Separation Techniques

• The components of a mixture can be separated based on the difference in physical properties of each component.
• To choose a suitable method of separation and purification, the type of mixture must first be identified.

There are different types of mixtures such as solid–solid, liquid–liquid, and solid–liquid mixtures. The components of a mixture can be separated and purified based on differences in their physical properties.

1. Hold a magnet close to the mixture to attract magnetic solids (e.g. iron spheres), while leaving behind other non-magnetic solids (e.g. sand).

• Strong magnets are used to separate iron and steel from waste products at recycling plants.

1. The solid mixture is poured through the sieve. The solids with smaller particles will pass through the pores of the sieve while the solids with larger particles will remain in the sieve.

• To sieve flour during baking
• To sieve soil samples to recover artefacts
• To perform particle size analysis of granular matter (e.g. soil samples)

• Sublimation: the direct change in the physical state of a substance from solid to gaseous state.
• Vapour deposition: the direct change in the state of a substance from gaseous to solid state.

• Examples of substances that sublime are iodine, dry ice, and naphthalene (found in mothballs).
• Sublimation is a method used to separate a mixture of solids, of which only one sublimes.
• It can be used to obtain a pure sample of the solid which sublimes. The resulting solid deposit is known as the sublimate.

1. Place an inverted filter funnel above an evaporating dish containing the solid mixture.
2. Heat the mixture gently.
3. As the solid mixture in the evaporating dish is heated, iodine sublimes to form iodine vapour.
4. When the iodine vapour touches the cooler inner surface of the inverted filter funnel, it undergoes vapour deposition to form iodine solids.

• A solution is made up of solids (solute) that are soluble and dissolved in a liquid (solvent).
• A suspension is made up of solids that are insoluble in a liquid.

• Filtration is used to separate a suspension into its components, the liquid and the insoluble solids, and can be used to collect either of the components.
• The use of a filter allows the filtrate (liquid) to pass through but retains the residue (insoluble solids).
• The components of a suspension are separated based on their difference in particle size.

1. Set up a filter funnel with filter paper over an empty beaker.
2. Pour the suspension into the filter funnel through the filter paper.
3. Rinse the residue with distilled water and pat dry between sheets of filter paper.

• Evaporation to dryness is used to obtain soluble solids (solute) from a solution based on the difference in boiling points of solute and solvent.
• The solvent has a lower boiling point and will evaporate first, leaving the solute behind.
• It is used to obtain solids that do not decompose on heating.

1. Heat aqueous sodium chloride in an evaporating dish until all the water has evaporated.

• Evaporation to dryness is used to harvest sea salt from seawater. A shallow hole is dug in the ground and filled with seawater. Water evaporates due to heat from the sun, leaving behind solid sea salt that can be collected by raking.

• Crystallisation is used to obtain pure soluble solids (solute) from a saturated solution.
• A saturated solution contains the maximum amount of solute that can be dissolved in the solvent at a particular temperature.

Crystallisation is used to obtain solids with the following properties:

• Decompose on heating
• Solubility in the given solvent decreases with decreasing temperature

1. Heat the solution gently until a saturated solution is obtained. Allow the hot, saturated solution to cool at room temperature to form pure crystals.
2. Filter to collect the crystals as the residue.
3. Rinse the crystals with cold distilled water and pat dry between sheets of filter paper.

When the hot saturated solution is cooled to room temperature, the dissolved solids will be formed as pure crystals. This is because as the temperature of the solution decreases, the solubility of the solids in the solution decreases. Solids that can no longer remain dissolved at a lower temperature start to form crystals.

• Crystallisation can be used to purify substances in the manufactured food industry and pharmaceutical industry.

• Simple distillation is used to separate a pure liquid (solvent) from a solution based on the difference in boiling points of solute(s) and solvent.
• The solvent has a lower boiling point and will vaporise first and be collected by condensation while the solute(s) remain(s) in the distillation flask.

1. Pour the mixture into a distillation flask. Add boiling chips to ensure smooth boiling.
2. Heat the solution until a constant temperature reading is obtained.
3. Collect the distillate in the conical flask.

