Separating Mixtures (WJEC GCSE Chemistry)

Separating Mixtures

• 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

Simple Distillation

• 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 apparatus

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, 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

Chromatography

• 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  =