Separating Mixtures (DP IB Chemistry): Revision Note

Separating mixtures

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

• All methods rely on some kind of difference in properties of the substances being separated

  • This is usually in a physical property such as boiling point or solubility

Separating a mixture of solids

• Differences in solubility can be used to separate solids

• For a difference in solubility, a suitable solvent must be chosen to ensure the desired substance only dissolves in it and not other substances or impurities, e.g. to separate a mixture of sand and salt, water is a suitable solvent to dissolve the salt, but not the sand

Solvation

• Solvation is the process where a solute dissolves in a solvent because the solvent particles surround and attract the solute particles

Example use(s):

• Separating sand from water

Steps:

1. Add the mixture to a suitable solvent (e.g. water)

2. Stir to help the soluble substance dissolve

3. Insoluble substances will remain undissolved

4. The soluble substance forms a solution with the solvent

Notes:

• In water, this process is called hydration

• Solvation is essential for separation techniques like filtration and crystallisation

• The solvent must be carefully chosen to dissolve only the desired component

Filtration

• Filtration is used to separate an undissolved (insoluble) solid from a mixture of solid and liquid or solution using filter paper

Example use(s):

• Separating sand from water

• Collecting crystals after crystallisation

Steps:

1. Place a filter paper in a funnel above a clean beaker

2. Pour the mixture into the funnel

3. The liquid (filtrate) passes through the filter paper

4. The solid (residue) remains on the paper

5. This works because solid particles are too large to pass through the pores in the filter paper

Filtration diagram

Notes:

• Vacuum filtration can be used for very fine solids that clog filter paper under gravity filtration

• Centrifugation can also be used to separate solid–liquid mixtures, especially when particles are too small or dense for standard filtration

• Filtration is often used after solvation to remove undissolved solids

Crystallisation

• Crystallisation is used to separate a dissolved solid from a solution when the solid is more soluble in hot solvent than cold

Example use(s):

• Recovering copper(II) sulfate crystals from solution

Steps:

1. Heat the solution gently to evaporate some solvent

2. Allow the solution to cool until saturated

3. Test saturation by using a cold glass rod (crystals form on the rod)

4. Leave the saturated solution to cool slowly

5. Crystals will form as the solubility decreases

6. Filter to collect the crystals

7. Wash the crystals with distilled water and dry on filter paper

Crystallisation technique diagram

Notes:

• Do not boil to dryness – this may prevent crystal formation

• Cool slowly – slower evaporation allows larger crystals to grow

Recrystallisation

• Recrystallisation is used to purify an impure solid by dissolving it in hot solvent and allowing it to crystallise as the solution cools.

Example use(s):

• Purifying benzoic acid

• Removing solid and soluble impurities from a sample

Steps:

1. Add a minimum amount of hot solvent to the impure solid until it dissolves

2. If solid impurities remain, perform hot filtration to remove them

3. Allow the solution to cool slowly to room temperature

4. Crystals of the purified product will begin to form as solubility decreases

5. Collect the crystals by filtration (preferably using Büchner apparatus)

6. Wash the crystals with fresh cold solvent to remove soluble impurities

7. Dry the crystals on filter paper

Recrystallisation equipment diagram

Notes:

• Always use the minimum amount of solvent to avoid loss of product

• Cooling the solution slowly helps form larger, purer crystals

• Büchner filtration under reduced pressure is faster and more effective than gravity filtration

• This method separates based on differential solubility in hot and cold solvent

Simple distillation

• Simple distillation is used to separate a solvent from a solute or a pure liquid from a mixture, based on differences in boiling point

Example use(s):

• Separating water from a salt solution

• Recovering pure solvent from a chemical reaction mixture

Steps:

1. Heat the solution in a round-bottomed flask

2. The liquid with the lowest boiling point evaporates first

3. Vapour passes into the condenser, where it cools and condenses

4. The distillate (purified liquid) is collected in a clean beaker

5. The solute or other components are left behind in the flask

Simple distillation diagram

Notes:

• This method is suitable when the liquid being collected has a significantly lower boiling point than the other components

• A more effective separation of liquids with similar boiling points (e.g. ethanol and water) is achieved using fractional distillation

Fractional distillation

• Fractional distillation is used to separate two or more miscible liquids with similar boiling point

Example use(s):

• Separating ethanol and water

• Separating components of crude oil on an industrial scale

Steps:

1. Heat the mixture in a round-bottomed flask

2. The liquid with the lowest boiling point evaporates first

3. Vapour passes up the fractionating column, allowing better separation of components

4. Vapour enters the condenser, cools and condenses into a liquid

5. The distillate is collected in a beaker

6. As the temperature rises, the next component evaporates and is collected in turn

7. Stop heating when the target components have been separated

Fractional distillation diagram

Notes:

• For ethanol (78 °C) and water (100 °C), heat to 78 °C to distil the ethanol

• Use an electric heater when flammable liquids are present

• Crude oil is separated by fractional distillation in industry, but this cannot be safely done in school labs

• Some schools use synthetic crude oil to simulate the process in demonstrations

Paper chromatography

• Chromatography is used to separate dissolved substances in a mixture based on differences in solubility and adsorption to the paper

Example use(s):

• Analysing dyes in black ink

• Testing food colourings or plant pigments

Steps:

1. Draw a pencil line near the bottom of the chromatography paper

   • Pencil is used because it doesn't dissolve in the solvent

2. Place small spots of the sample mixture on the line

3. Suspend the paper in a container with a shallow layer of solvent

   • Ensure the spots stay above the solvent level

4. Let the solvent move up the paper by capillary action

5. As the solvent travels, it carries components of the mixture at different rates

6. Allow the paper to dry and observe the separated spots (the chromatogram)

Paper chromatography equipment diagram

Notes:

• Substances that are more soluble in the solvent travel further

• Substances that are more strongly adsorbed to the paper move more slowly

• The resulting chromatogram shows a pattern of spots representing different components in the mixture

Summary of separation techniques

• Filtration – separates an insoluble solid from a liquid using a difference in solubility

  • Example: separating sand from salt solution

• Crystallisation – separates a dissolved solid based on solubility decreasing with temperature

  • Example: recovering copper(II) sulfate crystals from solution

• Recrystallisation – purifies a solid based on solubility in hot and cold solvent

  • Example: purifying benzoic acid

• Simple distillation – separates a solvent from a solute using a difference in boiling point

  • Example: separating water from salt solution

• Fractional distillation – separates two or more miscible liquids with different boiling points

  • Example: separating ethanol and water

• Chromatography – separates dissolved substances using differences in solubility and adsorption

  • Example: analysing dyes in ink or food colouring

• Magnetic separation – uses differences in magnetic properties

  • Example: removing iron filings from a mixture