A mixture contains more than one substance. The substances are just mixed together, and not chemically combined. For example: air is a mixture nitrogen, oxygen, and small amounts of other gases
- Filtration for separating a solid from a liquid suspension
- Centrifuge for separating a solid from a liquid suspension
- Evaporation for separating a solid from its solution
- Crystallization for separating a solid from its solution
- Distillation for separating a solvent from a solution
- Fractional distillation for separating two liquids of different boiling points
- Chromatography for separating different substances of different solubilities
This is a way to obtain the solvent from a solution. It could be used to obtain water from salt water, for example. Like this:
- Heat the solution in the flask. As it boils, water vapour rises into the condenser, leaving salt behind.
- The condenser is cold, so the vapour condenses to water in it.
- The water drips into the beaker. It is called distilled water. It is almost pure.
The liquid that is collected after passing through the condenser is called the distillate.
AIM: To separate salt from a salt solution using simple distillation.
MATERIALS: a round bottomed flask, condenser, a thermometer, a beaker, a clampstand, rubber tubing, a Bunsen burner, a salt solution.
- Setup the apparatus shown above.
- Turn on the water tap to cool the condenser.
- Heat the flask until all the liquid evaporates.
- Record the temperature at which steam is collected
- At what temperature is steam collected?
- What is the residue at the bottom of the flask after all the liquid has evaporated?
- What is the advantage of distillation over evaporation?
If miscible liquids are to be separated, then this can be done by fractional distillation. Fractional distillation relies upon the liquids having different boiling points.This is used to separate a mixture of liquids from each other.You could use it to separate a mixture of ethanol and water, for example.
AIM: To separate a mixture of ethanol and water using fractional distillation.
MATERIALS: a round bottomed flask, fractionating column, condenser, a thermometer, a beaker, a clampstand, rubber tubing, a Bunsen burner, ethanol solution.
- Heat the mixture in the flask. At about 78°C, the ethanol begins to boil. Some water evaporates too. So a mixture of ethanol and water vapours rises up the column.
- The vapours condense on the glass beads in the column, making them hot.
- When the beads reach about 78°C, ethanol vapour no longer condenses on them. Only the water vapour does. So water drips back into the flask. The ethanol vapour goes into the condenser.
- There it condenses. Pure liquid ethanol drips into the beaker.
- Eventually, the thermometer reading rises above 78 °C – a sign that all the ethanol has gone. So you can stop heating.
A mixture contains aqueous sodium chloride and insoluble powdered charcoal (carbon).
(i) Suggest how the powdered charcoal can be separated from the mixture. ….
(ii) Suggest how water can be removed from aqueous sodium chloride. …
- By filtration
- By distillation
Fractional distillation in industry
Uses of fractional distillation:
- in the petroleum industry, to refine crude oil into petrol and other groups of compounds. The oil is heated and the vapours rise to different heights, up a tall steel fractionating column.
- in producing ethanol. The ethanol is made by fermentation, using sugar cane or other plant material. It is separated from the fermented mixture by fractional distillation. Ethanol is used as a solvent, and as car fuel.
- to separate the gases in air. The air is cooled until it is liquid, then warmed up. The gases boil off one by one.
In fractional distillation, the liquids collected are called fractions
Chromatograhy is used to separate two or more substances that have different solubilities. This technic is used when you have to seprate mixtures of coloured materials such as inks and dyes. There are several types of chromatography; however, they all follow the same basic principles. The simplest kind is paper chromatography. To separate the different coloured dyes in a sample of black ink, a spot of the ink is put on to a piece of chromatography paper. This paper is then set in a suitable solvent.
As the solvent moves up the paper, the dyes are carried with it and begin to separate. They separate because the substances have different solubilities in the solvent and are absorbed to different degrees by the chromatography paper.The most soluble one travels fastest. As a result, they are separated gradually as the solvent moves up the paper
Uses of chromatography:
- identify substances
- check the purity of substances
- identify pollutants in air, or in samples of river water.
It is used on a large scale to:
- separate pure substances (for example for making medical drugs or food flavourings) from tanks of reaction mixtures, in factories
- separate individual compounds from the groups of compounds (fractions) obtained in refining petroleum.
- How would you obtain pure water from seawater? Draw the apparatus, and explain how the method works.
The method is called distillation. Setup the apparatus as shown in the diagram above. Heat the flask containing the seawater until the water evaporates. The water vapour will pass through the condenser where it is cooled and condensed back to liquid. Collect the pure water in a clean flask.