Mixtures (AQA GCSE Chemistry)

Types of 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

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

Crystallisation

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 

Simple distillation

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

Fractional distillation

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

Paper chromatography

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