A pure substance can be obtained from a mixture,
if the correct physical separation technique is used.
1. Filtration is the separation of particles of different size by trapping the larger ones against a porous barrier while allowing the smaller particles to pass through.
The larger particles cannot pass through because they are too big to fit through the holes in the barrier. Filtration is frequently used to separate insoluble solids from a liquid. e.g. separating the clay particles of muddy water to obtain clear water.
1. Fold the filter paper into the shape of a cone and place it in a funnel.
2. Pour the suspension of muddy water into the filter paper cone.
3. The water passes through the fine ‘holes’ in the filter paper.
4. The clay particles are too big to pass through so are trapped in the filter paper cone.
5. The material that passes through the filter is called the filtrate.
6. The material that does not pass through the filter is called the residue.
Everyday example: water filtration for use in the home at the Council ‘water works’; using tea bags to make tea.
2. Evaporation is the conversion of a liquid into a gas at a temperature below the boiling point.
It can be used to separate soluble and insoluble solids from a liquid by allowing the liquid to escape as a vapour.
Place the seawater into a beaker. Heat the beaker with the flame of a Bunsen burner. The liquid water turns into a gas and escapes from the beaker. The temperature is too low to melt the salt. The salt stays solid and remains in the beaker.
If the water is vapourised by boiling at 100˚C, the separation is quite fast however it may spit hot salt.
Examples the commercial extraction of salt from seawater; when the water of your sweat evaporates, a fine layer of salt remains on your skin. Desalination in the Middle East gets drinking water from seawater
3. Distillation is the vapourisation of a liquid at one location followed by its condensation at another place.
(a) Simple Distillation can be used to purify a liquid from insoluble and/or soluble solids e.g. obtaining pure water from sandy seawater and it can also be used to separate different miscible liquids of different boiling points.
(i) Using Simple Distillation to Obtain Pure Water from Sandy Seawater.
1. Attach a Liebig condenser to the side arm of the flask containing the sandy seawater.
2. Circulate cold water through the outer tube of the condenser – lead the water in at the lower connection.
3. This will keep the wall of the inner tube cool and so more condensation will take place.
4. Heat the seawater in the flask to boiling point – 100ºC.
5. The salt and sand remain solid at this temperature and stay in the flask.
6. The water vapourises and passes into the cool condenser where it condenses becoming a liquid.
7. The condensed water flows down the condenser into the collecting beaker.
8. The collected water is pure: it is colourless, tasteless and without sand.
9. The liquid collected from the condenser is called the distillate.
10. The solids that remain in the flask are called the residue.
Everyday example of simple distillation: obtaining pure drinking water from seawater.
(ii) Using Simple Distillation to Obtain Pure Alcohol from a Mixture of Alcohol and Water
1. Same set up as in (i) above.
2. But heat the mixture only to 78°C – this is the boiling point of Alcohol.
3. The alcohol vapourises rapidly at its boiling point but water vapourises slowly because the temperature is below its boiling point.
4. A very high proportion of alcohol vapour in comparison to water vapour passes into the condenser and so the distillate will have a high concentration of alcohol.
5. Further simple distillations of the ‘distillate’ will increase the proportion of alcohol.
(b) Using a Fractionating Column to Get Pure Alcohol from a Alcohol and Water fractional distillation.
1. Place the mixture of alcohol and water in a flask.
2. The vapours of the different liquids will condense at different locations.
3. The water vapour condenses in the fractionating column dripping back into the flask. The alcohol vapour passes into the condenser where it will be converted into a liquid and flows into the collecting beaker.
4. Everyday example: separating alcohol from water in the production of whiskey, brandy and potìn.
5. Refining crude oil to get different fuels such as petrol, kerosene and diesel is called fractional distillation.
4. Paper Chromatography is a method of separating a mixture of different soluble solids.
The solids are in solution in a particular solvent. The solvent is allowed to travel along the paper. The different soluble solids move with the solvent but at different speeds. If given enough time the different solids will separate completely being present in different parts of the paper strip.
Put a dot of black ink from a non-permanent marker 2 cm above the bottom of a strip of filter paper. Allow the spot to dry and place another dot on the same spot. Repeat several times on the same spot – now this spot has a high concentration of the mixture of soluble solids..
Then place the strip into a gas jar of the solvent (water) but the ink spot must be above the surface of the water.
Support the paper strip so it will stand upright even when wet.
Remove and dry the strip when the ‘solvent front’ is close to the top. The different coloured substances making up the black ink will be clearly separated. This colour picture of the different coloured solids is a chromatogram.