• Simple distillation is used to obtain pure water from seawater using desalination.

• Reverse osmosis makes use of pressure to force a solution through a partially permeable membrane.
• The membrane acts as a filter, allowing only solvent particles that are small enough to pass through, while retaining solute particles which are larger. As a result, the solvent is purified.
• The partially permeable membrane serves as an extremely fine filter capable of retaining bacteria, salts, sugars, proteins, particles, dyes, and other constituents.
• The solvent is purified and collected on the other side of the membrane.

• Reverse osmosis is used to obtain pure water from wastewater.

• Miscible liquids can mix with each other to form a solution.
• Immiscible liquids cannot mix with each other and hence, separate into distinct layers.

• Fractional distillation is used to separate mixtures of miscible liquids based on differences in boiling points.
• Upon heating, the liquid with the lowest boiling point will be the first to boil, rises and enters the condenser, where it is cooled and condensed to form liquid which is collected as the first distillate.

1. Pour the mixture of water and ethanol into the round bottom flask. Add boiling chips to ensure smooth boiling.
2. Heat the solution until a constant temperature reading is obtained.
3. Collect the distillate in the conical flask.
4. Change the conical flask once the temperature reading begins to rise to collect the next distillate.
5. Repeat Steps 2 to 4 until the mixture in the round bottom flask has completely boiled off.

• A fractionating column contains many glass beads which provide a large surface area for vapours of substances that have not reached their boiling points to condense and fall back into the round bottom flask.
• This ensures that only the substance that has reached its boiling point is passed through the condenser.

• To separate crude oil (petroleum) into its components (e.g. petrol, diesel)
• To separate liquefied air into its components (e.g. nitrogen, oxygen, argon)
• To extract ethanol produced by glucose fermentation in breweries

• A separating funnel is used to separate immiscible liquids based on differences in densities.

• A tap is used to control the flow of liquid out of the separating funnel.
• The liquid with the higher density will form at the bottom and be collected first.

• Chromatography is a method used to separate and identify components that have different solubilities in a given solvent.
• It can be used to determine whether a sample is pure. Only a small amount of the sample is required for analysis.

• The solvent and solutes move up from the bottom of the chromatography paper.
• As the solvent travels up the paper, the solutes present in the unknown mixture will dissolve and travel up the paper at different speeds based on their solubility in the solvent.

1. Draw a start line near the bottom of the chromatography paper using a pencil.
2. Place and label drops of the mixture and known dyes along the start line.
3. Secure the chromatography paper with tape onto a watch glass that covers a beaker containing the solvent. Ensure that the solvent level is below the start line. This ensures that the substances do not dissolve in the solvent directly but travel up the paper.
4. Allow the chromatogram to stand. Remove it once the solvent is close to the end of the paper.
5. Mark out the solvent front with a pencil.
6. Measure and record the distance travelled by the dyes when the solvent front is reached.
7. Identify components of the unknown mixture by comparing their positions with known substances.
8. Determine the retention factor (Rf) value of each component.

• Colourless substances on the chromatogram can still be identified through the use of locating agents and/or ultraviolet (UV) light.
• Locating agents are chemicals that react with the substances on the chromatogram to form coloured products.
• Some colourless substances are visible under UV light. Hence, by spraying a locating agent on a chromatogram and/or examining the chromatogram under UV light, colourless substances can be made visible for analysis.

• If a sample is a mixture, chromatography can be used to identify the individual components that are present in the sample based on their respective Rf values.
• Rf values cannot exceed 1 and are usually expressed to two decimal places.
• Rf values of components are affected by properties such as their solubilities in a given solvent. The basic principle is that a component with a higher solubility in a given solvent will travel a further distance.
• The Rf value of a particular component is constant for the same solvent, chromatography paper, and temperature. Hence, substances can be identified by comparing their Rf values with values of known substances.

• To determine the purity of substances
• To separate components (e.g. DNA fragments) in forensic samples
• To separate and identify colourings, additives or unauthorised substances in food
• To separate and identify banned substances (e.g. drugs) in urine or blood samples