured into the filter funnel

• Filter paper will only allow small liquid particles to pass through in the filtrate

• Solid particles are too large to pass through the filter paper so will stay behind as a residue

Selective solubility

Alcohol is flammable, therefore it cannot be heated directly. To heat alcohol, we should

use a steam bath or an electric heater.

If you need to collect sugar from sugar alcohol solution heat the solution using an electric

heater to crystallization point. Leave the solution to cool and crystals to form. Filter off the

crystals.

Crystallization

Crystallisation

• Used to separate a dissolved solid from a solution, when the solid is more soluble in hot solvent than in cold (e.g. copper sulphate from a solution of copper (II) sulphate in water)

• The solution is heated, allowing the solvent to evaporate and leaving a saturated solution behind

• You can test if the solution is saturated by dipping a clean, dry, cold glass rod into the solution

  • If the solution is saturated, crystals will form on the glass rod when it is removed and allowed to cool

• The saturated solution is allowed to cool slowly and solids will come out of the solution as the solubility decreases, and crystals will grow

• Crystals are collected by filtering the solution

• They are then washed with distilled water to remove any impurities and then allowed to dry

Sublimation: Examples of solids that can sublime at room temperature:

1) Solid iodine, I2 (s)

2) Dry ice or solid carbon dioxide CO2 (s)

3) Any ammonium compound as ammonium chloride, NH4Cl and ammonium bromide,

NH4Br

Simple Distillation

Fractional distillation

Distillation: Simple & Fractional

Simple Distillation

• Used to separate a liquid and soluble solid from a solution (e.g. water from a solution of saltwater) or a pure liquid from a mixture of liquids

• The solution is heated and pure water evaporates producing a vapour which rises through the neck of the round-bottomed flask

• The vapour passes through the condenser, where it cools and condenses, turning into pure water which is collected in a beaker

• After all the water is evaporated from the solution, only the solid solute will be left behind

Diagram showing the distillation of a mixture of salt and water

• Simple distillation can be used to separate the products of fermentation, such as alcohol and water

• However, more effective separation is to use fractional distillation where the liquids are closer to boiling point and a higher degree of purity is required

 Fractional distillation

• Used to separate two or more liquids that are miscible with one another (e.g. ethanol and water from a mixture of the two)

• The solution is heated to the temperature of the substance with the lowest boiling point

• This substance will rise and evaporate first, and vapours will pass through a condenser, where they cool and condense, turning into a liquid that will be collected in a beaker

• All of the substance is evaporated and collected, leaving behind the other components(s) of the mixture

• For water and ethanol: ethanol has a boiling point of 78 ºC and water of 100 ºC. The mixture is heated until it reaches 78 ºC, at which point the ethanol boils and distills out of the mixture and condenses into the beaker

• When the temperature starts to increase to 100 ºC heating should be stopped. Water and ethanol are now separated

Fractional distillation of a mixture of ethanol and water

• An electric heater is safer to use when there are flammable liquids present

• The separation of the components in petroleum is achieved by fractional distillation on an industrial scale

• Fractional distillation of crude oil is not carried out in school laboratories due to the toxic nature of some of the components of the crude oil, but it can sometimes be simulated using a synthetic crude oil made specially for the demonstration

fractional distillation of

liquefied air and fractional distillation of crude oil.

Separating funnel

Adsorption: means sticking to the surface.

Adsorption: sticking of the particles of one material on the surface of another. Examples

of adsorbing substances:

Silica gel: adsorbs water vapor

 Charcoal: adsorbs gases with strong odor and removes colored impurities from a solution

Chromatography

It is the technique used to separate different compounds, especially those that can be easily

destroyed by heat or chemicals.

 It can be used to separate colored components as:

1) Green liquid obtained by squashing green leaves.

2) Black ink

 The property that carries the liquid up the paper is capillary action.

Study of a chromatography paper after it dries:

Chromatography could be used to check the purity of a dye as well as a separation technique.

Dyes 1 and 2 are soluble pure substances . The spot moved up the paper with the solvent and

did not split.

Dyes 3, 4, 5 and 6 are mixtures of soluble components. The spot moved up with the solvent

and split into more than one spot. The greater the solubility of a component the greater is its

Rf value the larger is the separation from the base line.

Dyes 2 and 5, 3 and 5, 3 and 6 have a common component same Rf value. Rf value is a

physical constant.

If a dye sticks to the x on the base line this dye is considered to be insoluble in the solvent

aper Chromatography

Principles of Paper Chromatography

• A solution is a mixture that cannot be separated by filtration, therefore an alternative separation technique must be used

• Paper chromatography is used to separate a mixture of liquids that have different intermolecular interactions in a given solvent (e.g. separate the coloured inks that were used to make black ink)

• The substances can be identified by the distance they travel across a sheet of chromatography paper

• The stronger the interaction with the solvent, the farther the distance the component will travel up the chromatography paper

• The solvent will travel up the paper because the liquid molecules will have adhesive and cohesive forces with the pores of the chromatography paper

  • Since the solvent moves up the paper, the solvent is called the mobile phase

  • Since the chromatography paper stays in a fixed position, it is called the stationary phase

• The different components of the mixture will be separated because of the different intermolecular interactions with the mobile and the stationary phases

Conducting a Paper Chromatography analysis

1. Draw a pencil line at the bottom of the chromatography paper. A sample of the mixture is placed as a spot on the top of the line. Additional spots of known reference compounds are placed to identify the components of the mixture

2. Place the paper into the solvent container, making sure that the line sits above the level of the solvent. This will prevent the samples from washing into the solvent container

3. Analyze the chromatography paper by comparing the sample with the reference compounds

Diagram to Show the Method of Chromatography

Analysis of the composition of ink that can be used using paper chromatography. Blue ink travels the most distance showing the strongest interaction with the solvent and the weakest interaction with the chromatography paper

• The component that travels the greatest  distance along the paper has the strongest intermolecular interaction with the mobile phase and the weakest interaction with the stationary phase

• The component that travels the shortest distance along the paper has the weakest intermolecular interaction with the mobile phase and the strongest interaction with the stationary phase

• These values gives important information about the identity of the components and their polarities

• Since the stationary phase is polar, the higher the Rf values the more polar the component

• The Rf value and the difference between the separated substances will also depend on their solubility in the solvent that acts as the mobile phase

Calculating the Rf value