Separating Mixtures (WJEC GCSE Chemistry: Combined Science)

• A mixture is a combination of two or more substances that are not chemically joined together
• These substances can be elements and / or compounds

Examples of mixtures

Mixtures can be just elements, just compounds or elements and compounds, but the substances must not be chemically joined together

• This means that mixtures can be easily separated by physical processes such as:
  • Filtration
  • Evaporation
  • Distillation
  • Chromatography
• The physical process that is used for separation depends on the substances being separated
• All processes rely on a difference of some sort between the substances being separated
  • This is usually in a physical property such as boiling point, solubility, magnetism

 Filtration

• 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

 Evaporation

• 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
• A more common application of this is sometimes called crystallisation 
  • 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 evaporation / crystallisation

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

Distillation

• 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

The process of chromatography

The diagram shows how to complete a chromatography experiment and gives a basic analysis of the results

Chromatographic data analysis

• If two or more substances are the same, they will produce identical chromatograms
• It is common practice to include a known compound as a reference spot
  • This can help match up to an unknown spot or set of spots in order to identify it
• If the substance is a mixture, it will separate on the paper to show all the different components as separate spots
• An impure substance will show up with more than one spot, a pure substance should only show up with one spot

R_f values

• These values are used to identify the components of mixtures
• The R_f value of a particular compound is always the same
• Calculating the R_f value allows chemists to identify unknown substances because it can be compared with the R_f values of known substances under the same conditions

Calculation

• The formula to calculate the retention factor is:

Retention factor, R_f =

• The R_f value is a ratio and therefore has no units

Analysing a chromatogram

The key pieces of information from a chromatogram for calculating the R_f value are the distance travelled by the substance and the distance travelled by the solvent

• The R_f value of the substance in the chromatogram above can be calculated by:

                  R_f  =