Mixtures (AQA GCSE Chemistry: Combined Science)

How does filtration work?

• Filtration is used to separate an insoluble solid from a mixture of the solid and a liquid / solution
  • For example, sand from a mixture of sand and water

Method

• A filter paper is placed in a filter funnel above a beaker
• The mixture of insoluble solid and liquid is poured into the filter funnel
• The filter paper will only allow liquid particles and soluble solids to pass
  • The liquid that passes through into the beaker is called the filtrate
• Insoluble solids do not pass through the filter paper and are left behind as a residue

The filtration process

For a mixture of sand and water, the insoluble sand would be left as the residue and the water would be the filtrate

How does crystallisation work?

• This method is used to separate a dissolved solid from a solution
  • A simple application of this is to heat a solution to boiling, remove the heat and leave the solvent to evaporate
  • This is when the solid is more soluble in hot solvent than in cold, e.g. copper sulphate from a solution of copper(II) sulphate
• 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
  • Solids will come out of the solution as the solubility decreases
  • This will be seen as crystals growing
• The crystals are collected by filtration
• They are then washed with distilled water to remove any impurities
• Finally, they are allowed to dry
  • Common places to dry crystals are between sheets of filter paper or in a drying oven

The process of crystallisation

The solution is slowly heated to remove around half of the liquid. The remaining liquid will evaporate slowly 

How does simple distillation work?

• Distillation is 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
• Simple distillation can be used to separate the products of fermentation, such as alcohol and water
• However, fractional distillation is a more effective separation technique, commonly used when the boiling points of the liquids are close and/or a higher degree of purity is required, such as crude oil

Simple distillation apparatus

Diagram showing the distillation of a mixture of salt and water

How does fractional distillation work?

• This is 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 apparatus

Fractional distillation of a mixture of ethanol and water

How does paper chromatography work?

• This technique is used to separate substances that have different solubilities in a given solvent (e.g. different coloured inks that have been mixed to make black ink)
• A pencil line is drawn on chromatography paper and spots of the sample are placed on it
  • Pencil is used for this as ink would run into the chromatogram along with the samples
• The paper is then lowered into the solvent container
  • The pencil line must sit above the level of the solvent so the samples don´t wash into the solvent container
• The solvent travels up the paper by capillary action, taking some of the coloured substances with it
• Different substances have different solubilities so will travel at different rates
  • This causes the substances to separate
  • Those substances with higher solubility will travel further than the others
• This will show the different components of the ink / dye

Analysis of the composition of ink using paper chromatography